Binquet and Lee

Critical Review on “Bearing Capacity Analysis An alysis of Reinforced Earth Slabs” by Jean Binquet and Kenneth Lee Lokit Khemka CE14M032 This paper addressed an analytical solution for bearing capacity of a strip footing on a granular soil containing horizontal layers of flat metal strips or ties with relatively high tensile strength. The paper also compared the analytical results r esults with experimental data, besides giving a cost analysis. This paper was one of the first to use the concept of Bearing Capacity Ratio (Ratio of contact pressure of the footing on the reinforced soil to unreinforced soil at the same vertical settlement). BCR can be used to express the effect of the reinforcement. The analysis was almost entirely theoretical. The following assumptions are made during analysis: (i)

As the footing load increases, the footing and the soil beneath move down while the soil to sides move outward. The boundary between the downward moving and outward moving soil is the locus of the points of maximum shear stress at every depth. It has been observed in experimental study that tie breaks coincided approximately with these assumed slip surfaces.

(ii)

The slip surfaces are assumed to be symmetric on both sides of the footing. However, the actual failure will almost always tend to develop on only one side, if the footing is free to rotate. Nevertheless, up to the point of a major c atastrophic failure, the footing will probably settle more or less uniformly. Hence, this assumption should be valid.

(iii)

The stresses are independent of whether or not the foundation soil is reinforced. The

(iv)

extra load carrying capacity is from the tensile strength of the reinforcement. The central zone of soil moves down with respect to the outer zone along the slip surface and it drags the ties along with it. At the slip surface the ties are assumed to undergo two right angle bends around two frictionless rollers.

(v)

It is also assumed that the tie force per layer l ayer varies inversely with the number of layers, N, in the foundation. As mentioned in the paper, this assumption is arbitrary with no explanation behind it.

(vi)

Forces are evaluated for the same size of footing and the same settlement for a footing

(vii)

on unreinforced and reinforced soil. The shear and normal stresses at any point are calculated from elastic theory. This assumption is valid since granular soils can be considered to be elastic to some extent.

From the experiment data, it is clear that there is a remarkable agreement between theory and experimental data both for the bearing capaci ty failures and for the mode of failure. This is method however, have some limitations. For instance, this method is useful only for granular soils and for strip footings. Moreover, upon analysis, we find that tie resistance in same in every layer. The length of the reinforcement obtained from this method is much higher than that from other theories. Hence, this method c an give a somewhat uneconomical design.