The Aloft dam diversion tunnel with length of 450 m and diameter of 9 m is to be excavated in left abutment of Aloft dam and the currently under study. The lithology mostly comprises of jointed Asmara and Arum limestones. The tunnel passes the AS2 zone of Asmara formation belongs to dolomitic limestones of Miocene age. The rock mass discontinuities orientations were studied by a field survey and a statistical analysis. Also the boreholes logs and outcrop surveys were used to determine the basic characteristics of the discontinuities. The rock mass surrounding the tunnel is classified using the Q, RM and GI methods. The engineering properties and temporary tunnel support systems were determined based on the methods. A series of 3D analytical (stress-strength method) and numerical continuum (FEM) stability analyses were conducted for the rock mass surrounding excavation face based on the excavation scheme proposed by the empirical method based on RM for supported and unsupported tunnel. Based on the analyses, there is no need for intense support installation. For verification and to determine the influence area of the face, the longitudinal displacement profile values were extracted, drawn and compared with the available literature.

1. Introduction

The Aloft dam is to build on Aloft River located 77 km from Shah record city, Chaharmahal-Bakhtiary Province, is still under feasibility studies. The dam diversion tunnel has the length of 450 m, diameter of 9 m and inlet invert elevation of 1037 above sea level.

2. Geology of the study area

The lithology mostly consists of Avrom formation limestones, dolomitic limestones and limestones of Asmara formation which belong to lower Miocene age. Structural geology studies did not show any major fault in the tunnel route area, but a few minor and small local faults are observed in river bed and right abutment which are represented by vertical movement of less than a meter along with surface typical morphological evidences.

2.1. Orientation of discontinuities

To classify the discontinuities of the left abutment where the tunnel is to be excavated, 300 data pairs of discontinuities dip/dip directions were measured from the available outcrops.

2.2. Discontinuities characteristics

Basic characteristics of discontinuities including aperture, weathering, in fillings, spacing, roughness, persistence(continuity), Rock quality Designation (RED) and also ground water flow state and quantity, were extracted from 3 available boreholes logs (BH1 nearly inlet portal, BH2 in middle part and BH3 nearly outlet portal) and longitudinal geological tunnel profile. The data ranges used in the frequency analysis were uniquely determined from the ranges utilized for classifying the parameters used in famous classification systems such as Q, RM and GI.

3. Geotechnical

Evaluation and classification of rock mass along the tunnel route Considering longitudinal geological profile of tunnel route, structural geology of the tunnel, 3 available boreholes logs and discontinuities orientations and characteristics, the tunnel route was divided into 3 parts as inlet portal, middle part and outlet portal. The rock mass along the 3 parts were individually classified using well-known classification systems such as RM, Q, and GI.

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