Injection of A Collapsed Rock Mass and the Installation of 100 Ton Strand Anchors at the PHEP II Project-Paper

Paper

Pdf Summary

In March 2016, a major rock mass collapse caused by an active shear zone filled the downstream surge chamber of Bhutan’s Punatsangchhu-II Hydroelectric Project. Approximately 65,000 m³ of fragmented rock and soil accumulated inside the cavern, with a large, initially poorly defined cavity remaining above it. The unstable material had to be strengthened to form a stable mass capable of supporting reinforced-concrete walls and permanent anchors.<br /><br />Starting in 2020, BAUER carried out a two-stage cement-grouting program through 524 boreholes, totaling about 19,960 m of drilling. The first stage used low-pressure grout delivered through hoses to fill larger voids. The second used higher-pressure injection through specially designed TAM pipes with fabric bags, targeting smaller spaces and compacting softer material. Drilling was particularly difficult because the debris contained large boulders, collapsed steelwork and abandoned equipment, and the work area was confined.<br /><br />Quality checks indicated that the treatment met the project’s requirements: tested samples had an average unconfined compressive strength of 28 MPa, exceeding the specified 10 MPa, while water-pressure testing also produced satisfactory results.<br /><br />The subsequent anchoring program involved 359 permanent cable anchors, each designed for a working load of up to 1,000 kN, with approximately 19,900 m of drilling. Anchors were tested and tensioned in accordance with project specifications. Despite logistical challenges, including COVID-19 restrictions and shortages of materials, the grouting work was completed within the contractual period. The treatment was judged successful, while anchor installation was ongoing at the time of the paper.

Keywords

Punatsangchhu-II

hydroelectric project

rock mass collapse

cement grouting

shear zone

TAM pipes

permanent cable anchors

drilling boreholes

grout injection

unconfined compressive strength