Abstract
Different from sand and clay, loess has special engineering properties; hence, existing soil grouting theories are not suitable for the disaster treatment of shallow loess tunnels. In this study, a fine grouting reinforcement test system was developed, and the Yuhan Road tunnel overlying loess was used as the injection medium. An orthogonal test based on slurry dry density, moisture content, water-to-cement ratio, and grouting pressure was conducted. Results revealed that the loess samples have high integrity after grouting, and the cohesion and compressive strength improved significantly. The stress-strain curve showed that the strengthened samples have greater ultimate and residual strengths than samples before grouting. Through a range analysis, it was determined that water-to-cement ratio and moisture content are the main factors affecting loess cohesion and compressive strength. Therefore, a comprehensive test of the water-to-cement ratio and moisture content as a single variable was conducted. It was found that their influence on loess cohesion and compressive strength is not a single linear relationship but a combined balance. To characterize the joint effect of water in loess and in slurry on reinforcement, the concept of a comprehensive water-to-cement ratio is proposed, and the cohesion and compressive strength curves with respect to this ratio were drawn. An optimal comprehensive water-to-cement ratio, which corresponds to the maximum cohesion or compressive strength, was found. Based on this ratio, we further propose a method to calculate the water-to-cement ratio of slurry and suitable grouting amount for the Yuhan Road tunnel reinforcement project, in which all solution parameters can be measured via field tests. In the project, a surface layered grouting scheme, based on the optimal comprehensive water-to-cement ratio, was designed. After grouting, loess strength was improved significantly, permeability was reduced greatly, and the overall reinforcement effect was suitable; these results provide a reference for similar projects.
Highlights
In the recent years, many urban underground tunnels have been built, and the possibility of encountering complex geology in engineering construction projects is increasing
Grouting is often used to reinforce the overlying soil of tunnels [5], but it is limited by the lack of research on loess grouting reinforcement theory
According to previous engineering experience and research results, the dry density of soil ρd, moisture content w, slurry water-to-cement ratio (W : C), and grouting pressure P were selected as the orthogonal test basic factors
Summary
Many urban underground tunnels have been built, and the possibility of encountering complex geology in engineering construction projects is increasing. There are essential differences between the mixing and grouting methods: (1) the cement soil formed by the mixing method is a type of mixed engineering material, and its internal physical and mechanical properties are uniform; (2) the grouting method eliminates the overhead pores in the soil by splitting or compaction, and the loess mechanical properties after reinforcement depend on the soil-slurry interface [12]; (3) as discussed before, grouting pressure plays an important role, but its influence is not considered in the mixing method; (4) grouting vein distribution in the grouting sample is irregular, and shear strength errors measured by direct shear methods are large, while the triaxial shear test is more suitable. A series of laboratory tests were conducted to study the influence mechanism of water in grouting on loess reinforcement, thereby allowing us to draw a more suitable conclusion for practical engineering
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