Chemical reinforcement is mostly used for handling collapsibility accidents. The method of injecting certain chemical solutions into the foundation soil to generate cementitious substances or activate the surfaces of soil particles through chemical reactions, cementing and solidifying at contact points to enhance the bonding between soil particles and improve the mechanical strength of the soil mass is called the chemical reinforcement method. In China's collapsible loess regions, many foundation treatment applications have been made, and the chemical reinforcement methods that have gained practical experience include the silicification reinforcement method and the alkali solution reinforcement method. Their reinforcement mechanisms are as follows:
1. Silicification reinforcement method: By driving perforated metal injection pipes, sodium silicate (commonly known as water glass) solution is injected into the soil under a certain pressure. Alternatively, sodium silicate and calcium chloride solutions are injected into the soil successively.
The physicochemical process of silicification reinforcement of collapsible loess is based on one hand on the smooth penetration of low-concentration, low-viscosity sodium silicate solution into the loess pores, and on the other hand on the mutual coagulation of the solution and the soil, with the soil acting as a coagulant.
2. Alkali solution reinforcement: The use of sodium hydroxide solution to reinforce collapsible loess foundations in China began in the 1960s. The reinforcement principle is: after sodium hydroxide solution is injected into loess, it first undergoes replacement reactions with soluble and exchangeable alkaline earth metal cations in the soil, resulting in the formation of alkaline earth metal hydroxides on the surfaces of soil particles.
The reinforcement effect of alkali solution on soil differs from other chemical reinforcement methods. It does not precipitate cementitious substances from the solution itself; rather, after the chemical reaction between the alkali solution and the soil, the surfaces of soil particles are activated and self-cement, thereby enhancing the mechanical strength and water stability of the soil. However, alkali solution reinforcement is not suitable for the following conditions:
① For loess foundations with groundwater level or saturation greater than 80%.
② For loess foundations that have been infiltrated with asphalt, grease, and other petroleum compounds.