Engineering geological consideration of a deep pump house foundation floor: Case study of lift irrigation scheme

Geological mapping of deep foundation floor is essential to provide permanent data set for geological interpretation. A deep pump house (94 m long x 20 m width x 78 m deep) of Mahatma Gandhi Kalwakurthi Lift Irrigation Scheme-II (MGKLIS-II) wasconstructed at Mahaboobnagar District in TelanganaState for the irrigation of drought prone upland areas. Engineering geological mapping on 1:200 scale of foundation floor of pump house was carried out in order to evaluate the basic design parameters. All the discontinuities in the rock mass of pump house area with the zone of influence of the foundation were identified and mapped. Mapping was done to assess the requirement of any ground improvement by adopting suitable engineering measures. Safe bearing pressure of the foundation was estimated by rock mass rating, rockmass characteristic parameters. Based on investigations geotechnical problems were identified and suitable engineering measures for rock support were suggested to make foundation monolithic. Keywords—foundation floor; pump house; lift irrigation; rock mass rating; safe bearing pressure;Telangana state I.INTRODUCTION Mahatma Gandhi Kalwakurthy Lift Irrigation Scheme (MGKLIS) is proposed to provide irrigation water to an extent of 3.40 lakhs acres and utilising 25 TMC of water including drinking water facilities to 3,20,000 population (0.73 TMC) to chronically drought prone upland areas in Mahaboobnagar district covering about 303 villages in erstwhile taluks of Kollapur, Nagarkurnool, Achampet, Jadcherla and Kalwakurthy constituencies in 19 Mandals. This scheme (MGKLIS) was constructed having the three stages (I, II & III) for lifting the Krishna water from Srisailam reservoir (back water) to Gudipallygattu balancing reservoir through channels and tunnels(Fig. 1). Water will be lifted to a total height of 289 m in three stages from level +244.40 to +502.00m. The present investigations were carried out for the foundation of pump house of MGKLIS-II. This deep pump house (94 m long x 20 m width x 78 m deep) is design to install heavy pump machine (~220 tons each pump machine) for the lifting of water[1]. This scheme was constructed near village Sathapur of Mahaboobnagar District, located 195 km away from Hyderabad city, for lifting the water from Singotam balancing reservoir to Jonnalaboguda balancing reservoir. The FRL of the Singotam and Jonnalaboguda balancing reservoirs are RL +334.680 m and RL +407.000 m respectively. The capacity of Singotam and Jonnalaboguda balancing reservoirs are 0.55 TMC and 2.14 TMC. The major components of the project are: one 4 km long gravity canal from Singotam balancing reservoir having bed width of 19.15 m, one 4.53 km long and 6.85 m finished diameter ‘D’ shaped tunnel, one surge pool (94 m long x 40 m width x 75 m height), five 50 m long draft tube tunnels, one pump house (94 m long x 20 m width x 78 m height) and five (3 m finished diameter), 15 m length horizontal and 305 m length inclined (45°) main delivery tunnels. Lift height is 86 m and five pumps installed in the pump house cavity (30 MW capacity each). Design discharge of the pumps is 113.2 Cumec. This study pertains to large scale engineering geological mapping on 1:200 scale of excavated floor foundation of the pump house. The objective of this study was to advise suitable protective engineering measures of excavated foundation floor of pump house based on detailed engineering geological investigations. Pump House is the key role in Lift Irrigation Projects, which are facilities including high speed electric pumps and other equipment for pumping fluids from one place to another. The pump machine shall be of the vertical shaft, single stage, Francis Turbine Pump, suitable for direct coupling to motor of (5x30 MW) with 10% over load rating. The direction of rotation shall be anti-clockwise when viewed from top. Each pump shall be so designed of ~220 tons weight including all removable parts including impeller, shaft, guide bearing, shaft seal, guide apparatus etc. They are used for a variety of infrastructure systems, such as the supply of water to high level reservoirs, canals, the drainage of low-lying land, and the removal of sewage to processing sites. II.METHODOLOGY Geological mapping on 1:200 scale of the pump house foundation was carried by using the Total Station. Grids were prepared for mapping and the size of the grid was 2 m x 2m, which was decided based on the mapping accuracy and resolution required for such investigations. All the lithological Recent Advances in Rock Engineering (RARE 2016) © 2016. The authors Published by Atlantis Press 253 Fig.1. Plan and elevation of schematic drawing of MGKLIS [1] and structural features were observed and mapped by using Total Station surveying equipment. Detailed examination of rock types in each grid were carried out which includes mineralogical composition, texture, classification and nomenclature and degree/grade of weathering. Fracture filling that have taken place in the study site were examined and recorded. The attitude and structure of the rocks, fractures and joint pattern present in the floor and walls were determined for mapping. ISRM [2] classification for weathered mass was used to characterize the rock mass into different grades. The assessment of Rock Mass Rating [3] for granite rock masses was done based on the rock joints and their nature. III. GEOLOGY OF THE MGKLIS-II PROJECT SITE AND AREA AROUND MGKLIS–II project area forms a part of Eastern Block of Dharwar Craton mainly comprised of Archaean granites which are intruded by mafic dykes age ranging from Archaean to Upper Proterozoic [4]. Granites and gneisses are exposed in and around the project site over a large area. In these formation many shears, fractures and faults have affected the terrain controlled the invasion of quartz, pegmatite, apatite and basic dykes along NW-SE, NNW-SSE, WNW-ESE, N-S, NNE-SSW and ENE-WSW directions [5,6,7]. The reactivation is seen especially in WNW-ESE trending features associated with basic dykes and quartz reefs exhibit intense fractures and shears [5]. The regional study of Mahaboobnagar District area based on geology, photogeology and Landsat imageries [8],identified NE-SW, NW-SE, WNW-ESE, NNESSW and nearly N-S trending lineaments and major faults trending in NW-SE, WNW-ESE, nearly E-W directions in the eastern and southeastern part of the area. No major lineament or fault is passing through the pump house area of MGKLIS-II project site. The pump house area is excavated through medium to coarse grained pink granites, traversed by dolerite dykes. Pink granites are coarse grained, hard and jointed and shows phenocryst of alkali feldspar and quartz. Main minerals composition is alkali feldspars, quartz, mica and amphiboles. Three to five prominent joints set are developed and along the joint plane clay coating/filling was also recorded. Joints are irregular in pattern. Pink granite is generally fresh to slightly weathered (WI–WIII). Dolerite dykes are fine grained and greenish black in colour. The width of the dykes varies from 30 cm to >100 cm and their strike length is more than 50m in excavated part of pump house. Dykes are generally sheared along the contact at the pump house area. Plagioclase and clinopyroxene (augite /titanaugite) are the main minerals occurring in ophitic to sub-ophitic textures in dolerite. Quartz, epidote and opaques occur as accessories. Amphibole, biotite, sericite occur as alteration products. In a regional perspective E-W trending dykes show greater degree of alteration and are relatively older as established from intersecting features. Sulphide disseminations are reported in some dykes. Dykes are generally moderately to highly weathered (WIII – WIV).