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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