Strength characteristics of grouted sandy soil

Abstract

Seepage control is a big concern for embankments, dams, and other hydraulic structures constructed with sandy soil. Grouting, especially permeation grouting is one of the appropriate solutions for controlling seepage. However, cement-based permeation grouting is still a trial and error-based process both in the laboratory and field. In this study, attempts have been made to determine the strength characteristics of permeation grouted sandy soil. The properties of sand is Fineness Modulus (F.M.) 1.12, Permeability 8.84 × 10-6 cm/sec, Cohesion (c) 4.24 kN/m2 and Angle of internal friction (φ) 350.

A simple method has been devised to prepare grouted samples with water-cement ratios (W:C) of 2:1, 3:1, 4:1, 5:1 and the percentage of cement is respectively 11%, 8%, 6.5% and 5.5%. It is found that compressive strength of the samples prepared with 2:1 W:C ratio varies in the range between 200 kPa and 250 kPa after 28 days of curing. After 120 days of curing, the strength of the same samples increases up to 1000 kPa. The samples show ductile behavior and the failure strain is 9%. For low cement contents, i.e., 5:1 and 4:1 (W:C), the compressive strength varies from 28 kPa to 40 kPa, after 7 to 14 days of curing period. A significant increase was noticed in the compressive strength after 28 days of curing and it reached its peak after 90 days. Dry density increases whereas moisture content and void ratio decrease with the increase of cement content and curing age. It is also observed that, cohesion and Young’s modulus increase with the increase of cement content and curing age. Failure patterns of samples are column mode, collapse, axial split and shear for W:C of 5:1, 4:1, 3:1 and 2:1 respectively. However, failure pattern can change with increased curing age, e.g., for 2:1 (W:C) cured for 120, a failure pattern of slickenside was observed.

Consolidated Undrained (CU) triaxial tests were also performed on the grouted samples prepared with 11% (i.e., W:C of 2:1) cement cured for 7 days. It is observed that dilation occurred during the volume changes and pore pressure decreased with the increase of confining pressure. From the triaxial test, cohesion, c and angle of internal friction, ϕ are found to be 200kN/m2 and 37.4° respectively. In the case of triaxial test, the failure pattern is different, i.e., bulging failure pattern.

The changes of microstructures and chemical composition during hydration reaction were observed by the Scanning Electron Microscope (SEM), and Energy Dispersive Spectroscopy (EDS) respectively. Ettringite needles, C-S-H gel, C3S and C2S (2 Cao.SiO2) formation increases with the cement content and curing age, which means hydration and pozzolanic reaction increase as well. Ettringite is a needle-shaped crystal that contributes to the early strength development and works as the reinforcement, and C-S-H gel is like a glue binder that solidifies the samples. Particularly the cementitious products not only enhance the inter-cluster bonding strength but also fill the pore space. From these investigations, it is observed that the porosity decreases with the cement content and curing age, which means permeability decreases as well, without disturbing the soil microstructure. Permeation grouting is effective in increasing the strength and decreasing the permeability.