Observations of deformation and engineering geology in the Lam Ta Khong tunnel, Thailand

This paper briefly describes engineering geological observations, rock mass classification and deformation monitoring for the tunnel support in the Lam Ta Khong pumped storage project, Thailand. The area consists of Jurassic sedimentary rocks of the Khorat (Phu-Kradung) formation (sandstone and siltstone) and Quaternary clay/mudstone. The hydropower tunnels were constructed by the conventional method of drill and blast excavation. The main input parameters for engineering geological observations were the state of weathering, hardness, and joint spacing. Rock mass rating (RMR) values were also classified simultaneously along with engineering geological observations. A hardness parameter was used as an index for unconfined compressive strength and for RMR. The standard support pattern based on the rock mass classification was used and the performance in most tunnels was satisfactory. Deformations with respect to the time as well as the face advance after blast were regularly recorded. Rate of convergence was found to depend mainly upon rock mass class, face advance and time elapse after the support application. Typical tunnel movements over time and face advance are presented. An empirical time–deformation equation was approximated as d=a+b ln t, which is generally valid for face advance within one to five tunnel diameters and time elapsed over a day. Deformation d is in millimeters, a=0.8–0.9 and b=0.98–1.0 are functions of the rock mass and t is the time of elapsed measurement in days.