In the design of underground tunnels and caverns located in weak rocks, grouting plays an essential role. Grouting is a complex process demanding high degree of skill and knowledge in understanding of grout flow and requirements for certain geological conditions. This paper discusses a design of grouting in underground caverns with an aim to improve the stability of the structure and to reduce water inflow into the cavern through sealing of fractures. Estimation of grouting efficacy becomes an important part of any grouting since there is high degree of uncertainties involved in the interconnectivity and condition of fractures inside the rockmass. Therefore, two different trial grouting programs are devised, and their results are discussed in order to arrive at a robust grouting scheme. The systematic approach of both programs involves pre and post grouting investigations. One approach is based on multi stage grouting with efficacy check through Water Penetration Tests (WPT), Borehole Televiewer test and Seismic Refraction Test. Second approach is based on single stage grouting with efficacy check using WPT, Core logging and cross hole velocity tests. The analytical estimates of grout intake and penetration length are compared with actual grout intake in both trial programs. An understanding of the grout behavior is developed to arrive at a final grouting scheme for the given geological condition. This work shows the importance of state of art in the development of a grouting process based on geological and hydrogeological conditions and can be helpful in grouting design for underground tunnels and storage caverns

1. Introduction

It is well seen nowadays that due to decrease in land surface availability for mobility and storage, new tunnels and caverns are continuously being constructed all over the world. However, it may not be always possible to construct such underground facilities in a competent rock strata. Therefore, in weak rock strata, grouting becomes handy to have a safe excavation by reducing water inflow and improving the strength properties of the surrounding material. Grouting technique can be adopted for excavation through soil and rock strata; however this paper discusses grouting in rock strata only. Grouting is a complex process involving various dynamics in understanding the grout flow and requirements for a given geological conditions. The basic requirements for any grouting includes (i) decision on starting mix, (ii) decision on grouting pressure, (iii) spacing between grout holes, and (iv) methodology of grouting. All these basic requirements are discussed in the paper. The sealing of grout fractures in underground structures with the grout is an art as it is not possible to visualize the flow of grout inside the rocks. However, it cannot be a blind game given to those with high intuitive experience, which demands careful observations and analytical understanding of the grout behavior grout in the rockmass. As quoted by Houlsby [1], “Grouting can nowadays be classed as an engineered process, it has a pecularity that there is more art to it than in most engineering works”.

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