Settlement of strip footing on sand mat placed over thick soft clay deposit

Abstract

Shallow foundations on soft soils deposit without any improvement can undergo a high reduction in volume after consolidation and secondary settlement. For low and medium rise building projects on such soil condition, a deep foundation may not be economically feasible. For such a case an alternative to deep foundations may be shallow strip footings placed on a double layer foundation system in which the upper layer is untreated or cement treated compacted sand.

This research work deals with the specific case of the bearing capacity of a rigid plane-strain footing placed on the surface of a soil consisting of a uniform clean or treated sand layer overlying a thick, homogeneous bed of soft clay. The study considered both the cases where the thickness of the sand layer is thin or thick compared to the footing width. In all cases the ground surface and the interface between the two soil layers is taken as horizontal. It is assumed that the response of the clay layer is undrained for plastic loading stages and drained in consolidation stages and the response of the sand layer is drained in all loading stages.

The settlement of the shallow strip foundation resting on layered soils, where upper layer as untreated or treated sand layer and bottom clay layer has been analyzed in this research work. Parametric study has been conducted to determine the effect of thickness, density, cementation of sand mat and density, shear strength of the soft clay layer on the settlement of strip foundation. A better insight of elasto-plastic, consolidation and creep settlements of a strip footing on sand mat under different footing pressure equivalent to low or moderately loaded low to medium rise residential or commercial building loads has been developed. Guidelines has been established for designing shallow strip footing with sand mat on thick soft clay deposit determining the thickness of sand mat for different material characteristics to avoid punching shear failure and to limit the settlement to an allowable level. The relative settlement (S/So) at the center point of the footing for both untreated or cement treated upper sand mat was calculated, where So is the settlement for the case with Hi= 0.25m and S is the settlement for other thickness of upper sand mat. A larger value of relative settlement S/So indicates larger difference of settlement between the cases of small and larger thickness of sand layer. From this study it is concluded that the relative settlement S/S0 may be considered as the index of the effectiveness of sand layer. A guideline is developed for strip footing on soft inorganic NC soil of Bangladesh having void ratio from 1.0 to 1.45. The research work was limited to a single value of E' and ϕ' of soft clay layer and also a single ϕ' value of sand mat. These design charts may be used to obtain total settlement for a given value of footing pressure, sand mat thickness, footing width and initial void ratio. For untreated or cement treated sand as upper layer, settlement of strip footing may be calculated for particular value of Hi, q, B and γsat using developed design chart or equations. For a specific value of settlement the design thickness of sand mat may be obtained using the proposed charts. The permissible settlement as per BNBC 2015 is 50mm. Design thickness of sand mat for 50mm settlement may be directly obtained from these charts. For Hi/B =0.6 or above, settlement of Untreated Sand as upper layer into bottom clay layer is very small which represented the distribution of major deformation within upper layer and the influence zone of footing is limited in upper layer which indicates complete effectiveness of upper layer in bearing the foundation effectively. For Hi =0.75 or above settlement of footing with cement treated sand as upper layer and soft clay as bottom layer is very small which means the distribution of major deformation within upper layer and the function of footing is limited in upper layer which also indicates complete effectiveness of upper layer in bearing the foundation effectively. Brittle behavior of cemented sand and fracture or cracks is not considered in this analysis.