Methods for determination of the coefficient of consolidation and the time-rate settlement - an overview

The main objective of this paper is to review the state-of-the-art for determining coefficient of consolidation (cv), which is required for predicting the rate of settlement of structures founded on cohesive soil deposits in order to check the settlement criteria of foundation design. In the past five to six decades, attempts have been made to propose methods for determining more reliable values of cv. All these methods are critically reviewed in this paper in terms of their limitations and practical use. It is found that most of the current methods attempt to fit the characteristic features of average degree of consolidation (U) versus time factor (T) relationship from the Terzaghi consolidation theory to the observed compression versus time data obtained in the laboratory oedometer test with incremental loading. The two conventional methods, namely the Casagrande logarithm of time fitting and the Taylor square root of time fitting methods, are still having much greater acceptance and use in geotechnical engineering practice as the standard methods in spite of their limitations. Other alternative methods such as inflection point (Cour, 1971), velocity (Perkin, 1978), rectangular hyperbola fitting (Sridharan and Rao, 1981) and revised logarithm of time fitting (Robinson and Allam, 1996) methods are also based on rational approaches and have potential for predicting reliable values, sometimes even better than the values obtained by standard methods. Mainly, due to lack of comparisons with field observations of time rate of settlement, and also to some extent because of more data reduction effort and sophistication of the method, the alternative methods have not been found suitable for routine applications. These alternative methods do not appear to offer apparent advantages as expected in geotechnical engineering at the cost of extra effort. Among the methods available for estimating the field values of cv, the Asaoka method is preferred, mainly because of its rational and simple procedures. A comparison of cv values obtained from the laboratory data and the field observations are required for assessing the suitability of all the proposed methods, but unfortunately it has not been reported extensively. However, the limited comparisons in this paper clearly show that cv from the field is an order of magnitude higher than that from the laboratory. This overview will be useful to engineering students and practicing engineers, and at the same time, it highlights the need for further research