An idealized framework for the analysis of cohesive soils undergoing desiccation: Discussion

The discussers would like to congratulate the authors for a well-presented paper on an important subject area that has received relatively little attention in the past. The authors should also be commended for applying the proposed model to the analysis of carefully conducted field tests (Ayad et al. 1997). Recently, the discussers have studied several aspects of desiccation and crack formation in clays (Kodikara et al. 1998; Lau et al. 1991; Lau 1987). The cracking problem is complex and the model proposed by Konrad and Ayad (1997) provides an idealized framework for modeling desiccation cracking for a special class of problems. In order to stimulate further work on this area, the discussers would like to offer the following comments. The authors have assumed that the clay remains saturated during cracking and that K0 conditions are applicable up to the onset of cracking. The authors also indicate that the model is applicable to slurries, consolidated natural soils, and compacted clays. The discussers agree that the shrinkage behavior of unconsolidated or lightly consolidated heavy clays is generally dominated by normal shrinkage, during which the soil remains saturated. For example, slurried Regina clay does not desaturate until the soil is subjected to more than 1 MPa of suction. Hence, the proposed model provides a satisfactory approach utilizing the stress paths based on the effective and total stresses. However, there is evidence that some light clays deviate from this behavior and primarily show residual shrinkage as the soil desaturates (e.g., Bronswick 1988). Similarly, heavily overconsolidated clays as well as compacted clays predominantly behave in an unsaturated manner during desiccation. Since the tensile strength of these clays is higher than that for unconsolidated clays, suctions exceeding 100 kPa can develop at the point of initial cracking. Under these suctions, these soils are most likely to be unsaturated. For example, soil-water characteristics measured for compacted soil specimens of Indian Head till have shown that the air entry value is less than 100 kPa even for specimens compacted about 6% wet of optimum (Vanapalli et al. 1996). Also, compacted clays are placed in an unsaturated state (i.e., as low as 77% saturation) in engineering practice. In view of the above comments, it would appear that the proposed approach may be applicable for some clays, but it may be necessary to more thoroughly examine the deformation behavior of soil before applying the model to all clay soils. The authors refer to the paper by Morris et al. (1992) and indicate that their approach is not suitable for crack initiation primarily because it uses an unsaturated soils framework. Lau (1987) also used the same approach for the prediction of crack depths in soils. In this study, the stresses (i.e., σi – ua, where σ is the total normal stress, ua is the air pressure, and the subscript i denotes x, y, and z cartesian axes) degenerate to the traditional effective stresses at saturation. Using unsaturated soils framework and the K0 loading condition (Lau 1987; Morris et al. 1992), the net horizontal stress (σx – ua) can be expressed as