In many cases shallow foundations supported by geogrid-reinforced soil may be subjected to cyclic loading.

This problem will primarily be encountered by vibratory machine foundations. Das [13] and Das and Shin [14] reported laboratory model test results on settlement caused by cyclic loading on surface foundations supported, respectively, by reinforced sand and saturated clay.

The model tests of Das [13] were conducted with a square model foundation on unreinforced and geogrid-reinforced sand. Details of the sand and geogrid parameters were:

The laboratory tests were conducted by first applying a static load of intensity qs (= qu (R) /FS; FS = factor of safety) followed by a cyclic load of low frequency (1 cps). The amplitude of the intensity of cyclic load was qdc(max) . The nature of load application described is shown in Fig. 7.25. Figure 7.26 shows the nature of variation of foundation settlement due to cyclic load application

Sec with qdc (max) /qu (R) and number of load cycles n.  This is for the case of FS = 3. Note that, for any given test, Sec increases with n and reaches practically a maximum value  Sec (max)  at  n =  ncr . Based on these tests the followingconclusions can be drawn.

1. For given values of FS and n, the magnitude of Sec /B increases with
the increase in qdc (max) /qu (R) .

2. If the magnitude of qdc  (max) /qu  (R) and n remain constant, the value of Sec/B increases with a decrease in FS.

3. The magnitude of ncr for all tests in reinforced soil is approximately the same, varying between 1.75 × 105
 and 2.5 × 105  cycles. Similarly, the magnitude of  ncr  for all tests in unreinforced soil varies between 1.5 × 105  and 2.0 × 105  cycles.

The variations of Sec (max)  /B obtained from these tests for various values of qdc (max)  /qu (R)  and FS are shown in Fig. 7.27.  This  figure  clearly  demonstrates

the reduction of the level of permanent settlement caused by geogrid reinforcement due to cyclic loading. Using the results of Sec (max) given in Fig. 7.27, the variation of settlement ratio ρ for various combinations of qdc (max) /qu (R) and FS are plotted in Fig. 7.28. The settlement ratio is defined as

From Fig. 7.28 it can be seen that, although some scattering exists, the settlement ratio is only a function of qdc (max) /qu (R) and not the factor of safety, FS.

Laboratory model test results on continuous foundations with similar loading conditions as those described above (Fig. 7.25) in reinforced saturated clay were provided by Das and Shin [14]. General parameters of the test program were as follows:

Model foundation:  Continuous; B = 76.2 mm

Clay: Moisture content = 34%
Degree of saturation = 96%
Undrained shear strength, cu = 12 kN/m2
Reinforcement:  Geogrid; TENSAR BX1100

The general nature of the foundation settlement curve [for a given FS and qdc (max) /qu (R) ] obtained is shown in Fig. 7.29, which can be divided into three major zones. Zone 1 (for n = 1 to n = nr ) is a rapid settlement zone during which about 70% of the maximum settlement [Sec  (max) ] takes place. The magnitude of nr is about 10. Zone 2 (n = nr to n = ncr ) is a zone in which the settlement continues at a retarding rate reaching a maximum at n = ncr. For n! ncr, the settlement of the foundation due to cyclic loading is negligible. The magni-
tude of ncr for reinforced soil varied from 1.8 × 104 to 2.5 × 104 cycles.

Figure 7.30 shows the summary of the tests conducted, and it is a plot of Sec(max) /B for various combinations of qdc  (max)  /qu  (R) and FS. It is important to note that, for FS = 4.27, Sec (max) for reinforced soil was about 20% to 30% smaller than that in unreinforced soil.

 FIGURE 7.25 Nature of load application—cyclic load test

 FIGURE 7.26  Plot  of  Sec /B  versus  n (after  Das  [13])  ( Note:  For
reinforced sand u/B = h/B = 1/3; b/B = 4; d/B = 1-1/3)

 FIGURE 7.27  Plot  of  Sec(max) /B  versus  qdc(max) /qu(R) .  (after
 Das  [10])  (Note:  For  reinforced  sand,  u/B  = h/B = 1/3, b/B = 4, d/B = 1-1/3)

FIGURE 7.29  Nature   of  variation  of  foundation  settlement  in  clay      
due  to  cyclic  load  application


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