DFI CSCE Workshop April 12, 2013 Solutions for Embankments Over Soft Soils Embankments Over Soft Soils

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1 DFI CSCE Workshop April 12, 2013 Solutions for Embankments Over Soft Soils Embankments Over Soft Soils Construction over Unstable Soils focuses on methods to support embankment and embankment widening on the foundation, i.e., typically below-grade technologies. Methods include ground improvement and support over the unstable soils. Although the ground improvement is often below-grade, some atgrade technologies are also applicable to this application. 1

2 Embankments Over Soft Soils 1.Column-Supported Embankments a. Large number of column types can be used 2.Excavation and Replacement 3.Geosynthetic Reinforced Embankments 4.Lightweight Fill 5.Electro-Osmosis 6.Hydraulic Fill with Geocomposite and Vacuum Consolidation 7. Prefabricated Vertical Drains and Fill Preloading 8.Vacuum Preloading with and without PVDs Vertical Support Elements 1.Aggregate Columns 2.Combined Soil Stabilization with Vertical Columns 3. Continuous Flight Auger Piles 4.Deep Mixing Methods 5.Geotextile Encased Columns 6.Jet Grouting 7.Micropiles 8.Sand Compaction Piles 9.Vibro-Concrete Columns Selection tool within GeoTechTools 2

3 SOLUTIONS ABOVE GRADE EMBANKMENT EMBANKMENT UNSTABLE SOILS STABLE SOILS SOLUTIONS ABOVE OR BELOW GRADE Construction over Unstable Soils Construction over STABLE/STABILIZED Soils PAVEMENT SURFACE BASE SUBBASE WORKING PLATFORM SOLUTIONS UNSTABLE SOILS GROUND SURFACE SUBGRADE SOILS UNSTABLE SOILS OR STABLE SOILS GEOTECHNICAL PAVEMENT COMPONENTS (SOLUTIONS FOR BASE, SUBBASE, AND SUBGRADE) Geotechnical Pavement Components (Base, Subbase, and Subgrade) Working Platforms 3

4 DFI CSCE Workshop Column-Supported Embankments 4

5 Column-Supported Embankments Basic Function Column-Supported Embankments (CSE) enable construction of embankments over unstable soils by transferring the load to a stiffer underlying stratum. Column-Supported Embankments Advantages Accelerates construction compared to conventional methods Eliminate staged construction Reduces total and differential settlement Reduces stability problems Protects adjacent facilities from distress Can be used with a wide variety of columns to accommodate different site conditions 5

6 Column-Supported Embankments Description CSEs are used when the soil is too soft or compressible to support the embankment. The columns transfer the load to a firm stratum below the soft layer. The columns can be floating or end-bearing depending on the site geology, the project requirements, and the type of column used. For most CSE applications, the columns are end-bearing. When high-capacity columns with wide spacings are used, geosynthetic reinforcement is typically used at the interface between the top of the columns and the embankment to more efficiently transfer the embankment load to the columns. Column-Supported Embankments Geologic Applicability Typically used on soft compressible clay, peats, and organic soils where settlement and global stability are concerns Most cost effective when the compressible material thickness ranges from 15 to 70 feet (4.6 to 21.3 meters) Soft soil underlain by stiffer soil or bedrock 6

7 Column-Supported Embankments Construction Methods Columns of strong material are placed in the soft ground to provide the necessary support by transferring the embankment load to a firm stratum. Numerous types of columns that may be used A load transfer platform or bridging layer may be constructed immediately above the columns to help transfer the load from the embankment to the columns, and thereby permit larger spacing between columns than would be possible otherwise. Load transfer platforms generally consist of compacted soil and geosynthetic reinforcement. Column-Supported Embankments Potential Disadvantages CSEs can incur a higher cost than technologies that require more time before the embankment can be put into service. CSEs suffer form a lack of standard design procedures and lack of knowledge about technology benefits, design procedures, and construction techniques. 7

8 Design Methods No current FHWA design guidance. Limited information is available FHWA-NHI Ground Improvement manual (2006). SHRP 2 R02 project completed extensive research on design methods Developed recommended procedure QC/QA Methods No current FHWA QC/QA guidance. Limited information is available FHWA-NHI Ground Improvement manual (2006). QC/QA for a column-supported embankment project should include verification of the properties and placement of the LTP fill, embankment fill, the geosynthetic reinforcement, and the column type being used. Acceptance criteria are typically based on minimum total and/or differential settlement criteria. 8

9 CSE w/ Geosynthetic Load Transfer Platform Embankment Sand (Firm Foundation) Columns 9

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11 Historical Overview Developed in Europe in 1980 s as rapid construction technique First transportation application in 1984 for a bridge approach in Europe using concrete piles and one layer of reinforcement First US application was in 1994 for storage tank NJ Lightrail supported on CSE with VCC in

12 What type of columns may be used for CSE? Timber piles Steel piles Pre-cast concrete piles Soil mix columns Stone columns Geotextile encased columns-gec Geopiers VCC-Vibro concrete columns- Continuous Flight Auger (CFA) piles CSV-combined soil stabilization Load Transfer Platforms Reinforced Concrete Structural Mat Geosynthetic Reinforced Soil Mat 12

13 Applications Embankment support Bridge approach fill support Bridge abutment and foundation support Road widening, rapid construction Criteria for Successful Application Column spacing based on area replacement ratio 7-20% Clear span between columns < 3 m (~ 10 ft) Embankment height greater than clear span between columns LTP backfill friction angle > 35 Columns designed to carry full embankment load 13

14 Construction Materials Benefits of Pile Caps Reduce clear span between columns Reduce potential for punching failure through embankment 14

15 Load Transfer Platforms (LTP) Geosynthetic reinforced soil mass designed to transfer load from above the platform to VCC s, stone columns, etc. below the platform What type of geosynthetic reinforcement would you use for this application? High Strength Woven Geotextile Uniaxial Geogrid Biaxial Geogrid 15

16 Design Concepts Failure Modes Design Concepts Serviceability State 16

17 Column Design Steps Determine geotechnical characteristics of foundation soil Determine load in columns Select preliminary column type/types Calculate column capacity and length Consider deformation characteristics of column types (i.e., stone columns 30 50% settlement) Q r = (D e /2) 2 ( H + q) D e = effective diameter of column H = height of embankment q = live and dead load surcharge (typically 250 psf) = unit weight of the embankment soil Column Load 17

18 Preliminary Column Layout Column spacing 1.5m Column loads kn (25-50 kips) for 3-10m high embankment Column spacing 3.0m Column loads kn (90-250) for 3-10m high embankment Column costs typically represent 85-95% of construction cost Design Issues Edge Stability 18

19 Edge Stability L p = H (n-tan p ) n = side slope of the embankment p = (45- emb /2)] emb = effective friction angle of embankment fill Lateral Spreading 19

20 Lateral Spreading T ls = K a ( H + q)h/2 L e = T ls /[ H(c iemb tan emb )] Design Concepts Geosynthetic LTP Design 20

21 LTP Design Catenary Assumptions Soil arch forms in embankment Reinforcement is deformed during loading One layer of reinforcement LTP Design Beam Theory Assumptions Minimum three layers of reinforcement Spacing between layers cm (8-18 ) Platform thickness ½ clear span between columns Soil arch is fully developed within the depth of the platform 21

22 LTP Design Steps Select design method see GeoTechTools guidance Determine vertical load (W T ) from the embankment to be carried by the geosynthetic Determine the tensile force in the geosynthetic T RP Select reinforcement based on design requirements LTP Design Definition of Terms 22

23 What would you monitor during construction? Column installation Geosynthetic reinforcement material type Reinforcement seam/overlap Geosynthetic reinforcement placement LTP fill placement and compaction Embankment fill placement and compaction Quality Control Monitoring Column Installation Cast-in-place Grout/stone/sand/etc. quantity Strength testing (i.e, grout cubes, cores etc.) In-situ testing/verification Length,diameter spacing of columns Load test Driven Driving resistance Hammer efficiency PDA Length, blows per foot Hammer size Load test 23

24 Contracting Approaches Complete design & construction execution specified End result or performance approach with geometric & design criteria specified 24

25 DFI CSCE Workshop Aggregate Columns Aggregate Columns Basic Function Aggregate Columns are a ground improvement method that uses compacted aggregate to create stiff pier elements. Aggregate Columns help increase bearing capacity, shear strength, rate of consolidation, and liquefaction resistance; and reduces settlement. Aggregate Columns are either Aggregate Piers or Stone Columns 25

26 Aggregate Columns Advantages Rapid installation Cost effective compared to other foundations options Creates an additional drainage path and accelerates consolidation Allows for high level of compaction Efficient QC/QA procedures Aggregate Columns Description Aggregate Piers are a ground improvement system that places aggregate in predrilled holes to form stiff, high density aggregate piers. As the aggregate is rammed to form the piers, the aggregate is forced laterally into the sidewalls of the hole, partially densifying the surrounding soil. Stone Columns are columns formed with densified gravel or crushed rock in a pattern to create a composite foundation of the columns and the surrounding soil. The stiff columns carry a larger load than the surrounding soil to increase strength and capacity and reduce settlement. 26

27 Aggregate Columns Geologic Applicability Clays, silts, loose silt and sand, uncompacted fill, stiff clays, and medium dense sands. Recommended in soft clays with an undrained shear strength greater than 400 psf but has been used in clays with a strength as low as 150 psf. Bulging columns is a concern in soft clays. Particle sizes and shape of the column infill material depends on the construction technique used, but generally rangedfrom½into3in. Elevated water tables and cohesionless soils complicate the installation. Construction may be difficult in soft clays and loose sands, necessitating casing of the borehole Aggregate Columns Construction Methods Aggregate Piers: 24- to 36 inch (600 to 900 mm) diameter holes are drilled into the foundation soils. The holes normally reach depths of 7 to 30 feet (2 to 9 m) below grade. Casings are needed for cohesionless soils where the water table is above the depth of the pier. Lifts of well-graded aggregate are rammed into the holes. The first lift is open graded aggregate forms a bulb at the bottom of the pier. The subsequent compacted lifts are typically 12 inches deep. A high-energy beveled tamper mounted on excavator equipment is used to compact the aggregate. Stone Columns: Can be installed by water jetting, referred to as vibro-replacement or a wet, top feed method. The rock is densified by the vibratory probes as they are withdrawn from the ground. 27

28 Aggregate Columns Potential Disadvantages Limited treatment depth. Lack of bending resistance. Difficult to install in clean sands when the groundwater table is above the bottom of the pier. Not applicable of wide heavy load applications. With the wet technique of installation, the jetting water must be disposed. Uncertain whether all stone reaches the bottom of the hole using the dry-construction method. Soft soils may not provide adequate lateral support for the columns. Some techniques are proprietary Aggregate Columns-Applications Support of embankments Support of structures Improvement of slope stability 28

29 Stone Columns Introduction of backfill material into the soil to form dense columns that are tightly interlocked with the surrounding soil Related Technologies Rammed aggregate piers (Geopiers) Geotextile encased columns (GEC) Vibro-concrete columns (VCC) 29

30 Suitable Soils Stone Columns 100 Gravel Sand Silt Clay Percent Finer by Weight Stone Columns Vibro-compaction Particle Size, mm Stone Column Construction Methods Wet top feed - vibro-replacement Dry bottom feed - vibro-displacement 30

31 Wet Top Feed Vibro-Replacement Original stone column installation technique High pressure jet of water to open hole which probe follows into the ground Probe retracted in increments and stone introduced into void from the surface Best suited for sites with soft to firm soils (c u = 15 to 50 kn/m 2 (300 to 1000 psf) and high groundwater table Stone Placement 31

32 Dry Top and Bottom Feed Vibro-Displacement Jetting water effluent from wet top feed method causes environmental problems Dry top/bottom feed methods developed to solve these problems For shorter stone columns stone can still be feed down annulus For deep treatment stone is feed to the bottom of the vibrator through auxiliary tube Dry Bottom Feed Method 32

33 Applications Embankment stabilization Bridge approach fill stabilization Bridge abutment and foundation support Liquefaction mitigation Feasibility Evaluation - Stone Columns Typical column diameters m (1.5 4 ft) Allowable design load to approximately 500 kn (110 Kips) per column Most favorable outcome in compressible silts and clays (undrained shear strength kn/m 2 ( psf)) Depth 30 m (100 ft) max typical range m (35-50 ft ) Settlement reduction range 30 to 50% of unimproved 33

34 Feasibility Evaluation - Rammed Aggregate Piers Soils most favorable soft to medium stiff clays with undrained shear strengths > 15 kn/m 2 (300 psf) Typical lengths 5-10 m (15-35 ft) Typical allowable load capacity kn ( kips) Settlement < 25 mm (1 inch) Feasibility Evaluation VCC Vibro-concrete columns Typical column diameters cm (10-13 in) Typical allowable design loads kn/m 2 ( kips) Soils most favorable very soft clays and organic soils Typical lengths 5-10 m (15-35 ft) 34

35 Feasibility Evaluation GEC Geotextile encased columns Soils most favorable very soft clays and organic soils Typical lengths 5-10 m (15-35 ft) Typical column diameters m (28-36 in) Typical allowable load capacity kn (40-80 kips) Construction Equipment and Materials 35

36 Equipment for Stone Columns Vibrator Wet Dry 36

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38 Backfill Material for Stone Columns Vibro-replacement Stone or gravel: 25 to 60 mm (1 to 2½ inch) Vibro-displacement Top feed: 10 to 100 mm (3/8 to 4 inch) Bottom feed: 10 to 35 mm (3/8 to 1 3/8 inch) 38

39 Geopier Rammed Aggregate Piers Patented process (1993) Applications to date include support of structures and retaining walls on shallow compressible foundation soils Geopiers 39

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41 Design Methods FHWA design guidance for stone columns available in Barksdale & Bachus 1983 and FHWA-NHI Ground Improvement manual (2006). Geopier design provided by proprietary methods. A final design will usually consist of the number, diameter, length, spacing, and geometrical arrangement of aggregate columns and the required properties of the compacted stone after installation. 41

42 Aggregate Column Design Concepts Design for bearing capacity Design for settlement Design for uplift capacity Design for shear strength increase Design for seismicity Stone Columns Bearing Capacity 1. Cavity expansion theory (Barksdale and Bachus 1983a, and Elias et al. 2006a). 2. General bearing capacity and punching failure (Barksdale and Bachus 1983a). 3. Wedge failure method (Barksdale and Bachus 1983a) 4. Undrained shear strength method (Mitchell 1981b, Barksdale and Bachus 1983a, and Elias et al. 2006a). 5. Priebe s method (Priebe 1995). 42

43 Aggregate Piers Bearing Capacity 1. Cavity expansion theory (White and Suleiman 2004, and Wissman et al. 2001b). 2. A modified Terzaghi lower bound approach (Collin 2007a, Hall et al. 2002, and Wissman et al. 2001b). Aggregate Columns Bearing Capacity A conservative determination of the overall bearing capacity for both stone columns and rammed aggregate piers is obtained by neglecting the support contribution of the soft matrix soil. In this case the bearing capacity depends only on the vertical supporting capacity of the individual columns. 43

44 Aggregate Columns Bearing Capacity Using cavity expansion theory it can be shown that: 25 where F = factor of safety Stone Columns - Unit Cell Concept 44

45 Design Variables Area replacement ratio: S A stonecolumn A Stress ratio: n stone column cohesive soil Settlement ratio: N Settlement w / o columns Settlement w / columns Design Variables Typical spacing m ( ft) Typical diameter m (3 3.6 ft) Replacement ratio Stress ratio 2-6 Settlement ratio Column loads < 500 kn/column (110 kips/column) 45

46 Stress Distribution On unimproved soil: On improved soil: in situ soil stone column q 1 n 1 n q 1 n 1 Where: n = stress concentration ratio q = total stress on foundation = area ratio Design Process Embankment Stability Determine global stability without stone columns Assume area replacement ratio Assume stress ratio (2.5 for stability) Determine global shear strength Perform stability analysis Iterate if necessary 46

47 Design Considerations for Global Stability Global shear strength ( ) is function of the area replacement ratio (a s ) (spacing and diameter), the shear strength of the unimproved soil, and the frictional strength of the stone in the column = (1-a stone column )c + a stone column v tan Preliminary Estimate of Settlement Reduction Perform settlement analysis without stone columns Empirically determine for the area replacement ratio the settlement ratio Iterate if necessary with different area replacement ratio 47

48 Stone Columns Settlement 1. Equilibrium method (Mitchell 1981b, and Barksdale and Bachus 1983a). 2. Greenwood method (Barksdale and Bachus 1983a). 3. Finite element method design charts (Barksdale and Bachus 1983a). 4. Priebe s method (Priebe 1995). 48

49 Aggregate Piers Bearing Capacity 1. Two-layer approach (see references in technology product sheet). i. Specialized methods for upper layer ii. Conventional geotech methods for lower layer 2. Suleiman and White approach (Suleiman and White 2006). Bearing Capacity (ultimate load capacity) of Stone Columns q ult = cn c q ult = ultimate bearing capacity of stone column C = undrained shear strength of surrounding cohesive soil N c = bearing capacity factor for stone columns (18 Nc 22) 49

50 Liquefaction Mitigation Effective in densifying silty sands (<25% fines) Improvement of these soils is achieved by a combination of densification, reinforcement and drainage (increase density, increase soil confinement, control pore pressure development) QC/QA Methods FHWA design guidance for stone columns available in Barksdale & Bachus 1983 and FHWA-NHI Ground Improvement manual (2006). Geopier design provided by proprietary methods. Inspections, construction observations, daily logs, and record keeping are essential QC/QA activities for all technologies. These activities help to ensure and/or verify that: Good construction practices and the project specifications are followed. Problems can be anticipated before they occur, in some cases. Problems that do arise are caught early, and their cause can oftentimes be identified. All parties are in good communication. The project stays on schedule. 50

51 QC/QA Methods Stone Columns Gradation, specific gravity, loose density, and compacted density tests on the stone to be installed Minimum column diameter and compacted density of the stone During construction, stone consumption, in terms of buckets of a known weight or volume, monitored as a function of depth. For each stone column Location Measurement of rig verticality Elevation of top and bottom of each stone column Number of buckets of stone backfill in each stone column Amperage achieved as a function of depth. Time to penetrate and time to form each stone column Details of obstructions, delays, and any unusual ground conditions Digital data log of amperage, depth, and stone consumption QC/QA Methods Geopiers Gradation, specific gravity, hardness of stone Minimum column diameter and compacted density of the stone For each geopier Footing and pier location. Pier length and drilled diameter. Planned and actual pier elevations at the top and bottom. The number of lifts and time of tamping for each lift placed. Average lift thickness for each pier. Documentation of any unusual conditions encountered (e.g., sloughing). Type and size of densification equipment used Bottom STAbilization test (BSTA) is used to verify piers have an adequate stabilized bottom involving re-tamping the bottom of the piers to verify that displacement is within acceptable limits. 51

52 QC/QA Methods Aggregate Columns Post-construction QC/QA is dependent on the specific application and the type of ground in which the stone columns are installed. In-situ testing (SPT, CPT, or PMT) conducted at central points between the columns. Penetration resistance should be verified against values that were used to determine column spacing. The Dynamic Cone Penetrometer (DCP) is used in upper reaches of geopiers to verify density. If the columns are to support a structure or embankment, load tests are sometimes required to determine the short-term capacity and settlement of the column. Short-term load tests should be conducted (versions of ASTM D1143, Standard Test Methods for Deep Foundations Under Static Axial Compressive Load) on individual columns after all pore pressures induced by construction have dissipated. Budget Estimate Stone Columns Minimum cost of stone backfill can account for over 40% of total cost (will vary from site to site) Minimum cost of vibro-replacement $45/lm ($14 / lft) Minimum cost vibro-displacement starts at $60/lm ($18 / lft) Site specific load test costs Verification costs 52

53 Budget Estimate - Geopiers Mob/Demob minimum $5,000 - $15,000 /rig Cost per pier $75/lm ($23 / lft) Modulus test $5,000 each 53