5P System Of Tunnel Excavation For Extremely Weak Rock Mass With High Ingress Of Water
M M Madan
Chairman (Hydro) - National Council on Power and Advisor (Tunnelling Group) - National Council on Construction Equipment at (ASSOCHAM)
Tunnelling through Himalayan ranges is always a challenging job, especially in the weak rock formation with high ingress of water creating flowing ground conditions. The occurrence of extremely poor strata in the form of shear zones, thrust zones, or highly jointed rock mass while tunnelling in Himalayas makes the job of construction engineers most difficult during execution and has a greater degree of uncertainty, regarding time and cost. Thus, different methodologies are required to be adopted while excavating very poor rock mass (class V) and exceptionally poor (beyond class V) rock mass. Most of the time proximity of seepage water creates flowing ground. When such a difficult situation is encountered, it is managed by many methods ranging from primitive manual methods to modern methods using the state-of-the-art technology and the latest materials. Most of the hydro projects encounter such tunnelling problems in a small or big way.
A system was evolved while working in one of the difficult projects where extra ordinary geological conditions were encountered. The system was termed as ‘5P System of tunnelling’, which is performed through extremely weak rock formations. The 5P-System is a systematic way of dealing with extremely weak rock formation with high amount of seepage water.
This paper discusses 5P System in detail and the methodology to carry out the same for tunnelling through extremely weak rock mass with high ingress of water creating flowing ground. Further, the focus is on one of the projects in Himalayan geology, where this system was successfully used.
Introduction
During excavation of a tunnel, the rock strata encountered may include very hard rock with high stand-up time termed as class-I/II, a rock with good stand up time termed as Class-III, low stand up time termed as Class-IV, very low stand up time or no stand up time termed as Class-V or even Class-VI. The last categories when mixed with water create ‘flowing ground conditions’. The strata may include highly jointed rocks, shear zone material, thrust zone material, lake deposits, terrace deposits, bouldery strata or sandy, silty, clayey strata, overburden or glacial deposits, etc. If the above strata conditions encounter seepage water or a water body or artisan condition, the ground starts flowing.
The Old Process
If the material encountered in the strata is flowing or collapsible in nature, then a plug of concrete of about 3-4 m thick is normally constructed at the face where flowing ground is encountered. In the concrete plug, few 50 mm diameter nipples are also embedded for drainage of face. In addition, some nipples are embedded on the side walls and periphery of the tunnel so that in the periphery, at least 16-20 m long holes can be drilled in such a manner that the strata above the crown and the sides of the tunnel are well grouted to a height of 6-7 m. After solidifying the reach by grouting, full face excavation is carried out to a point where the grouted face of about 5-6 m thickness is left for further treatment. Grouting of the further strata is again resumed in the same way, but the nipples are re-fixed and grouted in the required direction; following this, the grout is allowed to set for three days before normal excavation work is started.
In highly waterborne strata, few small drainage tunnels of size about 2 sq. m in section are constructed in the side wall at a reasonable distance from the main tunnel section. The percolating water will automatically flow into these tunnels, from where this water can be pumped out. The small side parallel tunnels should be excavated much in advance to divert the flow of water into these openings. In this way, the strata though which the main tunnel is to be excavated will be rendered dry and excavation can be tackled by multi heading method very carefully by opening quarter heading at one time in a length of 0.5 m and installation of half side ribs provided with vertical support. Rib supports should be installed at a close spacing or zero spacing touching flange to flange, depending on rock strata and thereafter, immediately backfilling the space between ribs and the rock with concrete.
Review Of The Old Process
Making a concrete plug of 3-4 m itself is a time-consuming process and if the strata starts flowing, then it will be impossible to erect shutters and complete concreting. As the ground becomes unstable and slushy, such works are carried out manually; when heterogeneous soil, gauzy material or clayey material is encountered, it becomes difficult to grout and even drill holes through the conventional methods. Moreover, constructing a small tunnel is not an easy solution when working in constricted area and in flowing ground conditions. In such conditions, a long tunnel must be prepared parallel to the main tunnel to drain out water and only then, the normal work can be resumed. The other method is to divert the tunnel course and reroute the alignment (Tala and Allain), or reduce the diameter of tunnel into a smaller parallel tunnel (Giri Bata and Chhibro Khodri).
All these processes consume a lot of time and money. Also, the non uniform zig zag alignment causes friction losses in a hydro tunnel and the head is reduced to a great extent. Therefore, a need was felt that there should be a methodology by which the alignment is not disturbed, and the work progresses safely and smoothly. Therefore, an improved methodology was adopted.
Improved Methodology
The methodology to treat flowing ground and weak formation is required to be fool proof and systematic in treating such a ground. It is possible to systematically treat the strata with the help of modern machines and modern materials. The improved system is explained in the following section.
Plugging Of Face
The first activity, which is to provide a plug, is the most important part in the process. In the circumstance of flowing ground, as it is difficult to do the work, it becomes essential to plug the entire face. The same can be achieved by placing sand filled gunny bags to create a positive barrier. The bags will stop flow of muck and a stable face would be visible. With the gunny bags, it is also possible to drain out water from the gaps formed in between the bags and drainage pipes. To provide sealing of this plug, the whole area is synthetic fibre shotcreted. Shotcrete is a better solution as it provides quick support. In addition, steel girders can be provided for positive support. In case of weak formations, sealing should be done with FR shotcrete and long face bolts. The long face bolts shall either be fibre glass bolts or self drilling bolts through which grouting can also be carried out. Further operations can be carried out after plugging the face.
Pressure Relief
Pressure relief holes shall be provided from behind the tunnel face, preferably, 5 m short of heading. These pressure relief holes will allow the water at the face to drain from the crown of tunnel away from the heading. The machine which is being used for pipe roofing for providing pressure relief, can be used here as well, or a drill jumbo with special attachment can be used for providing pressure relief holes. These pressure relief holes should ideally be 25 m or more long, depending on the pipe roofing length. The pressure relief holes shall be longer than pipe roofing.
Probe Drilling
Probe drilling is carried out to identify the exact configuration of strata lying ahead of face. The probe drilling may be carried out with same machine with which drilling for roofing pipes is carried out. It is preferred to drill a hole of 100 mm or more. Further, a close watch by geologist should be kept on the pressure exerted during drilling and the cuttings obtained during flushing, with which prediction of the strata in the front of heading shall be made. The holes drilled should be a minimum of 30 m and may extend to 100 m length, depending upon the availability of machine and time. Three holes; one on the crown above pipe roofing, second and third on the 10 o’clock and 2 o’clock position are desirable. If high amount of water is encountered, then it will act as drainage pipe or if grouting is to be carried out through the same pipe, a decision can be taken accordingly at site.
Protection Of Roof
Protection of the roof in weak rock formations is the most important part. The protection of roof is to be done with pipe roofing or steel rebar forepoling. Steel rebar forepoling is not recommended as may not take the load of flowing ground, or it will need larger diameter of rebars which will be quite heavy and will create handling problems; therefore, it is advisable to do pipe roofing, which is more load bearing and also self draining. A special machine may be deployed to do the pipe roofing operation. The machine should be capable of drilling long holes of 100-150 mm diameter, upto a depth of 30-40 m at 5 degree lookout angle inclination depending on size of the tunnel. For smaller tunnels of 3 m diameter, a hole size up to 100 mm shall be enough. After drilling of each hole, a perforated pipe of suitable size is to be inserted up the drilled length. Depending on the type of strata, collapsing or firm, the depth of hole shall be decided. For small size tunnel, a depth of 10-15 m is enough; the pipes shall be seamless and perforated. Small pipes can be welded together to form a smooth joint. Since the same pipe shall be used for grouting operations also, it should be seamless, and the full crown shall be covered with these pipes. Further, the gap between the pipes can be decided as per the site condition; for worst conditions, no gap shall be left; for slightly better conditions a gap of 150 mm may be kept. Also, if the sides are also flowing, then pipes in the side wall above spring level or below may be provided. This procedure will fully secure the roof.
Pre-Grouting And Supporting
Pre-grouting of the strata beyond the pipe roofing length is carried out with OPC. The pre-grouting can be taken up through the specially drilled holes below the pipe roofing in the main heading. The strata shall be grouted with non-return valve type packer assembly and high pressure to allow the grout to spread. High pressure should not bulge or move the rock strata but should be sufficient to spread the grout on the entire heading at a depth of 10-20 m ahead of face, so that when excavation is carried out, a better geometry of rock is found and flowing nature is controlled. If the ground is not taking OPC grout then micro fine or ultra fine cement shall be used with silica fume or microsilica. In worst cases, PU grout is to be used, which may seal the water seepage instantly.
After all the above actions are performed, the face is ready for excavation up to a length 3 m short of the deepest point of pipe roofing. After excavation of the rock mass, the pipes, which are acting as a roof shall be supported with steel ribs and concrete back filling is carried in a normal manner. The spacing is decided based on the type of ground conditions. Further, the spacing between the ribs is filled with precast laggings or with fibre reinforced shotcrete.
Fig. 5: Weak Zone Excavation
Weak Zone Excavation Adopting ‘5P System Of Tunnelling In Weak Rock’:
The basic principal of weak rock tunnelling is dependent on quick support system as well as proper support by taking utmost care of supporting in parts so that the rock surface is not allowed to expose to the weathering affect. The operations shall be planned in such a manner that it improves the inherent capacity of the rock mass and restricts movement of rock mass or support system.
Ground Relaxation Ahead Of Face:
Full face construction shall be avoided and pilot heading in parts followed by benching should be adopted.
5P System Of Tunnel Excavation For Weak Rock Formations
The 5P System is very effective to carry out tunnelling through extremely weak rock mass with high ingress of water creating flowing ground. The system was successfully used in one of the projects in Himalayan geology where similar ground conditions were encountered.
P 5 System- Detailed Methodology
The system runs in five main steps which are detailed below. Further, the sequence must be followed as detailed.
1st P-Plug – (Plugging and Stabilization of Face):
To stop flow of ground material, the face is required to be plugged to sandbags and then covered with thick layer of shotcrete sprayed with dry system to stop leakage of water.
Immediately after flowing ground is encountered, the heading is required to be packed with sandbags.
Horizontal struts made from steel channels and I-beam may be welded to the ribs at springing level to prevent deformation of ribs.
Synthetic Fibre Reinforced Shotcrete (SyFRS) to be sprayed at heading covering the entire face (even on the gunny bags filled with sand) to prevent leakage of grout from the face and to facilitate channelising of water.
The ribs are supported with vertical steel columns and struts at the SPL and angular struts. The face is also shotcreted to seal the face completely to divert seepage water away from the face.
2nd P-Pressure Relief (Pressure Relief and Drainage Holes):
Pressure relief hole/drainage hole for channelising water flow are provided 3-4 m away from the face.
Three 89-100 mm diameter drainage holes about 10-15 m deep to be drilled in crown periphery at 10-15° upwards, followed by installation of perforated seamless pipes.
These help in reduction of hydrostatic pressure and minimising seepage from face ahead.
About 40% to 50% water on the face gets channelised and drained through these drainage holes at the crown.
3rd P-Probing (Probe Drilling):
To assess the ground conditions ahead of heading, a series of long probe holes of 100 mm diameter are drilled. The probe drilling is started by drilling one hole at the centre of the crown and two at 10 o’clock and 2 o’clock position. The holes are drilled to a depth as per requirement, which may vary from 10-20 m, or even longer.
Probe hole gives prediction about the strata in front of the face. During drilling of probe hole, the field geologist should plot the lithology based on probe hole drilling observations.
Geologist should be available all the time at the face during probe drilling.
The hole also acts as a drainage hole. These can be used for pre-grouting of the face ahead. Further, the holes should be inserted with perforated pipes and be provided with NRV (Non Return Valve) packers in case high pressure water is encountered, while hitting water charged shear zone with heavy flow of water with pressure.
4th P-Protection of Roof:
After drilling drainage holes to divert the water away from the face, roof is required to be protected and strengthened.
The protection is carried out by using large diameter (114 mm) Seamless Perforated Pipes Umbrella (SPPU) of 10-12 m, or even longer, length by creating an umbrella of pipes in the periphery of tunnel crown @ 10-15° angle upward at close spacing touching skin-to-skin or having a spacing @ 250 mm c/c (or 2D of pipe diameter) from spring level RHS to spring level LHS, depending on the rock and seepage conditions.
Special pipe fore-polling machine or attachment in drill jumbo is used to install pipes simultaneously with the drilling of holes, instead of installation of pipe after drilling of holes, which may not be possible later as the drilled hole may collapse.
As the pipes are perforated, cement grout spreads around the pipes to create a thick cement seam at the crown. Grouting is normally carried out at 3 to 4 bar pressure depending up on the ground condition. The water cement ratio of grout mix is kept as 1:1 to 2:1.
5th P- Pre-grouting (Pre-Grouting and Improving the Strength of Face):
Pre-Grouting of the face improves inherent capacity of the face and flowing ground can be avoided.
Drill long holes of 10-15 m length, 75-100 mm dia @ 15° angle in upward direction at the centre of top heading. About 3-4 holes should be drilled to perform pre-grouting of the strata by providing single or double packer assembly.
The grouting should be carried out at 40-50 bar pressure. Grouting should be carried out using micro fine cement/ultra fine cement with additives as micro silica/superplasticisers/ metakaolin. The doses are to be decided depending on ground condition, so that the face becomes impervious and water surrounding the cavity can be diverted from the main face.
Special self-drilling bolts with central hole (Equivalent to MAI bolts) may be used for stabilisation of face.
Roto-screw type grout pumps may be used for pre-grouting of the strata.
Pre-Grouting trials can be made and if grout intake is encouraging with high pressure, then number of holes can be increased to stabilise the rock.
Subsequently, excavation of the face is carried out very carefully. After installation of pipes (SPPU) in the roof, the next step is to excavate a niche in the crown (either RHS or LHS) for installation of part rib (1/4th set when strata is very poor or 1/2 Rib set in the upper portion with good stand up time).
Dummy support to the ribs in the form of vertical columns at the centre of tunnel heading and horizontal struts at spring level (as seen in the attached photo) and struts at an angle shall be provided to support the roof and to stop settlement of ribs.
The installed heading ribs should be touching skin to skin in extremely poor rock and/or @ spacing of 500 mm c/c with concrete lagging back filled with concrete immediately having good stand-up time.
The ribs should follow the periphery of Pipes in upward direction (as per sketch)
Before installation of columns, the top heading ribs shall be rock bolted at SPL and with horizontal struts welded with previously installed rib supports.
These supports are to be removed after columns are installed for the next set of rib installation.
The next cycle is started after leaving 1/3rd length of Pipes in the Rock for proper support. And the same cycle is followed for the second set of roof protection.
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Table 1: Tunnel Excavation Sequence |
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Stage 1a |
Plug the face with Gunny Bags filled with Sand |
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Stage 1b |
weld the struts; vertical, horizontal, inclined of suitable size channels, girders |
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Stage 1c |
Shotcrete the entire face with SyFRS 50 mm thick |
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Stage 2 |
Drilling of Pressure Relief Holes 3 nos X 10 m deep in the crown 4 m before the face-inclined in the crown – having perforated seamless pipe 10 m long |
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Stage 3 |
Drilling Probe Holes- 10 m long at the Face- 3 nos- 100 mm dia |
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Stage 4(a) |
Drilling for Pipe Roofing 114 mm dia Seamless Perforated Pipes 9 m long- 20 pipes |
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Stage 4(b) |
Cement Grout injections @ 2-4 Bar pressure through radial holes in order to strengthen the surrounding soil and minimize the water leakage. (increase consistency of grout, in case of more grout intake). |
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Stage 4(c) |
cutting niche in the heading on RHS for installation of Rib (if poor and weak rock, ½ Rib should be installed. The space between two ribs (in weak rock skin to skin rib be installed) be filled with shotcrete (Sy FRS) immediately. (Strata having seepage should be back filled with concrete lagging immediately). |
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Stage 4(d) |
Repeating Stage-4(c) for the other ½ side of tunnel heading (LHS). |
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Stage 4(e) |
Repeat Stage-4(c) and Stage-4(d) for next 2 m of tunnel. |
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Stage 4(f) |
support the First rib with Dummy central king post support and bottom strut at SPL. The fixing of precast segments shall be provided with the installation of rock bolts on the sides. |
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Stage 4(g) |
Excavate niche for the side column on RHS and fix column with either precast concrete lagging or shotcreting. |
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Stage 4(h) |
Repeat Stage-4(g) for the other side Column on LHS. |
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Stage 4(i) |
Excavate Bottom portion and install bottom Strut to complete the Ring. |
Standard Operating Procedure And Method Statement For Weak Rock Strata Excavation In Tunnels
Standard Operating Procedure and Method Statement is performed to support and excavate the disturbed rock mass, which is frequently destabilised by slow process of support. Tunnel excavation sequence is summarised in Table 1.
Total time for 2.0 m progress is 67 hrs + Loss of time (20%), i.e., 13 hrs + unforeseen 10 hrs = 90 hrs, say 0.5 m per day (15 m per month) - This can be improved with incentive and better management.
Discussion
Certain decisions are taken based on ground conditions observed, as there cannot be rigid rules for evolving a procedure. Some of the conditions which can be encountered are as follows:
Plug
During the plugging of heading, if water pressure is such that it cannot allow the plug to be put in place, then guide pipes may be placed in the plug. It will act as pressure relief for water and also act as a guide for drilling deeper holes for pressure relief and pre-grouting of the strata in the heading.
The plug should be leak proof as it may not be possible to do shotcreting because of leaking water. Further, the face should be provided with bulkhead by doing concreting and the pipes should always be left in the bulkhead.
If there is excessive pressure, the bulkhead should be reinforced by providing cross girders or steel channel sections welded together with ribs.
If it is not possible to reach the area of cavity because of fallen rock pieces and collapsed muck, the whole area which is filled with muck should be sealed with the plug, and grouted.
It is advisable not to remove the muck, and the consolidation should be done in situ.
Pressure Relief
Judgment must be made about placement of pressure relief holes. Sometimes it may happen that through the pressure relief hole, all the water starts coming with a very high pressure, and it becomes difficult to control the high-pressure water if prior preparations are not made. (Parbati-II TBM flooding)
A NRV should always be placed at outlet of pressure relief hole.
If the water cannot be directed away from the work area, then it is better to close the pressure relief hole by plugging with suitable material.
Probing
The mouth of the hole should be mounted with NRV type packer to prevent any high seepage water pressure.
The water pressure should be measured by installing a pressure gauge.
Geologist should always be present at the face and in liaison with machine operator, the fine cutting coming out of hole shall be examined for predicting the rock type and the pressure exerted during drilling will give an idea of strength of rock strata. Also, the geologist should be able to judge the quality of rock ahead in the tunnel face
Protection Of Roof
Depending on the strata strength, the spacing between the pipes be decided. In flowing ground, no spacing should be kept.
NRV suitable for the dia of pipes shall be kept always ready at site.
Dummy ribs may be required to be erected for supporting the pipes if it is observed that there is excessive pressure from the roof.
The pipes should have a support at both ends; one on the steel rib and other in the rock at the face. The minimum support length should be half of the diameter of tunnel.
Preferably heavy duty seamless MS pipes should be used.
The pipes should be able to take grouting pressure of 90 kg/sq.cm (9 kPa).
The pipes should be perforated so that it can take grout or drain out water.
Pre Grouting And Supporting
Pre-Grouting should always be attempted.
The grouting pressure must be controlled, otherwise it may bulge the face or start movement of the ground.
Close monitoring of the movement and leakages during grouting operation should be done.
If leakage of gout increases, then the operation should be terminated and given rest for setting time.
Water bearing jointed rocks shall always be pre-grouted.
The area under the supported roof should be excavated either by a low intensity blast or by hydraulic hammer.
The excavated area should be supported with steel rib supports and back filling with concrete or shotcreting immediately.
Micro Silica And Synthetic Fibres In Shotcreting
Fibre Reinforced Shotcrete (FRS) linings have been found effective for ground control in thousands of mining, tunnelling, and slope stabilisation projects around the world. The design of FRS linings for ground stabilisation has been subject to development for many years. Experience of steel FRS corrosion in Scandinavian tunnels has led to steel fibres being banned in sub-sea tunnels in Norway and restrictions have been placed on their use in Sweden and Japan. The durability of Synthetic Fibre as shotcrete reinforcement is economical and efficient making it more attractive in tunnelling and infrastructure industry.
The advantages of Synthetic FRS (SyFRS) are evident in both the wet and hardened states.
In the wet state, Synthetic FRS is easier to pump than steel FRS of similar performance.
Pumps and hoses are subject to less wear and tear, than while using steel fibre.
Synthetic fibres are safe to handle as they pose very little risk of human injury during handling. – Less quantity of fibres is used per cubic metre of shotcrete for post-crack performance.
Involves reduced environmental and transport costs results in low carbon emissions per cum of FRS
Potentially lower overall cost per cubic metre for most specified levels of post-crack performance.
If high performance is required across cracks wider than 0.25 mm, then Synthetic fibres are likely to result in a cheaper FRS mix
Synthetic FRS is highly suited for ground control in tunnels, caverns and mines for primary linings or even final lining.
Synthetic fibres can bridge much wider cracks than steel fibres and are thus more suited to the control of ground subject to high deformation.
Micro silica improves strength and reduces rebound of the placed shotcrete, thereby saving huge cost.
The low cost and adaptability of Synthetic FRS is the principal reason that this material has been accepted widely in the underground mining/tunnelling industry.
Conclusion
Although the tunnels are constructed in extremely weak strata, it takes much longer time to support the crown and proceed further. Sometimes it takes months of stabilisation process without an inch of movement. The 5P System explained above in paper has been tried successfully at many projects in various forms, but the sequence explained in the paper has been found to be most successful and regular progress can be achieved if planned in advance; all the materials and equipment should be made available in advance. Moreover, meticulous planning, quick decisions at the site and devotion of the engineers shall bring good results to expedite the excavation of tunnels with 5P System incident free.
From - CE&CR January 2020 Issue
