RAM GOPAL SAINI reports on the construction of a cross-passage for a twin tunnel system along with the problems encountered and solutions adopted, by using a case history of cross-passage in RK Puram Station, being constructed by L&T-SUCG JV for CC27 of Delhi Metro.
Cross-passages are reinforced concrete structures built in between either two tunnels (as in the case of twin tunnel systems) or a tunnel and the ground surface. They are provided to serve two primary purposes: emergency escape and maintenance work. In this article, the design, construction methodology and problems encountered during the construction of one such structure is described by analysing the case of a cross-passage being constructed by L&T-SUCG JV for twin tunnels between RK Puram and Munirka underground stations for project CC27 of Delhi MRTS Project, Phase-III.
The project consists of five underground metro stations (Hauz Khas, Vasant Vihar, Munirka, IIT and RK Puram), underground ramp at Shankar Vihar metro station (part of CC-27) and tunnel from end of this underground ramp to Hauz Khas metro station connecting the five underground stations listed above. Total tunnel drive length is 4.49 km. Keeping soil strata and time constraints in mind, five TBMs (Okumura TBM, THI-1 TBM, Herrenknecht TBM, THI-2 TBM and Mitsubishi TBM) were used to construct the tunnels. Of these, the Okumura TBM is particularly suitable for tunnelling in soft soil.
Of the total tunnel length under construction, the part of the tunnel connecting RK Puram station to Munirka station is the subject of consideration in this paper. The tunnel extends 1.32 km in length and has an internal diameter of 5.80 m and external diameter of 6.35 m.
As a standard practice, during the construction of a tunnel system, depending upon the length of the tunnel and soil characteristics, a pre-calculated number of cross-passages are planned. In this case, four cross-passages numbered CP-12, CP-13, CP-14 and CP-15 are being constructed between the two stations with an average gap of 400 m between them. The soil type found in this region is called ´Delhi Silt´, which contains silt and sand in varying proportions from 35 per cent to 80 per cent. It is lightly compacted, has only an insignificant proportion of clay and is largely non-plastic although its behaviour is transitional. The absence of rocks and stones lends this soil easily to the efficient use of the Okumura TBM.
Construction of the cross-passages is taken up only after completion of the tunnels on account of ease of working and efficiency. The below steps were followed for the construction of CP-15, the cross-passage under study:
The excavation work is further divided into two parts:
Heading: The upper half of the excavation is known as heading. Here, it was kept at a height of 1.8 m. After excavating for every heading, of 750 mm in length, arch-shaped ribs are fixed followed by fixing of wire mesh between two consecutive ribs, as shown in Figure 1. After this, shotcreting using M30 grade concrete mix was done to prevent collapsing of roof and side wall soil and strengthen these surfaces; 50-mm thickness of shotcrete layer was maintained throughout the procedure. The shotcrete mix contained a chemical named Master ROCS 540 for early setting of the mix (about three to four hours). Any further excavation was done only after checking the strength of the shotcrete mix, sprayed over the soil surface, which should be more than 10 kN per sq mm per 750 mm mesh layer.
Benching: The lower half of the excavation is known as benching. It was kept at a depth of 2.2 m below heading. Benching can be done up to a length short of heading, ie there should always be one extra heading than benching till the last part. Rib-fixing and shotcreting, etc, follows benching as mentioned above.
After this stage, permanent lining is constructed by using M40 grade waterproofing course concrete. Method of construction is similar to any RCC structure. Rebars are fixed first, as shown in Figure 2, followed by shuttering formwork before pouring the concrete mix. Note that the main reinforcement has to be provided as per the design, considering the overburden and lateral earth pressures of the location.
As per the drawing shown in Figure 3, the cross-passage contains a hydrophilic strip at the entry and exit, so as to prevent seepage from entering in the construction joint formed between the tunnels and the doors of the cross-passage. The cross-passage was 10.5 m in length, 1.31 m in width and 2.493 m in height. The total number of ribs used was 14.
During the construction of the cross-passage, a major problem was encountered owing to leakage of a water supply pipe located above the cross-passage, resulting in extensive seepage of water from the top strata of the soil into the cross-passage itself. This delayed work as it affected the shotcreting, which was not setting in the given timeframe, and lumps started forming on the surface. Although the leaking pipeline was fixed post haste, in order to prevent further leakage and possible failure of the shotcreted soil surface, pressure grouting using a grout mix of a composition of 3:3:2 (cement (OPC 53), sand and water) was done using grouting packers, along with pumping out of excess water. This filled up the voids and prevented any further seepage into the cross-passage.
The final settlement value as measured with the help of a total station came out to be 7 mm. As this value is much lower than the danger-level value (13.5 mm), specified by L&T and Delhi Metro Rail Corporation, the construction and design were deemed safe. Reduction in the level of vertical deformation was achieved owing to high rigidity caused by the arching action of the ribs-mesh arrangement. Also, the depth of excavation, which was nearly 11 m, offered well-compacted soil.
The entire construction process was done following the necessary safety measures, which were as follows:
This project is being undertaken on silty-clay type soil, a type of soft soil that limits the construction procedure. The construction method utilised in this case would not have been suitable for undertaking similar work in hard soil as the overburden pressure owing to the above strata would have been extremely large, exceeding the retaining limits of the ribs-mesh arrangement. However, in this project, the soil characteristics lent itself to the ribs-mesh method of construction.
The method used to prevent failures owing to seepage as encountered in this case can only be applied to situations where the water discharge is in limited quantity and can be easily pumped out in a short duration of time. If the quantity of water is large, say, owing to a nearby water body, excessive weight of the water may cause excessive settlements and, in the worst-case scenario, may lead to collapse of the cross-passage, resulting in fatalities.
About the Author:
RG Saini, Project Director, DMRC, L&T-SUCG JV CC-27, New Delhi, has dedicated service of 28 years involving 13 years in underground metro projects and 15 years in other projects of refinery and irrigation.