Investigation on performance of a large circular pit-in-pit excavation in clay-gravel-cobble mixed strata

A unique pit-in-pit (PIP) excavation, which comprised a large diameter circular pit outside and a smaller deeper rectangular pit inside, was constructed in clay-gravel-cobble mixed strata. To guarantee project safety as well as characterize behavior of PIP excavation, investigate influence of inner pit excavation on outer pit performance and explore lateral earth pressure mobilized in discrete geomaterials, this project was extensively instrumented throughout construction. Field data indicated that it performed distinctly from those excavations in literature. This excavation caused relatively smaller wall and ground displacements, both of which featured distinctive profiles; current empirical and semi-empirical approaches for predicting excavation performance were not applicable to this case any more. To control performance of PIP excavation, it is of paramount importance to limit lateral wall movement of inner pit. Contrary to previous recognition, basal rebound due to excavation took place not only in soft clay, but also in stiff to hard clay, clay-gravel-cobble mixtures, and even decomposed bedrock. Different from those in fine grained soils, magnitudes of lateral earth pressures against retaining wall in clay-gravel-cobble mixtures showed a wide range of scattering and their thrusts were transferred to wall mainly via contact force of rock particles. Classic Rankine theory substantially underestimated while current empirical apparent earth pressure (AEP) diagrams highly overestimated magnitude of lateral active earth pressure of clay-gravel-cobble mixtures. For design of PIP excavation, the ground between inner and outer retaining walls should be treated as the passive state during excavation of outer pit, while as the active state during subsequent excavation of inner pit when greater increment of deflection occurred to inner wall than outer wall. Casting head of inner wall inside base slab of outer pit prior to excavation of inner pit was a cost-effective solution for enhancing excavation performance; socketing wall toe into underlying decomposed bedrock significantly mitigated potential risk associated with wall kicking.