Soil Stabilization

artificially changing soil properties for construction purposes (by physical or chemical methods) at the natural site.

As a result of soil stabilization, the bearing capacity of the foundation of the structure is increased and its strength, watertightness, resistance to washout, and other properties are improved. Soil stabilization is widely used in the construction on sagging soils of industrial and civil buildings; for strengthening the banks adjoining highways in a hollow or the walls of a ditch, where the ground is saturated with water; for preventing landslides; in sinking shafts and in creating filtration-proof barriers in the foundations of hydraulic structures; for protecting concrete structures (foundations) from aggressive industrial effluents; and for improving the bearing capacity of piles and of large-diameter supports. Soil stabilization is achieved by injecting cementing materials or chemical solutions into the ground and also by applying electric currents to the ground or heating or cooling it. The basic methods of soil stabilization are cementation, argillization, bituminization, silicification, • resinification, methods using electrochemical or thermal action, and artificial freezing.

Cementation consists in injecting a cement suspension into the soil to be stabilized (such as fissured, rocky soil or soil containing sand and gravel) through a system of boreholes drilled in the soil. The suspension has a mass ratio of cement to water in the range of 0.1 to 2. To increase the mobility of dense cement solutions and cement-sand solutions a sulfitealcohol vinasse can be added, in the amount of 0.01–0.25 percent of the quantity of cement. Additions of calcium chloride, in the amount of 1–5 percent of the quantity of cement, can be used to regulate the acceleration of setting of the solution and increasing the initial strength of cement stones. The stability of the soil and its watertightness increase significantly after cementation.

Hot bituminization, as well as cementation, is used in cavernous rocky strata where subsurface flow attains a high speed. The purpose of hot bituminization is to seal those larger cavities that can not be sealed off by cementation because of the high speed of subsurface flow. Hot bitumen is injected into the cavities and fissures of cavernous rock through boreholes equipped with injectors. In cold bituminization a finely dispersed bitumen emulsion is injected into the ground. This method is used for very narrow fissures in rocky soil and for stabilization of sandy soils.

Argillization is used to reduce filtration capacity of fissured, rocky cavernous strata and of gravelly soils. In this method a clay suspension, with a small amount of a coagulant added, is inserted under high pressure into the fissures of the stratum.

The silicification method is based on the use of silicate solutions. To stabilize medium-grained sands the so-called two-solutions method is used. It consists of successively injecting into the ground solutions of sodium silicate and of calcium chloride. As a result of the reaction, silica gel is obtained. This gel imparts to the soil considerable strength and watertightness. Fine-grained sands are stabilized using the one-solution method of silicification, that is, using a solution of sodium silicate with phosphoric acid added (Figure 1). In loess soils only a solution of sodium silicate is injected into the ground. The role of the second solution is played by the salts of the soil itself.

Figure 1. Diagram of equipment used in silicification of soils: (1) tank containing binder, (2) tank containing acid, (3) pump “ND,” (4) mixer, (5) control console with recording instruments, (6) injector, (7) pneumatic drill used to sink injectors into the soil, (8) stabilization contour

Resinification consists in injecting into the ground an aqueous solution of carbamide resin, with additions of hydrochloric acid, oxalic acid, or ammonium chloride. This method is used to stabilize and to improve the strength and watertightness of fine-grained sandy soils.

In argillaceous soils, where injection of solutions into the soil is not feasible, an electrochemical stabilization method is used. This method is based on passing a direct electric current through the soil, into which a solution of calcium chloride has been introduced. As a result, the soil is dehydrated and packed. Exchange reactions occurring in the regions adjacent to electrodes are also conducive to the packing and stabilization of the soil. Electrochemical stabilization can be subdivided into electrodrying, electropacking and electrostabilization.

Thermal stabilization is used to strengthen sagging loess soils. In this method the soil to be stabilized is sintered by gaseous products of fuel combustion, having a temperature of 700° to 1000°C. Combustion is most effective if conducted within the stratum of the soil to be stabilized (Figure 2). Stabilization of unstable water-bearing soils can be achieved by artificially freezing the soil.

In the USSR all-Union symposia on stabilization and packing

Figure 2. Diagram of equipment used in thermal stabilization of sagging loess soils by combustion of fuel directly within a borehole: (1) sagging soil, (2) nonsagging soil, (3) compressor, (4) pipeline for cold air, (5) container for liquid fuel, (6) pump feeding fuel into the borehole, (7) fuel feed pipeline, (8) filter, (9) nozzle, (10) gate and combustion chamber, (11) borehole, (12) region of thermal soil stabilization

of soils are conducted periodically. Records of such symposia are published in special collections.