INTRODUCTION:

1. CONSTRUCTION STAGE GEOTECHNICAL ASSESSMENT ON THE FOUNDATIONS OF EARTH DAM (SINGLE BUND), GURRAMGUMPU BALANCING RESERVOIR, PHASE-II, PACKAGE-03/06, GALERU NAGARI SUJALA SRAVANTHI PROJECT, KADAPA DISTRICT, ANDHRA PRADESH B.P. Rawat & G.J.S. Prasad Geological Survey of India, SR, Hyderabad

2. INTRODUCTION: • GNSS project was conceived by State Govt. of Andhra Pradesh in the year 1988 and decided to take up the work in phased manner viz. Phases-I & II. The project is intended to provide irrigation facilities to an extent of 3.25Lakh acre in the districts of Kadapa, Chittor and Nellore, A.P., besides providing drinking water to the villages and towns en-route to canal alignment. • The scheme envisages drawl of 38 TMC surplus flood water of river Krishna from the fore-shore of Srisailam Reservoir through Srisailam Right bank Canal (SRBC) system from Pothireddipadu head regulator to Gorakallu, Owk and Gandikota reservoirs. From Gandikota reservoir by means of GNSS main canal, the project envisages to store water in 10 balancing reservoirs en-route to provide irrigation and drinking water facility.

3. GNSS, Phase –I has already commenced including construction of Gandikota reservoir across the Penna river, Kadapa District, Head regulator, GNSS main canal and two subsidiary reservoirs with necessary distributary system. • GNSS, Phase –II, is proposed to provide water facility for about 91.056 Ha of land and to provide drinking water facility en-route. • Gurramgumpu Balancing reservoir is a part of GNSS, Phase-II, Package 3, includes GNSS Main Canal from Ch. 96.500km to 119.000km, distributary system, formation of Single Bund Reservoir between Ch. 106.450km and Ch. 108.900km with one sluice and two surplus weirs as per the necessity. • The project contemplates construction of 30m high and 2.45km long earth dam for the formation of Single bund across Gurramgumpu stream to store 1TMC of water near Kadapa, Andhra Pradesh. The scheme is intended for irrigation and drinking water facility to Kadapa District, A.P.

4. LOCATION AND ACCESSBILITY: • The earth bund is located near village Gurramgumputanda of C.K. Dinne Mandal of Kadapa District, about 12km away from Kadapa town and is accessible through motarable road. The area falls in TS Nos. 57J/11 & 15. • The District HQ Kadapa can be approached through rail and road link. It is 450km away from Hyderabad.

6. PREVIOUS WORK: • The preliminary geotechnical investigation of the earth dam alignment of Gurramgumpu balancing reservoir, GNSS, Phase-II, package-03/06 was carried out during F.S.2008-09 by G.J.S. Prasad. It was followed by another note on proposed and modified part of single bund alignment from Ch. 105.350 to 107.100km for Gurramgumpu balancing reservoir GNSS, Phase II, Kadapa District A.P. F.S.2008-09 by J. Srihari and GJS Prasad. PRESENT WORK • At the request of Executive Engineer SKD, GNSS Division, Proddatur District Kadapa, A.P., the geotechnical assessment and 3-D geological mapping of cut-off-trench of earth bund between Ch.106.450 and 108.900km, covering 2.45km was carried out on 1:100 scale.

7. PROJECT FEATURES:

8. Surplus weir is also proposed to be constructed between Ch.108.440 and 108.550km (110m).

9. REGIONAL GEOLOGY: • The project area is located in a gently undulating terrain with scattered hills, raising to a maximum height in SE of GurramgumpuTanda (RL+202m). • Small ephemeral streams arising from hilly region in a sub-dendritic pattern join the Chinnagandivagu, a tributary of the Penner River. A stream trending near E-W crosses the proposed dam alignment between the Ch. 108.200km and Ch. 108.400km. • Litho-units of Cuddapah Super Group of Middle-Proterozoic Age occupy a major part of the area comprising Nallamalai Groups. Nallamalai Group is represented by Bairenkonda quartzite along the proposed dam alignment, overlying Papagni Group of rocks.

10. It consists predominantly of quartzite with subordinate purple arenaceous shale and is fine to medium grained and cherty at places. The quartzite is fine to medium grained, flesh red to reddish brown to white bedded/laminated in nature. • The thickness of beds of quartzite and shales recorded at some exposures ranges from 0.3m to 2.5m. The regional strike of the formations is NNW-SSE with easterly dips ranging from 10° to 30° east, except in some places swinging to N20°-25°W, dipping towards 6°-10° east.

11. GEOLOGY ALONG EXCAVATED CUT OFF TRENCH: • The cut-off-trench of earth dam in general, exposes predominantly thickly to thinly bedded, quartzite with subordinate purple arenaceous shale (thinly bedded/ partings) of Bairenkonda Formation of Nallamalai Group of Cuddapah Super Group. • The quartzite is fine to medium grained, flesh red to reddish brown to greyish white and bedded/laminated in nature. The thickness of beds of quartzite and shales ranges from 0.3m to 2.5m. • The average cut-off profile show the thickness of different litho-units in the order of superposition as i) 0.5-6.0m thick reddish brown silty clay and sandy silt with pebbly zone near gorge portion. ii) 0.5 - 4.2m Pebbles with clay (RBM) in gorge portion. iii) 0.5 -7.5m weathered quartzite and shale. iv) 1.5-4.5m fresh, hard quartzite with inter-bands of arenaceous shale.

12. Bedding (S0) in quartzite trends N200to 300W-S200to 300 E/ dips 80-150 due northeast. • Rockmass is traversed by three sets of prominent joints viz. i) N500to700E-S500to700 W ii) ii) N300 to 550W-S300to550 E iii) N-S toN200E-S200 W with vertical to sub vertical dips. However, bedding joints trending N200to300W-S200to300 E/ with 80-150 dip towards northeast. • Few vertical to sub vertical joints trending E-W are also recorded. The joints are moderately close to widely spaced, discontinuous to continuous, slightly open to tight, stained to unstained planar surfaces, however in general, the bedding joints are slightly open. • The quartzite shows ripple marks along the bedding planes at places.

13. GEOTECHNICAL ASSESSMENT OF CUT OFF TRENCH: • A 2.45km long and about 30m high earth dam is under construction between Ch. 106.450 and 108.900km with five IP points at chainages 106.725km, 107.375km, 107.640km, 107.875km and 108.125 respectively. • The fore- bearing of the alignment of this earth bund is N20ºW up to IP1; N 5°W up to IP3; N55°E up to IP4; N20°E-S20°W upto IP5. Previously a part of the dam alignment was considered on left flank, which was later modified to the present alignment. • Present work is confined to the newly proposed alignment between Ch. 106.450km and Ch.108.900km. The average ground levels along the alignment vary from +174m to +202m with a maximum water column of 27m. • 3-D geological mapping and geotechnical assessment of cut-off-trench of earth dam between Ch. 106.450km and Ch.108.900km, covering a total length of 2.45km was carried on 1:100 scale.

14. TABLE: GEOTECHNICAL ASSESSMENT OF THE FOUNDATION OF COT

15. 3-D geological map of part of COT, Gurramgumpu Balancing Reservoir, GNSS

16. CONCLUSION AND RECOMMENDATIONS: • The assessment of the foundations of cut-off grade is satisfactory and acceptable for filling up with suitable impervious soil and compacted as per design standards between Ch.106.450km and Ch.108.900km; only after implementing the suggestions given below. • It is suggested to: • remove muck, dislodged rock fragments present at the base in the cut-off trench in general and between Ch.108.195km and 108.200km; and between Ch. 108.845km and Ch. 108.850km in specific. • clean the base of the cut off trench thoroughly before filling with the suitable impervious soil.

17. trim and ease the side slopes of the cut off trench in general and more specific in the pebbly zone (RBM) as per the design drawings. • maintain the width of COT bed all through as per design, specifically between Ch. 108.840 and Ch.108.850km • eliminate the pebbly zone (overburden) present on both u/s and d/s sides of the cut off trench in the earthen embankment for proper bondage between casing soils and the ground as per standards. • seepage points/ drainage lines, recorded on u/s wall and d/s wall at certain chainages. Seepage zones/ drainage lines in the cut- off-trench are to be diverted or arrested so as to make the entire affected reach water tight. • excavate the reach between Ch.108.420km to Ch.108.570 and Ch 108.850 and 108.900km lying above FRL to a depth of 1m for key trench.

18. excavate the cut off trench further 0.5m to 1.0 m between certain chainages as per given elevation in accordance with design drawings. • eliminate the pebbly zone (overburden) present on both upstream and down-streams sides of the cut off trench in the earthen embankment for proper bondage between casing soils and the ground as per standards. • Based on the permeability data furnished by the project authorities, it has become necessary to erect a grout curtain below the bed of the cut off trench. • Grout holes going down to a depth of 6-9m at certain reaches spaced at 3m interval centre to centre along the central line of the earth dam. • Post grouting permeability data provided by the project authorities indicated ‘K’ values less to be 3 Lugeons, fulfilling the water tight condition of the foundation strata.

19. Fresh hard quartzite with shale interbands, exposed at COT bottem

20. Slightly weathered to fresh hard quartzite with shale interbands, exposed at right wall of COT

21. Deepening of Cut-off-trench in gorge portion.

22. Disposition of joints in hard, fresh quartzite foundation at COT bottom

23. Thank you