Tunnelling Techniques

Addressing geological challenges

Tunnelling for hydro projects, especially in the Himalayan region, has always been challenging. This is owing to the weak rock formation with a high ingress of water, which creates flowing ground conditions. In addition, it has extremely poor strata in the form of shear zones, thrust zones or highly jointed rock masses, which not only makes construction difficult but also involves a greater degree of uncertainty in terms of time and cost.

In India, tunnelling with conventional drill and blast method remains the first choice even for small stretches. Other solutions include tunnel boring machines (TBMs), the new Austrian tunnelling method (NATM), the 5P system, artificial ground freezing, DRESS (drainage, reinforcement, excavation, support solution) methodology and road headers.

A look at some of these methods, their impact and the challenges involved…

TBM

Over the past few years, TBMs have been used in the Himalayas for long hydro tunnels, with  mixed results. There are various TBM modes of tunnelling, including open mode, single shield, earth pressure balanced shields, slurry shield and double shield. With a 25 per cent success rate in India, concerns remain about the economical use of TBMs in Himalayan terrain. However, proper planning and evaluation of risk factors backed by sound geological and geotechnical investigations, and selection of the right type of TBM can mitigate the challenges to a large extent. Another issue in the tunnelling process is that the contractor avoids probe drilling as it involves a considerable amount of time. It is recommended that probe drilling should be carried out as an independent activity during the maintenance of the TBM to increase its utilisation time. The planned use of the TBM method at the Kishanganga hydroelectric project (HEP) proved to be successful and now, a bigger diameter TBM has been planned to be used for the Vishnugad-Pipalkoti HEP and the Pakal Dul HEP.

NATM

This method is commonly used in India. It provides optimised support based on observed ground conditions, and  is termed the “design as you monitor” approach. It is based on the observed convergence and movement in the rock mass, which is found out by mapping the prevailing rock conditions. Full-scale NATM was used for the first time in India for the Loktak project tunnel. The tunnel strata was predominantly shale with a high overburden and the tunnel had squeezing ground conditions. Thereafter, NATM was successfully used for the Uri HEP in Jammu & Kashmir.

The 5P system

The 5P system has been very successful for tunnelling through extremely weak rock mass with high ingress of water creating flowing ground conditions. The 5Ps comprise  p-plug, p-probe, p-pressure relief, p-protection of roof, and p-pre-grout and support. The process includes the creation of a plug for face stabilisation followed by plugging and sealing with synthetic fibre-reinforced shotcrete. Once the face is sealed, pressure relief holes are drilled, which help reduce the hydrostatic pressure of the face ahead and channelise water at the face. The roof is protected using large-diameter, long seamless perforated pipes, by creating an umbrella of pipes in the periphery of the tunnel and crown at very close spacing. Depending on the type of strata, pre-grouting is carried out using micro fine cement/adding micro silica through all the pipes installed on the roof. Finally, the supported area is excavated slowly, and steel ribs are installed at close spacing for advancement. The same cycle is followed for the second set after which roof protection is started.

Artificial ground freezing

Artificial ground freezing is based on the fact that ground strength increases when the ground freezes or saturated ground becomes waterproof when frozen. The strength increases to a level where it can be compared to concrete. Freezing is done by installing freezing pipes in the ground, usually into drilled holes. The most common methods for artificial ground freezing are brine freezing and nitrogen freezing. In brine freezing, brine circulates in a closed system in the periphery of the tunnel. The temperature of the brine is usually between (-)20 °C and (-)40 °C. In nitrogen freezing, liquid nitrogen is used for direct freezing. Due to the low temperature (-196 °C), the freezing is faster as compared to brine freezing.

The way forward

Tunnelling in the Himalayas has to be planned very carefully and modern techniques used. During excavation, progressive investigation should be carried out by drilling probe holes and the project should be prepared to deal with unforeseen conditions by keeping alternative equipment and material ready at the site. In addition, there should be a judicious increase in the penetration rate by increasing the thrust and speed of the cutter head rotation. In the TBM method, proper building of rates of excavation and mobilisation of equipment along with the TBM would be helpful in quick recovery from catastrophic conditions. For extremely weak zones, tunnelling by special methods should be planned well in advance.

Based on a presentation by M.M. Madan, Chief Executive Officer, Hydro and Renewables Business, Jindal Power Limited, at a recent Power Line conference

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