RMR and Q derived support measures for three tunnels were compared with those derived by rock wedge analysis. A numerical model was also run to assess the effect of internal water pressure in one of the tunnels. The study showed that although the empirical support recommendations are adequate in general, they do not meet some of the project specific requirement.


The most widely used rock mass classification methods for tunnel support design are RMR [1] and Q [2]. Over the years these methods have been revised to incorporate the experience gained subsequent to their initial introduction, RMR [3] and Q [4]. Notwithstanding the revisions, these methods still have limitations and room for improvements [5, 6, 7, 8, 9]. To identify their limitations and to suggest improvements, where possible, these methods can be evaluated by comparing their support recommendations with the support derived by other applicable methods. This paper applies RMR and Q to three water conveyance tunnels using the data collected during construction. The support measures derived by the two methods are compared with the results of a tetrahedral rock wedge analysis undertaken using UNWEDGE [10]. The effect of internal water pressure on the rock mass around one of the tunnels was also assessed by a numerical analysis using UDEC [11].

The Tunnels Studied

The three case tunnels are:

  • 493 m long, 11.3 m span horseshoe-shaped Chiew Larn diversion (CLD) tunnel and

  • 240 m long, 13 m span horseshoe-shaped Chiew Larn power (CLP) tunnel of the Chiew Larn hydropower project in the Southern Province of Thailand, and

  • 732 m long, 3.5 m wide and 3.5 m high D-shaped Huai Saphan Hin power (HSHP) tunnel located on the eastern seaboard of Thailand. The three tunnels were driven by drill and blast methods. The CLD tunnel, initially a river diversion tunnel for dam construction to create a reservoir, was converted to an irrigation tunnel by plugging it at approximately 105 m from the inlet and providing an outlet valve in the plug. During excavation, the tunnel was mostly dry, except for some isolated areas of water inflow during the wet season. The groundwater level around the tunnel length downstream of the plug was later elevated by the reservoir, which has a maximum elevation of 95 m RL (tunnel invert is at ~10 m RL). The average tunnel alignment is NW-SE with a 0.2% gradient. The tunnel overburden thickness varies from 40 to 80 m with an average of 60 m. The CLP tunnel feeds three 80 MW power units through three steel penstocks. Located in a hill slope, it has an alignment of 140o E and a plunge of 10o. Its overburden thickness varies from 25 to 50 m, with an average of 30 m. The tunnel is above the regional groundwater level and was mostly dry, with water only dripping in some places during the wet season.

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