Paper Information

Journal:   MODARES CIVIL ENGINEERING JOURNAL   APRIL 2017 , Volume 17 , Number 1 #F0062; Page(s) 13 To 27.
 
Paper: 

PERFORMANCE OF STRESS VARIABLES IN PREDICTING THE SHEAR STRENGTH OF UNSATURATED SOILS

 
 
Author(s):  BAGHERIEH A.R.*, FARSIJANI A., FARPOUR R.
 
* DEPARTMENT OF CIVIL ENGINEERING, MALAYER UNIVERSITY
 
Abstract: 

Shear strength is one of the most important features in mechanical behavior of soils. The shear strength of unsaturated soils is still a controversial discussion among the researchers in this field. The methods of determining unsaturated shear strength are classified into two major categories. First, two independent stress variables known by matric suction and net stress are employed. Further, saturated and unsaturated strength parameters are considered to be independent. In other words, as soon as the pore water pressure becomes negative, the saturated effective friction angle and cohesion become invalid. This approach became significantly dominant since the validity of effective stress in unsaturated soils was questioned, as it was not clear how the collapse phenomenon can be described through effective stress concept. In the late 90s, some researchers referred back to effective stress concept and some ambiguity in explaining collapse was resolved. In this approach, effective stress is the main stress variable. Net stress and suction are combined into effective stress. The saturated and unsaturated shear strength parameters are assumed to be independent, and there is a smooth transition between saturated and unsaturated soil modeling. In this research these two approaches are compared by means of unsaturated direct shear experiments and some relevant experimental data from literature. The advantages and shortcomings of the mentioned methods are analyzed. In the direct shear experiments, a wide range of soil suction was applied to the samples. Therefore, it is possible to compare the effective stress and independent stress approaches in a wide range of suctions. The suctions of samples were measured by filter paper method. By plotting the failure envelopes in two approaches, the advantage of effective stress approach over the approach of independent stress variables is obvious. This advantage is especially drastic at higher suctions. The experimental data from literature similarly revealed this result. Thus, it can be stated that effective stress approach is simpler and less time consuming since the failure envelope is an identical unique line for all suctions and strength parameters of a soil at saturated and unsaturated states. Contrary to independent stress variable approach, it is not required to measure the strength parameters at various suctions. In other words, if the effective stress is properly estimated, the unsaturated shear strength can be predicted straightforwardly. Effective stress parameter is the key factor for appropriate evaluation of effective stress in unsaturated soils. One of the highly cited proposed equations for effective stress parameter is verified by experimental data. The values of predicted effective stress parameter and the values measured from experiment are plotted versus suction. There is a good agreement between the effective stress parameters calculated by the equation and those measured from experimental data. Therefore, it can be concluded that the empirical equation can accurately predict the effective stress parameter. It is worth mentioning that by normalizing the suction through dividing it into air entry suction, the effective stress parameter versus normalized suction becomes a unique line, regardless of soil type. Thus, the effect of soil type and its structure is normalized by means of using suction ratio.

 
Keyword(s): SHEAR STRENGTH, SUCTION, EFFECTIVE STRESS, UNSATURATED SOILS
 
References: 
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