Detailed design and PMC for construction of road bed including two tunnels from km 17/00 to km 35/00 of the new Ovulavaripalle- Venkatachalam railway line in Andhra Pradesh for South Central Railways" was awarded in 2010. Funding for the project was provided by the Ministry of Railways, through Rail Vikas Nigam Limited (RVNL), an entity created to undertake rail projects on behalf of the Indian government.

The ground engineering and tunnelling department of Ayesa Engineering was placed in charge of tunnel design engineering, and developed much of the associated geological and geotechnical study material for the project.

The proposed railway line passes through 18km of deep forests, and includes the construction of two single-track tunnels (named Tunnel T1 and Tunnel T2). The main physical characteristics of these tunnels are listed in Table 1, below.

Due to the high difference existing between the maximum overburdens of these two tunnels, and in spite of the fact that the same geological units were passed through in both cases, the heaviest tunnel supports proposed for Tunnel T1 had to be modified for Tunnel T2 in order to cope with the possible appearance of high squeezing risks.

Geological and Geotechnical studies
The most significant difficulty emerged with the start of the design phase, and was related to the development of the geological mapping and geotechnical investigation campaign, which was necessary to obtain geotechnical and survey reports. Namely, the practical impossibility of transporting drilling equipment further than tunnel portals due to the steep and unforgiving topography.

On the other hand, there was a lack of adequate equipment available in the area, and it was also impossible to obtain information from stereographic flights.

Consequently, the results obtained were not as good as it had previously been hoped or expected, and consequently design had to be based only on geophysical surveys and geomechanical stations.

Lithologies consist mainly of metamorphic rocks like phyllites and shales (Cumbum Formation), quartzites and shales (Bairekonda Formation) and quartzites with conglomerates (Pulivenda Formation) or with shales (Grandikota Formation), all affected by tectonic episodes. Nearly all RMR index values in the area vary within a range from 35 to 75.

Three geotechnical units have been described, based on lithological differences: quartzites (PCq), ferrogenious quartzites and quartzitic phyllites (PCp) and phyllites with shales (PCs). The transition from the bottom lithologies of the strata (PCs) to the quartzites (PCq) at the top occurs in a gradual way.

Phyllites are weaker and have less consistency than quartzitic materials. Therefore, they have been more intensively weathered by previous erosive cycles. Areas marked out by phyllites (PCs) and quartzitic phyllites (PCp) have a more elongated and steep morphology than the marked out ones by the quartzites, which have rounded and almost flat tops.

The geological structure is quite simple, since everything dips between 30 and 50 degrees toward the northeast. Neither faults nor important fracturing zones can be observed.

The main values considered for the parameters of intact rock properties are included in Table 2, above right.

The area is characterised by northsouth preferential direction faults at Tunnel T1 and northeast-southwest and northwest-southeast preferential direction at Tunnel T2.

The main problems associated with these faults would be related to the presence of clayey milonites or cataclasites presenting poor geomechanical conditions, due to the fact that this could create high flows to the excavation depending on its hydrogeological conditions.

Construction Both tunnels are to be bored with full face excavation, following the principles of NATM. The possible option of a TBM was dismissed once the possibility of opening an intermediate cut and cover zone for Tunnel T2 was finally considered.

A functional section with an aerodynamic section of 38m2 was finally considered. It was also necessary to design some special wider functional sections for construction recesses, and for trolley refuges.

Six different types of support sections were considered. The first three support sections were alike in both tunnels, while the rest had to be adapted due to the high discrepency existing between the maximum overburden of each tunnel.

The use of an invert arch was recommended when RMR values were lower than 35. Specific support sections were also designed for fault crossings or squeezing zones.

Expansion bolts were proposed for support sections SC-I and SC-II, while corrugated steel bars were chosen for support sections SC-III and SC-IV. Self-drilling bolts umbrellas of 10m length were also proposed for fault crossings.

Excavation method by drilling and blasting was suggested for sections SC-I, SC-II and SC-III.

The rest of support sections could easily be excavated by mechanical means. Lattice girders types 50-25-20 and 50-32-20 were proposed for Tunnel T1 and heavier types 70-25-20 and 95-25-20 for Tunnel T2, due to the presence of higher overburdens. Support sections were checked by numerical modelling using finite differences method with FLAC3D.

Ground improvements proposed are mainly face reinforcements with fibreglass nails, provisional earth slope pillars, drainages on face, sprayed concrete in face and pipe umbrellas when required.

A special support section for squeezing zones was designed for Tunnel T2. Support proposed consisted of self-drilling bolts surrounding the whole excavated section, TH-36 steel ribs, LSC elements (lining stress controllers) and 25cm of shotcrete. This design could finally be modified during construction once that real geotechnical information at highest depths is obtained, due to the lack of information available during design, as it was mentioned earlier.

This section was developed considering that similar lithologies at tunnel depths higher than 250m have developed squeezing problems in Indian tunnels in the last years.

Cut and cover tunnels and evacuation shafts Since Tunnel T2 is excavated through a low overburden zone, a cut and cover tunnel was proposed in order to create two intermediate attack points for tunnel excavation, consequently reducing total excavation time. An emergency evacuation shaft was also included within this zone.

Conclusions
The two tunnel design calls for length of 6,670m and maximum overburden of 500m.

Tunnel designers had to face with difficulties mainly associated with the lack of some basic information like previous geological studies or available aerial photographs (stereo pairs). Also coping with logistics and the difficulty of placing borehole drilling equipment further than tunnel portals due to the steep landscape was a key issue.

Geophysical investigations were of great help to designers, Nevertheless some uncertainty remains regarding deeper sections of main tunnel