This paper presents the stability analysis and support design for two parallel diversion tunnels of the Manicure multi-purposed dam project in Myanmar by using empirical approaches and numerical method. The geological evaluation of the Manicure diversion tunnels is relied on exploratory data, field observations and laboratory test results. Rock mass rating system (RM), GI tunneling quality index (Q system), rock mass index (RI), and geological strength index (GI) are applied to assist in the support designs. Traditional guidelines for the rock support are used based on the results of the site characterizations. A series of numerical simulations (using Phase2 code) is performed to assess the stability conditions of the tunnels with and without the support systems. The required input parameters for numerical modeling (Phase2 code) are evaluated by empirical equations. After support installation, the extent of yielded zone and the radius of plastic zone significantly decrease as shown in the numerical results. Optimization between the empirical and numerical results is made to obtain the suitable support design for the tunnels. The results indicate that the use of empirical methods together with numerical method can provide the reasonable support systems for the underground openings.
The Manicure multi-purposed dam project is located on the Manicure River, 52 km from the Kala Town, Kala Township, Sagging Division in Myanmar (Figure 1). The project is a part of the Multi-purposed Government Funded Scheme and has been implemented under the supervision of Irrigation Department, Ministry of Agriculture and Irrigation, Myanmar. The dam will provide irrigated water for 20242 hectares and hydro-electricity to the local area. The two parallel tunnels are 1050 m long with 12 m wide modified horseshoe shape. They are driven underneath the south bank mountain to divert the water whilst a massive rock-fill dam is built. Within the confined space of a tunnel, it is difficult and dangerous to deal with stability or water problems which are sometimes encountered unexpectedly. In the case of Manicure diversion tunnel constructions, some tunnel stability problems are expected based on geological investigations. To classify the rock mass quality, rock mass classification systems including rock mass rating system (RM) , GI tunneling quality index (Q system) , rock mass index (RI) , and geological strength index (GI)  are applied. Their rating values are used to determine tunnel support systems and to evaluate the rock mass parameters. These empirical methods have been originally obtained from many underground case studies and they have been applied to many construction designs. These empirical methods cannot however adequately calculate stress distributions, support performance and deformational behavior around a tunnel. Therefore, 2D finite element code, Phase2 version 6.0, has been used.
The tunnels are in the Chin Flush aged of Tertiary as a part of the Rankine(Arawakan)-Chin-Naga Hill ranges. Most of the main Rakhine-Chin-Naga Hill ranges are thick monotonous series of apparently un fossiliferous marine flysch-type sediments including salty shales, Phillie, slates and poorly-graded calcareous sandstones.