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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

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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.

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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

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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.

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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.

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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).