Excavation Techniques for underground Drinking water pipe line with micro piles special application-Paper

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The paper describes the use of micropile shoring to enable installation of a large underground drinking-water pipeline in Mumbai. Conventional sheet piling was impractical because of the ground conditions. The project required a 5.5 m-deep, 5.1 m-wide excavation through soil overlying weathered and fresh basalt, with design based on the weakest encountered ground conditions.<br /><br />The solution used closely spaced, grouted steel-tube micropiles arranged as a retaining wall, connected by a capping beam and supported by a steel waler and a single level of cross-struts. The design incorporated a 25 kPa construction or traffic surcharge and a design water level 0.86 m below ground. Micropiles were approximately 8 m long, providing 2.5 m of embedment below the excavation base. Staged excavation and wall behavior were analyzed using WALLAP. The selected steel sections were checked for bending and axial stresses and found adequate.<br /><br />Construction involved surveying and drilling at 300 mm spacing, installing permanent steel casings, grouting, and building the capping beam. After excavation to the strut level, the waler and struts were installed before excavation continued to full depth. A concrete bed was then placed, the pipeline installed, and the structure completed and backfilled. Grout quality was controlled through mix specifications, sampling, and compressive-strength tests.<br /><br />Key challenges included protecting nearby trees and utilities, avoiding existing underground services, adapting drilling equipment to changing strata, and restrictions on night work in the residential area. The authors conclude that micropile walls with walers and struts offer a safe, practical alternative where sheet piling is not feasible. The project planned approximately 1.1 km of wall, comprising about 5,000 micropiles.

Keywords

micropile shoring

underground pipeline

Mumbai excavation

retaining wall

basalt strata

cross-struts

waler beam

grouted steel-tube micropiles

construction surcharge

WALLAP analysis