Interlocking, and peak and design strengths

A. N. SCHOFIELD (2006).Ge´otechnique56, No. 5, 357–358J. B. Burland, Imperial College London, UKProfessor Schofield discusses two issues: (a) the peakstrength of remoulded overconsolidated fine-grained soil andits post-peak behaviour; and (b) the inclusion of interlocking(dilatancy) in plastic design strength. The critical stateframework embraces the state boundary surfaces on the‘wet’ and ‘dry’ side, with the critical state line acting as a‘watershed’ between the two. It has brought a most valuablecoherence to the understanding of the mechanical behaviourof soils. Its practical value is enhanced if its limitations areunderstood as well as its many advantages.Professor Schofield argues that the Hvorslev cohesiveintercept for remoulded overconsolidated fine-grained soilsresults from interlocking, or the rate of dilation, rather thanfrom cohesion between the soil particles. While this inter-pretation seems reasonable, it still requires experimentalverification. Also, an explanation is needed as to why, priorto the formation of slip surfaces, undrained (constant vo-lume) stress paths rise to the same Hvorslev boundingsurface as drained tests that are dilating (see Fig. 8.13 ofSchofield & Wroth, 1968).The uniqueness of the critical state line for a given soilis a concept that is based on the results of experiments ongranular soils and on remoulded clay soils (Roscoe et al.,1958). A close study of the 1958 paper reveals howcircumspect the authors were in putting forward thishypothesis for heavily overconsolidated clay soils. Theystressed the inconclusiveness of the experimental evidence,which was due to the limitations of the triaxial test, therelative magnitude of the corrections that had to be appliedat the low stresses involved, and the difficulty of determin-ing the moisture content in a localised thin region ofconcentrated shearing—a point emphasised by ProfessorSchofield in his letter. The discussion that followed thepublication of the 1958 paper concentrated on the interpre-tation of the experimental evidence for critical states forclay soils on the ‘dry’ side.In the time since the publication of the 1958 paper theinfluence of the structure and fabric of clay soils has beenstudied extensively. For remoulded fine-grained soils there isevidence to suggest that, within a localised thin layer, pro-gressive orientation of clay particles can take place, resultingin stress ratios that fall below critical state values andculminating in residual strengths. For example, the discusserpresents experimental and microscopic evidence for theformation of thin slip surfaces in an undrained test onremoulded normally consolidated kaolin in which a signifi-cant reduction in stress ratio takes place subsequent to theformation of the slip surface (Burland, 1990, Figs 45–47).The fact that clay particles can orientate progressively duringshearing is undeniable, and the possibility of this occurringwithin a thin layer of gouge cannot be overlooked. It is notobvious that a soil element within such a thin slip surfacemust pass through a critical state (in p9, q, e space) prior tothe particles within it commencing progressive orientation.Thus the critical state line appears to form a useful target,but the discusser believes that it would be wise to retain thecaution that was evident in the original 1958 paper, in thelight of recent understanding of the influences of structureand fabric.The influences of structure become particularly significantin natural soils that have both bonding and fabric. Thecritical state framework for reconstituted samples of the soilprovides a valuable frame of reference for understanding andinterpreting the effects of structure for the natural soil(Burland, 1990).Can dilatancy be included in plastic design strength?Professor Schofield argues that it cannot, and recommendsusing CS friction in limiting equilibrium design methods.The logical conclusion seems to be that, for granular materi-als, we should ignore the very significant increases in thepeak angle of shearing resistance that increases of densitygive. Geotechnical engineers and clients would need muchconvincing if this suggestion was implemented for the de-sign of foundations, retaining walls, tunnels and—to a lesserextent—slopes, as the cost would be significant. An impor-tant factor that seems to have been largely ignored in thedebate about the use of critical state strengths in design isthe rate at which strength reduces beyond peak, and hencethe assessment of the potential in a given problem forprogressive local failure to take place during or subsequentto loading. Such an assessment is problem and soil depen-dent, and the writer urges caution in the use of blanketassertions about the use of critical state strengths in design.There is also a need for guidance as to how precisely thecritical state line and critical strengths should be determinedin natural structured soils.E. T. R. Dean, University of the West Indies, Trinidad andTobagoProfessor Schofield raises interesting questions on thenature of the strength of fine-grained soils, and on theselection of strength parameters for design. In this contribu-tion I should like to add some practical considerations, andone further question.Professor Schofield’s question (a) refers to remoulded,reconstituted soil. In practice we often deal with soils thatare ‘undisturbed’, in the sense that they have not beenremoulded or reconstituted. Examples would include thefoundations of many structures, which essentially rest ononly undisturbed, in situ material. In other cases we dealwith material that is partially disturbed, such as when stiff orhard clay is trucked to a site and compacted as a foundationmaterial or an embankment. For some of these materials a20% shear strain may perhaps not be sufficient to achievethe same kind of remoulding, but might well constitute‘failure’ in an engineering sense. In Trinidad and othertropical regions there are highly structured clays formed bythe weathering of rock. Offshore, worldwide, it is commonto find ductile clays whose remoulded strengths may be onehalf of their undisturbed strengths: the undisturbed strengthsare typically used in offshore pile design. Tailingsdams retain fine-grained materials formed from mechanical-chemical processes. They may be very different from inert,remoulded, reconstituted ‘laboratory’soils.It would be very interesting to this reader if the following