Improvement of selected coastal soils of Bangladesh using cement

Abstract

This study is carried out on three coastal soils by cement mixing collected from Matarbari (Coxbazar), Dhankhali (Potuakhali), and Mirshorai (Chittagong) where three different ground improvement techniques namely Deep Cement Mixing (DCM), High Vacuum Densification Method (HVDM) and Prefabricated Vertical Drain (PVD) were implemented respectively.

Using Unified Soil Classification System (USCS), the soils from Matarbari, Dhankhali and Mirshorai, are classified as Clayey Sand (SC) of medium plasticity, Sandy Silts (ML) of low plasticity, and Poorly-graded Sand with Silt (SP-SM) of no plasticity, respectively. The strength and deformation properties of untreated soils were determined by the Unconfined Compressive Strength (UCS) tests on specimens (38 mm diameter and 76 mm height) prepared by Standard Proctor Compaction at Optimum Moisture Content (OMC). The unconfined compressive strength (qu) of the untreated specimens of Dhankhali and Mirshorai soils are found in the range of 76 kPa to 71 kPa. Their failure pattern is observed as Y-shaped for Dhankhali soil and axial splitting for Mirshorai soil. However, Matarbari soil shows barreling failure pattern and the strength is 42 kPa considering at 15% strain.

Cement mixed specimens were prepared in the laboratory considering the field implementation of DCM process and five governing factors that affect the properties of soil-cement column in the field. Using plastic molds, specimens of 50 mm diameter and 100 mm height were prepared for UCS tests for each of four assumed in-situ dry densities (12,13,14 and 15 kN/m3) mixed with four different cement dosages namely 100, 150, 200, 240 kg/m3. Separate sets of specimens were prepared using fresh-water and saline-water. To prepare a specimen, predetermined quantities of the ingredients i.e. soil, cement and water were mixed manually and poured in the mold which was then covered with plastic bags and cured for either 28 days or 56 days. For all the three types of soils, it has been found from the results of lab-treated specimens that the strength increases and deformation decreases with the increase of cement dose, decrease of water content, and increase of curing periods at a specific untreated dry density. Clay content (≤ 0.002 mm) is found to have a significant role in strength development. Matarbari soil with the highest clay content (5.6%) is found to have the least qu whereas Mirshorai soil with least clay content (1.5%) shows the highest qu at each in-situ dry densities and specific cement dose. The higher strength development of Mirshorai soil may be attributed to greater amount of sand fraction (93%).

As for characteristics of molding water, the strength of freshwater mixed specimen is found to be greater than that of saline water mixed specimen at all cement doses and in-situ dry densities for Matarbari and Dhankhali soils. However, no conclusive comment can be made on the effect of molding water type for Mirshorai soil.

A few core samples of the field-treated soil for UCS tests were collected from Matarbari site, where the DCM technique was applied using slurry prepared by sea water with a fixed cement dose of 240 kg/m3 and water/cement ratio of 1.0. These specimens were different in size from the laboratory prepared UCS specimens but the length to diameter ratio were the same (2:1). The qu values were around 1740 kPa that is 1.24 times greater than the qu value of Lab-treated specimen at the same cement dose and 2.54 times greater than the qu value of untreated soil. The field specimens show a significant brittleness with deformation around 0.85mm.