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89 800 700 600 Horizontal Load (kips) 500 400 300 200 Measured Full-Scale 100 Simplified (Load minus Rankine & Adhesion to D=12.5ft) Simplified (Load minus Rankine & Adhesion to D=7.5ft) 0 0 0.5 1 1.5 2 2.5 3 Pile Cap Deflection (in) Figure 7-19. Comparison of simplified procedure to measured results. Note that the simplified procedure as presented requires evaluated in the FE parametric study are slightly different to adjustment at lower magnitude loading because full passive those constructed and tested in-situ. The effect of increasing the resistance is not mobilized at small deflections. To compute depth of treatment on lateral resistance is shown in Figure 7-20 the contribution of the treated block at less than full pas- where depth is relative to the ground surface. As indicated in sive resistance, the method requires that the passive resis- Figure 7-20, the relative magnitude of the improvement in lat- tance be reduced to the mobilized resistance as a function of eral resistance does not increase linearly in proportion to the deflection. This condition would require an iterative approach depth of treatment. These results suggest that the proposed use using estimates of the mobilized passive resistance versus of a truncated depth is a suitable simplification. Recall that the deflection. simplified procedure truncates the treated soil block at a depth A comparison of the measured and modeled (simplified equal to the outside-to-outside dimension of the piles in the procedure in conjunction with GROUP) behavior of the cap group perpendicular to the direction of loading. and center pile in the leading row is provided in Table 7-2. The proposed simple design procedure appears to provide reason- able agreement with full-scale test measurements, sufficient for 7.6 Design Recommendations general design purposes. Pile Group Improved with Cemented Soils The results shown in Figure 7-19 indicate that the proposed or Flowable Fill procedure for truncating the depth of treated soil for design purposes produces a design that is slightly conservative. This Assuming that the treated soil has a compressive strength of slight conservatism seems appropriate for design, particularly at least 75 psi, the following simplified design approach is rec- when a simplified method is employed. Furthermore, the ommended for the purpose of estimating the additional lateral reduced benefit of treatment with depth is indicated by param- resistance provided by the ground improvement: etric analyses conducted using an FE model. The results from the parametric FE model study are shown 1. Based on the proposed geometry of the treatment area, in Figure 7-20. Note that the ground improvement dimensions compute the magnitude of passive resistance acting on Table 7-2. Comparison of results from full-scale test and simplified procedure. Measured Simplified Pile Cap Rotation 0.76 0.96 Pile Deflection at a Depth of 7.5 ft beneath Ground Surface 1.14in 1.22in Maximum Bending Moment in Leading Row, Center Pile 100 to 160 k-ft 195 k-ft Depth to Maximum Moment beneath Ground Surface 15.5ft 13.5ft

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90 Figure 7-20. Summary of results from FEM parametric study of jet grout treatment. the leading face of the block using Rankine earth pressure 3. Compute the total lateral resistance contributed by the theory with the following constraints: treated block as the sum of the passive resistance and adhe- The surface area of the leading face for passive resis- sion acting on the projected area of the treatment block. tance calculations should be the projected area defined Reduce the external horizontal loading force magnitude by a 45 angle projecting from the pile cap (in all applied to the pile cap by the lateral resistance contributed directions) if ground improvement is adjacent to the by the treated block. pile cap. 4. Use the reduced horizontal forces described in Step 3 in The depth dimension of the projected area should be GROUP (or a similar software package) to estimate the truncated at a depth below ground surface equal to the contribution of the pile group to the total foundation per- outside-to-outside dimension of the piles in the group formance (e.g., deflection, moment, shear, etc.). perpendicular to the direction of loading. 5. The total foundation resistance to force effects is the sum 2. Compute the magnitude of adhesion acting along the sides of the lateral resistance provided by the treated soil block of the treatment block using pass = 0.9. The sides of the and the lateral resistance provided by the pile group, (i.e., contributing treatment block (defined by a 45 angle the unreduced foundation forces from Step 4). from the pile cap) are trapezoidal in shape and limited 6. If force-deflection calculations are necessary, the side shear to the geometric constraints noted in Step 1. A three- can be assumed to develop with a displacement of approx- dimensional weighted average is used to estimate the imately 0.25 in. and the passive force-displacement curve undrained shear strength acting along the sides (assuming can be computed using a hyperbolic curve as described by the shear strength profile varies with depth). If the treat- Duncan and Mokwa (2001). ment block projected area contains a horizontal base, the adhesion acting on the base of the block also should be Example 1--Soil Cement Wall Adjacent included. If the treatment depth is deeper and/or the base is to Pile Group not horizontal, a tension crack is expected to develop behind the trapezoidal block beneath the bottom of the pile Consider a typical existing foundation for a bridge pier cap, thereby nullifying any base adhesion. that includes a 3 4 group of 1-ft diameter piles spaced at

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91 12ft 12ft 3ft 9ft 7ft (a) Plan View (b) Elevation View Figure 7-21. Existing foundation prior to ground improvement. a 3-ft center-to-center spacing in both directions as shown mass of treated soil is created that substantially exceeds the in Figure 7-21. The piles are structurally fixed (full moment in-situ strength of the virgin soil. connection) to a 3-ft-thick reinforced concrete pile cap. To model the geotechnical benefit provided by the ground The top of the pile cap is at the ground surface. The direc- improvement, the simplified method described herein is tion of horizontal loading induced by the structure is as used to compute the additional resistance provided by the shown. ground improvement. This additional resistance is then sub- The soil is soft to medium clay with a uniform undrained tracted from the lateral demand imposed on the foundation. shear strength of 500 lb/ft2. The total unit weight of the soil is Upon determining the reduced lateral demand, the problem 110 lb/ft3 and the water table is at a depth of 10 ft beneath the can be modeled as one would typically do using commercially ground surface. available software such as GROUP, or similar. Note that when Due to an anticipated increase in structural demand applied using GROUP, the option for considering an "embedded pile to the foundation, the lateral resistance (geotechnical) pro- cap" should not be activated. vided by the existing foundation requires enhancement. There- fore, ground improvement is being considered as shown in Analysis Procedure Figure 7-22. The ground improvement involves introducing 1. Compute the magnitude of passive resistance acting on and mixing Portland cement into the virgin soil such that a the projected area at the leading face of the ground 5ft 8ft 5ft 20ft (b) Elevation View (a) Plan View Figure 7-22. Existing foundation after ground improvement.

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92 7ft 19ft 1ft (b) Elevation View (a) Plan View Figure 7-23. Existing foundation after ground improvement with projected areas. improvement block. The projected area is defined by 45 b. Adhesion along Sides, Fs: angles projecting from the pile cap edges (in all directions) and is truncated a depth below ground surface equal to the Fs = A s pass c u = 54 ft 2 0.9 500 lb ft 2 = 24 k outside-to-outside pile spacing perpendicular to the direc- tion of loading as follows (see Figure 7-23): c. Projected Area of Horizontal Base, Ab: a. Projected Area of Leading Face, ALF: A b = 19 ft 1 ft = 19 ft 2 A LF = 19 ft 7 ft = 133 ft 2 d. Adhesion along Base, Fb: b. Average Effective Vertical Stress at Leading Face of Projected Area, v: Fb = A b pass c u = 19 ft 2 0.9 500 lb ft 2 = 9 k v = 110 lb ft 3.5 ft = 385 lb ft 3 2 e. Cumulative Adhesion along Sides and Base, Fa: c. Passive Pressure Acting on Leading Face of Projected F = Fs + Fb = 24 k + 9 k = 33k Area, pp: 3. Compute the total lateral resistance contributed by the p p = 2c u + v = ( 2 500 lb ft ) + 385 lb ft = 1, 385 lb ft 2 2 2 treated block, F, as the sum of the passive resistance, Fp, and cumulative adhesion, F, acting on the projected area of the d. Passive Force Acting on Leading Face of Projected treatment block: Area, Fp: F = Fp + Fa = 184 k + 33k = 217k Fp = A LF p p = 133 ft 1, 385 lb ft = 184k 2 2 4. Compute the reduced horizontal load to be used in foun- 2. Compute the magnitude of adhesion acting along the sides dation analyses, Preduced, by subtracting the total lateral and horizontal base (if applicable) of the projected area of resistance contributed by the treated block, F, from the the treatment block using an adhesion factor, pass, equal external horizontal loading force magnitude applied to the to 0.9. foundation, P: a. Projected Area of Sides, AS: Preduced = P - F = P - 217k A s = 2 sides [(5 ft 3 ft ) + (1 ft 4 ft ) + ( 1 2 4 ft 4 ft )] 5. Use Preduced to analyze the foundation using commercially = 54 ft 2 available software such as LPILE, GROUP, FBPier, etc.

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93 14ft 11ft 10ft (a) Plan View 14ft (b) Elevation View Figure 7-24. Existing foundation after ground improvement. Example 2--Soil Cement Wall Around a Pile Group b. Average Effective Vertical Stress at Leading Face of Projected Area, v: Consider the same pile group foundation and soil profile v = 110 lb ft [ 3.5 ft + ( 7 ft 2 )] = 715 lb ft described in Example 1. Due to an anticipated increase 3 2 in structural demand applied to the foundation, the lat- eral resistance (geotechnical) provided by the existing c. Passive Pressure Acting on Leading Face of Projected foundation requires enhancement. The ground improve- Area, pp: ment involves introducing and mixing Portland cement p p = 2c u + v = ( 2 500 lb ft ) + 715 lb ft = 1, 715 lb ft 2 2 2 into the virgin soil beneath the cap and around the piles as shown in Figure 7-24 such that a mass of treated soil is cre- ated that substantially exceeds the in-situ strength of the virgin soil. To model the geotechnical benefit provided by the ground improvement, the simplified method described 11ft herein is used to compute the additional resistance pro- vided by the ground improvement (see Figure 7-25). This additional resistance is then subtracted from the lateral demand imposed on the foundation. Upon determining the reduced lateral demand, the problem can be modeled as one would typically do using commercially available soft- (a) Plan View ware such as GROUP, or similar. Note that when using GROUP, the option for considering an embedded pile cap should not be activated. Analysis Procedure 7ft 1. Compute the magnitude of passive resistance acting on the projected area at the leading face of the ground improve- ment block. The projected area is defined by 45 angles pro- jecting from the outer edges of the piles and is truncated at a depth below ground surface equal to the outside-to- outside pile spacing perpendicular to the direction of load- ing as shown in: (b) Elevation View a. Projected Area of Leading Face, ALF: Figure 7-25. Existing foundation after ground improvement with A LF = 11 ft 7 ft = 77 ft 2 projected areas.

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94 d. Passive Force Acting on Leading Face of Projected Area, resistance of the group. To apply the simplified method to this Fp: case, we recommend that an equivalent "pile cap" geometry be constructed to the outside edges of the pile group as illus- Fp = A LF p p = 77 ft 2 1, 715 lb ft 2 = 132k trated in Figure 7-26. In this case, the base of the equivalent pile cap would be the top of the cemented soil zone. Using this 2. Compute the magnitude of adhesion acting along the sides equivalent pile cap geometry, the resistance can be estimated and base of the projected area of the treatment block using using the same approach as that described for the case with the an adhesion factor, pass, equal to 0.9. piles in Examples 1 and 2. a. Projected Area of Sides, AS: Construction Considerations A s = 2 sides (14 ft 7 ft ) = 196 ft 2 For new construction, ground improvement can be performed most economically and efficiently if it is performed prior to b. Adhesion along Sides, Fs: foundation construction. Clearly, if piles or drilled shafts are not in place, then a wider variety of treatment methods can Fs = A s pass c u = 196 ft 2 0.9 500 lb ft 2 = 88 k be employed to create a soilcrete block. For example, in the absence of piles, the soil could be treated to a depth of 5 meters c. Projected Area of Base, Ab: using mass soil mixing. For high-moisture-content soils, the dry A b = 11 ft 14 ft = 154 ft 2 method could be used while the wet method could be used for other soils. Wet or dry mixing by column methods could be d. Adhesion along Base, Fb: performed to much deeper depths, if necessary. Piles could be driven through the treated soil if driving was performed within Fb = A b pass c u = 154 ft 2 0.9 500 lb ft 2 = 69 k a day or two after treatment, otherwise, pre-drilling might be required. If new piles were driven prior to soil treatment, soil e. Cumulative Adhesion along Sides and Base, Fa: mixing techniques could still be used to treat the soil around the periphery of the pile group prior to construction of the F = Fs + Fb = 88 k + 69 k = 157k cap so that the treated soil would be in direct contact with the exterior piles. However, jet grouting would be required to treat 3. Compute the total lateral resistance contributed by the the soil between the piles to create intimate contact between the treated block, F, as the sum of the passive resistance, Fp, interior piles and the treated soil. Because jet grouting is more and cumulative adhesion, F, acting on the projected area of the treatment block: expensive than other treatment methods and the piles would restrict access, the cost of treatment would be high. F = Fp + Fa = 132 k + 157 k = 289k 4. Compute the reduced horizontal load to be used in foun- dation analyses, Preduced, by subtracting the total lateral resistance contributed by the treated block, F, from the external horizontal loading force magnitude applied to the foundation, P: Preduced = P - F = P - 289k 5. Use Preduced to analyze the foundation using commercially available software such as LPILE, GROUP, FBPier, etc. Adaptation for Pile Groups with Bent Caps Some states use pile bents where piles extend from the ground to a support beam under the deck without including a pile cap in the ground. The piles then extend down below the bent cap, often through water, and into the ground without having a cap that comes in contact with the ground. The results of the field tests conducted during this study indicate that even Figure 7-26. Equivalent pile cap though a pile cap is not present, the piles and cemented soil geometry for use with piles with still function as a composite block, which increases the lateral a bent cap.

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95 For retrofit of existing structures where piles and pile caps or abutment back wall. In addition, the granular zone should are already in place, soil mixing methods could be used to cre- be compacted to a minimum of 95% of the modified Proctor ate a soilcrete wall adjacent to the pile cap, but contact would maximum unit weight. The passive resistance for the pile cap not be achieved between the underlying piles and the treated can then be computed using the following procedure: soil. Jet grouting would allow the creation of a soilcrete zone around the periphery of the pile group that would also be in 1. Compute the ultimate passive force per width (Ep) for the contact with the piles below the cap to improve lateral resis- dense granular zone using the pile cap or abutment back- tance. Core drilling through the pile cap would be required to wall height (H) and unit weight () of the dense granular allow jet grouting to be performed below the pile cap that soil using the equation: would again increase the cost of treatment. E p = 0.5 H2K p cos (12) Pile Group Improved by Excavating Clay Use the log-spiral method to compute the Kp value with and Replacing with Compacted Granular Fill the friction angle of the dense granular zone and a wall Based on available test results and limited numerical friction () equal to 0.7 times the friction angle of the results, the compacted granular zone should extend at least soil. 6 pile diameters below the ground surface and 10 pile diam- 2. Compute the passive force ratio (PFR) for geometry and eters beyond the edge of the pile face in the direction of load- frictional properties of the dense and loose granular soils ing. The lateral resistance can then be analyzed using the using Equation 1. properties of the compacted fill with a computer program 3. Multiply the passive force computed in Step 1 by the PFR such as GROUP. to obtain the horizontal passive force per length for the combined geometry. 4. For long abutments, multiply the passive force per length Pile Group Improved by Compacting by the actual length of the abutment wall to obtain the total Narrow Dense Granular Soil Adjacent horizontal passive force on the wall. For pile caps, multiply to the Pile Cap in Loose Sand the passive force per length by the effective width of the cap, The dense compacted granular zone should have a mini- which is equal to the actual width of the cap multiplied by mum width of 3 ft and extend 2 ft below the base of the pile cap the R3D factor to account for 3D end shear effects.