CPT-Based Assessment of Densification After Ground Improvement with Rigid Inclusions and Rammed Aggregate Piers® (RAP)

Within the scope of this manuscript, the mitigation performance of a composite ground improvement solution, which is composed of 18 m long 40 cm diameter GeoConcrete® Column (GCC) and 50 cm diameter Impact® Rammed Aggregate Pier® (RAP) along with 40 m long 80 diameter piles is assessed by pre- and post-cone penetration testing (CPT). These elements are designed for controlling excessive total and differential settlements, and liquefaction triggering at a paper mill site. In this paper, the site geology, geotechnical model, design aspects of GCC and Impact® RAP patented systems and QA/QC measures are discussed. As a mitigation solution, 18 m and 40 m long elements are designed to be constructed in the soft to medium stiff silty clay with scattered silt and sand interlayers. Improvement expectations from GCC and Impact® RAP elements are partially verified by pre- and post-CPT data, and are listed as: (i) densification of cohesionless silt and sand layers, (ii) shear stress transfer to rigid columns during cyclic (seismic) loading, reducing seismic demand from foundation soils (iii) increased horizontal stresses, leading to increased soil (and column) stiffness and strength, (iv) vertical drainage through aggregate columns to dissipate cyclically – induced excess pore water pressures. The results show that due to ramming and vibration induced-densification, cone tip resistance has increased by a factor of 1.3–1.6 in cohesionless layers.