Vinod Kumar Singh

This study presents an enhanced hydraulic fixed-piston (HNS-N) sampler that reflects a local practice. This sampler was employed at a deltaic site along with two other types of samplers: a similar hydraulic (HNS-K) type with no vacuum breaker and an oil-operated (ONS) sampler that is equipped with a vacuum breaker and is fixed at the center of a casing. The generation of a slowly but erratically increasing pressure in the hydraulic samplers often does not ensure constant speed and full penetration; thus, the pressure in these samplers was increased rapidly after the drill rods were filled with water. By contrast, the pressure in the ONS sampler was increased instantaneously. The hydraulic samplers exhibited slightly discrete penetration behavior, whereas the ONS sampler induced constant penetration. The sample qualities of the samplers are ordered based on an evaluation via two nondestructive methods and a consolidation test, as follows: ONS > HNS–N > HNS–K. The sample quality was affected signifi...

This research studied the application of samplers under three fixity conditions during sampling tube penetration and evaluated the quality of the retrieved samples. Original and portable types of oil-operated fixed-piston samplers with identical sampling tubes were employed at a site in the Nakdong River Delta. Suction, shear wave velocity and consolidation tests were conducted to assess sample quality. The soil fabric, including holes, nodules, roots and shells, rather than soil sensitivity (4 to 28) was found to significantly affect sample quality, particularly suction. The sample quality indicators, except for the suction-based values, near the centre of each sampling tube tended to vary depending on the bulk density of the ground. The original type of sampler fixed at the centre of the steel casing produced samples of slightly better quality than those produced by the same sampler freely located in the borehole and those produced by a portable sampler tightly located in the steel casing. This finding was also supported by different qualities of samples obtained using the oil-operated and mechanical samplers at a different site. It is thus concluded that the fixity of samplers is a minor factor that influences sample quality.

The deformation modulus of sands below 30 m in the Nakdong River Delta was estimated by using various in-situ tests, such as the piezocone penetrometer (CPT), seismic dilatometer (SDMT), and pressuremeter tests (PMT). Disturbed sand samples retrieved at several depths of two boreholes were reconstituted to simulate the in-situ deposited condition using a slurry method. Thereafter, the drained triaxial compression (TX) and the resonant column (RC) tests were also conducted. The elastic modulus obtained through the application of the TX and PMT results to conventional methods led to the underestimation of the in-situ values, because the inherent characteristics of the natural sands were not considered. Thus, the non-linear modulus degradation curves were constructed using the modulus degradation from the TX, RC, PMT, and the in-situ maximum shear modulus from SDMT. Thereafter, the modulus at 25% failure stress strength was re-evaluated. The re-evaluated values from the TX were in good agreement with those from the PMT, using the unloading curves and with the existing empirical equation based on the seismic CPT. Finally, a CPT-compatible empirical formula for the deformation modulus at 25% failure strength was proposed.

An empirical method of estimating the shear strength of deep sands from the Nakdong River Delta was developed. The piezocone penetration test (CPT/CPTu) and sampling of disturbed sand at several depths were performed at two sites. A total of 39 isotropically consolidated–drained triaxial tests on reconstituted samples provided by a slurry displacement method were conducted to estimate the shear strengths. The sand deposits are classified into three types based on the physical properties and the soil behavior type index (I_c). The six existing empirical formulas considered in the present study, which are based on the overburden stress and cone resistance, either underestimate or overestimate the shear strengths of the sands obtained from the triaxial tests. By contrast, the newly developed empirical formula, which accounted for the types of sands in addition to the two other factors, matches well with the triaxial results.