TBM- THE FUTURE OF TUNNEL EXCAVATIONS IN INDIA; WITH SPECIAL REFERENCE TO PARBATI PROJECT (STAGE-II)
M M Madaan, General Manager, Parbati Hydroelectric Projects, National Hydroelectric Power Corporation, India
SYNOPSIS
With the increase in development work of hydroelectric projects in India, the amount of tunneling work has increased many folds. Besides to achieve time bound programs of construction of mega projects in a period of 4 to 5 years modern methods of tunnel driving are being considered as only solution to achieve a high rate of progress. In eighties a progress of 75m per face per month was considered as a high rate of progress whereas now a days even 150m per face per month is not considered as a good progress. Most of the tunnels now under construction are quite long and normal DBM are failing to achieve a high rate of progress. The only solution is use of TBMs to achieve a time bound program of tunnel excavation in long reaches. To gain confidence level with TBM working in Indian and Himalayan conditions the systems are yet to be established. Not many tunnels in India have been bored with TBMs, nevertheless it has proved a success in tunnels of India also except one odd case. The paper specifically deals with selection of TBMs for Parbati Hydroelectric Project, dealing with potential risks and expectations. Some of the projects where TBM has been used have also been discussed in brief.
INTRODUCTION
The Parbati Hydroelectric Project Stage II, located in the Kullu district of Himachal Pradesh, envisages utilisation of water from the Parbati River to obtain a gross head of 862 m between the dam site at Pulga and the powerhouse site on the right bank of the Sainj River near Suind village. A 31.371 km long headrace tunnel and twin pressure shafts with a length of 1542 m convey the water to a power house located above ground. The powerhouse will have an installed capacity of 800 MW. Construction Lot PB.2 includes civil works for 21.233 km of headrace tunnel (HRT) from RD 3500 m to RD 24733 m. Tunnel from Adit 1 and Adit 3 is being excavated by drill-and-blast method (DBM) whereas 9.050 km of HRT upstream from Adit 2 is to be bored with Tunnel Boring Machine (TBM) from RD 19354 m to RD 10304 m. The excavated diameter of the TBM tunnel has been designed as 6.80 m. Depending on the rock classifications rock support is envisaged using primarily rock anchors, shotcrete and steel sets. Final lining will be with Telescopic Folding Shutters. Whereas twin pressure shafts as part of Lot PB.3 having length of 1542m are inclined at 30° to the horizontal have been planned to be excavated with TBM. Pressure shafts shall have an internal diameter of 3.5 m after placement of steel lining. TBM with a nominal excavated diameter of 4.88 m shall be used. According to the Technical Specifications provided as part of the Tender documents the TBM shall be either an open-type TBM or a single shield TBM. Subsequently contractor has proposed a variant using a double-shield TBM for excavation of the pressure shafts. Concrete segmental lining has been proposed for rock support over the entire length of the TBM drive. The first TBM for 9km long HRT face has arrived at site and erection has been started at Adit 2 site. Excavation of Adit 2 having a length of 976m stands completed.
TUNNELLING CONDITIONS
HRT face
The TBM section of the headrace tunnel will be excavated mostly through granite/gneissose granite (RD 19354 – 15700 m) followed by quartzite (RD 15700- 10300 m). Bands of biotite schist, talc chlorite schist or metabasics can be expected along the entire length of the TBM drive. The granites are hard and massive exhibiting a well developed foliation in some areas. The quartzites are moderately to extremely hard. The schists, varying in thickness from one to tens of meters, are softer with altered clay being observed in places. No exploratory drilling was done along the TBM section of the headrace tunnel due to the high cover. No major faults were recognised along the headrace tunnel but numerous lineaments representing large joint fractures or faults crossing the headrace tunnel (including TBM section) have been interpreted from satellite imagery and aerial photos. Total overburden along the TBM section of the headrace tunnel varies between approximately 100 m at Hurla Nala (near Adit -2) to 1300 m. The average compressive strength of the granite / gneissose granite is assessed as 54 MPa parallel to the foliation and 91 MPa perpendicular to the foliation. Average Brazilian tensile strength for granite / gneissose granite is assessed as 9 MPa parallel to the foliation and 15 MPa perpendicular to the foliation. For quartzite the average compressive strength is assessed as 121 MPa parallel to the foliation and 267 MPa perpendicular to the foliation, while the average Brazilian tensile strength is given as 16 MPa parallel to the foliation and 25 MPa perpendicular to the foliation. The orientation of the foliation planes in granite / gneissose granite is given as 060/045 (dip direction / dip slope) and the foliation surfaces are described as very persistent (10 –50 m) and as rough/planar. The chlorite schist bands generally occur parallel to the foliation. With the roughly north-south alignment of the headrace tunnel (ca. N190° in drive direction) the schist bands will show an apparent dip transverse to the tunnel which could have an unfavourable effect locally on wall stability, requiring immediate rock support. Wedge formation in the face with sliding towards the tunnel opening can be considered to be a relatively low risk in the granite / gneissose granite section. The formation of roof wedges can however be expected. The chlorite schist bands are generally seen parallel to the foliation. Wedge formation in the face with sliding towards the tunnel opening can be considered to be a relatively low risk in the quartzite section. The formation of roof wedges can however be expected.
Pressure Shaft
The inclined shafts will be excavated through moderately foliated to massive metavolcanics with chloritic phyllite zones and intermittent bands of chlorite schist. The metavolcanics are strong and massive whereas the phyllites and schists are moderately to closely jointed and moderately strong to weak. Quartz veins and quartzite bands are encountered in places. Talc lenses are observed within the chlorite schist. As a consequence of shearing, the strength of the rock mass varies from moderately strong to weak. In the pressure shaft area 5 drillholes have been completed and logged. According to the drilling logs and the revised geological section moderately foliated to massive metavolcanics dominate in the upper section of the inclined shafts while closely foliated and jointed zones occur more frequently in the lower section. No faults are expected along the alignment of the inclined shafts. Total overburden along the inclined shafts varies between 100 (locally in upper section) and 300 m with up to 80 m of soils and rockfall debris. The rock cover varies between around 70m (locally in upper section) and 250 m. The average compressive strength of the metavolcanics is given as 41 MPa parallel to the foliation and 74 MPa perpendicular to the foliation. Average Brazilian tensile strength is given as 8 MPa parallel to the foliation and 13.5 MPa perpendicular to the foliation. The chlorite schist bands are generally orientated parallel to the foliation. The roughly east-west alignment of the inclined shafts (dipping at 30° to east) is such that it will be driven at an acute angle with respect to the foliation and the schist bands which must be considered as unfavourable and requiring immediate rock support.
ROCK CLASSES
HRT
As per RMR classification, almost 79% of the HRT rock strata is expected to fall in class II and III, about 8% in Class-I and Class-III and Class-IV constitute the remaining 13%. In the TBM portion Manikaran Quartzite is expected to cover 60% of the reach and rest 40% granite gneiss.
Rock classes in Manikaran quartzite section:
– About 8 % of TBM tunnel length expected to be Class I
– About 66 % of TBM tunnel length expected to be Class II
– About 13% of TBM tunnel length expected to be Class III
– About 13 % of TBM tunnel length expected to be Class IV
Rock classes in granite/gneissose granite section:
– About 18 % of tunnel length expected to be Class I
– About 71 % of tunnel length expected to be Class II
– About 11% of tunnel length expected to be Class III (including Class IV)
In general dry to dripping conditions, locally moderate inflow of water is anticipated along the headrace tunnel. Water inflow in talc chlorite bands may require special measures due to the low rock strength.
Pressure Shaft
The pressure shafts run in Meta basics, which are traversed intermittently by weak chlorite schist. Whereas the meta-basics are having feebly foliated massive bands with some jointed portions also, the chlorite schists are thinly layered with a low compressive strength. However the foliation direction is oblique to the pressure shaft, which makes tunneling through them slightly less problematic.
Rock classes:
– About 20 % of shaft length is expected to be Class II
– About 60% of shaft length is expected to be Class III
– About 20 % of shaft length is expected to be Class IV
In general dripping to moderate flow of water (in jointed metavolcanics) is anticipated along the inclined shafts. Water inflow in talc chlorite bands may require special measures due to the low rock strength.
BRIEF DESCRIPTION OF TBM
TBM for HRT
The TBM to be used for HRT is Robbins TBM MK 27 of 6.8m dia. The main structure of the Jarva MK 27 consists of the main body, torque tube with front and rear bearing housings, together with the drive shaft and bearings. This is a refurbished machine. Maximum machine thrust is 18'550 kN and considered suitable for hard rock machine. The machine is equipped with ring-mounted probe drilling equipment, which can cover 360 degrees of tunnel. Maximum probe length is considered to be 120 m. The probe drills are also intended for use in the installation of drain holes and for cover grouting. This machine is open type-high performance with six 525 kW main drive motors. The conveyor has a straight alignment without the down dip in the cutter head location. The machine is equipped with 49 number 432mm dia cutters. Maximum operating load per cutter is recommended as 267 kN maximum (17” cutters). The cutter head is a closed, backloading type, with recessed cutters and equipped with low profile muck buckets and replaceable scrapers. Manhole access (2 x 60 cm dia.) is provided. Nominal cutter spacing is 65 mm. The installed cutterhead capacity is 3150 kW and Stroke length is 2.050 m. Cutterhead drive includes six variable speed drive motors (VFD). Maximum cutterhead rotation speed is 5.77 rpm. Maximum total gripping force is 55600 kN carried over 4 gripper pads with H=3.6 m, W=1.4 m resulting in maximum rock pressure of 3.22 Mpa. TBM conveyor of 1000mm width has normal capacity of 875 cum per hour. Normal estimated power consumption is 2525 kVA. TBM has arrangement of rock bolting, wet & dry shotcreting and ring beam erector for erection of heavy steel arches. The machine is also equipped with high performance injection grouting plant. In the event of unexpected geological conditions, drilling into rock ahead of face through cutterhead would be possible in upper arc. The machine has reached the site. Erection out side the adit –2 has been started in the month of Dec 2003. The TBM after assembling out side the adit portal along with its back up system shall walk up to the Launch chamber. Based on past experience of Himalayan geology, an average progress of 300m per month has been estimated in the type of geology to be encountered.
TBM for Pressure Shaft
Mitsubishi TBM MHI-NFM-BORETEC shall be used for boring two number inclined pressure shafts. TBM of 4.88m dia is a refurbished machine. This is a double shield TBM. Main drive motors consists of 6 units of 250kW each. Machine is equipped with 33 number 432mm cutters. Maximum operating load per cutter is 267 kN. Normal operating power consumption is estimated as 1500 kVA. The machine shall be erected inside the tunnel. TBM for inclined pressure shaft shall be erected inside for which two chambers of 35m long and 12m high at the end of the adit shall be constructed. The TBM after boring one pressure shaft shall be dismantled at the top and taken out again to be assembled in the second pressure shaft at the bottom. Two chambers for dismantling at top shall also be constructed. Precast segmental linning shall be used for supporting the excavated area. Thereafter the pressure shaft shall be steel lined. The segmental linning is hexagonal ends type. The back thrust of the TBM shall be transferred to the segmental linning. In addition the TBM has gripper pads and anti skidding device. The excavated muck shall be transported through a steel trough and flushed with a jet of water from the working face down to the adit bottom in a hopper where water and muck shall be separated out. The dry muck shall be transported from hopper to out of tunnel through conveyor belt. The machine is equipped with probe drilling machine and segment erector.
Dewatering will be accomplished using 2 x 150 mm holes through the invert of the forward shield. A drainage channel is provided through the gripper shield. Drilling ahead of the face (maximum 80 m) is possible along the crown and tunnel wall through sleeves (3”) in the gripper shield. A grouting plant will be provided in the chamber at the shaft bottom. The telescopic shield is provided with windows which can be opened in order to examine rock conditions or to perform rock treatment (e.g. grouting or rockbolting) in exceptional circumstances. Stabilisers (2 x 20 ton) and auxiliary grippers (2 x 7.8 ton) are provided in the forward shield. The cutter head is a closed, backloading type, with recessed cutters and equipped with low profile muck buckets and replaceable bucket lips. Manhole access is provided. The annular gap between the rock and shield will normally be 63 mm. An overbore of 50 mm is possible if squeezing rock is encountered. The main bearing seals are being upgraded to the “EPB type” (3-way labyrinth). Variable speed drive motors (VFD) with a safety release coupling in the drive system are provided. The segment erector is capable of handling segments (approx. 2 tons) at a 30° shaft inclination. The backup decks will be cabled together to prevent any risk in the inclined shafts due to breakage of a coupling. Normally any necessary ground treatment (dewatering, grouting measures) will be performed from the gripper shield using the probe drilling equipment. In exceptional circumstances drilling through the cutter head into the face and/or radial drilling from the telescopic shield can be accomplished using an auxiliary drill. Polyurethane foam can be applied through the cutter head to seal the vicinity of the cutter head followed by a grout umbrella in the tunnel crown extending 5 to 15 m in front of the cutter head. Advance rates with selected TBM has been estimated as 10 m per working day (14 – 16 hr) per TBM.
POTENTIAL RISK SITUATIONS
HRT
The following potential risk situations have been considered relevant for the evaluation of TBM equipment:
–Development of roof wedges along schist bands leading to breakouts or larger overbreak; possibly causing overbreak in the tunnel crown adjacent to the face;
–Roof wedges due to joint sets and foliation;
–Rock fall from tunnel crown in closely foliated and/or moderately to heavily jointed rock;
–Sidewall overbreak particularly in the vicinity of schist bands;
–Moderately squeezing rock conditions in schists and phyllites;
–Water inflow in chlorite bands.
In addition machine capabilities in the event of unexpected or extraordinary geological occurrences have been considered in the choice of TBM equipment, as the occurrence such conditions over shorter distances along the headrace cannot be ruled out.
Pressure Shaft
The following potential risk situations have been considered relevant for the choice of TBM equipment:
–Development of roof wedges along the foliation and schist bands leading to breakouts or larger over break;
–Sliding of rock wedges out of the tunnel face, possibly leading to overbreak in the tunnel crown adjacent to the face;
–Roof wedges due to joint sets and foliation;
–Rock fall from tunnel crown in closely foliated and/or moderately to heavily jointed rock;
–Sidewall overbreak;
–Moderately squeezing rock conditions in schists and phyllites;
–Water inflow in chlorite bands.
These risk situations have mostly been considered in a risk assessment carried out. In addition machine capabilities in the event of unexpected or extraordinary geological occurrences have been considered in the choice of TBM equipment, as the occurrence of such conditions over shorter distances along the shaft cannot be ruled out.
An analysis of the capabilities of a double-shield TBM is summarised in the table:1 annexed
DISCUSSIONS
Major features of the double-shield TBM include:
–Capability to excavate hard rock using the gripper to provide thrust reaction;
–Capability to excavate concurrently with erection of support in order to achieve higher advance rate;