Evaluation of Consolidation Degree from CPTu using Rahardjo (2016) Method – Case Study of Consolidating Soil in East Kalimantan

Abstract

A geotechnical investigation comprised 13 CPTu and 23 dissipation testing were performed on a site in East Kalimantan due to landslide triggered by 15 to 30m thick soft clay underlying 75m high overburden embankment. The investigation covers an area of about 250m x 275m wide on the area next to the toe of the embankment. From geological map, the site situated at borders of Balikpapan Formation (Tmbp) and Kampung Baru Formation (Tpkb) with Alluvium (Qa) formation from Heliosen period nearby. Measurement from penetration showed high pore-water pressure higher than the hydrostatic pressure. Result from dissipation testing showed that the soft clay is still consolidating with residual excess pore pressure (u_f) still exist. Rahardjo (2016) Method was developed using CPTu data as its basis especially soft clay data to determine overconsolidation ratio (OCR). However, it can also be used to determine the degree of consolidation. The method uses Pore Pressure Ratio (Bq) value, excess pore pressure divided by net cone resistance – Bq = Δu/(q_t – σ_v0), which was obtained when performing CPTu test. The formula proposed was 1/(1.2Bq+0.1). The method stated that the value of Bq=0.75 equals to OCR=1 which is showing a normally consolidated clay. Bq value higher than 0.75 will show a degree of consolidation of a consolidating soil and for Bq value lower than 0.75 will show an OCR value of overconsolidated soil. Applying Rahardjo (2016) Method to the dissipation data performed showed an agreement on soft clay where the soil is still consolidating. However, when applied to over consolidated soil near surface this method will show a greater OCR value.