APPLICATION OF GROUND ANCHORS TO SUPPORT DEEP EXCAVATION AND COMPACTION GROUTING FOR NATM TUNNEL CONSTRUCTION FOR DELHI METRO RAIL CORPORATION (DMRC)

Transcription

1 APPLICATION OF GROUND ANCHORS TO SUPPORT DEEP EXCAVATION AND COMPACTION GROUTING FOR NATM TUNNEL CONSTRUCTION FOR DELHI METRO RAIL CORPORATION (DMRC) Authors: Jitendra Tyagi, CPM, Delhi Metro Rail Corporation, Mohan Gupta, Divisional Director, Mott MacDonald Pvt Ltd, Ashit Shah, Project Director, Mott MacDonald Pvt Ltd, Y. H. Krishna and B.C. Kanth, Keller Ground Engineering India Pvt. Ltd. ABSTRACT Metro rail construction is planned and is underway in several cities in India, including New Delhi, Mumbai, Chennai and Bengaluru. In New Delhi, the Delhi Metro Rail Corporation (DMRC) has successfully completed and commissioned the 1 st phase of the metro network covering 65 km. As part of its 2 nd phase, construction of about 121 km of metro network is almost complete. This includes an exclusive Airport Metro Express Line, which is under final phase of commissioning. As part of the Airport Metro Express Line, an underground metro station is planned next to the existing New Delhi Railway Station, which required deep excavations in range of 11 m to 19 m. A retaining wall system comprising of soldier pile walls and multi-level ground anchors was adopted to support the deep vertical excavation., A different geotechnical challenge was faced at one of the underground metro corridors near Saket Station. Here the presence of loose sandy silts along an abandoned Nallah channel posed problems with regard to effective soil arching, which is necessary for tunnel construction using the proposed NATM method. Compaction Grouting was used to increase the stiffness in the in-situ soils and to enable effective soil arching above the tunnel crown. This paper presents the construction methodology, QA/QC measures and performance testing results related to Ground Anchors and Compaction Grouting.

2 1. INTRODUCTION Delhi Metro Rail Corporation (DMRC) has successfully completed the 1 st 65 km phase of metro network in New Delhi. As part of its 2 nd phase, construction of about 121 km of metro network is almost complete. This includes an exclusive link, namely the Airport Metro Express Line, which is under final phase of commissioning. As a part of this project, an underground metro station, a multi-level car park and a cut-andcover tunnel have been constructed, which required 11 m to 19 m deep excavation. The designed retaining wall system includes soldier pile walls and multi-level ground anchors to support the vertical deep excavation. Fig. 1 shows part of the DMRC network and the location of the Airport Metro Express Line. Fig. 1: DMRC network showing Airport Metro Express Line and Saket Station On the link connecting Central Secretariat Station with Gurgaon, at one of the underground tunnel section (near Saket Station, Fig. 1) connecting Central Secretariat and Qutub Minar, the New Austrian Tunnelling Method (NATM) was adopted for tunnel construction. The presence of loose sandy silts along an abandoned Nallah channel posed problems with regard to effective soil arching, which is required for safe tunnel construction. To address this problem, compaction grouting was chosen to increase the stiffness of the in-situ soils to allow effective soil arching above the tunnel crown. 2. CASE STUDY 1: GROUND ANCHORS AT NEW DELHI METRO STATION 2.1 Introduction to Ground Anchors A ground anchor is a structural element installed in soil or rock that is used to transmit an applied tensile load (as a result of horizontal earth pressure) into the ground. The basic components of the anchor include (a) the anchorage, (b) the free (or un-bonded) length and (c) the fixed (or bonded or grouted) length. Depending on the application, the anchors may be classified as (a) permanent anchors, (b) temporary non-retrievable anchors or (c) temporary retrievable anchors.

3 Fig. 2 illustrates the schematic of the types of anchors according to method of installation (BS 8081, 1989). Fig. 2: Type of Anchors according to method of installation (BS 8081, 1989). (a) Straight shaft gravitygrouted anchors, (b) Straight shaft pressure grouted anchors, (c) Post grouted anchors (d) Underreamed anchors. In general anchor capacity and performance are influenced by four main factors, namely (a) the number of strands to achieve the desired structural capacity, (b) ground characteristics, especially shear strength, to achieve the desired geotechnical capacity, (c) installation techniques and (d) workmanship attained in the field. 2.2 Soil conditions In general site consists of silty clay (Delhi Silt Alluvium) with depth of bedrock varying from as low as 5m to as deep as 18m. The rock can be described as highly to moderately weathered Quartzite. The following profiles (Fig. 3) describe the general stratigraphy with respect to varying depth of bedrock. (a) Fig. 3: a) Typical soil profile at New Delhi Station b) Typical soil profile at cut & cover tunnel site (b)

4 2.3 Geotechnical problem For the construction of the underground metro station, multi-level car park and cut-and-cover tunnel on Airport Metro Express Line stretch, an 11 m to 19 m deep excavation was required. The site is next to the existing New Delhi Railway Station and is surrounded by other structures like hotels and hospitals. Therefore, deep vertical excavations were necessary. Fig. 4 shows the layout of station building & car park location. Fig. 4: Layout showing station building and multi-level car park To retain the soil of the 11 m to 19 m deep vertical excavation, a retention system comprising of soldier pile walls in combination with multi-level soil and rock anchors was proposed. Two to three levels of ground anchors (60 tons and 80 tons) were installed depending upon the depth of excavation. Where the rock level was high, only a single level of anchors was installed. Fig. 5 shows the schematic of the three levels of soil anchors and rock anchors. (a) (b) Fig. 5: a) Schematic showing three levels of soil and rock anchors at New Delhi Station site b) Schematic showing one level of strut followed by rock anchors at cut & cover tunnel site

5 2.4 Structural and Geotechnical Capacity of Anchors Structural Capacity To achieve the desired structural capacity of the anchors i.e., say 80 tons, the anchors are fabricated using 6 Nos. of each 12.7mm diameter steel strands (7 ply) LRPC confirming to IS: , clause-ii, were used as per the following calculations: Design capacity of the anchor = 80 T Capacity of each strand of 12.7mm dia. = T (As per IS 14268:1995 For 7 ply, 12.7 mm nominal dia., LRPC strands, clause II) No. of strands = 6nos (say) Total Structural capacity of the anchor = 18.74T x 6nos = T Factor of safety against STRUCTURAL capacity of the anchor = Theoretical capacity / design capacity = T / 80T = 1.41 > 1.4 (as per BS 8081: 1989) Geotechnical Capacity The main components of the geotechnical capacity of the anchor are free and fixed length, which are arrived at based on the following calculations: Design Capacity of the anchor Length of anchor in the active wedge zone Free length of anchor Fixed length of the anchor, L Geotechnical capacity of the anchor = 80T = 10.5m (as per to failure wedge analysis) = 12.5m (incl. 2m additional buffer length) = 9.5m (say) = π x D x L x τ f (Sandy silt) D, Dia of drill hole = 0.152m τ f (Sandy silt) is theoretical skin friction (> 400 kn/sq.m) for Sandy Silts having Consistency Index, I c =1.25, according to BS 8081: 1989, clause , Fixed Length in Type C Anchorages Considering, theoretical skin friction τ f (Sandy silt) = 400 kn/sq.m The ultimate geotechnical capacity of anchor = (π x x 9.5) x 400 = 1,815 kn ~ 182T Factor of safety against geotechnical capacity of the anchor = Theoretical capacity / design capacity = 182 T / 80T = 2.3 > 2 (as per BS 8081: 1989) Total length of the anchor = Free length + Fixed length = 12.5m + 9.5m = 22m

6 2.5 Installation method Installation of ground anchors consists primarily of drilling, installation of fabricated anchor, cement grouting and finally by pre-stressing after a curing period of 7 to 10 days. Fig. 6 shows anchor installation works in progress and construction of underground New Delhi metro station in full swing after the successful excavation to the desired depth. (a) (b) Fig. 6: a) Installation of ground anchors using Casagrande C6 Hydraulic drill rig b) Construction of underground New Delhi metro station after successful excavation to the desired depth (New Delhi railway station is also seen in the back ground) 2.6 Testing results After a curing period of 7 to 10 days, each and every anchor was tested / pre-stressed using a 100 ton multi-strand pre-stressing jack. The anchors are stressed to a test load of 1.1 times of the working load. The working loads were 60 tons at station building and 80 tons at cut-and-cover tunnel location. Every anchor was tested to confirm the respective design capacities. Fig. 7 shows the stressing activity in progress. Fig. 7: Pre-stressing using 100T capacity multi strand jack

7 2.7 Quality Assurance and Control State-of-the-art of anchor installation includes appropriate QA-QC procedures throughout the construction process. The following QA-QC parameters were monitored and recorded on site, during the installation of anchor: Drilling - Drilling logs consisting of type of soil encountered with depth were kept - (It must be ensured that the soil / rock encountered is not significantly different from the assumptions made during the design. Any significant deviations would trigger a design review.) Anchor fabrication parameters - Components such as fixed length, free length, length of grout pipes, etc. were checked before the anchor was installed in the drill hole Primary Grouting - Volume of the grout pumped in and the flow rate was recorded Secondary Grouting - Volume of the grout pumped in and the flow rate was recorded - Grout pressures (> 20kg/sq.cm) at which the grout is pumped was recorded Pre-stressing - All the ground anchors were pre-stressed (100% frequency) - All anchors were tested to 1.1 times the design load - Elongation of steel were recorded and checked to be under acceptable limits - Staged loading and deformations were recorded 2.8 Performance of Ground Anchors For the New Delhi Railway station excavation, multi-level carpark and cut-and-cover tunnel, a total of 600 ground anchors were installed. Excavation was completed successfully in before the middle of CASE STUDY 2: COMPACTION GROUTING FOR NATM TUNNEL AT SAKET 3.1 Introduction to Compaction Grouting The compaction grouting technique uses displacement and compaction to improve ground conditions. A very viscous (low-mobility), aggregate grout is pumped in stages, forming grout bulbs, which displace and densify the surrounding soils. Significant improvement can be achieved by correctly sequencing the grouting work from primary to secondary to tertiary grids. The compaction grouting method may be used for the improvement of non-cohesive soils, especially in cases, where soils of loose to medium density are encountered. This method is also used in fine-grained soils in order to install elements of higher strength and bearing capacity, thus improving the load bearing behaviour of the soil. 3.2 Soil conditions The soils at site generally consist of sandy silt fill to 5m depth. The abandoned Nallah channel was excavated and filled with locally available sandy silt to level the ground. SPT N values in the sandy silt fill were in the range of 4 to 17, indicating loose to medium dense. This was followed by medium dense to dense Delhi Silt alluvium layer, with SPT N values between 20 & 30 to about 26 m depth. This is underlain by moderately weathered Quartzite bedrock.

8 3.3 Geotechnical problem Tunnel excavation by NATM was proposed at a depth of about 9 m below existing ground level. The soil above the tunnel crown is fill material (along the Nallah alignment) consisting of sandy silt/silty sand in the top 5 to 6 m. This was followed by Delhi silt alluvium down to the tunnel crown. Fig. 8 illustrates the layout of the NATM tunnels and the alignment of Nallah channel. Fig. 8: Layout of the NATM Tunnels and abandoned Nallah channel NATM is a method where the surrounding rock or soil formations of a tunnel are integrated into an overall ring-like support structure, thus the supporting formations will themselves be part of this supporting structure. But the pre-improvement soil conditions (loose to medium dense sandy silt/silty sand) was not expected to allow effective arching. 3.4 Geotechnical Requirement Hence, in order to permit safe and stable NATM tunnel excavation and primary lining construction, it was necessary to carry out a combination of shallow and deep ground treatment by compaction grouting. An theoretical SPT N value profile between 10 and 18 with respect to depth was proposed by using the correlation, SPT N = Z, where, Z is depth, to form effective soil arching during tunnel construction. Fig. 9 illustrates the proposed NATM tunnels under a filled up soil strata at abandoned Nallah channel location along with the existing and required SPT N value profile. Fig. 9: Schematic of the NATM tunnels under an abandoned Nallah channel; the required and existing SPT N values are plotted on the right

9 3.5 Installation method Generally construction consists of drilling, installation of stinger rods and pumping the low slump grout mix from the bottom of the treatment depth to the working platform in steps. For compaction grouting, a low slump cement with a mix proportion of 1:3, water-cement ratio of 0.5 and admixtures like Bentonite and Glenium are used as a plasticizer to increase the workability of grout mix. The slump value of grout mix is about 120 to 150 mm. A truck mounted hydraulic drill rig was used to drill a nominal diameter hole of 90 mm to a depth of about 8 m through the over burden soils. After drilling, the grout mix was pumped through the stinger rods, to form a bulb like element in the loose soils, in stages (0.5 m each) from bottom to the top of the working platform. Fig. 10 shows the compaction grouting works at site. (a) Fig. 10: a) Picture illustrating the progress of compaction grouting works at site b) Measurement of slump as QA-QC procedures 3.6 Testing results Field trials were carried out to establish a suitable grid pattern to achieve the intended post compaction grouting SPT N values. Trials were carried using 2 m and 4 m square grids. Pre and post compaction grouting SPT N values were recorded and analysed. Fig. 11 illustrates the typical layout of the compaction grouting 2m and 4m square grid: (b) Fig. 11: Layout illustrating the compaction grouting grid 2 m and 4 m

10 Pre and post treatment analysis are also done to find the strength of the improved ground. Post treatment SPT N values in the filled up soil increased and ranged between 20 & 30. Both 2 m and 4 m grids were generally able to achieve the required design SPT N values. Fig. 12 shows the Comparison between Design SPT values with Pre and Post-improvement SPT values. Comparision of Design SPT values with Pre and Post SPT Values Depth (m) SPT N values Design SPT Idealised Post SPT on 4m grid Idealised Pre-SPT Idealised Post SPT on 2m grid Fig. 12: Comparison of Design SPT values with Pre and Post SPT Values 3.7 Quality Assessment and Control As with other ground improvement techniques, proper quality assurance and quality control (QA-QC) procedures were adopted. The working parameters (e.g depth, pressure, grout volume, heave etc) were maintained and recorded at each stage of compaction grouting process to determine the appropriate termination point. Termination of particular grouting stage was considered when one of the following conditions achieved: Pre-determined grout volume is achieved (in accordance with bulb diameter i.e., 0.5 m) Pre-determined grout pressure is achieved (in accordance with depth of treatment i.e., 12 kg/sq.cm to 18 kg/sq.cm) Mortar is overflowing from same grout hole collar Excessive ground heave is measured i.e. greater than or equals to 15 mm Working parameters (grout volume, pressure, depth, etc.) were monitored using automated quality control systems, which are recorded and printed real-time during the installation of the compaction grouting columns. 3.8 Performance of improved ground A total of 296 grout points were drilled with over 420 m 3 of grout pumped. 19 preimprovement and 17 post-improvement SPT boreholes were drilled. The NATM tunnel excavation was successfully completed in the middle of 2009.

11 4. CONCLUSIONS Soldier pile walls in combination with ground anchors as a retention system was successfully carried out to support the 11m to 19m deep excavations for the underground station and tunnelling works Delhi Metro Rail Project sites. This retention system facilitated the space for construction activities of the underground station and cut & cover tunnel. The construction of these underground structures is now complete (Fig. 13). (a) Fig. 13: Pictures illustrating the regular traffic movement over completed underground a) New Delhi Metro Station Building b) Cut & Cover Tunnel at Ram Manohar Lohia Hospital Similarly, Compaction Grouting proved to be effective in densifying the loose silty sandy deposits above the tunnel crown. This facilitated the construction of tunnel by NATM method as the loose silty sandy soils densified after the compaction grouting there by forming self arching which is required for NATM method of construction. This was for the first time Delhi Metro Rail Corporation has constructed a tunnel in loose deposits using NATM method. (b) References BS 8081: 1999, British Standard Code of Practice for Ground Anchorages, British Standards Institute, London. IS 14268: 1995, Specification for uncoated stress relieved low relaxation seven ply strand for prestressed concrete, Bureau of Indian Standards, New Delhi.