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OCR for page 71
71
500
450
400
Lateral Force (kips)
350
300
250
200 Virgin Soil
Length = 3 ft
150
Length = 4 ft
100 Length = 5 ft
Length = 6 ft
50
Length = 7 ft
0
0.0 0.5 1.0 1.5 2.0
Displacement (in)
Figure 6-17. Results of parametric study on the length of the
jet grout treatment zone beside the pile cap on the computed
load-displacement curve.
crete, the increase of the lateral capacity is mainly caused by the on the increase in lateral pile group resistance. For the first of
increase of the shear area of the jet grouting soilcrete. There- these studies, the improved soil block is assumed to be at the
fore, these results indicate that a lower strength soilcrete, which side of the pile cap and have a length of 4 ft in the direction of
could be produced with a lower cost treatment method, could loading, a width of 9 ft perpendicular to the loading direction,
produce the same adhesive resistance as that obtained with jet and a depth of 12.5 ft (see Figure 6-19).
grouting. This finding increases the potential that soil improve- The compressive strength of the improved soil (f c) is
ment methods can be a cost-effective approach for increasing assumed to vary over a wide range between 21 and 7700 psi.
lateral pile group resistance. Young's modulus is assumed to abide by the same relation as
that of conventional concrete [E = 57000(f 0.5
c) ] and is in the
range of 260 to 5000 ksi, which covers the typical strengths of
6.7 Material Strength Effect on mass mixed and jet grouted soil (see Table 6-1). Since the
Lateral Pile Group Resistance improved soils with the Young's moduli shown in Table 6-1
Based on the results from the previous parametric studies, are much stiffer than the virgin clay, the improved soils have
another set of parametric studies was performed to investi- been modeled as linear elastic materials.
gate the effect of the strength of the soil improvement zone The load-displacement curves computed using the FEM
model are presented in Figure 6-20 and the improvement ratio
is plotted versus compressive strength in Figure 6-21. As shown
2.0 in Figure 6-20, the load-displacement curves all plot on top of
each other for the range of compressive strengths investigated.
1.8
As a result, the improvement ratio is essentially constant rela-
tive to the compressive strength of the treated zone as shown
Improvement Ratio
in Figure 6-21. These results show that the lateral capacity of
1.6
the pile group is not sensitive to the compressive strength of the
treated zone, which is as expected since all of the improved soils
1.4
are much stiffer than the virgin clay. Therefore, for practical
y = 0.0595x + 1.1592 purposes, the improved soils can be considered to act as a rigid
1.2 2
R = 0.9976
block for the range of material properties in Table 6-1. The
numerical model suggests that the lateral capacity of the pile
1.0 group is sensitive to the geometry of the improved soil, but not
0 1 2 3 4 5 6 7 8
to the material strength (or Young's Modulus), provided the
Length (ft) improved soil is much stiffer than the virgin clay. The typical
Figure 6-18. Improvement ratio as a function of mass mix and jet grouting soilcrete are much stiffer than the
jet grout treatment length adjacent to the cap. soft clay.
