Tunnelling in the Himalayas is rapt with complex geological and geotechnical issues. Water ingress further worsens the excavation process by reducing the shear strength of the surrounding rockmass. This inadvertently leads to tunnel collapses and failures, resulting in undue delays; accidents & injuries to the manpower and heavy economic losses to the organisation. Sealing of fractured rockmasses using DrucHyd-2C (PU) injection is one of the better alternative to the conventional cement grout. DrucHyd-2C’s low viscosity, expandable nature, quick reaction time, impermeability, non-toxicity, and ability to enter finer voids are the crucial benefits over other grout material. The present paper deals with the experience of using DrucHyd-2C in a railway tunnel (T5) in the Himalayan state of Jammu & Kashmir, India.
Geologically, the region is composed of Sirban Group (predominantly Dolostone, Limestone & Shale).The rock mass in the concerned stretch is highly jointed grey dolomite altered with three sets of joints, shear seams and shear zones charged with water. Tunnel T5 had intersected a thick shear zone at chainage 44+508, along which heavy water inrush (400L/sec) was taking place. Due to the delay in mitigation, a huge cavity had formed on the left crown of the tunnel excavation. Afterwards, a two component DrucHyd-2C (PU) injection was applied to divert the water away from the excavation, together with the consolidation of the debris material. To tackle the problem, a methodology was adopted taking into account the geo-environment. The performance of the grouting material was optimized, in terms of reaction time and strength, based on the prevalent geological and hydrological conditions. The execution was closely monitored to yield the best results.
The present study area is a part of the construction of balance tunnelling works of Tunnel T-5-Twin tube (between km 42/980 to km 48/940 approx.) on the Katra Dharam section of Madhubabu et al. (2016)INDOROCK 2016: 6th Indian Rock Conference 17-18 June 2016 USBRL Project, in the State of Jammu & Kashmir, Udhampur-Srinagar-Baramulla Rail Link Railway line is the biggest project in the construction of a mountain railway since independence. From Jammu to Baramulla, length of the new rail line is 345 km. It passes through the young Himalayas, tectonic thrusts and faults. The stretch is situated in high mountains and deep gorges requiring construction of large number of tunnels and major bridges. Generally, the geology of the area is Sirban Dolomite and Murree formations. Start of the project is in Shiwalik formation. The strata in most of the tunnels is fragile and charged with water particularly in tunnels passing through shear Zones (Kamal Dev Ralh).
The most common problem in tunnelling is the existence of ground water. If the flow of water into the tunnel cannot be controlled, work and environment related safety problems are likely to occur. Namely, a great reduction in the level of the groundwater table can be induced if the Water flow into the tunnel is not prevented continuously. Therefore, the pore pressure will continuously drop and consequently large amounts of ground settlements will occur by the closure of pores and fractures, causing significant damage overlying structures (Kucuk et al.,2009). An uncontrolled drop in the pore pressure will trigger the closure of pores and fractures near the tunnel and cause damages to the adjoining area as a result of ground settlements. In order to prevent this, the ground water with high inflow rate should not be allowed to drain through the interior of the tunnel.
Groundwater inflows into tunnel excavations must be evaluated relative to tolerable risk. Any tunnel intersecting a water bearing formation is exposed to some level of risk associated with water inflows. The magnitude of this risk is highly variable and dependent upon the site conditions, impacts of inflows on third parties and the means and methods undertaken to Construct the tunnel. Understanding the magnitude of these risks and developing appropriate measures to mitigate these risks is one of the keys to successful construction of a tunnel.
Repeatedly, tunnel projects encountering high groundwater inflows have suffered severe cost overruns, schedule delays, and environmental impacts when the consequences of these risks were not fully recognized and there was no contractual mechanism to mitigate these ground watering flows. Estimating groundwater inflows into rock tunnels is far from an exact science.
Several authors have investigated this issue including Goodman et al. (1965) and Heuer (1995).
There are also several numerical models, which can be used to estimate potential inflows. In
general, the uncertainty with groundwater inflow estimates greatly increases with increasing
hydrostatic head and complexity in the geological conditions.
INDOROCK2016110 418 Also, due to continuous water seepage, the surrounding rockmass has become weak because of reduced shear strength of the material. The stoppage of water may be seasonal factor but the danger of tunnel collapse still persists. From the Chainage KM 44+517 to KM 44+527, as the heavy water seepage has damaged the left wall of the tunnel and benching. Hence, to make the further excavation safe it is highly recommended to proceed with pre grouting to avert any untoward incident.
Himalaya is young and active mountain chain with complex geology and the underground excavation is hampered due to diverse geological problems viz., difficult terrain, thrust zones, shear zones, folded rock sequence, in-situ stresses, rock cover, ingress of water, geothermal gradient, ingress of gases, high level of seismicity etc. All these challenges may end up in extended delays and shooting up the cost of the project. Proper tactics to avert the accidents and delays need to be devised. These can reduce time, cost overrun and enhance safety and stability of the structures, if scientifically implemented (Mauriya, Yadav, & Angra, 2010) Excavation through jointed rock mass along with ground water faces major problems such as:
(i) Heavy ingress of water in tunnel hinders the construction activities inside
(ii) Failure of rock mass near crown and spring level due to reduced shear strength of saturated rock mass For weak rocks and high water pressure it can virtually turn into flowing ground condition.
(iii) The support system get affected due to high pore water pressure behind the tunnel periphery
(Mauriya et al., 2010).In the Himalaya, the rock masses are highly fractured, folded, sheared and deeply weathered,due to active tectonics (Panthee et al. 2016). Tunnelling through numerous zones of weakness, fractures and faults is thus a risky business. Moreover, the majority of these weak zones are in highly conductive, signifying possible sources of ground water aquifer as well as possible sources of water leakage from the completed unlined/shotcrete lined tunnels. Hence, treating the rock mass with chemical injection grouting in the Himalayan region against water leakage is a cost effective and environmentally friendly solution (Panthi & Nilsen, 2002) .
DrucHyd-2C (PU) is one of the more popular material used for chemical injection in rocks masses with high water ingress. It swells 21 times and strengthens the rock mass (Bhawani & Singh 2006) Madhubabu et al. (2016) INDOROCK 2016: 6th Indian Rock Conference 17-18 June 2016
DrucHyd-2C is typically a two component system that has several advantages over conventional grout. It has the ability to chemically bond to the rock, unlike other supports which rely on frictional contact. As it is injected under pressure, it inherently targets rock discontinuities and large pores, which are the paths of least resistance. PU also has a low viscosity, allowing it to penetrate cracks as small as 0.05 mm wide (Knoblauch 1994). It has engineered expansion properties (1:1 to 1:12) which also allow for penetration and cost effectiveness (Molinda, 2004).
The use of PU injection and stabilization is most commonly used in difficult ground conditions
characterized by fractured, broken rockmass that is progressively undergoing failure or actively
caving. The injection of the PU material into the rock discontinuities reinforces the fractured
rock to the point where it can support itself and the weight of overlying weak rock by forming
a grout-reinforced beam. The beam structure then bridges the weaker or more fractured rock to
adjacent abutments having greater supporting strength. The use of easily-mobilized injection
systems has made DrucHyd-2C (PU) resin stabilization a common practice, especially for stabilisation of underground excavation in mountainous regions. DrucHyd-2C (PU) injection,
employing a range of PU mix designs, also acts as a sealant to prevent groundwater inflows
(Federal & Highway, 2008).
T5P1 tunnel alignment passes through hill slope of dolomite of the Trikuta formation (Mesopriterozoic, 1000 million years of age) aligned sub parallel to Main Boundary Thrust (MBT) otherwise known as Reasi fault. Passing above the MBT the alignment encounters shear band, seams and the slope induced deformations (Fig. 1).
Fig-1: Geological Section along Tunnel T5 with location of the present face Ch 44+520INDOROCK2016110 420
3.1 Shear bands and seams
In the dolomite of Trikuta formation of Sirban group, shear bands and seams are monotonously
encountered in a group or isolated and are the main cause of instability in T5P1 tunnelbasically
caving and rock fall behavior. The extension of the zone is not always regular or linear and can take various forms of dilation and pinching. In fact, from what we can see and estimate the cross section area of the present cavity in the sheared zone is less than 1.0m3, however it can be more in different location depending on the rockmass quality. The rockmass of the zone suffers intensive deterioration during shearing whereby the rockmass is mylonitized, sugar cubed, turned densely jointed. The situation of the sheared zone also provides a very suitable venue for the subsurface water to accumulate and to serve as saturated zone. Weathering- chemical or physical both occurs if the terrain is of limestone and dolomite.
Generally the central part of the sheared zone is filled with thick whitish gouge which may act
as an impervious layer. During chemical weathering whitish precipitates occurs through the joints and leaves the rockmass degraded – density and strength wise and turn entire rockmass looking whitish. In the presence of high hydrostatic pressure, the heavily crushed material in the sheared zone behaves as a ravelling and flowing ground.
Heavy water seepage occurred at the current face Ch. 44+520 in the T5P1 Main tunnel.Overburden in this chainage is approximately 360m. Face Mapping between CH 44+503 to 4+520 (Shear bands in cross hatch):
3.2 Summary of Ground conditions from Ch 44+490 to Ch 44+520:
Ch 44+491 to 44+500: The shear zone shown in the face mapping above running diagonally from left SPL to right crown was first appeared around Ch 44+491 and up to Ch 44+500. The Ch. 44+520 Ch. 44+518 Madhubabu et al. (2016)
INDOROCK 2016: 6th Indian Rock Conference 17-18 June 2016
rock mass was characterized as Fair with RMR 42-45; the ground water condition was recorded
as wet. The rock mass was assigned GDE support class C1.
Ch 44+501 to 44+510: Beyond Ch 44+501 and up to Ch 44+510 the rock mass was
characterized as Poor with RMR below 40 and the ground water condition was reported as dripping to flowing, mostly concentrated above the shear zone. The rock mass was assigned GDE class C2. In this 10m section, two cavities were formed on the right side (Ch 44+503 &44+508) above SPL generally associated with the shear zone but were dealt with effectively.Additional strengthening measures were recommended in the vicinity of the shear zone with consolidation grouting and longer anchor bolts. Ch 44+510 to 44+520: From Ch 44+510 and up to the current face at Ch 44+520 the rock mass had deteriorated slightly more but still characterized as Poor with RMR below 40 and ground water conditions between dripping and flowing up to Ch 44+518, concentrated mostly above the shear zone. The shear zone had gradually moved close to the crown. The rock mass was assigned GDE support class C2.
Ch. 44+516 Ch. 44+514
Ch. 44+512 Ch.44+510
- Shear band and the cavity
- Debris at the back chainage floor
- Water and Debris 4. A slight deformation occurred on the
4. Specific Problem Identification
During the tunnelling from KM44/516.5 to KM 44/520, there has been huge amount of water ingress observed because of the formation of shear zone. a cavity formed at KM44/516.5 at the crown portion along the direction of shear zone as the sheared rock mass/shear zone material washed out due to the high ingress of water. One more cavity was formed at KM44/520 at the left crown portion, 4 to 5m along the tunnel direction and is again due to the wash out of the shear zone/ sheared rock mass material under the influence of high ingress of water. After backfilling the cavity portion it is decided to drill some drainage holes to tap the water but while doing the same there is again high sudden outburst of water which further leads to the wash out of shear zone material along with the back filled concrete. After this, it is decided to remove the accumulated rock debris and to drill some drainage holes to tap the water but again, water outburst has been witnessed from the left side of the face (Fig.2) along with huge outflow of sheared rock debris. Due to which approximately 20m of excavated portion of the tunnel length was filled with washout/debris material with time (approximately upto crown level). Due to the accumulation of huge rock debris material Madhubabu et al. (2016)INDOROCK 2016: 6th Indian Rock Conference 17-18 June 2016 at the face (KM44/520) the water seepage has stopped from the face but made its channel approximately up to 17m behind the face and started gushing out from mid crown portion between the lattice girders of KM44/503-KM44/504.
No further advancement has been done at the face due to the high ingress of water and accumulated volume (approx. 6000 cubic meter) of huge debris/washout material has been removed till now.
Cement grouting has been carried out for the present face. However, the refusal has been as high as 80% of the cement material.
Fig. 2: Heavy water ingress from the left crown portion, KM 44/520.
5 .Use of DrucHyd-2C (PU) grout to mitigate the problem
As suggested by client and consultant, it is decided to carry out pre excavation grouting to consolidate the debris present at the crown to make the further excavation safer. Grouting with
DrucHyd-2C [PU] resins represents an effective method of improvement of mechanical and sealing properties of soil and rockmass in constructions. The principle of grouting technologies is injection of liquid grouting material into the rock environment under pressure.
5.1 Injection hole length
Maximum hole lengths that can be effectively grouted depends upon the type of ground being grouted (including the number and range of hydraulic conductivities in the zones or fractures INDOROCK2016110 424 to be grouted), the grout hole spacing, and their location relative to the tunnel, the grout pressure that can be applied and the capacity of the grouting equipment and the set or gel time of the grout (Crouthamel et al. 2005). For the present case, a hole length of 9 m is recommended for the injection operations (Fig. 3). The grouting length is also affected by the drilling capability using perforated SDAs’.
Figure 3. Scheme of T5P1-treatment plan-Pre injection-CH-44+520 (Not to scale; Plan view)
5.2 Injection hole orientation
A half umbrella pattern is recommended for the present case, reason being more spatial extension, which will help to create an envelope of consolidated mass. The holes should be spaced at 0.5 m. The injection should start at 44+515, from the right side, below the excavation line, directed towards the right side (200-300). The lower holes will be drilled through the unconsolidated debris (Fig. 4), which will be challenging and may lead to slight change in orientation/strategy. The aim will be to consolidate the weak sheared mass/debris above the crown periphery and face, diverting the water and bridging the consolidated mass with the sound rock (Fig. 5). The injection holes should be inclined at an angle of 10-20 degrees, with respect to the tunnel periphery. The injection should be carried out with 32 mm perforated SDAs to form a cylindrical mass and the perforations should be made at a distance of 0.5m. As it is difficult to drill with perforated SDA rods, we have come to conclusion that, a 9m injection hole will comprise of only the middle 3m portion having perforation (spaced at 0.5m), while Madhubabu et al. (2016)
INDOROCK 2016: 6th Indian Rock Conference 17-18 June 2016
the first and the last 3m SDAs’ will be non-perforated. The grouting should commence at right to the centre line and proceed toward the left spring level.
Figure 5: L-section of the proposed injection (not to scale).
Figure 6: T5P1 – MT – Cross Section – Pre-grouting/injection – CH44+520 (not to scale).
5.3 Injection pressure
Occurrences of leakage to the face greatly diminish the ability to adequately permeate and treat
the rock mass. This will negatively impact the performance of the pre-excavation grouting
operation in meeting its design criteria. Selection of grout pressures are normally based upon
the amount of ground cover, and rock mass quality. Typically in moderate rock mass quality,
grout pressure should begin at about 0.25 to 0.5 bars per meter of ground cover. More competent and stonger rock may allow higher grout pressures to be applied.
As the overburden thickness of at the concerned face is about 360m, it is recommend having a
maximum grouting pressure of 15 bars for the sound rock, while for the debris material the injection pressure will be 4 bars. Injection pressure will be increased intermittently, with in the safe limit, till the foam doesn’t appear on the adjacent surface. If the grout flows freely from the adjacent crack/region, injection will be stopped and a few minutes will be given to let the grout set.
Rigorous hard work was done to inject the PU chemical according to the methodology under the severe hydrological conditions to densify and divert the water. As a result, we could be able to densify the strata successfully (Fig.7). A total chemical quantity of 15 tons was required to finally stabilise the rock strata. The client was recommended to go ahead with the help of steel ribs erected at the distance of 0.5 meter.