Sprayed concrete for underground excavations - A status report

B.C Viljoen

doi:10.17159/2411-9717/2015/v115n11a18

Abstract

Synopsis Commencement of the next phase of the Lesotho Highlands Water Project is a year or so away from becoming reality. Some 50 km of tunnels in the form of access adits, shafts, diversion tunnels, and main waterways will be constructed. Sprayed concrete will form an integral part of the temporary and permanent rock support systems that will be installed. Furthermore, the use of sprayed concrete as a final tunnel lining solution was proposed as a viable alternative to concrete linings in the feasibility studies that preceded the forthcoming implementation phase. The aim of this paper is to make the reader aware of all the sprayed concrete related issues that engineers tend to grapple with during the design and construction of tunnel projects and other underground works. Primarily, these issues would include selection of applicable design parameters, technical specifications, construction application methods, quality control and, most importantly in the context of final or permanent linings, the durability of sprayed concrete. The International Tunnelling Association (ITA) Working Group (WG) 12 has been tasked specifically to publish a set of recommendations on the Quality and Control of the Application of Sprayed Concrete and the Testing of Sprayed Concrete for Durability. These tasks were also further explored at the recent World Tunnel Congress (WTC 2015) that was held in Dubrovnik, Croatia at the end of May 2015.

Highlights

Sprayed concrete is presented in a variety of shapes, designs, forms, application methods and accompanying terminologies including shotcrete, gunite, pneumatically applied concrete, and the like
Durability is dependent on selecting the appropriate materials and the mix design, and on the standard of workmanship employed during application
For this type of application durability would not be an issue, and quality and control would be focused on environmental conditions rather than long-term performance criteria because this type of sprayed concrete application would be later included in a permanent concrete or sprayed concrete lining

Summary

Introduction

Sprayed concrete is presented in a variety of shapes, designs, forms, application methods and accompanying terminologies including shotcrete, gunite, pneumatically applied concrete, and the like. Durability is dependent on selecting the appropriate materials and the mix design, and on the standard of workmanship employed during application. Mix design can be nullified if the end product lacks homogeneity, has voids or laminations, and is unduly permeable. Such deficiencies can usually be attributed to poor workmanship of the sprayed concrete operator. ITA Working Group (WG) 12, under the leadership of Professor Eivind Grøv, Chief Scientist and Professor at the University of Science and Technology in Trondheim (NTNU), has been tasked to publish a set of recommendations on Quality and Control of the Application of Sprayed Concrete and the Testing of Sprayed Concrete for Durability. Sprayed concrete soil nail walls in road cuttings or linings in metro subways are good examples where durability is an essential characteristic

Working environment

Quality and control

Remedial works

General considerations

Quality control testing

Available reference specifications