Cover Image

Not for Sale



View/Hide Left Panel
Click for next page ( 80


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 79
79 60 55 50 45 Passive Resistance (k) 40 35 30 25 20 15 10 GROUP Model Herbst (2008) 5 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 Pile Cap Deflection (in) Figure 7-5. Computed passive resistance acting on pile cap in virgin soil. to displace the cap the same distance. A comparison between the field load test and the applied horizontal force in the cali- the passive resistance reported and that computed by the brated model. GROUP model is shown in Figure 7-5. The good agreement The pile cap rotation predicted by the GROUP model was appears to further validate the GROUP model. then compared to that which was measured, as shown in Fig- ure 7-7. Although not perfect, adequate agreement between the two exists. These results again seem to confirm the applicabil- 7.3 Comparison with Results from ity of the GROUP model. Tests Involving Mass Mixing The additional resistance provided by the mass mix is equal The calibrated model and the procedure for estimating the to the difference between the load applied during the test and contribution of the pile cap to lateral resistance described the load applied in GROUP to displace the cap the same above provides a means of directly evaluating the benefit of distance. A comparison between the virgin soil (passive) resis- using mass mix soil improvement adjacent to a pile cap. Mass tance and that observed during the mass mix test (computed mix soil improvement was performed adjacent to Pile Cap 1. by the GROUP model) is shown in Figure 7-8. The magnitude In plan view, the treatment was 4 ft parallel to, and 11 ft per- of additional resistance provided by the mass mix is consid- pendicular to, the direction of loading. The treatment depth ered to be a combination of passive resistance acting on a por- was 10 ft, which resulted in improved soil adjacent to and tion of the leading edge of the mass mix block and adhesion 7.5 ft beneath the bottom of the pile cap. Details of the mass along the sides of the block. mix procedure are provided in Section 3.8 and Herbst (2008, Appendix 3). The unconfined compressive strength of the 7.4 Development of mass mix material was reportedly on the order of about 130 psi Simplified Method and was therefore substantially stronger than the in-situ fine-grained soil. The simplified method proposed for design is based on Similar to the approach used for Test 1 (virgin soil, pile cap estimating the contribution of the treated ground around the embedded), the magnitude of the applied horizontal load pile group using a limit equilibrium analysis of the treated soil was reduced in the GROUP model until the field-measured mass. This analysis includes passive resistance acting against deflection and GROUP model deflection were nearly equal, as the face of the treated soil mass and adhesion acting on the shown in Figure 7-6. This procedure was performed for each sides of the treated soil mass as lateral displacement mobilizes loading increment. The passive resistance provided by the the passive soil resistance. pile cap plus mass mix at various deflections was computed as To model the effect of ground treatment using generally the difference between the actual applied horizontal force in available computer software, the contribution of the treated

OCR for page 79
80 500 450 400 350 Horizontal Load (kips) 300 250 200 150 100 Measured Full-Scale 50 GROUP Model 0 0 0.25 0.5 0.75 1 1.25 1.5 Pile Deflection at Bottom of Cap (in) Figure 7-6. Comparison of load-deflection curves for pile group test involving mass mixing. soil to the lateral resistance of the foundation is modeled by subject to limitations on the projected area and the depth of reducing the magnitude of horizontal load applied to the the treated area relative to the width of the group. foundation by the estimated amount of passive and adhesive resistance provided by the improved (treated) area. The limit Passive Resistance Acting on the Face equilibrium analysis is used to estimate the magnitude of pas- of the Treated Soil Mass sive resistance acting on the leading face of the treated block and the amount of adhesive resistance acting on the sides and Based upon the computed magnitude of passive resistance the base of the block. The geometry of the treated soil mass is acting against the leading face of the pile cap (50 kips at 1-in. 500 450 400 350 Horizontal Load (kips) 300 250 200 150 100 Measured Full-Scale (low) Measured Full Scale (high) 50 GROUP Model 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 Pile Cap Rotation (degrees) Figure 7-7. Comparison of measured and computed load-rotation curves for test involving mass mixing.

OCR for page 79
81 250 200 Passive + Adhesion (kips) 150 100 50 Mass Mix (GROUP Model) Virgin Soil (GROUP Model) 0 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 Pile Cap Deflection (in) Figure 7-8. Additional lateral resistance provided by mass mixing estimated by the GROUP model. deflection), and the dimensions of the embedded cap (8.75 ft this relationship so as to provide a total lateral resistance against by 2.5 ft), a unit value for passive resistance acting against the the cap equal to that determined from the previous compar- cap in virgin soil can be computed as follows: isons of measured contribution of the pile cap. Since pp has been determined to provide an average value of 2.29 ksf and v p p = 50 k ( 2.5 ft 8.75 ft ) = 2.29 ksf (9) can be estimated from the unit weight of the soil, cu can be determined in the upper foot. The undrained shear strength Because of observed desiccation of the crust and associated profile used to estimate passive and adhesive resistance is shown cracks in the soil matrix, it is considered likely that the effective in Figure 7-9, along with the effective vertical stress and unit shear strength contributing to passive lateral resistance in the passive resistance (Rankine 2-D) profiles. upper 1 ft of the soil mass would be slightly less than the strength measured using the relatively small cone penetrometer or lab- Adhesion Acting on the Sides oratory tests on relatively small samples. and Base of the Treated Soil Mass With the computed unit passive resistance determined above, Rankine earth pressure theory can be used to determine The parametric study using finite element analyses provided the undrained shear strength profile over the depth of the pile an evaluation of the effect of treatment depth on the lateral cap. Note that the details of the strength profile within this resistance contributed by the treatment. One of these analyses depth range do not change the calibrated GROUP model dis- considered treatment only to the bottom of the cap (depth of cussed previously because the piles are embedded beneath 2.5 ft) and adjacent to the leading face of the cap. The zone of this zone and are not affected by the shear strength surround- treatment in the model was 4 ft in the direction of loading and ing the pile cap (cap not embedded in the GROUP model). 9 ft perpendicular to the direction of loading, slightly smaller The following expression for Rankine passive resistance was than actually tested due to limitations in the geometry of the used to correlate the relationship between the undrained shear model. The results indicate that 45k of additional resistance strength of the soil and the lateral soil resistance acting against resulted from creating this treated block above the magnitude the cap: of the resistance provided by virgin soil. This additional resis- tance is likely to have been developed by adhesion along the two p p = 2c u + v (10) sides and the base of the treatment block, assuming the unit passive resistance acting on the leading face of the block is equal Below the top 1 ft of soil, this relationship was used with the to that of the pile cap with identical dimensions. shear strength profile used in the GROUP model. In the upper Because the dimensions of the treated block are known and 1 ft of soil, the shear strength profile was back-calculated using the magnitude of additional resistance provided by adhesion

OCR for page 79
82 ksf 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 0 1 2 Depth below Ground Surface (ft) 3 4 5 6 7 8 9 10 11 12 Undrained Shear Strength Effective Vertical Stress 13 Rankine Unit Passive Resistance Bottom of Pile Cap 14 Figure 7-9. Variation of undrained strength, effective vertical stress and Rankine unit passive resistance vs depth at the virgin soil profile. on the block is known, an adhesion factor (pass) can be deter- Limitations Related to the Geometry mined based on a comparison of the adhesion acting on the of the Treated Soil Mass block with the undrained shear strength of the soil. This adhesion factor represents the ratio of the adhesion to the The parametric FE studies generally indicate a linear increase undrained shear strength, a parameter that is similar but not in lateral resistance when the improvement dimension paral- identical to the alpha factor () used for the axial resistance of lel to the direction of loading increases. Intuitively, this linear the driven piles (which was 0.85). Of course, considering the increase should continue to an upper bound that is defined by different construction methods and materials, there is no rea- the entire soil layer around the pile cap and having the prop- son to assume that the adhesion on the treated block would erties of the treated soil. be the same as the adhesion on a driven steel pile. From Fig- Alternatively, the relative amount of improvement with ure 7-9, the average undrained shear strength acting along the increasing depth of treatment is observed to decrease. Beyond depth of the pile cap (0 to 2.5 ft) is 1.07 ksf; the undrained a depth of about three times the pile cap embedment depth shear strength adjacent to the bottom of the pile cap is 0.79 ksf. (equal to 7.5 ft for this study), relatively little increase in lat- The adhesion factor (pass) can then be estimated as follows: eral resistance is observed. The small amount that is observed is likely a result of the treatment affecting the lateral behavior 45 k = ( 2 sides 2.5 ft 4 ft 1.07 ksf pass ) of the piles as opposed to only providing additional resistance to the cap. The simplified design approach presented herein + (1 base 9 ft 4 ft 0.79 ksf pass ) conservatively neglects the slight additional resistance that pass = 0.9 (11) may exist beneath a depth of three times the pile cap embed- ment depth. Accordingly, a simplified approach to estimate the addi- Iterative analyses indicate the geometry of the constructed tional lateral resistance provided by a block of improved soil block (treated soil) is not identical to the constructed dimen- adjacent to the pile cap in the direction of loading has been sions with regard to surface area available for passive and adhe- developed. This simplified approach has been calibrated to sive resistance. For this purpose, a "projected area" is proposed. the parametric studies conducted using an FE model and is The projected area is defined by a line projecting at a 52 angle presented in Chapter 6. The parametric studies, which utilized from the heel of the leading edge of the pile cap through the a model calibrated to the full-scale load test results, investigated treated block. All surface area above this projected line is the relative improvement of lateral resistance gleaned by vary- available for either passive resistance or adhesion. This pro- ing both the width and depth of treatment. The results of these jected area is shown relative to the FE parametric study in Fig- studies are shown in Figures 7-10 and 7-11. ures 7-12 and 7-13.

OCR for page 79
83 Figure 7-10. Results from FE parametric depth study for soil mixing. Figure 7-11. Results from FE parametric length study for mass mixing.

OCR for page 79
84 0 Adhesion -1 Mass Mix (2 Sides) Pile Cap Treatment -2 Passive -3 52 Resistance -4 Pile -5 -6 -7 Projected Area Contributing to Additional Lateral Resistance -8 (Typical) -9 -10 -11 -12 -13 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Figure 7-12. Projected area available for passive and adhesive resistance (1 of 2). The 52 angle defining the projected area was determined defined by a 52 angle, the benefit of treatment and applica- to provide a best fit with respect to a comparison between the bility of this simplified approach should be truncated at a results of the parametric study and the simplified method. depth equal to 3 times the pile cap embedment depth assum- For design purposes, the projected area can conservatively be ing the depth dimension of the treatment controls. defined using a 45 angle, which is recommended. Further- Note that the sides of the contributing treatment block more, based on results of the FE parametric study as well as defined by the projected area are trapezoidal in shape. Accord- the investigated treatment geometries using a projected area ingly, a three-dimensional weighted average is necessary for 0 Adhesion -1 Mass Mix (2 Sides) Pile Cap Treatment -2 Passive Resistance -3 52 -4 Pile -5 -6 Adhesion - Base (where applicable) -7 -8 Projected Area Contributing to Additional Lateral Resistance -9 (Typical) -10 -11 -12 -13 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Figure 7-13. Projected area available for passive and adhesive resistance (2 of 2).