Pore Pressure Ratio

1. Discuss the relevance of the pore pressure ratio in the context of Bishop’s method of slope stability analysis. Include in your discussion practical ways of obtaining values of the pore pressure ratio.
ANS:
According to Bishopslope stability problems which includes reduction in pore water pressure , the loss in shear strength can be categorised into two types:
(i). Pore pressure is an independent variable. The total stress acting in the soil doesnot af-fect the pore pressure in this class. In this cse, the pore pressure is governed by the water level or ground water flow pattern or seepage. The long term stability problems, slopes of sand or gravel which are subjected to rpid drawdown of water comes in this class

ii. Pore pressure is a

Then porepressure ratio is equal to pore pressure coefficients B.
The knowledge of pore pressure parameters is essential for the determination of effective stress from total stress. Skempton gave the pore pressure parameters which express the response of pore pressure due to changes in the total stress under undrained conditions. These parameters are used to predict pore water pressure in the field under similar condi-tions.
Pore pressure coefficient A fluctuates with both the stress value and the rate of strain, due mainly to the variation of Δud with the deviator stress for a given soil. The value of A must be quoted for some specific point, e.g. at maximum deviator stress or at maximum effective stress ratio (σ1′/σ3) at maximum deviator stress it can vary from 1.5 (for a highly sensitive clay) to −0.5 (for a heavily over consolidated clay). So the value of A is required for deter-mining the consolidation of soil.
The value of B is needed to determine whether the soil is saturated or not. When B=1 the soil is saturated and if B<1 the soil is partially saturated.
3. For a soil that has cohesive component to its strength, describe the mode of slope failure and the general approach used to determine the factor of safety for such a

A railway cutting in London Clay was excavated in 1920, but now engineers are concerned about its stability. A senior engineer has suggested that the existing slope be analysed in terms of Bishop’s rigorous method. Discuss the reasons why you think Bishop’s rigorous method has been specified and explain all the relevant information that you would need to undertake the analysis.
The formula for the conventional method of analysis gives 15% error though it is on the safe side. However it gives the value less than the case. In the construction of new embankments or railway cutting this error can lead to high cost which aren’t even needed. So the rigorous method should be used.
This method satisfies all three conditions of equilibrium i.e force equilibrium in horizontal and vertical direction and moment equilibrium condition. The results from rigorous method are more accurate than that obtained from non rigorous methods.
In this method, the slope is cut into slices. The weight and weight components of all slices are determined. The angle of shearing resistance should be known. The value of factor of safety is determined based on the value of the pore pressure