Assessment of Rock Mass Quality and Support Estimation along Headrace Tunnel of a Small Hydropower in District Mansehra, Khyber Pakhtunkhwa, Pakistan

The main purpose of this study is to classify the rock mass quality by using rock mass quality (Q) and Rock Mass Rating (RMR) systems along headrace tunnel of small hydropower in Mansehra District, Khyber Pakhtunkhwa. Geological field work was carried out to determine the orientation, spacing, aperture, roughness and alteration of discontinuities of rock mass. The quality of rock mass along the tunnel route is classified as good to very poor quality by Q system, while very good to very poor by RMR classification system. The relatively good rock conditions are acquired via RMR values that are attributed to ground water conditions, joint spacing, RQD and favorable orientation of discontinuities with respect to the tunnel drive. The petrographic studies revealed that study area is mainly comprised of five major geological rock units namely quartz mica schist (QMS), garnet mica schist (GMS), garnet bearing quartz mica schist (G-QMS), calcareous schist (CS), marble (M). The collected samples of quartz mica schist, marble and garnet bearing quartz mica schist are fine to medium grained, compact and are cross cut by few discontinuities having greater spacing. Therefore, these rocks have greater average RQD, Q values, RMR ratings as compared to garnet mica schist and calcareous schist.


Introduction
Government of Pakistan is highly committed to curtailing the energy crises of the country by developing various hydropower projects especially in northern area of Pakistan. In this regard, a small hydropower project with 2.8 km long headrace tunnel was proposed in Mansehra district, Khyber Pakhtunkhwa. In many engineering projects including tunneling, ground excavation, foundations and slope stability analysis, geology plays a key role. Among these projects tunnel constructions require a considerable geological input [1]. The overall engineering design weakens by numerous fractures and cracks developed due to tunneling in hard ground conditions leads to deteriorating the physical condition of the ground and rocks. It is necessary to study the geological behavior (spacing and degree of jointing, strength, and attitude) of the strata for safe and economic design of the tunnel [2]. Rock mass classification systems have been effectively applied in many engineering projects especially tunneling projects [3] [4]. To estimate the required support measurements the rock mass classification systems have been proved supportive in assessing the rock mass parameters [5] [6]. Following parameters are used to describe and classify the rock mass into Various researchers [7]- [21] studied the rock mass characterization along tunnel route by using empirical rock mass classification systems e.g. Rock mass rating (RMR), Tunneling quality index (Q system) etc. to classify the rock mass with different rock classes according to the physical characteristics of rock mass.
This study was conducted to characterize the rock mass along proposed tunnel route by using RMR and Q system. Field studies revealed that study area comprised of five major rock units that have strong impact on the various properties of discontinuities including rock mass strength.

Geological Mapping and Discontinuity Surveys
The study area lies in the Hazara Nappe that is disjointed from Besham Nappe to the West by dextral Thakot shear zone [22] [23]. The Hazara Nappe dominantly constitutes of metapelites and metapsammites of Pre-Cambrian Tanawal Formation ( Figure 1). The Late Cambrian Mansehra porphyritic granite intruded into Tanawal Formation [24]. At some places dolomites of Paleozoic Abbottabad Formation unconformably overlie the Tanawal Formation [25].
Quantitative readings of joints parameters (joint spacing, persistence (length), roughness and joints frequency etc.) were collected at different stations using

Tunnelling Quality Index (Q) of Study Area
The joint set number (J n ), roughness (J r ), alteration (J a ), water reduction (J w ) and Stress Reduction Factor (SRF) values were assessed during scanline surveys. The Q value varies between 0.01 and 13.33. The rock mass classified as good (B) to very poor (E) rocks. The rock type, rock mass parameters, calculated Q values and rock class are given in Table 2.

Rock Mass Rating (RMR) of Study Area
The parameters including the Uniaxial Compressive Strength (UCS) [30], RQD,  Table 3 with rock quality ranged between very good (I) to very poor (V) rock. orientation of discontinuities conditions were observed.

Garnet Bearing Quartz Mica Schist (G-QMS)
The average values UCS for G-QMS is 95 MPa with average RQD 75%, joints spacing ranges from 10 -50 cm, joints are undulating, planar and smooth, persistence of 3 -10 m and joint apertures are <1 mm in width with soft filling materials.
Discontinuities attitude reveals fair to unfavorable tunnel drive conditions.

Garnet Mica Schist (GMS) and Micaceous Schist (MS)
The GMS has average UCS of 80 MPa and RQD of 50%, joint spacing range is 10 -22 cm, joint apertures <1 mm in width with soft filling materials with undulating, planar, smooth & slickensided joints, persistence of 0.3 -3 m.

Marble (MB), Siliceous Marble (SM) and Micaceous Marble (MM)
The average UCS for these rocks is 80 MPa with RQD of 60%, joint apertures are <1 mm -5 mm with non-softening fillings and clean also, rough, irregular, un-

Estimated Support Recommendations
The support estimation was calculated by both RMR and Q system. The  (Table 4).

Correlation between Petrography and Rock Mass Parameters
The detailed petrographic studies of rock samples revealed that tunnel majorly     and are cross cut by few discontinuities having greater spacing. As a result, these rocks have greater average RQD, Q values, RMR ratings ( Table 5). The remaining two rock types such as GMS and CS are relatively medium to coarse-grained with many closely spaced discontinuities. The average RQD, Q values and RMR ratings of these segments are comparatively low (Table 5).

Conclusion
The main objectives of the current research work are the rock mass categorization along proposed tunnel alignment using RMR and Q system and comparison of rock mass quality with mineralogical composition of rocks. The results were further materialized to predict and assess applicable rock reinforcement requirements for tunnel. Tunnel was divided into five major geological segments on the bases of rock type: quartz mica schist (QMS), garnet mica schist (GMS), garnet bearing quartz mica schist (G-QMS), calcareous schist (CS), marble (M).
Q values vary between 0.01 and 13.33 that depicted rock masse quality ranges from good to extremely poor in case of Q-system, while very good to very poor according to RMR. Geological segments comprised of following rock type quartz mica schist, marble and garnet bearing quartz mica schist having fine to medium-grained texture, compact and having large spacing. Therefore, these geological segments have greater average RQD, Q values, RMR ratings as compared to garnet mica schist and calcareous schist.