DECISION TREE FOR GROUND IMPROVEMENT IN TRANSPORTATION APPLICATIONS. A Thesis. Presented to. The Graduate Faculty of The University of Akron

Transcription

1 DECISION TREE FOR GROUND IMPROVEMENT IN TRANSPORTATION APPLICATIONS A Thesis Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Master of Science Aileen Yenco December, 2013

2 DECISION TREE FOR GROUND IMPROVEMENT IN TRANSPORTATION APPLICATIONS Aileen Yenco Thesis Approved: Accepted: Advisor Dr. Robert Y. Liang Dean of the College Dr. George K. Haritos Faculty Reader Dr. Junliang Tao Dean of the Graduate School Dr. George R. Newkome Faculty Reader Dr. Anil Patnaik Date Department Chair Dr. Wieslaw Binienda ii

3 DEDICATION This thesis is dedicated to my wonderful fiancé who has supported me through every step of this process. In addition, this thesis is dedicated to my parents who have supported me my entire life. Thank you. iii

4 TABLE OF CONTENTS Page LIST OF TABLES... vi LIST OF FIGURES... vii CHAPTER I. INTRODUCTION Terminology Statement of Objectives Organization of Thesis...5 II. LITERATURE REVIEW Dynamic Compaction Polyurethane Injection Column Supported Embankment Soil Mixing Controlled Modulus Columns Drains...51 III. DEVELOPMENT OF DECISION TREE Purpose Factors Considered Result...62 iv

5 IV. APPLICATION OF DECISION TREE Overview Example Summary and Conclusion...67 V. SUMMARY AND CONCLUSIONS...69 REFERENCES...71 v

6 LIST OF TABLES Table Page Summary of advantages and disadvantages of dynamic compaction Dynamic Compaction Parameter Summary Summary of advantages and disadvantages of polyurethane injection Polyurethane Injection Parameter Summary Summary of advantages and disadvantages of column-supported embankment Column-Supported Embankment Parameter Summary Summary of advantages and disadvantages of soil mixing Soil Mixing Parameter Summary Summary of advantages and disadvantages of controlled modulus columns Controlled Modulus Columns Parameter Summary Summary of advantages and disadvantages of drains Drains Parameter Summary Summary of ground improvement allowable locations Ground improvement maximum treatment depths Ground improvement minimum treatment depths...61 vi

7 LIST OF FIGURES Figure Page Dynamic compaction dropping weight mechanism Dynamic compaction typical grid layout Polyurethane injection installation Polyurethane injection soil stabilization Column-supported embankment setup Deep soil mixing Shallow soil mixing Controlled modulus column installation Installation of PVDs with mandrel Movement of water in PVDs Deformation of PVD after consolidation Decision Tree Example embankment cross-section...66 vii

8 CHAPTER I INTRODUCTION Problematic soils can damage structures residing on them, such as roadway pavement, if left untreated. The damage occurs due to a multitude of reasons: low bearing capacity, low shear strength, collapse of voids, uplift or consolidation of the soil. Consequently, the damage that occurs should be fixed. This is done one of two ways. One way is to treat the soil in-situ to maintain the existing structure. The second way involves removing the damaged element, applying soil treatment to strengthen the soil or removing and replacing the problematic soil, and then rebuilding the structure entirely. Ideally, these problematic soils are identified and treated prior to construction so that damage of the structure can be avoided altogether. Soils that can cause damaging problems include organic materials, soft cohesive soils, loose granular soils, expansive clays, collapsible soils, and uncontrolled fill. Many ground improvement treatments exist to strengthen these soils so that they can adequately bear the loads applied to them without undergoing failure or producing substantial postconstruction settlement. By applying the appropriate ground improvement method prior to construction, structural repair costs can be avoided and thus money will be saved. Problematic roadway subsurface soils, including difficult organic and expansive soils, can be treated successfully with a variety of ground improvement techniques. 1

9 However, not every ground improvement method works in every situation. Each technology has its limitations in the types of soil it can treat, the depth at which it can be applied, the environment it can be used, the strength it provides, the time it takes and the cost it requires. There therefore exists a need for a process which can narrow down only the compatible methods for a particular project. A decision tree has been developed to satisfy this need so that incompatible ground improvement methods can be ruled out and only appropriate treatments are considered for the situation at hand. 1.1 Terminology It is important to understand terms used often in the geotechnical and ground improvement literature while reading this paper. This information helps paint a clearer picture about the ground improvement methods covered in the subsequent chapters. Some terms have specific meaning within the realm of this paper. Key terms have been identified and briefly explained below. Consolidation volume change or settlement due to the dissipation of pore water pressure in clay (Das, 2006). Organic material soil with high organic content consisting of decayed vegetation; typically has high water content; continued degradation of this soil over time causes substantial settlement. Expansive soil soil that exudes swelling pressure that may cause an overlying structure to lift, bend, or tilt (Buzzi et al., 2010). 2

10 Collapsible soil a loose but stiff soil when dry that loses strength when wet, which subsequently results in substantial settlement (Rollins & Kim, 2010). Uncontrolled fill man-placed soil consisting of varying materials; typically not compacted. Soft cohesive soil low-strength clay that undergoes consolidation when subjected to loads. Loose granular soil cohesionless material; gravel, sand or silt in an uncompacted state. Urban for the intent of this examination, this term indicates locations where sensitive structures exist nearby that could be damaged from soil movement. Also, it includes locations that consist of areas where noise pollution is unfavorable due to the proximity to populated areas. Rural - for the intent of this examination, this term indicates locations which do not have sensitive structures nearby and where noise pollution is not an issue. Depth of influence the range of depths under the ground surface where treatment can be applied effectively. 1.2 Statement of Objectives This paper has three tasks: performing a literature review, developing a decision tree based on the review, and testing the decision tree. Structurally, this paper is then divided into three respective sections. From this, each section seeks to accomplish a specific goal. 3

11 The literature review provides an overview for six types of ground improvement: dynamic compaction, polyurethane injection, soil mixing, column-supported embankment, controlled modulus columns, and drains. These technologies were specifically chosen for their varied attributes, as well as their high rate of success. Each form of technology has been analyzed in terms of advantages, disadvantages, applicable soil types, and technology-specific conditions. Below is a list of typical sections covered under the literature review. Overview introduction of the technology including specifics on installation and mechanisms that cause improvement of the soil. Applicable Soil Types typical types of soil that can be treated with the ground improvement method. Depth/Height Limitations the range of depth at which the method can treat the soil most effectively; the maximum depth that can be treated with the technology. Ground Water Conditions some technologies work under soils with certain water content; this section discusses whether the method works under saturated and/or non-saturated conditions. Theoretical Work some technologies have extensive theoretical research, which are mentioned under this section. Environmental Aspects discusses whether the technology is compatible with contaminated soils; alternatively, it may discuss whether the material used in the technology is environmentally-friendly or not. 4

12 QC/QA Assessment Methodologies stands for Quality Control/Quality Assurance; covers methods that measure the quality of results and procedures which ensure the finished ground improvement meets project requirements. General cost provides general fees/rates associated with the technology Advantages lists the benefits of the ground improvement method Disadvantages lists the known problems associated with the ground improvement method Summary compiles the disadvantages and advantages of the technology as well as supplies a table summarizing its parameters. Secondly, a decision tree based on the material from the literature review has been created. The decision tree provides a process for selecting a specific ground improvement technology. The process focuses on narrowing down technologies based on specific restraining factors such as location and depth of soil treatment. Finally, an example is given in order to test the decision tree process. This theoretical project is then examined using the decision tree, comparing various forms of ground improvement technology. This process, therefore, acts as a framework for how the decision tree may be used practically. 1.3 Organization of Thesis This paper is divided into chapters as follows: Chapter 1 Introduction Chapter 2 Literature Review 5

13 Chapter 3 Development of Decision Tree Chapter 4 Application of Decision Tree Chapter 5 Summary and Conclusions 6

14 CHAPTER II LITERATURE REVIEW 2.1 Dynamic Compaction During dynamic compaction, a large weight is dropped, or tamped, multiple times by a crane onto the ground, which compresses the soil. This hammering effect creates a dense mass of soil immediately below the area of impact and also sends high-energy compression waves downwards and outwards (Pan & Selby, 2002). This action increases the soil bearing capacity and reduces post-construction settlement. This process, also known as dynamic consolidation, can be used with several types of problematic soils in order to satisfy project requirements. In accordance with compliance criterion, numerous parameters must be determined in order to use dynamic consolidation for a project. Some of these parameters include existing soil characteristics, weight size, drop height, number of optimal blows, grid spacing, applied energy and depth of improvement. Calibration testing is required prior to performing the dynamic consolidation technique on an entire project. The testing reveals the optimum number of blows per compaction point, the amount of compaction energy and number of passes required, spacing to resist heave interference, and whether or not the tamping induces compaction instead of volume displacement (Ali et al., 1997). With this information, dynamic compaction can be applied more efficiently and 7

15 successfully to projects in the field. In Figure 2.1.1, the dropping weight mechanism during dynamic compaction is shown. This tamping motion is repeated for a set number of blows on one spot before moving to the next point. Typically points are set up in rows and delivered in passes along the treatment site. Depth of Influence : 3 to 39 Figure Dynamic compaction dropping weight mechanism Figure shows a typical layout for dynamic compaction points. The first round, or pass, is done with enough space between the rows to do the next pass. The second pass then covers this space. More passes can be added depending on project requirements to fill in leftover areas. During ground improvement treatment using dynamic consolidation, a pass or two of high energy tamping is performed followed by a couple of passes of lower energy tamping. The high energy tamping usually consists of a heavier weight dropped at taller 8

16 heights while lower energy tamping is completed with a lighter weight at shorter heights. The higher energy passes compact deeper layers of soil while the lower energy passes compact shallower depths. It has been found that a combination of ground improvements in addition to dynamic consolidation can help meet project requirements. Some common techniques combined with dynamic compaction include dynamic replacement, surcharging and wick drains. Both surcharging and wick drains help induce settlement. First pass Second pass Figure Dynamic compaction typical grid layout 9

17 2.1.1 Applicable Soil Types Several types of soil conditions may benefit from the dynamic consolidation ground improvement. In particular, landfills have been known to benefit from this technique, as the subgrade may have large voids from the variable fill which can be condensed with tamping. As available land for new roadways dwindles, treating old landfills to bear these loads utilizes space that would otherwise be avoided. Granular, cohesionless soils are typically considered to be the most favorable type of soils for dynamic consolidation. This is because these soils have high permeability, which allows the pore water that rises during tamping to dissipate almost instantaneously. The dissipation of the pore water pressure allows settlement to occur. In impervious soils such as clay, the excess pore water pressure takes more time to dissipate, and the Federal Highway Administration (FHWA) guidelines indicate that dynamic consolidation improvement is generally minor (Lukas, 1995). The FHWA also mentions that semi-pervious soils such as silt or silty sands can be improved using dynamic compaction, but that vertical wick drains may need to be installed to facilitate pore water pressure dissipation (Lukas, 1995). Not only is this an extra step to consider but it is also extra money spent on improvement for these soils. The dynamic compaction technique then may not be the most efficient or economical choice for silty or clay materials. A study of 15 case histories in the U.S. found that dynamic compaction successfully improved collapsible soils; however, lenses of cohesive soil in the ground reduced the effectiveness of dynamic compaction on mostly granular soils (Rollins & Kim, 2010). In the FHWA guidelines, it is mentioned that layers of either hard or soft soil 10

18 may impede compaction efficiency in layers deeper in the soil profile (Lukas, 1995). It is noted that the degree to which these layers absorb the compaction energy is dependent on the thickness of that layer. These findings suggest that combination profiles consisting of layers of both cohesive and granular materials may not benefit from dynamic compaction as much as desired. Cohesive soils are not typically considered for dynamic compaction, and several articles and general literature state that these soils should be treated with dynamic replacement or other techniques instead. Although dynamic replacement is typically recommended for cohesive soils, Menard and Broise (1975) suggested that dynamic consolidation can improve clay strength properties as long as liquefaction is not reached. Once the clay becomes liquefied, remolding occurs, and the soil loses structural strength which cannot be regained with further compaction. However, it was discovered that clay that has not yet reached liquefaction can benefit from dynamic consolidation, as the action compresses micro-bubbles of gas in the soil and induces settlement. Menard and Broise (1975) also notes a fissuring or splitting phenomenon in saturated, cohesive natural soils which increases the permeability during dynamic consolidation. This increase in permeability dissipates pore-water pressure rapidly, which is a favorable condition in dynamic compaction ground improvement. It is likely difficult to determine how much tamping would cause liquefaction of the soil in the field; testing would be required to determine whether the cohesive material would benefit from the treatment or not. This extra step along with the uncertainty suggests that other types of techniques outside of dynamic compaction should be considered for the treatment of cohesive soils. 11

19 2.1.2 Depth/Height Limitations Dynamic compaction affects mostly deeper layers of soil. The first m ( ft) of soil does not benefit from tamping and must be ironed or roller-compacted (Ali et al., 1997). The depth of influence for dynamic compaction is commonly in the range of 3 to 9 m ( ft) (Lu et al., 2009); although a depth exceeding 10 to 12 m ( ft) can be achieved using high energy dynamic compaction which requires specialized equipment to reach a high degree of efficiency (Menard, n.d.). This range of treatment depth is not as large as other types of available soil improvement methods which will be covered in other sections Groundwater Conditions Literature shows that dynamic consolidation can work in both saturated and nonsaturated soils. The former will usually require a dewatering program such as a trench for drainage or a pump to remove water that draws up to the surface during tamping. The latter is the preferred situation as the build-up of pore-water pressure in saturated soils may require time to dissipate before proceeding with treatment. Once the pore-water pressure has dissipated, the soil reorganizes into a denser state. This thereby increases the strength of the material QA/QC Assessment Methodologies QA/QC assessment usually consists of performing in-situ testing before and after ground treatment. For dynamic consolidation, the most commonly used tests include cone 12

20 penetration tests (CPTs), pressuremeter tests (PMTs), and standard penetration tests (SPTs). The testing performed after treatment is then compared to determine if the soil parameters are sufficiently compliant for the project. One important parameter to check is the depth of influence. Ground Penetrating Radar (GPR) and sand cone testing was used to estimate the depth of influence on the Chongqing, China project site (Lu et al., 2009) General Cost The cost of implementing dynamic compaction varies depending on such factors as the project size, depth of treatment, requirements, location and year of construction. Mobilization estimates for dynamic compaction from the Strategic Highway Research Program 2 (SHRP 2) range from $20,000 to $40,000. SHRP 2 also lists the treatment cost ranging from $10.00 to $25.00 per square yard for treatment areas greater than 50,000 square yards. And finally, the SHRP 2 mentions that backfill material to fill in the craters formed during compaction may be an additional expense. The cost of this material is listed from $3.00 to $10.00 per cubic yard (SHRP 2, 2012d) Advantages One advantage of dynamic consolidation is that it is able to treat weak soil at considerable depths beneath the surface without the contractor having to perform costly excavation and replacement. On one project in Chongqing, China, an alternative to dynamic compaction was excavating and replacing 2 to 8 m of randomly-placed mountainous fill throughout the entire site. Another alternative was to use deep 13

21 foundations, but it was considered more economical to use dynamic compaction and shallow foundations (Lu et al., 2009). This shows that dynamic compaction can be a costefficient ground improvement technique, especially at this depth of treatment. Menard and Broise (1975) discussed the idea that dynamic compaction also reduces secondary consolidation as well as primary consolidation. They attributed this reduction in secondary settlement to the dissipation of internal stresses in the soil from the dynamic consolidation tamping. Menard and Broise (1975) also noted that the reduction in secondary settlement appeared to be in proportion to the reduction in primary settlement. It should be noted that secondary settlement is more important in organic and highly compressible cohesive soils than in other types of soils (Das, 2006). This information is important for projects that are dealing with clay or organic soils, which are not typically associated with dynamic compaction. Another advantage of dynamic compaction is that it has a design guideline. The FHWA has a guideline manual for using dynamic compaction in the US (Publication FHWA-SA ). Not every type of soil improvement method has a set of design guidelines to follow, but the existence of such makes design easier for the engineer and more reliable in the field. Dynamic compaction has also been successful at treating many types of soils. Landfills, granular soils, collapsible soils can all be treated with this method. There is some potential for soil improvement in cohesive soils using dynamic compaction, but testing is required to verify this possibility. Dynamic compaction is also versatile in that it can be used in both saturated and non-saturated soil conditions. In saturated conditions, however, wick drains are 14

22 recommended to help dissipate pore water pressure when used in soils that do not dissipate the pore water pressure quickly. In non-saturated conditions, there is no pore water pressure to dissipate and wick drains are not required Disadvantages The impact of the weight in dynamic compaction sends waves of vibration through the ground. These vibrations could cause disturbance and possible damage of surrounding structures such as underground utilities and foundations, making this technique unfavorable in urban environments. Consequently, this limits the amount of projects dynamic compaction can be used on considerably. The review on dynamic compaction did not find information on treatment in organic or expansive soils. This suggests other techniques are more favorable for use with these soil conditions. Dynamic compaction should not be considered for projects with organic or expansive soils. Another disadvantage experienced when using dynamic compaction has occurred with combination soil profiles. Lenses of cohesive soils may impede the effectiveness of dynamic compaction on deeper layers of soil. For this reason, caution should be used when considering this method on combination profiles including cohesive soils. And finally, the mobilization cost of the dynamic compaction equipment to the project site is high (SHRP 2, 2012k). This means that the method is not economical for small projects but becomes more economical as the size of the project increases. The high cost of mobilization to the site limits dynamic compaction to large projects for costeffectiveness. 15

23 2.1.8 Summary A list of the advantages and disadvantages of dynamic compaction for use in roadway projects is summarized in Table below. The main disadvantage of this technology is its limitation to areas without sensitive structures nearby. This means that it can only be used in areas of completely new construction. Table Summary of advantages and disadvantages of dynamic compaction Advantages o Costs less than traditional methods o May reduce secondary settlement in cohesive materials o Works in both saturated and nonsaturated conditions o Successful in landfills, granular soils, collapsible and potentially in cohesive materials Dynamic Compaction Disadvantages o Could damage nearby structures o Limited to rural locations o Not possible to use in organic/expansive soils o Reduced efficiency in combination profiles (cohesive above granular) o High mobilization cost Table summarizes the attributes of dynamic compaction. Overall, this technology works with most soil types, although caution should be taken if considering its use with cohesive materials; dynamic replacement may be a better option in that case. Dynamic compaction, unlike many ground improvement techniques, has a standard guidance document. Compared to other reviewed technologies, the range of treatment depth is more limited. Because dynamic compaction induces settlement through impact vibrations, it can only be used in areas away from other structures. Despite this, the effectiveness of dynamic compaction is what makes it relevant in ground improvement today. 16

24 Table Dynamic Compaction Parameter Summary Type of Soil Uncontrolled Fill Cohesive Granular Organic Expansive Collapsible Contaminated Dynamic Compaction Depth of Influence Existing FHWA Design Manual? Location Settlement >50 Urban Rural Inducing Reducing High Mobilization Cost? Yes No 17

25 2.2 Polyurethane Injection Polyurethane injection is a form of ground improvement which is typically used to reduce differential settlement produced by existing structures such as buildings and pavement. Tubes are inserted into areas of weak soil and polyurethane resin is pumped into the ground. The resin flows into the soil and expands to a minimum of 5 times its original volume, compacting the surrounding soil while filling in voids. The compacting action of the polyurethane occurs due to the resistance of the overburden soil pressure (Brown, 2010).This compacting action consequently strengthens the soil. The polyurethane resin starts as a liquid when initially permeating the soil, solidifying as the reaction between two components occurs. More specifically, this liquid injection contains a liquid isocyanate compound and a liquid polyol compound, which react together and form a solid polymer with high strength (Chun & Ryu, 2000). After injection into the ground, the resulting soil-mass has improved strength and stability. Polyurethane injections are also used to level out structures. The first vertical movement of the structure after injection begins indicates that soil stability is achieved; however, further movement may be necessary for bringing the structure to grade. The injection continues until the desired height is reached (Brown, n.d.). 18

26 Figure Polyurethane injection installation (Stable Soils of Florida, 2011) Typically, when polyurethane is injected into the ground, it is injected near the ground surface first to stabilize the upper soils as well as create a platform. Then, voids deeper in the profile are filled while the polyurethane pushes against the soil, lifting the platform and thus the settled structure vertically. Figure shows the upper soil stabilization and subsequent lifting of the pavement structure experiencing settlement issues Applicable Soil Types Polyurethane injection can be used on many types of soils and conditions. Both cohesionless and cohesive soils can benefit from this ground improvement. For organic materials such as peat, the results are inconclusive but encouraging. 19

27 Figure Polyurethane injection soil stabilization (Uretek Holdings Inc, 2013) In the case of granular soils, such as aggregate base and coarse sand, the liquid polymer displaces the water and binds to the soil as well as compacts it during expansion (Brown, 2010). For saturated fine sands, flowable soil as well as water is displaced and the leftover soil is encapsulated by the polyurethane (Brown, 2010). Although it was found that the upper sand layer increased in stiffness in some areas of a study (Applied Research Associates [ARA], 2009) using polyurethane grout, there were also areas that experienced softening. The source of this problem was not clear, but the author indicated equipment capability and limitations of the study parameters as probable causes (ARA, 20

28 2009). Overall, these problems are not typically anticipated to occur and strengthening of the granular soils can increase with polyurethane injection. For silts and clays, the polyurethane fills in weak lenses in the layers and compacts the soil (Brown, 2010). By filling in the voids in the soil, the polyurethane increases the bearing capacity while also reducing the potential for settlement. The pressure of the expanding resin against the soil compacts it, which improves the overall strength of the soil as well. Treatment of organic soils with polyurethane injection results is not definitive. For organic soils such as peat, the reaction time of the polyurethane is accelerated so that vertical shear walls form, which can develop a honey-comb structure within the soil (Brown, n.d.). This honey-comb polyurethane structure has the potential to support loads, though conclusive evidence of the success of this method is still needed. Polyurethane walls formed in peat during a study by Popik, Trout and Brown (2010); they observed that the polymer did not mix with the peat but rather fractured it. It was suggested that the resulting honey-comb structure could possibly transfer pavement loads through the peat layer. It should be noted, however, that the cone penetration data in the study showed little to no increase in the stiffness of the peat layer after polyurethane injection (ARA, 2009). A different approach may be required to utilize the potential strength of the honey-comb structure. Popik et al. (2010) suggested grouting above and below the peat with polyurethane first before treating the peat to avoid punching shear failure and settlement between the walls. While this suggestion seems helpful, researchers have yet to test this 21

29 approach. Further research of polyurethane grouting to remediate peat layers is needed in order to prove the success, if any, of this method in organic soils. Polyurethane injection has also been tested in the treatment of expansive soils. Some concern exists, though, that overlifting of the structure after polyurethane treatment could occur due to the expansive nature of the soils. The results from a study by Buzzi, et al. (2010) suggested that this phenomenon is limited and that the swelling pressure can be alleviated by injecting below the cracked soil zone. The option to use polyurethane injection in expansive soils should be considered with caution Depth/Height Limitations Polyurethane has a limited range of depth in which treatment can be applied. The maximum treatment depth is 30 feet, according to the Strategic Highway Research Program 2 (2012m). Projects which have problematic soils deep than 30 feet, then, cannot be treated with polyurethane injection QC/QA Assessment Methodologies The components of the polymer reaction are considered to be mildly toxic. Care should be taken to keep the materials in separate concealed and labeled drums attached to the pump system (Ardnt et al., 2008). The grout is highly flammable before and after curing, although this is unlikely to be an issue once installed in the ground (Ardnt et al., 2008). With proper procedures in place during installation, these issues should not pose a problem. 22

30 2.2.4 Environmental Aspects Polyurethane in its uncured form has the potential to be mildly toxic, and the solvents used to control the viscosity could possibly pollute the groundwater (Ardnt et al., 2008). The isocyanate portion can cause irritation to skin, eyes and mucous membranes while the polyol may cause skin irritation (Ardnt et al., 2008). Once the reaction between the components occurs, however, the resultant material is inert and environmentally benign. Other environmental aspects that could cause problems for the polyurethane include ultraviolet lighting and microbial attacks. Ultraviolet lighting may cause degradation of the material; however, this should not be a problem for ground improvement applications (Ardnt et al., 2008). As for potentially degrading microbial attacks, it has been found to occur with polyester-based materials (Ardnt et al., 2008). For this reason, it is suggested that polyester-based polyurethane material be avoided in ground improvement Ground Water Conditions Polyurethane can be used in saturated soil conditions, but only when selected with water-soluble diluents (Brown, n.d.). The inclusion of water in the polymer reaction can cause a higher volume expansion, which in turns causes a decrease in compressive strength of the material (Chun & Ryu, 2000). Experiencing a reduction of strength defeats the purpose of adding the material to improve the soil. This decrease in strength, however, can be avoided with knowledgeable selection of the polyurethane by the engineer; the components used in the injection can be formulated to resist the intrusion of 23

31 the water from the soil (Brown, 2010). This suggests that all polyurethane injection soil improvement applications use a water-resistant formula General Cost The cost of implementing polyurethane injection varies depending on such factors as the project size, depth of treatment, requirements, location and year of construction. Mobilization estimates for injected foam fill from the SHRP 2 range from $10,000 to $20,000. SHRP 2 also lists the treatment cost ranging from $3.50 to $8.00 per pound for material amounts greater than 1,000 pounds (2012f) Advantages An advantage of polyurethane injection is that it can be used to spot-treat problem areas. Without having to treat an entire project site, satisfactory improvement and soil rehabilitation is achieved efficiently. Polyurethane resin also helps with differential settlement in particular, since that issue is caused by weaker soils consolidating more than other areas of the site. By injecting polyurethane in only the areas of weak soil, the structure is lifted; thereby physically decreasing the amount it has settled while simultaneously strengthening the soil to prevent future settlement. Another advantage of polyurethane treatment is that it can be applied in-situ postconstruction. Many ground improvement techniques must be planned and implemented prior to construction of the structure. If these ground improvements are not installed and a soil issue arises, such as differential settlement, then polyurethane can be used to strengthen the soil so that it can bear the existing load and prevent further settlement. 24

32 This is an advantage over other types of soil improvement techniques which require planned execution prior to construction. The polyurethane injection technique also has advantages over cement-based grouts for transportation applications. Some of these advantages include strengthening the soil while adding less weight, resisting degradation from traffic loads, better infiltration due to lower viscosity, having tensile strength, and faster cure rates (Brown, n.d.). These advantages should be taken into consideration when deciding between a polyurethane injection and a cement-based grout ground improvement method. Polyurethane injection also has a variety of applications in tunneling, roadway pavements, residential houses, buildings and airline runways. In a case study discussed by Brown (n.d.), a runway for aircraft was treated with polyurethane to stabilize the soil. The treatment was monitored over 11 years until the runway was replaced. When the polyurethane was extracted, it was found that it was still intact and performing as designed. This case has shown that the resultant structure of the polyurethane injection can be durable for many years Disadvantages There is limited documented research on the use of polyurethane injections for soil improvement. Some suggestions of possible applications of this method, such as use with peat, are not currently backed up with success in the field or lab. Decisions to use polyurethane in such applications may, therefore, carry more risk than other types of well-documented ground improvement techniques. 25

33 Furthermore, the cost of polyurethane injection is higher compared to alternative materials (Abu al-eis & LaBarca, 2007). Because of this, polyurethane becomes less economical as project size increases. Polyurethane is then limited to small projects only for cost-effectiveness. In addition to the high cost from the material, polyurethane injection also has associated mobilization costs (SHRP 2, 2012f). Typically when high mobilization costs are associated with a treatment method, it is suggested that the method be used for large projects only. Since the material cost of the polyurethane is also high, this is not recommended. Polyurethane injection is a costly operation, so the advantages must be weighed to determine whether or not the cost makes up for the benefits gained from the method Summary A list of the advantages and disadvantages of polyurethane injection for use in roadway projects is summarized in Table below. The main disadvantage of this technology is its material cost, which makes it less economical in large projects. A big advantage of the polyurethane injection is its very fast cure time: treatment occurs in minutes. 26

34 Table Summary of advantages and disadvantages of polyurethane injection Advantages o Spot treatment o Lifts settled pavement/structures to grade o Applied in-situ post-construction o Strengthens soil without adding much weight o Resists traffic load degradation o Infiltrates the soil easily o Has tensile strength o Cures quickly o Used successfully in roadway and other traffic applications o Durable o Can be used with wide range of soils Polyurethane Injection Disadvantages o Mildly toxic in uncured form o Not as much documented use; risk involved o Susceptible to UV light and microbial degradation o High material cost Table summarizes the attributes of polyurethane injection ground improvement. Overall, this technology works with most soil types; potentially including organic materials, although more research is required to confirm this. The range of treatment depth is more limited than in other reviewed technologies. Polyurethane injection is mostly used for small roadway pavement projects. 27

35 Table Polyurethane Injection Parameter Summary Polyurethane Injection Type of Soil Uncontrolled Fill Cohesive Granular Organic Expansive Collapsible Contaminated Depth of Influence Existing FHWA Design Manual? Location Settlement >50 Urban Rural Inducing Reducing High Mobilization Cost? Yes No 28

36 2.3 Column Supported Embankment Column-supported embankment (CSE) is a type of ground improvement which is designed to transfer an embankment load through underlying columns to a stiffer soil layer (i.e. foundation) below. Because of the mechanism of soil arching, which transfers the load from the soil to the columns, embankments can be supported while residing over soft soils. Often, geosynthetic reinforcement is placed above the columns and below the embankment fill to create a load transfer platform (LTP), which facilitates load transfer to the columns (Collin et al., 2005). Figure shows a typical setup for the CSE. An embankment is placed on top of the columns or piles with an LTP between the bottom of the embankment and above the columns. The LTP is typically made up of layers of sand and geosynthetic material which helps to distribute the load of the embankment evenly to the columns. The columns bear in a stiff layer of soil instead of bedrock. Deep foundations, which are a common traditional method, may be anchored to transfer loads directly to the bedrock. This means that more column/pile material may be needed in order to reach the bedrock, as opposed to the amount of material used to reach a stiff soil layer above the bedrock. The deep foundations therefore typically cost more than installing the shorter length columns or piles. CSEs are versatile in the material that can be used to support them. The CSE can be made with driven piles, stone columns, jet-grouted columns, soil-mixed columns, vibro-concrete columns, composite columns or other types of columns (Zheng et al., 2009; Filz et al., 2012). The type of column used in the CSE depends on the project strength requirements and material costs. Caution is taken when selecting the type of 29

37 column to use, as some columns exhibit much less strength than others. For example, while stone columns may cost less than other types of columns, stone columns have low bearing capacity and shear strength and no tensile strength (Zheng et al. 2009). Columns that can drain the soil should be avoided if the embankment is meant to reduce settlement instead of induce it. Column-Supported Embankment Load Transfer Platform Weak Soil Stiff Bearing Layer Columns Figure Column-supported embankment setup It should also be noted that granular columns dissipate pore water pressure, in which case the intention is rapid consolidation of the soft soil. This in turn will create large settlements on the embankment (Zheng et al. 2009).This differs from the purpose of using columns to transfer loads away from the soft soil, which reduces settlement on the 30

38 embankment. Reducing settlement is important in areas where sensitive structures are nearby and could be damaged from the movement of soil QC/QA Assessment Methodologies Confirmation of the placement and properties of the LTP, including the geosynthetic reinforcement, and the embankment fill should be a part of the QC/QA for a CSE. For large projects, a test CSE should be considered. Settlement monitoring instrumentation should also be included to determine if the CSE performs as designed (SHRP 2, 2012c). Although these methods are not standardized, they are highly recommended since the design of the CSE is complex Applicable Soil Types CSEs have shown to work in a wide range of soil types. A study near West Point, Virginia concluded that a CSE built with prestressed concrete piles sufficiently supported a roadway over marshland. The soil profile included silty sand, peat, organic clay, and marine clay layers (Hoppe & Hite, 2006) Depth/Height Limitations The CSE depth limitations correspond to column material and construction limitations. For example, soil mixed columns, reviewed in 2.4 Soil Mixing, can reach depths of up to 150 ft, while controlled modulus columns (discussed in the next section) can treat depths up to approximately 82 ft (Pearlman & Porbaha 2006). The SHRP 2 31

39 indicates that CSEs are most cost-effective when the soil to be treated ranges from 15 to 70 feet (2012j). Overall, the selection of column or pile type in a CSE will determine the range of depth limitation Theoretical Work A numerical analysis performed by Huang and Li (2009) examined a CSE with different column lengths. The study focused on alternating short and long column lengths in order to test bearing capacity. The findings suggested that the long piles resisted settlements more while the shorter piles had more influence over bearing capacity. Another paper studied cyclic loading on a CSE with a numerical analysis. It was found that the geosynthetic reinforced CSE underwent 25% of the deformation experienced by the unreinforced CSE under cyclic loading (Han & Bhandari, 2009). This study suggests that embankments that undergo cyclic loading, such as highway embankments, might experience less deformation when geo-reinforcement is used General Cost The cost of implementing a CSE varies depending on such factors as the project size, depth of treatment, requirements, location, material of the columns used, and year of construction. As the material of the columns can vary, the cost of the column material for a CSE should be determined based on the material expected to be used on the project. The cost given by SHRP 2 lists the geosynthetic reinforcement for the LTP from $2.50 to $12.00 per square yard and for the working platform as $1.00 to $3.50, both for material 32

40 amounts greater than 5,000 square yards. SHRP 2 also lists the granular fill material ranging from $7.00 to $20.00 per ton for quantities greater than 2,500 tons (2012b) Advantages In the instance of preloading the ground with a heavy load to consolidate the soil, it could take months for the majority of the settlement to occur, using valuable time in a construction schedule. One of the advantages of using a CSE is the faster construction time (Chen et al., 2009; Huang & Li, 2009; Filz et al., 2012). Once the CSE system is in place, loads are transferred through the columns to the foundation layer. Therefore, the consolidation of the soft soil does not need to occur prior to construction. Because consolidation of the soft soil is avoided, the amount of total and differential settlement is reduced. CSEs cause less deformation compared to unsupported embankments on the soil. This is preferable for serviceability reasons as well as to protect adjacent existing structures (Filz et al. 2012). For this reason, CSEs can be used in both rural and urban environments. Another advantage of CSEs is that they can be used in a wide range of soil types. These soil types included organic materials, cohesive and granular soils. Combination profiles have been treated successfully with CSE Disadvantages Although the CSE has plenty of advantages, it also has its disadvantages. These include the high initial cost and uncertainties pertaining to its design (Filz et al. 2012). It has been noted that the embankment height versus the column spacing has significant 33

41 influence over the success of the CSE in the field. One study suggests that an embankment height times the length of spacing between columns is necessary in order to develop soil arching and decrease potential settlement (Chen et al. 2009). Without this height, adequate transfer of the load to the columns will not occur and failure of the embankment may result in the way of excess settlement. Embankment height is not the only parameter to consider when avoiding failure of CSE. A highway embankment using CSE on marshland in South Carolina experienced large differential settlements so significant that it was closed off immediately after construction. Although the embankment followed design guidelines, it was later found that the geosynthetic reinforcement was under-designed and caused this failure to occur (Camp & Siegel, 2006). This shows that there exists a level of risk when incorporating a CSE into a project from lack of understanding. CSEs continue to be researched, and numerous studies have suggested guidelines and improvements for designing this complex system (Collin et al., 2005; Chen et al., 2009; Huang & Li, 2009; Zheng et al., 2009; Filz et al., 2012). Although there are suggested guidelines, no FHWA design manual currently exists for CSEs. These cases show that there are several key parameters to take into account when designing a CSE. Firstly, the height of the embankment must be higher than the spacing of the columns. Secondly, the geosynthetic reinforcement used in the LTP must be designed to properly transfer soil loads to the columns evenly. 34

42 2.3.8 Summary A list of the advantages and disadvantages of CSE for use in roadway projects is summarized in Table The main disadvantage of this technology is its complexity, which makes it difficult to design and heightens the risk of failure. The most notable advantage of the CSE technique is it reduces total and differential settlement. This means it can be used in urban areas without causing damage to nearby structures. Table Summary of advantages and disadvantages of column-supported embankment Advantages o Fast construction o Reduced total and differential settlement o Works on wide range of soils Column-Supported Embankment Disadvantages o High initial cost o Design uncertainties o Risk of failure from not understanding complexity Table summarizes the attributes of CSE ground improvement. Overall, this technology works with most soil types, reducing settlement during and after construction. However, the initial high costs as well as the lack of understanding the complexities of the technology can make it challenging to use in the field. 35

43 Table Column-Supported Embankment Parameter Summary Column- Supported Embankment Type of Soil Uncontrolled Fill Cohesive Granular Organic Expansive Collapsible Contaminated Depth of Influence Existing FHWA Design Manual? Location Settlement >50 * Urban Rural Inducing Reducing High Mobilization Cost? Yes No *Up to 70

44 2.4 Soil Mixing Soil mixing is a form of ground improvement which consists of mixing the soil with a binder, such as lime or Portland cement, to improve its strength and stability. There are two depth-related types of soil mixing consisting of Shallow Soil Mixing (SSM) and Deep Soil Mixing (DSM). These two methods differ in several ways but both consist of mixing binder into the soil to increase its strength. The process for SSM consists of a crane mounted with a large auger of three to twelve feet in diameter which drills into the soil. SSM is typically used for loose and soft soils while DSM is typically used for harder and deeper soils (Jasperse, n.d.). For DSM, auger diameters range from twenty-four to thirty-six inches. The binder is injected through the tip of the augers as soil drilling occurs (Jasperse, n.d.). This process simultaneously breaks the soil apart and mixes it with the binder to form a column of strengthened soil material. Once the auger or augers are removed, the drilling procedure is repeated over the project site in an overlapping pattern (Jasperse, n.d.). Figures and show the deep soil mixing and shallow soil mixing technique, respectively. Noise and vibrations caused during construction can be detrimental in urban areas. Vibrations in particular can cause damage to surrounding structures. Soil mixing is favorable in urban conditions since the amounts of noise and vibrations induced during construction are low (Porbaha & Kim, 2003). Sensitive structures, then, will not experience damage from the incorporation of this method nearby. In addition, soil-mixed barriers have been used to help reduce high-speed train vibrations in Sweden (Porbaha & Kim, 2003). 37

45 Figure Deep soil mixing (Zetas Zemin Teknolojisi A.S., n.d.) Figure Shallow soil mixing (Omran Ista Co., 2013) Applicable Soil Types Commonly, soil mixing is used to treat soft soils which can benefit from pozzolanic hardening, such as clays. Soil mixing has been used on several types of soils successfully, although cobbles and boulders should be avoided (SHRP 2, 2012l). Difficult 38

46 soils such as expansive clay and organics have been soil mix treated with encouraging results. Soils that swell or shrink due to seasonal moisture changes can cause problems for structures that lie above them. Pavements built on expansive soils can incur surface cracks and performance issues. A two year study by Madhyannapu et al. (2009) in Fort Worth, Texas on expansive clay under pavement loads found that DSM-treated soil experienced less vertical and lateral shrink-swell related movements when compared to untreated soil. It is suggested that DSM is potentially a successful ground improvement method for soils that experience swelling and shrinking (Madhyannapu et al., 2009). Soil mixing could reduce the problems experienced by these types of soils. A study in Thames estuary, UK showed organic soils could be strengthened by soil mixing. The site consisted of soft clay and peat layers with high moisture contents. After treating the soil with binder, the resultant shear strength was on average two to three times the design shear strength (Dahlstrom, 2012). This shows that soil mixing improves organic soil strength Depth/Height Limitations The two types of soil mixing, SSM and DSM, have two different ranges for soil treatment depth. SSM can reach depths up to 30 feet while DSM can reach depths of up to 150 feet (Jasperse, n.d.). This technology has one of the largest ranges of soil treatment depth reviewed. 39

47 2.4.3 QC/QA Assessment Methodologies Laboratory work prior to field implementation, also known as bench scale testing, is required in order to determine the best combination of and the amount binders for the soil being treated. Bench scale testing, monitoring of each soil-mixed column, and postconstruction verification, such as strength testing on core samples, should be included in the QA/QC program (SHRP 2, 2012e). Instrumentation and monitoring with inclinometers, settlement plates, moisture probes, and total pressure cells have been used to evaluate the success of DSM in the field (Madhyannapu et al., 2009) Environmental Aspects Soil mixing has been used successfully in soil remediation as a way to contain or treat pollutants in the soil. The resultant soil-mixed columns have no environmental influence (Jasperse, n.d.). During the drilling process, spoil may be produced when using a wet-slurry for the binder, which will need to be removed afterwards (SHRP 2, 2012l). If contaminated soils are encountered, disposing of the spoil will cost extra money General Cost The cost of implementing soil mixing varies depending on such factors as the project size, depth of treatment, requirements, location, and year of construction. The cost provided by SHRP 2 lists deep soil mixing treatment from $60.00 to $ per cubic yard for material amounts greater than 5,000 cubic yards. SHRP 2 also lists the mobilization fee ranging from $75,000 to $125,000 (2011). 40

48 2.4.6 Advantages The main advantage of soil mixing comes from its versatility. It has been applied to raising bearing capacity, decreasing settlement, averting sliding failure, supporting excavations, containing water or pollutants, addressing seismic issues, and reducing vibration (Porbaha & Kim, 2003). The adaptability of soil mixing means that it can be used successfully in a variety of settings. Also, another advantage of soil mixing is that it has a design manual. FHWA released a design manual recently for deep mixing for embankment and foundation support (Bruce et al., 2013). With this information available, engineers have a valuable resource for soil mixing design Disadvantages There are two main disadvantages with soil mixing. Firstly, mobilization of the equipment required for soil mixing to the project site is expensive. This means that soil mixing may not be an economical choice for small projects (SHRP 2, 2012l). As the project size increases, soil mixing becomes a more cost-efficient method. Secondly, softening has been observed in areas surrounding soil-mixed columns in marine clay. However, testing shows that the surrounding soil stiffened over time and within 70 days increased to 50% over the original strength (Shen et al., 2008). Due to the increase in strength over time, the disadvantage soil mixing presents is only temporary in this case. 41

49 2.4.8 Summary A list of the advantages and disadvantages of soil mixing for use in roadway projects is summarized in Table below. The main disadvantage of this technology is its high mobilization cost, which makes it less economical in small projects. The most notable advantage of the soil mixing technique is it produces little vibration. This means it can be used in urban areas without causing damage to nearby structures. Table Summary of advantages and disadvantages of soil mixing Advantages o Versatility o Low vibrations o Can be used in both urban and rural environments o Used in soil remediation o Successfully used in organic and expansive soils o Large depth range Soil Mixing Disadvantages o High mobilization cost o Temporary softening in marine clay Table summarizes the attributes of soil mixing ground improvement. Overall, this technology works with most soil types, including expansive and organic soils. Soil mixing has the largest range of treatment depth of the techniques reviewed, up to 150 ft. 42

50 Table Soil Mixing Parameter Summary Type of Soil Uncontrolled Fill Cohesive Granular Organic Expansive Collapsible Contaminated Depth of Influence Existing FHWA Design Manual? Location Settlement Soil Mixing >50 * Urban Rural Inducing Reducing High Mobilization Cost? Yes No *Up to

51 2.5 Controlled Modulus Columns The controlled modulus column (CMC) is a method of ground improvement originally developed in Europe. The CMC is designed to both reinforce and densify soft soils. A hollow-stem auger is drilled into the soil, displacing soil laterally, down to the bearing layer, and cement-based grout is then injected at a low pressure through the auger to create a column. The lateral soil displacement compacts and strengthens the surrounding soil around the column. As the auger is retracted, the grout is pumped without mixing with the soil (Pearlman & Porbaha, 2006). The process is repeated to create more columns in the soil. See Figure for a representation of the CMC installation. The result is a composite material which has improved global stiffness (Pearlman & Masse, n.d.). Both the soil and the CMCs are designed to carry the load while producing little settlement. The LTP, which is also discussed under the CSE section, is paired with the CMCs to promote soil arching, transferring 50-95% of the load to the CMCs (Pearlman & Masse, n.d.). Because the CMCs carry a portion of the load, they should bear in a stiff soil layer to reduce the overall settlement experienced. CMCs require specific tools and skills to implement in the field. Specialized equipment is used to create the CMCs, which provides large torque capacity and high downward thrust (Pearlman & Porbaha, 2006). Skilled and experienced operators are also needed, then, to install CMCs. 44

52 Figure Controlled modulus column installation (Vibro Menard 2013) Applicable Soil Types CMCs have been used in all types of soils. This includes uncontrolled fills, loose sands, soft clays and organic peat (Masse et al., 2004). In particular, CMCs were used successfully to support a roadway embankment in which very soft clay and fibrous peat were encountered over chalk and silty sand layers (Pearlman & Porbaha, 2006). Although CMCs have shown to work successfully in a wide range of soils, their usefulness with expansive soils remains unknown Depth/Height Limitations CMCs can reach a large range of depths for soil improvement treatment. The maximum length of treatment noted is 25 m or about 82 ft (Pearlman & Porbaha, 2006). 45

53 This range is smaller than that of deep soil mixing but larger than other types of methods reviewed including polyurethane injection and dynamic compaction QC/QA Assessment Methodologies The equipment to install CMCs has an automatic QC mechanism built-in unlike other types of soil improvement technologies. Quality control during CMC installation is accomplished with real time monitoring of the following: speed of rotation, rate of advancement or withdrawal of the auger torque, depth of the column, time, grout pressure and volume versus depth (Pearlman & Masse, n.d.). The real-time monitoring results in a higher quality in the end product. In terms of quality assurance, testing the strength and properties of the grout as well as performing load testing in-situ is also recommended (Pearlman & Masse, n.d.; Pearlman & Porbaha, 2006). This helps verify the CMCs are performing as designed. There are no FHWA design guidelines for this technology so it is important that testing be performed to validate the design Environmental Aspects The installation of CMCs is environmentally friendly. Virtually no spoils, or excess soils, are brought to the surface when CMCs are installed, which is important if contaminated soils are encountered at a site. Bringing contaminated soils to the surface results in adding cost to remove it from the site. Also, since CMCs do not drain the soil, the migration of contaminated groundwater is not induced (Pearlman & Masse, n.d.). Overall, CMCs do not actively cause harm to the environment. 46

54 2.3.5 General Cost The cost of implementing CMCs varies depending on such factors as the project size, depth of treatment, requirements, location, soil parameters and year of construction. The cost of CMC treatment ranges from $17.00 to $30.00 per linear foot but is typically $19.00 to $25.00 per linear foot. Mobilization costs range from $30,000 to $150,000 but are typically around $50,000 (J. Griffin, personal communication, November 13, 2013) Advantages CMCs provide benefits typically not found in other alternative ground improvement technologies. CMCs have been found to reduce settlement more than stone columns (Pearlman & Porbaha, 2006). CMCs are also a faster alternative to wick drains (Pearlman & Porbaha, 2006). The quality of soil mixed columns depends in part on the soil encountered, which makes it harder to control, while the CMC does not mix with the soil and therefore the quality is more easily controlled (Miao et al., 2009). Traditional rigid deep foundations such as piles are typically more expensive than CMCs because they require a structural connection, such as a mat or pile cap (Pearlman & Bloomfield, n.d.; Pearlman & Masse, n.d.). Also, the installation of CMCs is vibration-free, so they are a good choice for urban areas where adjacent structures must be protected (Masse et al., 2004; Pearlman & Bloomfield, n.d.). This shows that CMCs have many benefits over other types of ground improvement. Quality control of the CMCs is achievable through real time monitoring, which reduces the possibility of necking. Monitoring the pressure as the grout is injected can also help identify areas of weak zones in the soil and the lengths of the columns can be 47

55 adjusted accordingly in the field. This is an advantage over other types of columns where real time monitoring does not occur (Pearlman & Masse, n.d.). Having this control heightens the reliability of the method as well as the overall strength of the treated soil Disadvantages Like many of the other ground improvement technologies, the mobilization cost for this method is high. This means it is less economical in small projects than it is for larger projects. Although the initial cost is high, the method is effective at improving the soil. Another disadvantage of CMCs is the technology s lack of availability in certain areas of the world. The introduction of CMCs into the USA has been slow (Masse et al., 2004). The need for technologies that can treat problematic soils such as peatland is more important in areas such as Europe where available land is dwindling. The CMC technology requires new skills and equipment, which is an extra cost and effort that a contractor has to make but may not be as urgently required as it is in other countries. Therefore, it may be difficult to acquire the specialized equipment and skilled operators to perform such an installation for projects in the USA Summary A list of the advantages and disadvantages of CMCs for use in roadway projects is summarized in Table The main disadvantage of this technology is its lack of availability in the USA. The most notable advantage of the CMC technique is its real- 48

56 time monitoring during installation. This allows greater control over the quality of the columns produced, which in turn may make this technique more reliable than others. Table Summary of advantages and disadvantages of controlled modulus columns Advantages o High level of quality control o Cost less than traditional deep foundations o Faster construction than wick drains o Reduces settlement Controlled Modulus Columns Disadvantages o High initial cost o May not be available in USA Table summarizes the attributes of CMC ground improvement. Overall, this technology works with most soil types, reducing settlement. However, the initial high costs as well as the lack of availability are obstacles for engineers who wish to utilize this technology. 49

57 Table Controlled Modulus Columns Parameter Summary Controlled Modulus Columns Type of Soil Uncontrolled Fill Cohesive Granular Organic Expansive Collapsible Contaminated Depth of Influence Existing FHWA Design Manual? Location Settlement >50 * Urban Rural Inducing Reducing High Mobilization Cost? Yes No *Up to 82

58 2.6 Drains A common ground improvement technique involves draining the water from the soil in order to accelerate consolidation. This can be done with a variety of materials, including sand columns, stone columns and prefabricated vertical drains (PVDs). First, the drains are installed by augering for granular materials or mandrel for PVDs. See Figure for a picture of installed PVDs using a mandrel. For the natural materials, stone or sand is placed and compacted after augering to create columns which work as drains. The drains are then preloaded with a surcharge such as an embankment fill to encourage settlement prior to construction. The shorter drainage path allows water to move out of the soil at a faster rate and the surcharge provides overburden pressure to help this process. The water then rises up and out of the soil at the ground surface, where it can be diverted elsewhere. This dissipation of the pore water pressure then increases the shear strength of the soil (Barron, 1948) Applicable Soil Types Soils with higher permeability transfer water more efficiently than in soils with higher fine-grained content. A combination of wick drains and stone columns in one study increased the Standard Penetration Test blow count values of silty sand and sandy silts but were less effective in layers with more fines (Rollins, Quimby, Johnson, & Price, 2009). This shows that granular materials undergo more settlement from drains than fine-grained soils experience. Both PVDs and stone columns are typically used for inorganic clays and silts (SHRP 2, 2012n; SHRP 2, 2012i). The Strategic Highway Research Program 2 also notes 51

59 that stone columns should not be used with peat; this is likely because that type of soil does not provide the lateral confinement necessary to avoid bulging failure (2012i). Figure Installation of PVDs with mandrel (Layfield Environmental Systems, 2007) Depth/Height Limitations The depth of influence for drains varies considerably depending on the material used. Stone columns have a maximum influence depth of 90 feet (SHRP 2, 2012i). PVDs, however, can extend 100 feet and on some projects have extended greater than 200 feet (SHRP 2, 2012n). Natural material columns are more limited than PVDs in terms of depth treatment ranges. 52

60 2.6.3 General Cost The cost of implementing drains varies depending on such factors as the project size, depth of treatment, requirements, location, and year of construction. For sand columns, estimates on material costs range from $11.00 to $24.50 per linear foot for material amounts greater than 1,000 linear feet; the mobilization fee ranges from $80,000 to $100,000 (SHRP 2, 2012h). For aggregate columns, material costs range $20.00 to $60.00 per linear foot for material amounts greater than 1,000 linear feet; the mobilization fee ranges from $20,000 to $40,000 (2012a). For PVDs, material range from $0.50 to $1.50 per linear foot for material amounts greater than 10,000 linear feet; the mobilization fee ranges from $10,000 to $20,000 (2012g) Advantages Preloading the soil alone will consolidate the soil over time. However, using drains can reduce the time needed to wait until an acceptable amount of consolidation is achieved. By inducing settlement prior to construction, the soil will settle less over time than it would without treatment. Another advantage of this method concerns the natural material drains. Sand and stone columns are considered to be economical and inexpensive compared to other column technologies (Zheng et al. 2009). Most other technologies use more expensive material such as polyurethane or grout. Both PVDs and stone columns have design manuals by the FHWA. For PVDs, the following are available: Prefabricated Vertical Drains, Volumes 1, 2 and 3 (FHWA- RD , FHWA-RD , and FHWA-RD , respectively). The Design and 53

61 Construction of Stone Columns, Volumes 1 and 2 are under publication numbers FHWA- RD and FHWA-RD , respectively. These manuals aid the engineer with the design of these drain-type improvement methods Disadvantages Although drains can reduce the time needed to consolidate the soil with preloading alone, consolidation with drains can still take months (Masse et al., 2004). Faster methods for ground improvement exist, such as CMCs and other types of CSEs. Projects which require a time efficient method of ground improvement may benefit from techniques besides drains. Besides requiring more time, stone and sand are structurally weaker than cementbased material. The granular columns have low bearing capacity, low shear strength and no tensile strength compared to other types of columns (Zheng et al., 2009). Also, stone columns can experience bulging failure if lateral confinement needed from the soil cannot be provided (e.g. soft soils) (Pearlman & Porbaha, 2006). Stone columns are then not recommended for use in soft soils which may experience this failure. The intent of the natural material columns should focus on inducing settlement rather than attempting to provide substantial reinforcement to the soil. Another type of drain, the PVD, is made up of synthetic material. Figure shows the movement of water entering the drain from the soil. PVDs cannot shrink, therefore as the soil consolidates it likely bends laterally which decreases the vertical permeability and thereby the efficiency of the drain (see Figure 2.6.3). Sand can resist this issue better and can continue to drain longer than PVDs (Aboshi et al., 2001). 54

62 However, this lateral displacement still occurs in natural material columns and may potentially reduce the efficiency of the drain. Surcharge (embankment load) Figure Movement of water in PVDs Ground surface after consolidation Ground surface before consolidation Figure Deformation of PVD after consolidation 55

63 2.6.6 Summary A list of the advantages and disadvantages of drains for use in roadway projects is summarized in Table The main disadvantages of this technology are its structural weakness and longer period of time it takes to consolidate versus other technologies reviewed. For this reason, it is suggested that the stone and sand columns be designed to induce settlement without completely relying on the structural strength they contribute to the soil. The most notable advantage of the drain technique is its low cost compared to other types of technologies. Table Summary of advantages and disadvantages of drains Advantages o Consolidates soil faster than preloading alone o Inexpensive Drains Disadvantages o Consolidation still takes time (months) o Structurally weak o Low bearing capacity o Low shear strength o No tensile strength o Failure may occur if soils are too soft to provide confinement o Efficiency decreases as soil consolidates o Cannot be used in urban environment with sensitive structures Table summarizes the attributes of drain ground improvement. Overall, this technology is inexpensive and will eventually induce settlement. However, this will still take time out of the construction schedule and will provide little extra strength. Also, because it induces settlement, the drain technique should be applied in areas without sensitive structures nearby. 56

64 Table Drains Parameter Summary Type of Soil Uncontrolled Fill Cohesive Granular Organic Expansive Collapsible Contaminated Depth of Influence Existing FHWA Design Manual? Location Settlement Drains >50 * Urban Rural Inducing Reducing High Mobilization Cost? Yes No *Over 200 using PVDs 57

65 CHAPTER III DEVELOPMENT OF DECISION TREE 3.1 Purpose The ground improvement methods reviewed treat a wide range of problematic soils under varying conditions. In the field, the engineer must determine the most appropriate treatment under specific project conditions. These conditions can rule out possible treatment methods which are not compatible and therefore not applicable in that situation. By ruling out incompatible ground improvement methods, the engineer can then select the most appropriate method from the remaining compatible treatment options. A decision tree can facilitate this process. In its nature, the decision tree allows the user to eliminate options which are not compatible with the situation at hand. This is done by offering branches or decisions regarding particular conditions of the situation. Once each branch is selected over another, incompatible solutions are ruled out and the remaining possible methods are presented at the end. Not every ground improvement technique is suitable for every project in the field. The location, type of soil in the profile, range of depth of the problematic soil, and project constraints such as cost, time and settlement restrictions can determine whether or not a ground improvement treatment can be used on a project. A selection process using 58

66 existing research can be useful when taking into consideration possible treatment options. A decision tree of possible ground improvement techniques for roadway projects over a range of problematic soils is required. The techniques previously reviewed and incorporated into the decision tree are as follows Dynamic Compaction Polyurethane Injection Column-Supported Embankment Soil Mixing Controlled Modulus Columns Drains The decision tree developed for this thesis does not go over every possible ground improvement method that exists, as there are many and not all of them exhibit the qualities desired. The purpose of this decision tree is to provide options that have been shown to succeed with a wide range of problematic soils such as organic materials. 3.2 Factors Considered Location The location of a project influences the decision-making process. For example, preloading with or without drains can cause considerable settlement. This and vibrations from methods such as dynamic compaction can cause damage to nearby buildings and utilities. For this reason, these methods are not suitable in an urban environment. They 59

67 are, however, suitable for use in a rural environment where sensitive structures are not in the surrounding area. Each method reviewed considered thus has been organized under rural or urban categories, although several methods are possible to use in both location types. Table summarizes the allowable locations for each ground improvement method. Table Summary of ground improvement allowable locations Ground Improvement Method Location (Urban or Rural) Dynamic Compaction Rural Polyurethane Injection Urban and Rural Column-Supported Embankment Urban and Rural Soil Mixing Urban and Rural Controlled Modulus Columns Urban and Rural Drains Rural As can be seen from Table 3.2.1, all ground improvement methods reviewed are allowable in rural environments. A project in an urban location limits the user to fewer potential treatment options Depth of Treatment Another important factor to consider is the depth to the soil that requires treatment. Each ground improvement technique has a range of depth at which the method is most effective. Outside of this range, the treatment method is not suitable for use. Below is Table which consists of the ground improvement methods reviewed and their corresponding maximum range of treatment depths. Some ground improvement 60

68 methods have minimum depths of treatment. These treatment methods are summarized in Table Table Ground improvement maximum treatment depths Ground Improvement Method Maximum Treatment Depth (ft below ground surface) Dynamic Compaction 39 Polyurethane Injection 30 Column-Supported Embankment 70 Soil Mixing 150 Controlled Modulus Columns 82 Drains (Stone Columns) 90 Drains (PVDs) >200 Table Ground improvement minimum treatment depths Ground Improvement Method Minimum Treatment Depth (ft below ground surface) Dynamic Compaction 5 Column-Supported Embankment Type of Soil The majority of the techniques reviewed have been shown to succeed with a wide range of soils including soft cohesive, loose granular and organic soils. A few techniques also have been used successfully with expansive soils, including polyurethane injection and soil mixing. It is also noted that soil mixing and CMCs can be used in contaminated soils. Since the majority of the techniques can treat similar soils, this category separation 61

69 does not appear in every branch of the tree. Techniques that can treat either expansive or contaminated soils are indicated with an asterisk and plus-sign, respectively Project Constraints Several general project constraints were taken into consideration while developing the decision tree. Some techniques are more economical than others based on the size of the project. For example, polyurethane injection can be more cost-efficient if used in a small project compared to other techniques with hefty mobilization costs. However, these high mobilization cost techniques may become more economical as the project size increases. Outside of cost, settlement allowances may restrict the decisions that can be made with regards to soil improvement. For example, embankment drains and preloading are designed to induce rapid consolidation, typically before the roadway construction begins; however, additional settlement may occur after the roadway is built on the embankment over time. This additional settlement may or may not be tolerable for the project constraints. Also, these techniques take additional time in the construction schedule (i.e. waiting for settlement to occur), so projects with time constraints may require faster types of ground improvement. 3.3 Result Taking into consideration the location of the project, depth of treatment, type of soil and general project constraints, a decision tree using the reviewed ground improvement techniques is presented in Figure 3.1. It should be noted that the decision 62

70 tree should be used as a quick reference in order to start examining the possibilities for ground improvement in a project. Further research on the users end is required to determine what ground improvement method would be the most economical and successful for the project at hand. 63

71 64 Figure 3.1 Decision Tree