Geogrid reinforcement can reduce the settlement of shallow foundations that are  likely  to  be subjected to impact loading.  This is shown in the results of laboratory model tests in sand reported by Das [13]. The tests were conducted with a square surface foundation (Df = 0; B = 76.2 mm). TENSAR BX1000 geogrid was used as reinforcement. Following are the physical parameters of the soil and reinforcement:

The idealized shape of the impact load applied to the model foundation is shown in Fig. 7.31, in which tr and td are rise and decay times, and qt (max) is the maximum intensity of the impact load. For these tests the average values of tr and td were approximately 1.75 s and 1.4 s, respectively. The maximum settlements observed due to the impact loading Set (max)  are shown in a nondimensional  form  in  Fig.  7.32.  In  this  figure,  qu and  Se  (u) , respectively, are the ultimate bearing capacity and the corresponding foundation settlement on unreinforced sand. From this  figure it is obvious that

1. For a given value of  qt (max) /qu, the foundation settlement decreases with an increase in the number of geogrid layers.

2. For a given number of reinforcement layers, the magnitude of  Set (max)
increases with the increase in qt (max) /qu .

The effectiveness with which geogrid reinforcement helps reduce the settlement can be expressed by a quantity called the settlement reduction factor R or

Based on the results given in Fig. 7.32, the variation of R with qt (max) /qu and d/dcr is shown in  Fig. 7.33. From the plot it is obvious that the geogrid reinforcement acts as an excellent settlement retardant under impact loading.

FIGURE 7.31 Nature of transient load

FIGURE 7.32   Variation  of  Set(max) /Se(u)  with  qt(max) /qu           
 and  d/B (after  Das  [13])

FIGURE 7.33  Plot  of  settlement  reductionfactor  with  qt(max)
/qu  and  d/dcr (after  Das  [14])


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