Management & Mitigation of Groundwater infiltration within underground excavation in Rock Angelo Indelicato angelo.indelicato@dragageshk.com Dragages Hong Kong Limited

Underground excavation often involves dealing with groundwater infiltration. The management and mitigation of water inflow is important during the tunneling work as it affects both the construction area and the surroundings. This session will focus on the followings key elements: Where groundwater comes from; How to measure the water infiltration; Mitigation measures; Past Cases; Conclusions.

Why groundwater infiltration is important? Groundwater related problems can be direct or indirect and subdivided into 5 categories (Loew S. et al. 2010): High water inflows causing excavation and safety problems; High water pressures and seepage forces causing instability problems; High water inflows causing impacts on surface springs and wetlands; High water inflows causing settlements of sensitive surface layers (soils) Deep tunnel drainage and pore pressure drawdown causing large scale rock mass deformation. Designer specification regarding water inflow (this do not affect the stability of the tunnel, however, if the water inflow exceed the Design limit, it has to be addressed).

Where Groundwater comes from? Groundwater find its own way inside the underground excavation through rock joints or crack and crevices present in the shotcrete, this is common on the shaft walls. At the bottom of the shaft the water can pop up during the excavation (example: during probe drilling operations or during the drilling of the blast holes). 50lt./min. water inflow during probing

Examples of How to measure the groundwater infiltration during rock excavation From the underground area (quantitative methods): Kibble method; Pumping method; Pipe channel; Probing; V-notch wier. From the surface area (qualitative methods): Piezometers; Settlement Markers. Specificherei meglio le differenze tra I diversi metodi

Kibble method This method consists on using a sealed bucket (kibble) to collect the water from the shaft bottom. During the weekend, when no operation was carried out within the shaft, the water pump was switched off to allow the groundwater to accumulate at the end of the current excavation. The water was then pumped into the kibble and carried out on the surface where the total volume was calculated. Sometimes more than one kibble was necessary to dry the shaft from the water from. Pro: Very effective in estimating the water inflow on the shaft Con: No operation can be carried out during the measurement

Example of volume calculation using the Kibble method As the water inflow is higher than the Design limits, the area needs remedial works

Pumping Method This method is carried out on the sinking stage inside the shaft, here a container of known volume is used to collect the water. Basically the water pump is moved inside the container and it is calculated the time the pump took to fill the container with 100 litres of water. Pro: the measurement can be carried out without switching off and on the water pump and slowing down the production Con: As the stage is not fixed but change in height the pump works not at a constant rate. If the stage is very high the water pump need to take into account the gravity force to pump the water out, this can affect the validity of the test and also the calculation of the volume of water infiltration.

Example of pumping method test

Pipe Channel Method This method uses a catchment pipe ring around the shaft walls which is embedded into the shotcrete layer. Any water leaking from the shaft wall surface can be collected within this structure and subsequently quantified. Pro: Same as pumping method Con: very short life of the catchment ring, most of water is not collected by this system, so water inflow calculation is not precise

Probing The main purpose of probe drilling is to identify potential geological or hydrogeological hazards. After the holes are completed an hydraulic inflatable packer is installed at 2-3 metres from the collar and the water inflow rate is calculated. If the water inflow is below the Design criteria the holes were filled with grout, in case the water inflow exceeded the Design criteria further holes were drilled. This operation was carried out untill the last holes had a water inflow acceptable (control holes). Material used to grout holes: Colloidal Silica and Microfine Cement

V-notch Wier V-notch wier uses a metal slab with a V shape opening. Water is collected in a pool and after the slab is open the water level is measured so that the water inflow is calculated. This type of measurement is used mainly to check if the post grouting work has been effective.

Measurements on Surface Piezometers are used to monitor changes in piezometric head which indicate changes in the groundwater condition. Their great advantage is that they are small in scale and provides useful information. Settlement markers are a series of points located in the buildings and street adjacent to the underground excavation. Their level have been measured before the excavation started and periodically during the excavation. They provide valuable information regarding any possible subsidence caused by the underground work. Other instruments can be also used to control both groundwater and the condition of the ground (i.e. Tiltometers)

Mitigation Measures Pre-excavation Grouting; Post Grouting; Concrete Lining

Pre-excavating Grouting If probing shows water inflow higher than the limit established by the Client, extra holes have to be drilled around the shaft. They are subsequently filled with grout and they will form a “fan’ around the excavation area which will reduce the water inflow to acceptable level. Once completed control holes will be drilled to make sure water infiltration has been reduced to an acceptable level. Sometimes the initial probing shows low level of water infiltration. So the initial probe holes are considered enough. However as they do not cover the entire excavation area, as such some zones are not treated and later on they might cause trouble so that post grout work is needed. Typical length of probe holes 25m with an angle of 8°

Pre-excavation Grouting in Tunnel

Post Grouting This technique is more a remedial work if the previous did not achieve a substantial results in mitigate groundwater infiltrations. This system involved the injection of grout into semi-horizontal holes drilled in specific areas where the water infiltration was more severe in order to reduce it.

Concrete lining Concrete lining is a permanent structure that can be cast-in-place or made by precast unit. The type is chosen according to the ground conditions of construction areas, tunneling method, the use of the underground areas. A layer of waterproof material is placed between the rock excavated and the concrete lining. Sometimes around the concrete other materials such gravel and mortar can be placed to facilitate the movement of the groundwater to the level below.

Hazard related to mismanagement of groundwater Case 1 Loetschberg Tunnel – Switzerland Tunnel completed in 1912, is 13.7 Km long. The tunnel was flooded after the excavation reached the contact between limestone and alluvium of the Kander torrent 201m above. The excavation was filled in second with water, boulders, silt mud and sand causing the death of 25 miners. The main cause of the accident was a poor knowledge of hydrogeologic conditions of the area.

Seoul Metro Line 5 – Phase 2, South Korea Jan. 1993 Case 2 Seoul Metro Line 5 – Phase 2, South Korea Jan. 1993 Construction of Metro Tunnel with Drill & Blast Method Tunnel Collapse after removing spoil and water inflow up to 300 lt./min recorded. Among the possible causes: High groundwater pressure. Remedial work included backfilling and grouting works Sinkhole in Yongdungpo Seoul from Shin J. et al 2006

Ping Lin Pilot Tunnel – Taiwan 1996 Case 3 Ping Lin Pilot Tunnel – Taiwan 1996 The Ping Lin is a 12.9Km long tunnel excavated by TBM. In 1996 during the excavation of the pilot tunnel, groundwater infiltration reached 750 Lt./s which caused a severe collapse with a huge volume of debris flow. The TBM was trapped with a consequent delay of the work. Freeing the TBM and Water sealing/consolidating grouting lasted several months, affecting considerably the budget (Tseng D. et al. 2001, Kipko E. et al. 1998) Plan view of the 10th stoppage of pilot tunnel TBM, Tseng D. et al. 2001

Conclusion The combination of the methods above manage and mitigate the groundwater infiltration within shafts, tunnel and in general any underground excavation. Keeping in mind the Burland’s triangle and its principle we can say that the ground investigation can help to identify any potential hazard, generating ground models and helping with the design. Field test and observation can help to verify the hydrogeological model and the relationship with the actual ground profile. Important: A contingency plan is seen also as wise way out, in case of high infiltration of water exceeding the Design or for more serious issues Idealization, conceptual, physical or analytical model Ground profile Appropriate model Observed behaviour Precedent Experience Ground exploration and description Observation, measurement, Lab & field testing

References: Indelicato A. 2012 – Management & Mitigation of Groundwater with Deep excavation Shaft – The HATS2A project experience in 32nd Annual Seminar, Geotechnical Division, Hong Kong Institution of Engineers, pp.111-117; Kipko E.,Spichak Y., Pozolov Y. 1998 – Water Sealing of Fault Zones at Ping Lin Pilot Tunnel in Taiwan in Proceedings of International Mine Water Association Symposium, Johannesburg 1998, pp.141-148; Loew, S. Barla,G. and Diederichs M. 2010 - Keynote Paper: Engineering Geology of Alpine Tunnels: Past, Present and Future. In Proceedings of the International Association of Engineering Geologists. Aukland, New Zealand. Sept, ’10. 34pgs; Shin J., Lee I., Lee Y., Shin H. 2006 – Lessons from Serial Tunnel Collapse during Construction of the Seoul Subway Line 5 in Proceedings of the World Tunnel Congress and 32nd ITA Assembly, Seoul, Korea 22-27 April 2006 Stratskraba V. 1984 – Ground-water as a nuisance in International Journal of Mine Water, Vol.3 (1), pp. 25-40; Tseng D., Tsai B., Chang L. 2001 – A case study on ground treatment for a rock tunnel with high groundwater ingression in Taiwan in Tunnelling and Underground Space Technology Vol.16 Issue 3, pp. 175-183;