Development, Procedure, and Results for the Constrained Modulus of Gravel and Crushed Rock Test

Constrained modulus (M_s) has recently found use in predicting deflection and buckling of buried pipe. Crushed rock and gravel are preferred materials for buried pipe support and previously only presumptive values of M_s were available. A one-dimensional compression test procedure to measure the M_s of coarse-grained soils (maximum particle sizes ranging from 0.5 inch to 1.5 inch) has been developed. A test specimen is placed in a 17.5 inch inside diameter, 17.3 inch high Teflon^TM lined steel pipe operating as a floating ring. A series of increasingly greater axial loads is applied, and axial deformation is measured. M_s is then calculated for each incremental pressure. Samples may be tested at low, medium, and high densities at varying degrees of saturation. Thirty-five tests were completed on five clean, crushed rock samples, one river rock sample, and one crushed concrete sample obtained from different sources in the Colorado area. Proper testing procedures and equipment, including friction and scale effects adjustment factors, were developed. It is demonstrated that M_s of hard, uniformly graded and shaped crushed rock,river rock and crushed concrete: (1) Is not greatly dependent on stress level; (2) Is strongly dependent on placement compaction effort; (3) Is weakly dependent on placement moisture condition; (4) Varies significantly with changes in crushed rock particle characteristics.