Estimation of stiffness modulus and settlement of stone column improved composite sandy soil based on in-situ tests

• Main Author: Boneo, Ulysis B.
• Format: text
• Language: English
• Published: Animo Repository 2021
• Subjects: Columns; Composite materials; Soil liquefaction; Civil Engineering
• Online Access: https://animorepository.dlsu.edu.ph/etdm_civ/3
• Institution: De La Salle University

Considering foundations that are to be built on areas with weak soil and high seismic activities, primary and post liquefaction settlements become a major problem that engineers must resolve. Out of several techniques available for improving the weak subsurface, stone columns technique is one of the ideal option. Stone columns increase the strength of weak soil and at the same time eliminates the occurrence of soil liquefaction during seismic activities. But a relatively bigger foundation settlement is expected with the use of stone columns as compared to other deep soil improvement methods. While there are already a lot of studies on the estimation of settlement of stone columns-improved foundations, there are only few available data gathered and analyzed based on the actual in-situ foundation settlement. This paper presented and analyzed the calculated expected settlement versus the actual settlement of five 14m diameter tank foundations with stone columns. Quality of stone columns was maintained by ensuring that the crowd pressure on the vibratory hammer reaches 100 bars before pulling it up for the next lift. The expected settlement was calculated using PLAXIS 3D Finite Element Method (FEM) of design, utilizing the stiffness modulus of stone column-improved composite soil which was derived from the result of Standard Penetration Test (SPT) between the stone columns and Static Modulus Test (SMT) done on stone column. The actual settlement of foundations was taken during the hydrotest of the five tanks. It was observed that on the average, the calculated expected settlement is 50% bigger than the actual settlement during hydrotest. It was concluded that that the bigger values of calculated expected settlement or lower value of calculated stiffness modulus of stone column improved composite soil was due to: (1)design assumption of constant minimum cross-sectional diameter of stone columns along its whole length; and (2)the under-estimation of N-SPT values of soil between the stone columns where SPT was carried out midway between the installed stone columns. A numerical factor of 1.2 can be multiplied to the calculated composite elastic modulus to balance the conservativity of the design and economic implication.