Earthworks — if stiffness is important specify and test for it

Abstract

Civil and geotechnical engineers routinely develop specifications for earthworks associated with civil structures (industrial warehouses, buildings, embankments, pavements, dams, reinforced earth retaining walls etc.) with the intention usually of controlling the stiffness and strength of the material in its as placed condition. This paper focuses on the stiffness of the landform resulting from the earthworks, where the term stiffness is used to describe the deformation behaviour of the material when subjected to loading. It is the authors’ experience that the indirect methods of controlling stiffness rely on a combination of material, placement, compaction, moisture and earthworks control requirements. Furthermore, such requirements do not, under all circumstances, control the in situ stiffness of the soil. This in turn can result in earthworks requirements being specified which are not necessary to result in adequate stiffness i.e. over specification; or earthworks requirements which do not necessarily result in adequate stiffness being specified, i.e. under specification. Furthermore, the typical earthworks specification has not changed much in 40 years, yet the typical performance requirements for superstructures supported on the earthworks have changed. On this basis, what in the past may have been ‘stiff enough’, may not be adequate and then for other uses may be grossly over specified. It is the authors’ opinion that the above issues can be addressed by direct measurement of the stiffness. Such measurements need to be understood within the limitations of characterising complex soil behaviour using simple models, e.g. elastoplasticity. This paper presents six case studies where direct measurement of stiffness assisted the authors in addressing the limitations discussed above. Further, it presents results of stiffness measurements taken by means of plate load testing, settlement plates, large-scale oedometers and large-scale loading tests. The authors hope that the paper will encourage practitioners to ‘specify and test for stiffness where it matters’.