Laboratory Study of the Shear Strength and State Boundary Surface of a Natural Lumpy Soil

Landfills consisting of waste natural lumpy soils from open-pit mining are being deposited. The shear strength of excavated lumpy soils is important for designing the landfills. To this end, a series of oedometer and triaxial tests (including conventional, constant $p^{\prime }$, and undrained triaxial tests) were performed on a natural lumpy soil taken from an open-pit mine in Germany. In addition, reconstituted and natural intact counterparts were investigated as a reference. The test data reveal that interlump porosity is substantially reduced because of the rearrangement of the lumps at a stress level of 100 kPa, which is significantly lower than the yield stress of the natural lumps (4,380 kPa). The critical state line of the natural lumpy soil can be defined in terms of the $p^{\prime }\text{-}q$ relationship; however, it is not unique in the $v\text{-}{p}^{\prime }$ compression plane, which depends on the loading stress path. The critical state points of the natural lumpy soil ($p^{\prime }\text{-}q$ plane) are located above the critical state line of the reconstituted soil. The diagenetic soil structure of the natural lumps may be responsible for the difference in critical state between the natural lumpy soil and the reconstituted soil. After being normalized by the Hvorslev equivalent pressure, the fitting line of the critical state points of the natural lumpy soil crossed that of the reconstituted soil. The difference between the critical state of the lumpy soil and the reconstituted soil diminished with the increase of the consolidation stress.