starts once the first-stage grout is completed and has gained strength under secondary cooling (normally after two days). This is performed with micro cement, which penetrates the leftover small spaces of the first-round contact grout with OPC. In this, regroutable perforated injection tubes are embedded inside the plug. They are placed in three peripheral rows in contact with the rock at 1 m interspacing distance. These pipes are laid not more than 5 m in length. They are overlapped to cover the entire perimeter in each row. These pipes are made of PVC, which can withstand the high pressure created by the hardened concrete. They have an outer protective layer that prevents cement slurry entering the grout pipes during casting of the plug. Before and during second-stage contact grouting with microcement, water is injected through perforated grout-injection tubes to check the injection tube function and quantify the volume of leak and void space in the contact. This is called as tightness test and is carried out through the central tube at 3 bar pressure, as shown in Figure 9 using water. Grouting and testing is repeated as required till tightness is attained.
Fig. 9 : Sectional view of contact grouting pipe arrangement around a tunnel plug
7.
In view of uncertainty involved in the design of such large structures, a systematic and detailed approach for investigations along with the active design approach linked to construction progress is followed. Concrete plugs which are required to separate crude oil and water are designed as gas tight to prevent flow of any oil vapor outside the cavern. Because mass concreting is involved in the construction of these plugs, suitable arrangements for the cooling of concrete are carried out to prevent development of high temperatures inside the plugs along with grouting to ensure tightness.
8. References 1. ACI (American Concrete Institute). (1993). “Cooling and insulating systems for mass concrete.” ACI 207.4R-93, Farmington Hills, MI. 2. ACI (American Concrete Institute). (2007). “Report on thermal and volume change effects on cracking of mass concrete.” ACI 207.2R-07, Farmington Hills, MI. 3. Barton. N., Lien. R, Lunde, J. (1974). “Engineering classification of rock masses for the design of tunnel support, Rock Mechanics” Vol.6, N 4, pp 189-236. 4.
Indian Code IS 456. (2000). “Code of practice for plain and reinforced concrete.”
5.
Hoek, E., and Brown, E. T. (1980). “Empirical strength criterion for rock masses.” J. Geotech. Engrg. Div., 106(GT9), 1013–1035.
6.
IS 3370: Part 2. (2009). “Code of practice for design of concrete structures for storage of liquids.”
7.
Nanda A. (2012). “Design and construction of storage caverns” Keynote Lecture, Proceedings of Indian Geotechnical conference, New Delhi, India.
Discussion and Conclusions
This paper presents a study on different aspects involved in engineering and design of large underground storage caverns for storage of crude oil with specific emphasis on design of concrete plugs.
The Bridge and Structural Engineer
Volume 45 Number 4 December 2015 109