Research On The Structure Of The Rock Mass In The Dam Area Of The Songta Hydropower Station Along The Upper Reaches Of The Rapidly Uplifting Nu River

The Songta dam is being constructed on the upper reaches of the Nu River. Aconcrete double-curved arch dam with a maximum height of318m is planned, whichwill be the highest arch dam in the world. The station will have a hydroelectricgenerating capacity of3,600MW. With the rapid uplift of the Qinghai-Tibet plateausince the Quaternary period, the cutting ability of the Nu River increases. The damarea exhibits a mountain-canyon geomorphology, which is characterized by extremelynarrow valleys and steep slopes. The predominant lithology is Cretaceous biotitegranite. Affected by endogenic and exogenic geological processes, a lot ofdiscontinuities are present in the rock mass. The super-high dam has a strictrequirement for the stability of the rock mass. In large hydropower station projects,complicated rock structures are deemed to have a critical effect on the stability of thebasement and abutment of the dam, the underground structures, as well as the slopeengineering. Therefore, it is of great significance to investigate the characteristic ofrock mass structures at the Songta dam site from the perspective of the feasibility,safety, and economy of the project.Based on the “controlling theory of the rock mass structures”, this paper is aimedat studying the features of the rock mass structures and the approaches on theevaluation of the structures using data collected from33exploration tunnels at theSongta dam site. The main conclusions are listed as follows: 1. Research on the engineering geological behavior of rock discontinuities wasconducted using plenty of discontinuity data collected from the dam area, leading tothe following results:(1) In the dam area, discontinuities with steep dip angles mostlytrend NE-NEE, whereas discontinuities with middle-shallow dip angles exposed onthe left bank have a negative dip direction with those exposed on the right bank;(2)The distributions of aperture conditions for discontinuities exposed on both banks aresimilar, and most of discontinuities (occupying more than70%) are closed, and only avery few discontinuities have an aperture of more than10mm;(3) Discontinuitiesexposed on both banks have similar persistence distributions, and the discontinuitypersistence is mainly small and very small;(4) The roughness of discontinuities onboth banks are similar, and very rough, rough, and slightly smooth discontinuitiespredominate the dam area;(5) Discontinuities in the rock masses on both banks havesimilar water conditions, and the number of discontinuities with ‘completely dry’condition to ‘flowing’ condition decreases step by step;(6) Discontinuities with nofilling materials dominate the dam area, and the second large number ofdiscontinuities are clay-filled discontinuities;(7) Discontinuities on the right bank aremainly slightly weathered, whereas discontinuities on the left bank are mainlymoderately and slightly weathered;(8) The average spacing for discontinuitiesexposed on the right bank is smaller than that for discontinuities exposed on the leftbank, and the predominant discontinuities (i.e., NNW-SN, NNE and NE-NEEtrending classes) have smaller spacing than random discontinuities.2. Research on the key issues in evaluating the structures of rock masses wascarried out. Based on nonparametric tests, this paper presented a new method for theidentification of structural domain boundaries in rock masses using discontinuityorientations. Besides, a multivariate technique for evaluating the statisticalhomogeneity of jointed rock masses was developed, in which nine parameters,namely: orientation, spacing, aperture, roughness, trace length, trace type, filling,groundwater condition, and weathering, were considered. This paper made adiscussion on the selection of distance measures and the computation of discontinuitycluster centers in the grouping of rock discontinuity sets, and then presented a K-means algorithm based on particle swarm optimization method for theidentification of discontinuity sets based on orientations.3. Based on the description of the features of rock mass structures, a stochasticfracture pattern modeling program was developed to model three-dimensional (3-D)fracture network systems in a jointed rock mass using two-dimensional fracture datacollected from exploration tunnels. The spatial relationship between discontinuitiescan be obtained using a visualization technique.4. An algorithm for calculating the connectivity parameters was developed. Therelationships between the connectivity parameters and joint parameters wereinvestigated. The spatial and size effect of the rock mass in the study area wasdiscussed using the connectivity parameters. Based on the connectivity, therepresentative elemental volume of the rock mass was calculated.