240 Construction of Cross Passages Completely Submerged under Ground Water Ayush Raj Assistant Engineer, Delhi Metro Rail Corporation Limited, New Delhi, India E-mail : ayush_raj25@yahoo.com Ashwani Kumar Chief Project Manager-3, Delhi Metro Rail Corporation Limited, New Delhi, India E-mail : cpm3.track@gmail.com Rajesh Kumar Mittal Project Manager, Delhi Metro Rail Corporation Limited, New Delhi, India E-mail : rkmittaldmrc@gmail.com ABSTRACT: This paper is a case study of Construction of Cross Passages between South Extension and Lajpat Nagar Metro Stations of line-7 of Phase-III of Delhi Metro. The geological strata at the cross passages i.e. CP-18 and CP-19 mainly comprised of silty sand and were completely submerged under ground water. CP-19 was located below a vacant field and CP-18 was located just below a road junction surrounded by residential houses. Initially Forepoling and TAM grouting was carried at both the cross passages. At the time of excavation, flowing ground condition encountered at CP- 19 and some settlement was also observed at surface due to ground loss. For stabilising the ground above CP-19 and to fill any cavity formed because of ground loss, compensation grouting was carried from the top surface. Also during the excavation TAM grouting was carried on the excavated face of cross passages to ensure stability of ground inside the cross passage. At CP-18, due to space constraint and the presence of various utilities, compensation grouting was not possible from top. Therefore, to encounter the problem faced at CP-19 and to mitigate any risk to the buildings above CP-18, in addition to Forepoling and TAM Grouting, chemical grouting was carried on the crown of cross passage to avoid any ground loss. For restraining ingress of water and to provide compressive strength to ground, Polyurethane injection grouting with very low viscosity was carried. By adopting this methodology, CP-18 was successfully constructed with minimal ground loss and with maximum settlement of -11mm observed at ground surface. Keywords: Cross Passage Construction; Submerged Under Water; Flowing Ground Condition; Compensation Grouting; TAM Grouting; Polyurethane Injection Grouting 1. Introduction The total length covered by metro after the completion of phase-iii of project assigned to Delhi Metro Rail Corporation is 329 km. A total of 140 km of the total length is added to the existing network of 189 km in phase-iii. In this 140 km, underground twin tunnels of length 53 km (106 km tubes) are constructed in phase-iii of the deposit work. The current work discusses about the challenges faced and process used in constructing these underground tunnels. The network from Mukundpur-shiv vihar also known as line-7, constituted of 58.596 km. Line-7 has 39.479 km of elevated network and 19.117 is km underground network. There are total 38 stations out of which 26 are elevated and 12 are underground. In line-7, four numbers of cross passage have been constructed between South Extension and Lajpat Nagar stations by NATM methodology. Two cross passages namely CP-18 and CP-19 (Fig. 1) amongst the four constructed passages are under ground water posing the maximum challenge. Fig. 1 Schematic Diagram of Cross Passages between South Extension and Lajpat Nagar The area around the construction site is densely populated and logistically it is difficult to carry out the construction work in these areas. Therefore, the whole construction process requires optimization. Furthermore, any major settlement would have lead into damaging the structures and subsequently, endangering the safety of residents in in the vicinity of construction site. Fig. 2 shows the actual location of CP-18 and CP-19, superimposed on Google map. Cross Passage - 18 Cross Passage - 19 Fig. 2 CP-18 and CP-19 located in densely populated residential area The geological strata mainly comprised of silty sand with flowing ground condition at CP-18 and CP-19. The permeability of soil encountered at these locations is in range of 1 to 9 leugon. The levels of the top of crown portion of the two submersed cross passages, CP-18 and CP-19, from ground level are 21.2 m and 18.6 m respectively and levels of ground water table (GWT) at CP-18 and CP-19 are 18.9 m & 16.7 m below the ground level. The GWT, therefore, of 2.3 m & 2.2 m above the crown of CP-18 and CP-19 respectively are shown in fig. 3.
excavation line of the cross passages (Boehm (2014)). The main purpose of this umbrella of pipes and grout on top of crown portion is to avoid sudden ground loss from the top which can result in ground settlement. Fig. 3 CP-18 and CP-19 completely submerged under ground water 2. Initial construction methodology for CP-18 and CP-19 The formulization of the construction methodology of cross passage CP-18 and CP-19 between South Extension and Lajpat Nagar metro stations is carried in such a way that any kind of damages to the nearby buildings is avoided which can ensure the safety of the people residing near the locations of CP-18 and CP-19. It is achieved by minimizing the settlement of ground above cross passages locations. Ensuring the safety of workers is also of utmost importance in case of any collapse inside cross passages during construction phase. Initially, it is decided that forepoling (Boehm (2014) )and Tube a Manchette (TAM) grouting at CP-18 and CP-19 (as shown in fig. 4) are performed before commencing the excavation of these cross passages. Chu et. al. (2009) discuses the construction process in tunneling and Hausmann (1984) mentions the main principles used in ground modification. DMRC (2017) has compiled the method sequence of cross passage construction under water level which is widely referenced in the current work, apart from that general processes are dealt in brief by Babu in the NPTEL course. In TAM grouting, grout is prepared at site with water, cement and bentonite (Toth (2015)). The grout is inserted with the help of my pump. Fig. 4 Forepoling and TAM grouting scheme for CP-18 and CP-19 2.1 Forepoling Ground losses are avoided using insertions of single layered 76 mm Ø forepoles of 9 m in lengths, at an angle of 10-15 with 300 mm C/C spacing, during excavation of cross passages, as shown in fig. 5. These pipes are inserted in both the tunnels and afterwards, cement is grouted in the inserted pipes. As a result, an umbrella is formed above the crown Fig. 5 Forepoling at CP-18 and CP-19 2.2 TAM grouting It is necessary to ensure the safety of workers inside the cross passage by restricting collapses during the excavation. This is achieved by stabilizing the ground through TAM grouting at the excavation face. Grouting is carried by inserting TAM pipes of 50mm Ø from both the tunnels before commencing the excavation. C/C spacing between TAM pipes is kept less than 500mm. To ensure the overlapping between the grouting, length of TAM pipes is kept at 6m. Fig. 6 shows the TAM grouting scheme carried during the construction of CP-18 and CP- 19. Fig. 6 TAM grouting at CP-18 and CP-19 TAM pipes have 50 to 75 mm rubber sleeves fixed on them at every 400 mm. These sleeves act as non return member to the grout holes beneath them. Three grout holes are provided at 120 underneath the sleeves. After insertion of TAM pipes, consolidation grouting is carried. Grout mix of cement, bentonite and water with varying water-cement ratio is pumped in different stages (as shown in Table-1) using piston type grout pump (Hany pump). The water-cement ratio is in accordance with the strata and site requirement. Deformation in tunnel lining is avoided by gradually increasing the grout pressure from 0.0 to 3.0 bars. Table-1 Grout mix in different stages Grouting Stage Cement (kg) Water (litre) Bentonite (kg) Stage I 50 200 4 Stage II 50 150 4 Stage III 50 100 4 241
Stage IV 50 50 4 Fig. 7 shows the actual site photograph of forepoling and TAM grouting carried at various locations of cross passage. Pneumatic drilling machine is used in the drilling work for insertion of foreploes and TAM pipes. Fig. 8 Plan showing TAM grouting carried from ground above CP-19 Table-2 Number of bags grouted in different pipes GRID 1 2 4 5 A 30 29 30 13 B 50 49 71 C 64 18 19 79 D 54 11 16 E 24 39 20 21 Fig. 7 Forepoling and TAM grouting at site 3. Problem faced during construction of CP-19 The construction of these cross passages is very challenging. The excavation process of CP-19 stared first because it is located just below an open ground as compared to CP-18 which is surrounded by residential houses. Two lattices at 500mm c/c are successfully installed after the forepoling and ground stabilization by TAM grouting (as mentioned above). A Sudden slush ingress in huge quantity is encountered from the crown of cross passage and a cavity of around 200 mm is observed on the ground just above CP-19 location. Since, CP-19 is located beneath an open ground, no significant damage to any of the property is reported by the sudden ingress of slush but the excavation activity is stopped and sequence of excavation is again deliberated. 3.2 Change in the construction methodology of CP- 19 The detailed deliberation between DDC, site team and DMRC design department lead to a revised construction scheme proposal for both the cross passages after which the excavation recommenced. To reduce the flow of slush and to ensure safety of workers, excavation is carried in parts and lattice is erected and shotcreted in pieces. Central hump is always maintained in front of the excavation face to avoid face collapse and additional TAM grouting is carried from front face after every 3 m and also from the locations where slush ingress is encountered. During the entire excavation, a total of 7761 cement bags (50kg/bag) are grouted to support and stabilize the ground. Fig. 9 shows the sequence of excavation followed at CP-19 by applying the precautionary measure mentioned above. 3.1 Remedial measure for ground loss To avoid further ground settlement and to fill any cavity formed because of slush ingress, cement grouting is carried in the affected area by inserting 12 m long TAM pipes in a grid pattern from the top of CP-19 as shown in fig. 8. Approximately, 875 cement bags (50 kg/bag) are grouted above CP-19. Table-2 indicates the number of bags grouted in each pipe. Fig. 9 Revised construction sequence at CP-19 Also, prior to commencement of excavation for benching, 32 mm Ø forepoles of 3 m are inserted at an angle of 45 to support ground during excavation of benching. As shown in fig. 10, 32 mm Ø forepoles are inserted at the junction of overt shotcrete and temporary benching. 242
4. Modification in Construction methodology of CP- 18 The location of CP-19 is beneath an open ground and its entire construction is carried by supporting ground with forepoling, stabilizing the soil by TAM grouting and complying to additional precautions as mentioned above. CP-18 is located beneath a junction of public road and is surrounded by residential houses in every direction. Settlement like CP-19 can lead to major damage to the adjacent properties and can also prove fatal to the lives of residents. Fig. 10 Forepoling during Benching Excavation Further, minimal exposure of soil is ensured during the excavation apart from adopting above methodology for construction of CP-19. The entire process starting from excavation to lattice erection to final shotcrete is completed without any stoppage once the remedies are included in the construction plan. Fig. 11 is the site photograph taken after successful excavation of CP-19. Fig. 11 Successful excavation of CP-19 3.3 Contingency measures Before the start of any excavation, 1-2 m 3 of concrete is always kept at cross passage location to immediately seal and stop any slush ingress. Sufficient numbers of gunny bags filled with dry soil are also kept near cross passages which can be used in case of a sudden collapse from the face of crown. Fig. 12 CP-18 at Road Junction and Surrounded by Houses- Topographical map In view of the safety of people and infrastructure in the region adjacent to CP-18, apart for forepoling, TAM grouting and additional precaution adopted for construction of CP-19, chemical grouting is also performed to enhance the stability of the ground in the vicinity of CP-18. During the entire excavation, a total of 3112 cement bags (50kg/bag) are grouted to support and stabilise the ground using 10930 kg polyurethane grout. 4.1 Chemical grouting in CP-18 Ground near CP-18 mainly constituted of silty sand having permeability of 1 to 9 lugeon. The ground conditions are similar to CP-19 and have a flowing tendency. Considering the location constraint, very low permeability of soil and safety during excavation, polyurethane injection grouting with very low viscosity is carried. The resin of the grout used reacts with water to form rigid hydrophobic foam which restrains/diverts water from entering CP-18. Polyurethane resin is used with catalyst. Technical data of the material used and physical properties of end product are tabulated in Table- 3 and Table-4. Table-3 Technical data of product used S No Properties Value 1 2 3 Appearance (colour) Viscosity at 25 C (mpas) Density at 25 C (kg/l) Polyurethane resins Brown Liquid Catalyst Clear Liquid 25-50 25-35 1.1 0.97 243
Table-4 Physical properties of end product S No Properties Value 1 Compressive Strength @ 3 days (Mpa) 3.0 2 Flexural Strength (kpa) 1691 As tabulated in Table-4, apart from restraining water the resin used also provides 3-4 MPa compressive strength to the soil. The strength provided to the soil helps in ensuring the safety during excavation and also eases the excavation activity by reducing the flowing tendency of soil. For injecting polyurethane grout, self drilling anchors (SDAs) 32 mm Ø are inserted to a length of 7.5m from both the tunnels at an angle of 10-15 i.e. parallel to inclination of forepoling. The SDAs are provided with perforations of 2-3 mm Ø at an interval of 750 mm along its length, with a non-return valve at it end through which polyurethane grouting is carried. As shown in fig. 13, Spacing of SDAs is kept 300 mm c/c in 2 rows with zigzag pattern. 1 st row is inserted 300 mm above the already installed network of forepoling and 2 nd row is inserted 200 mm above the 1 st row. To ensure the safety of houses coming in the influence zone of cross passages CP-18 and CP-19, on the wall of the houses Bi-reflex targets, tilt meter, crack meter and building settlement markers are installed as shown in fig. 14. To monitor the settlement of the ground, ground settlement markers are installed. To monitor the movement of excavated surface of cross passages and tunnel lining segments, bi-reflex targets are installed on the lining segments and shotcrete surface of cross passages as shown in fig. 15. The readings are taken after every 12 hours by dedicated team of surveyors and depending on the situation encountered during construction, the frequency of monitoring is sometimes reduced to even 3 hours. Fig. 14 Instruments (BTs, CM, TM) on houses Fig. 13 Revised construction sequence at CP-19 4.2 Contingency Measures during construction In addition to the contingency measures adopted at CP- 19, 5 numbers of weep holes of 76 mm Ø are provided alongside the section of the cross passage from both the tunnels to divert ground water outside the curtain formed by polyurethane grouting. Also extra TAM grouting is carried outside the boundary of cross passage from TAM pipes inserted as shown in fig. 13. 5. Instrumentation Monitoring at CP-18 and CP-19 During the entire construction activity, intensive instrumentation monitoring is carried at ground surface near the vicinity of CP-18 and CP-19 and also inside the tunnel and cross passage. Fig. 15 BTs inside tunnels and CPs 6. Conclusion CP-18 and CP-19 are successfully constructed by adopting thr above methodology, and will be operational soon. The settlement observed above CP-19 is restored to its original condition. By using polyurethane grouting, the maximum settlement observed in ground settlement 244
marker above CP-18 is only -11 mm and no damage is noticed to the adjacent houses. References Babu G.L.S. Ground Improvement - Ground Treatment Using Grouting, NPTEL Course, Indian Institute of Science, Bangalore, India. Boehm K (2014) Advance Forepoling System, NATM Seminar, Sau Paulo, Brazil Hausmann M R (1984) Engineering principles of Ground Modification. Jian C., Varaksin S., Klotz U., Mengé P. (2009) Construction Processes, State of the Art Report, ICSMGE, Alexandria, Egypt. Toth A. (2015) Tunneling and Underground Construction Technology- Ground Improvement. Delhi Metro Rail Corporation Limited DMRC (2017) Method Statement for Cross Passage Construction under water level, Technical Report. 245