TYPICAL SOIL NAILING PRACTICES IN THE UNITED STATES

Transcription

1 TYPICAL SOIL NAILING PRACTICES IN THE UNITED STATES Walter G. Kutschke, P.E., URS Corporation, Pittsburgh, Pennsylvania, USA Fred S. Tarquinio, P.E., Nicholson Construction Company, Cuddy, Pennsylvania, USA Soil Nailing has been used in the United States (US) as a support of excavation technique for over 30-years. As a means to assess the current state of soil nail wall practice in the US, we have identified five significant soil nail wall projects. These projects were recently completed in the eastern US and have a combined area of approximately 17,900 square meters of finished shotcrete surface. They required the use of innovative design and construction methods in order to address various challenges, including slideprone back slope materials, perched water, highly erodible rock materials, curved wall alignments, very tight construction tolerances and unexpected subsurface conditions. Special focus is given to issues that developed during construction, such as bench excavation, soil nail installation methods, shotcrete mix design, anticipated shotcrete quantities, and shotcrete nozzlemen qualifications in order to provide an overview of typical soil nailing practices in the United States. INTRODUCTION Soil nailing has been used in a variety of civil engineering projects in the last 30-years throughout the United States. It is believed that this technology originated as an extension of the New Austrian Tunneling Method, which combines reinforced shotcrete and rock bolting to create a flexile support for underground excavations. Soletanche is credited with the first recorded application of soil nailing in Europe. The project involved an 18-m high cut slope excavated in Fontainebleau Sand, near Versailles, France. In the United States, the first US Patent for soil nailing was filed in 1970, with the first published application occurring in The 1976 project involved a 13.7-m deep excavation support for an extension of Good Samaritan Hospital in Portland, Oregon (FHWA, 1998). Since that time, soil nailing has grown to become common practice for temporary support of excavation in the United States. This technology is also gaining acceptance as a permanent earth retention system in the private sector. However, public agencies often tend to express a bias toward this technology and prefer other, more common retention systems. SOIL NAIL WALLS DRILL AND GROUT Soil nail work in the United States is generally governed by design and inspector guidelines presented in FHWA (2003) and FHWA (1994). These design guidelines are generally utilized by both the private and public sector in the United States. Traditional methods of installation involve drill and grout soil nailing techniques wherein a solid-bar soil nail is inserted into a predrilled hole and then tremie grouted. FHWA (2003) precluded the use of hollow-core soil nails. However, FHWA (2006) has begun research into this system given the potential cost savings of this technology. As a means to assess the current state of soil nail practice, this paper will present issues that occurred during construction of four significant drill and grout soil nail wall projects in the eastern United States (the fifth project will be discussed later in the hollow core bar section of this paper). Although each project presented herein is different with regard to geologic conditions and design, these projects each experienced some similar situations. These situations present a learning situation to assess the current state of practice. It should be noted that the review of soil nail wall design concepts and general construction methods are not the intent of this paper, as they are readily available from FHWA (2003). DRILL AND GROUT CASE HISTORIES The first soil nail wall project is located in the relatively mountainous terrain of western Pennsylvania. This project involved the construction of over 850 square meters of soil nail retaining structure with a total of 4,200 lineal 1

2 meters of soil nails and 330 cubic meters of shotcrete. In addition, the project also required construction of a shotcrete slope protection system which used very similar construction techniques as those employed for soil nail walls. This effort required the placement of approximately 8,400 square meters of slope protection with a total of 10,400 lineal meters of rock anchors and nearly 2,200 cubic meters of shotcrete. This system is believed to be the largest known application of such a system to date (refer to Kutschke et al. (2007) for further details regarding this work). The second project is located in Washington, D.C. and involved construction of a soil nail retaining wall used for the support of excavation for the new Chinese Embassy building. This project consisted of the installation of over 4,600 square meters of exposed shotcrete wall surface, approximately 1,600 soil nails, and over 1,200 cubic meters of shotcrete. The third project involved the construction of a soil nail retaining wall for the support of excavation for a new retail development in eastern Pennsylvania. This project involved 930 square meters of exposed shotcrete surface requiring 400 soil nails and 300 cubic meters of shotcrete. The fourth project also involved the construction of a shotcrete wall used for support of excavation for a new retail development in southwestern Pennsylvania. This project consisted of 1,500 square meters of soil nail wall with approximately 600 soil nails and 600 cubic meters of shotcrete. QA/QC The following sections present details regarding these issues. It is in these situations that lessons are learned to assess the current state of practice. Groundwater Soil nail wall construction is not appropriate for all subsurface conditions. Groundwater, surface runoff or perched water in the wall excavation are likely to cause stability and drainage problems during construction. In addition, the shotcrete may have a problem adhering to the excavated face if surface water is present. This was a major problem in a small area of the wall in eastern Pennsylvania. As a result, large quantities of shotcrete were required in order to maintain the alignment of this permanent wall. Excessive seepage can be detrimental to newly placed shotcrete because it acts to wash the cement off of the aggregate and to create additional load resulting in minor cracking and sloughing to reduction of shotcrete adhesion to the wall face. The most effective means to address seepage at the wall face is to control and direct the groundwater flow. The placement of additional drainage geocomposite and / or the use of PVC drain pipe to capture and direct the drainage away from the newly placed shotcrete have both been effective, as displayed by Figure 1, where drainage in excess of 310 liters per 310± liters per hr The first four projects identified were designed using conventional solid bar soil nails. The fifth project will be discussed later in the hollow core bar section of this paper. These projects were all successfully completed and are in service. The similar design and construction issues encountered with these projects were: Groundwater Bench Excavation Nail Installation Structural Materials Shotcrete Installation Nozzleman Experience 60± liters per hr Figure 1 Additional Wall Drainage for Seepage Control hour was occurring at select locations along the shotcrete face. Although this is an extreme event, it demonstrates the effectiveness of this approach. The geocomposite drains and PVC 2

3 drain pipe effectively collected and diverted the flow away from the slope face and allowed the placement of shotcrete in this instance. It is important to note that when these drainage measures are employed, they are self-supported by securing them to the slope face or steel reinforcement rather than relying on the shotcrete for support. lifts considering soil conditions and also the practical heights between lifts. Maximum permissible bench lifts may also be noted in the specifications. For comparison purposes, Figure 3 indicates an appropriate bench excavation that will be much easier for a nozzleman to work. Bench Excavation Bench excavation heights are not only limited to the stand-up time of the ground, but consideration must also be given to the nozzleman s abilities. In order for the proper application of shotcrete, the nozzle must be perpendicular to the slope face. As the angle between the slope face and nozzle increases, the degree of compaction decreases with a corresponding increase in rebound. Bench heights beyond about 1.5 to 1.8 meters place additional burden on the nozzleman and can result in quality problems as the upper reaches are difficult to shoot and finish properly. As such, it is the author s experience that limiting the bench height to 1.5 to 1.8 meters enables a nozzleman to properly and safely shoot the wall face and upper overlap area. Figure 2 displays a Figure 3 Appropriate Bench Height Excavation Nail Installation Air-track drilling is an economical drill method when drilling into materials that do not require casing to support the hole. Figure 4 illustrates a typical air-track drilling operation. Figure 4 Typical Air-Track Drilling Figure 2 Challenging Application of Shotcrete due to Excessive Bench Height Excavation typical situation when the bench height approaches this upper limit. The congested reinforcement zone in the overlap area requires particular attention that is difficult and burdensome for this experienced nozzleman. Also, it is difficult for him to blend-in the final shotcrete layer and rebound will significantly increase (note the significant rebound that has all ready accumulated at the base). Design must utilize appropriate vertical distances between Soil nail drilling production rates are highly dependent on equipment and driller, but rates of 0.3 to 0.6 meters per minute are typical values in hard clay or weathered rock. Nail hole diameters are generally limited to 100 to 125 mm with airtrack equipment. Although these hole diameters theoretically provide sufficient grout coverage between the nail and bonding strata, the ability of the driller to consistently create a straight shaft is debatable. This consideration, combined with the natural sag of the bar as it deflects under self-weight between the centralizer support points, will significantly reduce grout 3

4 coverage locally along the bar. Therefore, a centralizer spacing of less than the three meter US industry standard is warranted in environments that require long-term corrosion protection. Furthermore, centralizers should be secured to the soil nails by tie-wire; methods such a taping do not properly secure the centralizer and can result in bunching of the centralizers as the nails are inserted into the hole. Although air-track drills are an economical means of advancing a soil nail drill hole, the drilling operation can create significant disturbance at the slope face resulting in sloughing and soil break-outs. If this condition persists, a drill berm is highly effective, as shown on Figure 5. Figure 5 Prudent Application of a Drill Berm Figure 5 displays the slope face, in this case consisting of cohesive residual soil, during drilling. The dashed line noted in the figure represents the back-of-wall face. The use of a drill berm in this situation prevented the sloughing and general drill disturbance from impacting the back-of-wall face. There would have been detrimental impacts to this structure had a drill berm not be used. Telehandlers, or similar machines, are commonly used to lift a bundle of soil nails for the labor force to insert them into the grouted drill hole. Although this practice is acceptable, care should be taken as the nails are lifted from the forks. At no time should the inspector allow the nails to slide against the fork and into the hole. This needlessly exposes the nail to abrasion that can create holes in the epoxy coating. Nails should be manually lifted and inserted into the hole. Structural Materials The most important aspects of material design and quality control with respect to soil nail walls are the nail grout and shotcrete. Typical nail grout consists of cement and water combination with approximately 0.45 water/cement ratio, having 28-day compressive strength of at least 27.6 MPa. However, it is crucial in the timing of most soil nail projects to have three, two or possibly one day strength results. Shotcrete is generally applied using the wet-mix process (FHWA 2003). This process generally results in a higher volume throughput with less rebound. Wet-mix application rates for these projects were typically about 4.5 to 5.5 minutes per cubic meter of shotcrete. Similar to the nail grout, it is critical to have shotcrete compressive strength results at three, two or one day(s) in order to maximize production without comprising the integrity of the wall. Proper mix design and adequate drainage are paramount to the longevity of the shotcrete face due to freeze / thaw cycling. Shotcrete slump is largely self-controlling; too wet and it will slough, too dry and it will not pump. A combination of proper air entrainment and a low water:cement ratio help provide adequate freeze / thaw durability. Published literature indicates that loss of entrained air during the pumping and spray application is typically 4-5% (FHWA 1998). Typical wet mix shotcrete designs require a water:cement ratio no greater than 0.45 with minimum air entrainment of 7-10%, measured at the truck. Pozzolans, such as fly ash, improve pumpability and will produce a more durable shotcrete by mitigating the alkali-silica reactions, increasing resistance to sulfate attack, and reducing ingress of potentially deleterious materials such as chloride and water. However, fly ash has the potential to impact air entrainment. Hill (2006) indicates that as the loss on ignition value of fly ash increases, the dosage of air entrainment chemical will generally increase. Suitable material selection is essential. Proper aggregate distribution is very important with regard to strength and durability of the finished shotcrete face, but also is very critical with regard to pumpability and the ability of the shotcrete mix to adhere to the excavated face. Figure 6 (next page) represents the 4

5 Percent Passing by Weight Sieve Size (mm) Figure 6 Recommended Shotcrete Aggregate Proportions Shotcrete Installation On most projects, the general contractor performs the bulk excavation, and therefore is required to provide the finished cut soil/rock faces onto which the specialty geotechnical subcontractor will apply the shotcrete. In most cases, the general contractors on these projects found it challenging to excavate the weathered rock to the planned angles without significant overbreak, as exampled by Figure 7. As a result, it was necessary to completely fill the overbreak pockets, in some cases as much as 1 meter deep, with shotcrete in order to leave a fairly uniform finished surface. recommended range of aggregate size distribution for a good shotcrete mix, which is in general conformance with FHWA (2003). Shotcrete reinforcement is based on the structural requirements of the soil nail wall. In addition to this reinforcement, an additional layer of wire mesh-type reinforcement can be added. The wire mesh opening should be no smaller than 100 mm, since smaller openings will generally act to interfere with shotcrete placement. It is suggested that this mesh provides additional confinement to minimize shotcrete sag as a greater thickness of shotcrete is placed; however, its benefit is questionable and lift thickness should be limited to 150± mm even where it is used. Wall drainage is paramount to the longevity of a soil nail wall system. Geocomposite drainage panels or strips are often used to provide drainage. These materials are generally tacked to the slope face with reinforcing pins and installed in shingle fashion as the excavation is lowered. Drains are daylighted by means of a weephole, and care must be taken to avoid creating a low spot for water to collect. Weep holes must be covered during application of shotcrete to avoid clogging the drain. Extreme care must be taken by the nozzleman to avoid placing shotcrete behind the drainage panel and therefore render it useless. As such, it is extremely important that the drains are securely fastened against the slope face prior to shotcrete placement. Figure 7 Overbreak From this experience, it is suggested that contracts include a pay item for excess shotcrete. However, it is also important to note that this item can be a source of contention, and thus pay items should be reviewed and accepted by the owner as readily as possible. A separate pay item for plain shotcrete is advantageous because it does not include incidentals such as reinforcement, bearing plates, drainage strips, etc. It is emphasized that the owner should periodically review the work conditions in order to gain a level of confidence that additional shotcrete is necessary, and that quantities are not unjustifiably increased. Bid quantities should include a reasonable contingent value in order to minimize financial impact to the project. It is suggested that this value is approximately 30% of the overall estimated neat shotcrete quantity. 5

6 Experienced Nozzlemen For shotcrete installation, especially for permanent shotcrete walls or temporary walls with tight horizontal tolerances, it is extremely important to have experienced shotcrete nozzlemen. These individuals are ultimately responsible for the final product, and this work requires a high degree of craftsmanship. Preconstruction test panels are necessary to evaluate the nozzlemen qualifications, and the preparation of shotcrete test panels (Figure 8) is a standard Quality Assurance practice carried out in order to evaluate the qualifications of each nozzleman prior to the beginning of production. Figure 9 Shotcrete Test Panel Cores The cores taken from the unreinforced panels are generally tested for unconfined compressive strength and boiled absorption. It has been observed that, even among personnel that have been approved for a given project, different nozzlemen can produce a wide range of shotcrete quality depending on their individual experience and technique. Therefore, it cannot be assumed that just because a particular nozzleman demonstrated adequate qualifications per the project specifications that he will consistently produce high quality shotcrete in production. Figure 8 Nozzlemen Test Panels It is important to note in Figure 8 that the panels are at the same approximate angle as the slope panels and are prepared using the shotcrete mix proposed for use on the project. The reinforced panels are cored for visual observation to assess whether the nozzleman s technique results in uniform shotcrete distribution around the reinforcement. Figure 9 indicates shotcrete cores obtained from a test panel. Note that the left-most core exhibits a significant build-up of aggregate (rock pocket) behind the reinforcement. This test panel was created by an inexperience laborer and he was not permitted to serve as a nozzleman. Inspection staff should be aware of poor technique and the inferior shotcrete qualities that develop as a result. It should also be understood that even the best shotcrete nozzlemen will not produce a shotcrete face that looks like poured concrete. Shotcrete faces in general will be rough and non-uniformly colored unless they are specially troweled or floated and colored with pigmented sealers. The nozzleman and inspector must also pay close attention to the bearing plate area as this will act as a barrier if the plates are mounted prior to shooting. Figure 10 (next page) indicates an experienced nozzleman. Note how the nozzle is near perpendicular to the slope face and relatively low rebound. 6

7 soil nail grout cubes and proof/verification testing of the soil nails. 2. The shotcrete facing, specifically the unconfined compressive strength and boiled absorption testing of the shotcrete. Compressive strength testing of grout is relatively straightforward. Figure 12 indicates a Figure 10 Experienced Nozzlemen Proper curing of the shotcrete during cold weather is extremely important. Shotcrete not cured properly according to project specifications can result in low compressive strength and surface deterioration. In addition to curing, the receiving surface must be free of ice or other deleterious elements. Figure 11 indicates one method to pre-heat a receiving surface during inclement weather. Figure 11 Cold Weather Operations Obstructed from view under the tarp are a series of torpedo heaters. Also note the insulation blankets, adjacent to the blue tarp, which was placed on relatively fresh shotcrete. Test panels shot under similar circumstances confirmed the suitability of this approach. Quality Control/Quality Assurance There are two basic elements for quality control / quality assurance for a soil nail wall project, namely: 1. The soil nail elements, specifically unconfined compressive strength testing of UCS (MPa) TIME (DAYS) DATA CRITERIA Figure 12 Typical Grout Cube Test Data Plot typical scatter of nail grout data. It is important to note that some data points fall below the criteria line and could result in rejection of soil nails. Although this was cause for concern during construction, there is no trend to support low grout breaks, and the probable explanation for these outliers is a defective cube (i.e., improper curing, cracked cube, et cetera). Significant discussions could have been avoided had this cube been identified as defective and not suitable for testing. Handling, curing, storage and transportation of grout cubes is very important, and proper care must be adhered to for accurate results of test samples. Specific gravity testing of the mixed grout using a mud balance is important to confirm the mix design of the grout, especially when low compressive test grout break results occur. Soil nail testing generally involves verification and proof testing as outlined in FHWA (2003). Generally, soil nail tests should be performed to assess the overall nail resistance. Separating and testing various geologic strata within the length of a single nail is not recommended because it can create unnecessary complications. The important parameter is the overall resistance offered by the installed soil nail as compared to the design resistance required by the soil nail load diagram developed for a particular design. Figure 13 (next page) 7

8 DEFLECTION (MM) TEST DATA Figure 13 Typical Soil Nail Test Set-Up displays a typical soil nail test set-up. It is important that the inspector and contractor coordinate test activities. Typically, an observant inspector will select test nail locations based on drill rig response, review of cuttings, or some other geotechnical concern. The design engineer, inspector and contractor must understand the type of test and test loads and then use this information to select an appropriate sized soil nail bar to avoid overstressing the nail during a test situation, as might happen if a production nail was tested under a verification test load. The review and interpretation of the nail test data is done in accordance with the project specifications. Typically, two plots are generated, namely a movement vs. load plot, as exampled by Figure 14, and a movement vs. time plot, as exampled by Figure 15. MOVEMENT (MM) TEST DATA CRITERIA LOAD (kn) Figure 14 Soil Nail Test, Movement vs. Load TIME (MIN) Figure 15 Soil Nail Test, Movement vs. Time SOIL NAIL WALLS HOLLOW-CORE BAR A technique gaining acceptance in the United States is the use of a hollow-core bar (HCB) to replace the traditional drill and grout method. Although generally precluded for use on public funded projects, this system of self-grouting soil nails has been employed in the United States for approximately ten years (FHWA 2006). In the hollow-core soil nail construction technique, drilling, installation, and grouting of nails are combined into one step. This method is most advantageous for caving ground conditions and was initially employed for temporary shoring applications. However, improvements in field QA/QC procedures have expanded this technique to permanent retaining wall construction. In as much as this is a relatively new technology in the United States, a brief overview of basic construction and design concepts are presented. Basic Construction and Design Concepts The hollow-core bar is fitted with an over-sized, sacrificial drill bit that has two to five holes to allow grout to pass through. The nail is rotated and advanced with a percussion hammer. Grout is pumped through the hollow-core bar and exits through the drill bit. Grout pressure then forces the grout back along the outside of the bar to the ground surface. When the nail reaches the target depth, both the drill bit and nail are left in the hole to serve as the soil nail reinforcement. Hollow-core soil nails generally produce a better grout-ground interface bond as well as a stifferload deformation behavior than traditional drill and grout techniques. 8

9 Pullout resistance of the HCB is defined in units of force per unit length, rather than in terms of stress units traditionally used for drill and grout techniques. The reason is that the HCB often develops an irregular shaped grout column and depending on local enlargements, passive resistance may develop along the column. HCB must meet the requirements of ASTM A615; grade 60 is the minimum requirement. Two types of hollow-core soil nails are available at this time in the US, with the primary difference being the type of exposed trend on the nail (i.e., rope or R-thread and the Titan thread). The implications of the different thread type is that under tension and/or bending, the grout body surrounding a HCB with Titan threads appears to mitigate the development and propagation of cracks in the grout better than the R-thread at different loads (FHWA, 2006). However, it must be noted that the grout body, whether surrounding a solid-bar or HCB, will ultimately develop cracks once the load reaches a threshold value. Installation of HCB is performed effectively with drill rigs exhibiting the following minimum capabilities (FHWA, 2006): Rotation = 160 rpm Torque = 2,500 N-m Percussion Energy = 610 Joules. Two general grout mixes are used for HCB construction; both consisting of a neat cement grout. One is termed flushing grout, which is used during the advancement of the nail and typically involves a w/c ratio of 0.7 to 0.9 to prevent waste of cement and therefore reduce cost. The full strength final grout is used for flushing when the nail is within 1.5 m of its target depth for granular soils and 3 m for cohesive soils. Design 28-day compressive strengths are typically 20.7 MPa to 34.5 MPa with water / cement ratios between 0.6 and 0.45, respectively. Once the target depth is reached, the full strength grout will continue to be pumped until the grout exiting the hole is the same consistency as the grout entering the hole. Final grouting is generally achieved with a piston or plunger pump with about 120 liters/minute volume and 0.5 MPa to 1 MPa pressure, with pressures up to 10 MPa for clean out or dirt blockages. The water content of the grout mix is the prime control over grout properties and is generally checked through specific gravity measurements. An approximate relation to monitor the w/c ratio of the grout mix, based on measured specific gravity (SG), is (PTI 2004): w/c (by weight) = 4.4 (SG/1000) -3.6 Drill bit selection is critical for HCB. At a minimum, the drill bit diameter (D bit ) will equal the diameter of the grout body (D gb ). Common relationships between D bit and D gb are as follows (FHWA 2006): Weathered rock and clays, D gb D bit. Granular soils, D gb 1.5 to 2.0 D bit. Unlike solid bar nails, bulb type centralizers are not suitable for hollow-core soil nails because they tend to impede grout flow. HCB require star type centralizers made of steel and have an ID greater than the OD of the HCB, but smaller than the OD of the coupler. As such, these centralizers are free to rotate and move along the bar during installation. Since common length increments for HCB are 1.5 and 3.0 m, one centralizer within a given nail segment is deemed sufficient. HCB are predicated on a continuous flow of grouting exiting the mouth of the drill hole during the drilling operation. This basic concept raises concerns regarding a representative proof or verification test unless the grout to steel or grout to ground bond can be broken over a certain length. Testing frequency and procedures do not vary for HCB and solid-bars described in FHWA 1998 and Based on FHWA (2006), recent HCB walls in the US exhibited the following parameters: Wall heights = 3 to 20 m Wall Slopes = Vertical to 1/2 H:1V Nail Spacing = 1.5 x 1.5 m to 1.83 x 2.74 m Minimum Hole Diameter = 76 to 102 mm Nail Lengths = Up to 15 m Nail Corrosion Protection = Grout to fusion bonded purple marine epoxy-coated Measured Nail Adhesion Values = 11 to 117 kn/m 9

10 HOLLOW-CORE BAR CASE HISTORY The fifth case history project involved the construction of several soil nail retaining walls for the support of excavation for a new power plant sub station in New York, New York. This project involved 1,600 square meters of exposed shotcrete surface requiring 670 soil nails and 390 cubic meters of shotcrete. The design consisted of 40/20 and 52/26 TITAN hollow core bars which used 115 mm drill bits. Figure 16 Hollow Core Soil Nail Support SUMMARY AND CONCLUSIONS Five significant soil nail wall projects were recently completed with a combined retained area of approximately 17,900 square meters. The lessons learned from these projects were: 1. Bench Stability Although soil nail wall construction is extremely versatile, its use should be limited to appropriate soil types. Consideration must be given to soil stand-up time and groundwater conditions. Constructability reviews during design must consider nail spacing and address bench height limitations in the project specifications. Innovation is the key to success when encountering difficult conditions. 2. Shotcrete Over-Runs All soil nail wall projects will experience shotcrete overruns if the neat area/volume is used in the bid tabulations. Voids and slope sloughing are inevitable. Paying for shotcrete overruns can become a source of great contention between the engineer, owner and contractor. Project specifications should consider the use of a bid item with contingent quantities for extra shotcrete, with extra quantities in the order of 30% of the neat volume. The owner needs to understand and accept these quantities as they develop. 3. Nozzlemen Experience Nozzlemen are ultimately responsible for the overall quality of the finished shotcrete product. Their craftsmanship results in the final aesthetic appearance of a wall (when specifications require a gun finish) and their skill attributes to the structural continuity of the wall. From a contractor s perspective, the nozzelmen are given significant financial responsibility and they rely on their skill to apply the shotcrete in accordance with the tolerance noted in the specifications. Establishing their qualification prior to production is an industry standard that should always be adhered to. 4. Experience and Communication The experience that each team member brings to the project is vital to the success of a project. An experienced design engineer and contractor understand issues that are important. It is this experience and communication that can maintain schedules and limit financial risk. Regarding the use of Hollow-Core Bar soil nails, the FHWA indicates the following significant recommendations for further study (FHWA, 2006): 1. Possible modification of the current analytical model (FHWA 1998, 2003) to better reflect the behavior of the HCB installation process. 2. Determination of the extent of the permeation zone in dry cohesive soils when a diluted grout is used and its effect of the strength of the grout body after final grouting. 3. Evaluation of nail adhesion as a function of nail advance rate and grouting pressures. 4. Development of a cost-effective method to obtain an adequate unbonded zone for verification and proof testing and development of a method for protection of the unbonded length after conclusion of proof tests. 5. Determination of HCB installation method on the integrity of corrosion protection coatings and the effect of corrosion loss on the shape 10

11 and size of the thread and its implications on grout-steel bond. 6. Evaluation of the difference in load capacity between different bar/thread types. 7. Determination of water:cement ratios of the flushing and final grout as a function of soil gradation and type. The issues presented in this paper are those of the authors based on the referenced project experience. Other soil nail and shotcrete projects may not have experienced similar issues. ACKNOWLEDGEMENTS This document represents work originally developed by Kutschke, W.G., Tarquinio, F.S., and Petersen, W.K. (2007) and acknowledgement is hereby given to the Deep Foundations Institute. The authors would like to thank Mr. Barry Siel from the FHWA for providing valuable information regarding the use of Hollow-Core Bar Soil Nails. The authors would also like to thank the people from URS Corporation, Nicholson Construction Company and Weidlinger Associates, Inc. who were involved in the design and construction of the referenced projects. REFERENCES BONITA, G., TARQUINIO, F. and WAGNER, L., Soil Nail Support of Excavation System for the Embassy of the Peoples Republic of China in the United States, Proceedings of the Deep Foundations Institute (DFI) 31 st Annual Conference on Deep Foundations, October 2006, Washington D.C. FHWA, Manual for Design & Construction Monitoring of Soil Nail Walls, FHWA-SA R. U.S. Department of Transportation, Federal Highway Administration, Washington, D.C. FHWA, Soil Nailing Field Inspectors Manual Soil Nail Walls. FHWA-SA , U.S. Department of Transportation, Federal Highway Administration, Washington, D.C. HILL, R.L., The Impact of Fly Ash on Air- Entrained Concrete, High Performance Concrete Bridge Views, #43, National Concrete Bridge Council, Skokie, IL. KUTSCHKE, W.G., PETERSEN, W.K, AND MEYERS, J.R., Rock Slope Protection System for Differential Weathering Materials, Proceedings of Geo-Denver 2007, Embankments, Dams and Slopes: Lessons Learned from New Orleans Levee Failures and Other Current Issues, Geotechnical Special Publication No. 161 (CD-ROM), ASCE, Reston, VA. KUTSCHKE, W.G., TARQUINIO, F.S., AND PETERSEN, W.K., Practical Soil Nail Wall Design and Constructability, Proceedings of Deep Foundations Institute, 32 nd Annual Conference on Deep Foundations, Deep Foundations Institute, Hawthorne, NJ. PTI (2004). Recommendations for Prestressed Rock and soil Anchors. Post-Tensioning Institute, Phoenix, AZ. FHWA Hollow-Core Soil Nails: State-ofthe-Practice Unassigned, U.S. Department of Transportation, Federal Highway Administration, Washington, D.C. FHWA, Geotechnical Circular No. 7. Soil Nail Walls, Publication FHWA-IF , U.S. Department of Transportation, Federal Highway Administration, Washington, D.C. 11