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POST IMPOUNDING PROBLEMS AND MANAGEMENT MEASURES IN CARBONATE GEOLOGY AT KHANPUR DAM PROJECT, PAKISTAN.

Byline: Mohammad Saleem Khan, Tahir Ali Gilani and Mian Ali Gul

ABSTRACT: A number of problems were encountered during the first impounding of Khanpur Dam which took place accidentally due to extra ordinary precipitation just after the closure of diversion tunnel during 1983. The major problems were extremely high seepage, boiling and springing of water at a number of places downstream dam, uplift pressures at the toe of main dam near left abutment, turbid flow from relief wells and development of sinkholes in the reservoir area.

Immediately various remedial measures were adopted for the treatment of the problems. In the phase-1, installation of double line relief wells at the toe of main dam and left saddle were provided. Drain holes in diversion tunnel and downstream of right saddle and strengthening of grout curtain of left abutment of main dam were also carried out. Sinkholes were treated and barge dumping was made over seepage ingress areas.

After the remedial works of 1st phase, the filling of reservoir was restricted to the level of 1965 feet instead of 1982 feet of design elevation. Due to increased demand of water from Capital Development Authority Islamabad, phase-2 remedial works were initiated during 1999. These works were started to cut down the extra ordinary seepage to the tune of 16% of the base flow through fractured carbonate geology and to ensure the safety of the dam structures.

The major dam safety works carried out during 1999-2002, comprising of strengthening and deepening of grout curtain on right and left abutment of main dam and dental treatment of open joints in the reservoir area. After the completion of approximately 50% of designed dam safety works, the dam is being operated at full storage capacity with maximum reservoir level of 1982 feet and regulating 2,50,000 acre feet of water annually. However still approximately 50 cusecs of seepage water at reservoir level of 1982 feet is being destroyed which is great economic loss. It is also increasing the fractures by dissolving the limestone formation; suggestions are made to overcome the problem.

Keywords: Carbonate geology, seepage, uplift pressures, grouting.

1. INTRODUCTION

Khanpur Dam Project is located in Haripur District of Khyber Pakhtun Khwa about 50 kilometers Northwest of Islamabad at River Haro which is a tributary of Indus River. Haro River drains runoff from western slopes of Murree Hills and southern slopes of Nathiagulli Mountain and also receive partial runoff from Margalla Hills.

Khanpur Dam completed in 1985 was originally designed for irrigation of 35,000 acres of land and municipal and industrial water supply to meet the growing needs of municipal water of Taxila, Rawalpindi and Islamabad. Now a small hydropower station has recently started its test power generation and few more hydropower stations are under process of study. Its main features comprise a main dam, three saddle dams, (two on the right, and one on the left) of main dam, a gated spillway and irrigation canal system. Canal system consists of main out let sluice structure bifurcating into two branches just downstream dam.

The right bank branch having capacity of 110 cusecs which is purely meant for irrigation supplies to Haripur and Attock districts while the left bank branch with a capacity of 440 cusecs conveys water to irrigate Haripur and Rawalpindi districts partly as well as for municipal and industrial water supplies to the cities of Taxila, Rawalpindi and Islamabad.

The geology of Khanpur Dam is composed of highly fractured and jointed limestone of Eocene age belonging to Margalla Hill Limestone Formation which is porous and contributes a great amount of seepage through the dam. Hence a number of seepage control measures such as grout curtain, dental treatment and impervious blanket were provided at the time of construction which did not prove sufficient. After first impounding of reservoir significant seepage was encountered therefore additional arrangements were made including strengthening of grout curtain and double rows of relief wells down stream main dam toe to subsidize the excessive pressures.

After the operation of 25 year still one of the major problems being faced at Khanpur Dam is significant losses through seepage which is also weakening the various structures of dam i.e. abutments of main dam and spillway area by dissolution of limestone.

Many researchers have studied dams with reference to seepage and other problems and also suggested various remedial measures. Romanov et al., [1] reported that, "water flowing through narrow fissures and fractures in soluble rock, e.g. limestone and gypsum, widens by chemical dissolution. This process, called karstification sculptures subterranean river systems which drain most off their catchment. Close to dam sites, unusually high hydraulic gradients are president to drive the water impounded in the reservoir downstream through fractures reaching below the dam. Under such conditions the natural process of karstification is accelerated to such an extent that high leakage rates may arise, which danger the operation of the hydraulic structure."

The geological formations where the dams are constructed play very important role in post impounding performance of the dams and related structures. Carbonate geology generally contains joints, fractures and famous for solution cavities which are responsible for high rate of seepage through abutments of the dam. Uromeihy [2] while evaluating the seepage problem at Chapar Abad Dam, Iran, described," engineering geological properties in the ground of the Chapar Abad Dam was investigated in order to evaluate seepage problems and to select a proper method of water proofing prior to construction. The dam is located in the northwest of the Iran and is going under construction phase. The geology of the site consists of Early Cambrian Lime-stones and shale that crop out on the abutments and a valley that is filled by 60 m of alluvium deposits.

The presence of thick alluvium deposits with various coefficients of permeability along the foundation, demonstrate a possible seepage problem after water impoundment in the reservoir".

Turkmen [3] has analyzed the performance of grout curtain to treat the seepage problem at the Kalecik Dam, Turkey, and described the grouting procedure, "the main grout curtain is 200 m long and 60 m deep and was constructed on the right bank. After reservoir impounding, new springs were observed in the downstream area. Therefore after the construction of the dam, remedial curtain grouting was required and carried out at three stages" and concluded," as a result of the additional grouting measures, the spring discharge observed downstream of the dam embankment decreased".

In this paper various problems encountered at Khanpur Dam due to carbonate geological formation and their remedial measures are discussed to ensure the safety of the dams, constructed in similar geological conditions.

2. Post impounding problems

The first impounding of Khanpur Dam accidentally took place due to heavy rains during July 1983 just after the closure of diversion tunnel. As a result of impounding of reservoir a large number of problems were reported. The important few problems encountered as accidental impounding are, high seepage, boiling and springing, high uplift pressures, turbid flows through relief wells and development of sinkholes which are briefly described as follows.

Extremely high seepage was encountered through all the embankments i.e. main dam, right and left saddles (Fig. 1), The total amount of seepage was many times more than the base flow of the River Haro. At a large number of places, downstream of dam, boiling and springing started. Even in the houses of the New Khanpur Township water started oozing out which created great panic among the local population who left the area during the first filling of the dam.

The instruments installed at various structures for monitoring indicated high degree of uplift pressures at the toe of main dam near left abutment. Piping was also reported through a couple of locations near by left abutment of main dam, which is very serious for dam safety view point. Turbid flow started from all the relief wells and drains holes indicating direct ingress of flood water from the reservoir. This type of event is directly related to the entry of flood water from upper horizons of the reservoir. On immediate lowering of the reservoir to the minimum level of 1910 feet, a large number of sinkholes appeared in the reservoir area. These sinkholes are responsible for high seepage through various structures of the dam.

3. Pre impounding seepage control measures adopted during construction

Keeping in view the carbonate geology which is composed of highly fractured limestone and shale beds in the abutments and upstream area (Fig. 2), it was expected that seepage would be the major problem. Hence to minimize the seepage a number of controlling measures such as grout curtains at abutments, impervious blanket and dental treatment were adopted during construction phase, which are discussed as follows.

At right and left abutment of main dam and a part of right periphery a grout curtain was provided to cut down the seepage path. Another grout curtain was also provided under the spillway. Impervious blanket was laid upstream main dam to check the seepage path through valley fill gravel present in original river bed. The clay blanket extends to 3000 feet upstream main dam and its maximum thickness was 25 feet. The open joints near the major structures and abutments of main dam and saddle embankment were cleaned and dentally concreted to check the ingress of water.

4. Immediate measures adopted to control the problems, Phase - I (1983 - 1987)

Installation of double lines of relief wells at the toe of main embankment dam downstream of existing row of relief wells were installed to reduce the pressure on the toe of main dam. These relief wells also play important role in elongating the seepage flow path and in providing stability to the embankment. Additional relief wells were also provided downstream of left saddle embankment to reduce the seepage pressures at elevated levels of the left saddle embankment.

To provide the relief and reduce stresses, drain holes were installed at few locations such as downstream of right saddle embankment, left abutment of left saddle, in diversion tunnel and right saddle embankment.

Placement of weight berm over the up lift pressure area near left abutment downstream main dam. A couple of instruments were also installed in the weight berm to monitor the behaviour of up lift pressures.

The existing grout curtain was further strengthened by deepening and providing more drilling and grouting and drainage curtain was also constructed downstream of grout curtain.

All the sinkholes exposed in the reservoir area were treated by placing inverted filler and dumping impervious material over them.

Dye tests proved the entry of flood water from contact of right periphery of main dam and clay blanket. Hence barge dumping at whole of the contact of blanket and right periphery of main embankment dam was provided to stop the direct entry of flood water which is responsible for turbid flow event through seepage from all the drain holes and relief wells as well as from fractures exposed downstream of abutments.

5. Dam safety works Phase - II (1999 - 2002)

Keeping in view the high seepage losses to the tune of 16% of base flow and to ensure the safety of dam and its abutments a scheme was prepared by the Project Authority as Dam Safety Works and partially it was completed from 1999 to 2002. A number of measures have been taken as Dam Safety Works, such as extension of existing grout curtain to a deeper extent under the abutments and dental treatment of open joints and fractured rocks which are major carrier of seepage. Locations of the various work areas are shown in Fig. 1.

Seepage and uplift problems observed just after the first impounding in 1983 through left abutment is mainly attributed to lower limb of the syncline which was not grouted earlier during construction phase or through the remedial measures after July 1983. The drilling and grouting works include 18200 feet drilling and 670 tons of grout to be injected to seal the seepage paths and strengthen the abutment (Fig. 3).

The grout curtain under the spillway and right abutment has been originally constructed, up to elevation +-1855 feet. The seepage through right abutment and under spillway measured at Plunge Pool is quite excessive as compared to other structures. The geological formations below the spillway and right abutment of main dam is favourable for seepage. Hence the existing over hanging grout curtain is to be tied up with shale by deepening it to Elevation 1730 feet. This work includes 19100 feet drilling and 795 tons of grout injection.

Table 1: Status of drilling and grouting after 44% completion of Dam Safety Works during Phase-2 (Khan and Qureshi, 2004)

###Structure###Proposed###Executed

###Drilling ft. Grouting Year Drilling ft. Grouting Bentonite % Sand 50 kg Silt 50 kg

###tons###tons###bag###bag

RightAbt.Of###19100###795###2000###1115###176###1###-###10

Main dam###

Right Abt.of Main###-###-###2001###9391###390###1###50###6

Dam

Left Abt.of Main###18200###670###2000###8296###422###1###-###7

Dam

Left Abt.of Main###5740###-###-###-###-###-###-###-

Dam

Total:###43040###1465###-###18803###988###-###50###23

Table 2: Reduction in seepage at various reservoir levels partial completion of works during Phase-2 (Khan and Qureshi, 2004)

Reservoir###Seepage before###Seepage after grouting

Level###grouting###Reduction

###Year 2000###Year 2001

Feet###Cusecs###Cusecs###Cusecs###%

1915###16.41###-###5.93###63.86

1920###20.64###15.67###8.50###58.81

1925###28.30###26.34###16.02###43.39

1930###36.78###32.35###26.12###28.98

1935###38.60###32.92###30.63###20.64

1940###50.93###45.40###40.79###19.91

The limestone at right abutment is highly fractured and jointed between RD-1200 to RD-1800 are providing direct paths for seepage. The area to be dentally concreted involves a quantity of 1120 cubic meters of concrete mix 1:3:6 including injecting cement and slurry ratio 1:2 in to the joints after cleaning and flushing before placement of concrete, as indicated in fresh dental treatment (Fig. 4).

At a number of locations, ingress points were established through dye testing. Some of these locations in reservoir are submerged under water column. Barge dumping with impervious material was made to effectively seal ingress points which were conduit carrier of high seepage. A large number of sinkholes were developed in the reservoir area. These were classified into two types on the basis of formations where in developed i e soft formation, silt and clays and hard formation, argillaceous limestone and massive limestone with cover of soil.Figure 4

The following methodology was adopted to treat these sinkholes which are one of the major cause of high seepage and possible safety hazards.Sinkholes were cleaned from silt and debris, poured fine sand and water to block inter-onnectivity of sinkholes, poured medium coarse grained sand up to 9 - 12 inches layer. Holes were grouted with cement and sand. Impervious blanket of 2-3 feet on the top of the grouting was laid down. The thickness of the blanketing is determined according to the site conditions and size of the sinkhole.

6. Evaluation of remedial works

On 50 % completion of the Dam Safety Works, Khan and Qureshi [5] made a comparison of seepage reduction due to remedial works which is given in Table .1 and Table 2.be tied up with shale by deepening it to Elevation 1730 feet. This work includes 19100 feet curtain drilling and 795 tons of grout injection.

The limestone at right abutment is highly fractured and jointed between RD-1200 to RD-1800 are providing direct paths for seepage. The area to be dentally concreted involves a quantity of 1120 cubic meters of including injecting cement and slurry ratio 1:2 in to joints after cleaning and finishing before placement of concrete as indicated in fresh dental treatment.(Fig. 4).

At a number of locations, ingress points were established through dye testing. Some of these locations in reservoir are submerged under water column. Barge dumping with impervious material was made to effectively seal ingress points which were conduit carrier of high seepage. A large number of sinkholes were developed in the reservoir area. These were classified into two types on the basis of formations where in developed i e soft formation, silt and clays and hard formation, argillaceous limestone and massive limestone with cover of soil.

The following methodology was adopted to treat these sinkholes which are one of the major cause of high seepage and possible safety hazards.er designed remedial works to further cut down the seepage.

CONCLUSION

Post impounding problems in dams constructed in carbonate geological formations are very critical and problems such as high seepage sink-holes development and high uplift pressures are associated with these dams. Monitoring of various structures and periodic treatment is directly related to the safety of dams. Sinkholes treatment through inverted filters and then local blanketing and grouting through drilling along the abutments ensure reduction of seepage to acceptable limits.

ACKNOWLEDGEMENT

The technical support and supervision of Project Director, (Water and Power Development Authority), Khanpur Dam Project and General.Manager. Tarbela Dam Project and other staff during the execution of dam safety works is greatly appreciated.

REFERENCES

1. Romanov, D., Gabrovsek, F. Dreybrodt, W. Dam sites in soluble rocks, a model of increasing leakage by dissolutional widening of fractures beneath a dam. Engineering Geology, 70 (2003) 17-35(2003).

2. Uromeihy, A., Barzegari, G. Evaluation and treatment of seepage problems at Chapar Abad Dam, Iran. Engineering Geology, 91: 219-228(2007).

3. Turkmen, S. Treatment of seepage problem at the Kalecik Dam Turkey. Engineering Geology, 68 159-169(2003).

4. `Ali, S.M. Third Periodic Inspection Report on Khanpur Dam Project. Published by Associated consulting Engineers, Lahore Pakistan(1993):.

5. Khan, M.S. Qureshi, M. S. Application of grouting for seepage reduction and dam safety in carbonate geology, its management and evaluation. J., Sci., Int., Vol. 16, No. 4. 265-270(2004).

6. Siddiqui, R.H. Ed. WAPDA Annual Report 2007-08. Published by Pakistan Water and Power Development Authority, Lahore(2008):

7. WAPDA, Khanpur Dam Project sinkhole treatment diagram prepared by Project Monitoring Organization, Tarbela Dam1999..

Department of Geological Engineering UET Lahore (E-mail: msaleemkhan1984@yahoo.com) Department of Geological Engineering UET Lahore WAPDA, High Head Power Projects, Beesham
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Author:Khan, Mohammad Saleem; Ali, Tahir; Gul, Mian Ali
Publication:Science International
Article Type:Report
Geographic Code:9PAKI
Date:Jun 30, 2011
Words:3029
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