STUDY OF MANGLA DAMRAISING,PAKISTAN
BY ZIAULLAH CE-30/2012-2013 1
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BACKGROUND The Mangla Dam in Pakistan serves the world's largest irrigation network, bringing water to 120,000 square kilometers of land and serving the 50 million people who live in the vast Indus River basin. It is the fifth largest earth-filled dam and has the largest capacity spillway, capable of discharging 28,600 m3/s, The Project is operated and maintained by WAPDA. The dams are inspected annually by the Dam Safety Organization, WAPDA and at five year intervals by independent inspection teams constituted for this purpose from outside or from within WAPDA.
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BACKGROUND, Cont. Since its completion, the overall performance of the various components has remained satisfactory. However, seepage and foundation pore pressures at some places in the sandstone beds are relatively high.
The original design made provision for future raising of the dam crest by 12 m. In 2000,reservoir capacity having been lost as predicted due to sedimentation. The Government of Pakistan decided to exploit the raising provision, thus regaining capacity. 5
History of Original Construction The original project was built from 1961 to 1967 with funding from the World Bank. The project was designed and supervised by Binnie & Partners of UK (now Black & Veatch), and built by Mangla Dam Contractors, a consortium of 8 U.S. construction firms. The original scheme formed what was then one of the world’s largest man-made lakes. Despite a series of events during construction (such as discovery of the sheared clays and Indo-Pakistan war of 1965), the contractor completed the works one year ahead of the schedule and claimed his completion bonus. 6
LOCATION The project is located on the Jhelum River at Mangla and its 260 square kilometer reservoir partly lies in Azad Jammu & Kashmir and partly in province of the Punjab.
The catchment area of the dam is 33,000 square kilometers of which about 28% lies more than 3,000 m above mean sea level.
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PROJECT COMPONENTS The Mangla Dam includes four major zoned earth fill embankments with a total length of over 13 km and a maximum height of 138 m. 110 million m3 of fill was required for the embankments, which was unprecedented at that time. The main spillway was designed for a discharge of 28 600 m3/s and performed well when tested close to its design capacity to route flood in September 1992.
An un-gated, overflow-type emergency spillway with a design capacity of 6 500 m3/s was also provided for operation when the capacity of the main spillway is exceed. The ten 100 MW hydropower units were installed in three stages 8
PROJECT COMPONENTS, Cont. Irrigation valves supplement flows when the discharge through the power units is less than the irrigation demand. Under normal operation the reservoir undergoes an annual cycle of filling and emptying, corresponding to the freeze/thaw cycle. This was an important consideration in planning construction activities for the raising project. It required existing freeboard to be re-established in time for the next seasonal filling in order to maintain dam safety.
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FEASIBILITY STUDY OF DAM RAISING Between design and construction of Mangla Dam in the 1960’s to 2000, understanding of the geology, seismicity and hydrology of the area around Mangla had advanced significantly. In addition, design standards had changed, communities had grown on the shoreline and a large volume of sediment had been deposited in the reservoir. Therefore, it was considered that the feasibility of the original provision for the dam raising be examined in the light of these changes.
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FEASIBILITY STUDY OF DAM RAISING In July 2000, the study was assigned to the consultants Mangla Joint Venture (MJV), which comprised (NESPAK), Associated Consulting Engineers (Pvt) Ltd, Barqaab Consulting Services (Pvt) Limited, Binnie Black & Veatch International Ltd. from UK, and Harza Engineering Company Int. (now MWH Americas Inc. ) from USA. The scope of the Feasibility Study included the following
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FEASIBILITY STUDY OF DAM RAISING 1- Review of the foundation conditions to check their adequacy for raising considering the present state of the art in geotechnical engineering, including the results of investigations carried out for Kalabagh Dam and operational experience of Mangla Dam. 2- Review performance data of the foundation and structures to evaluate whether they supported raising in accordance with the original provisions or to some other optimal level. 3- Re-evaluation of seismic conditions and their incorporation into the design of the raised dams. 4- Configuration of raising for the various embankments and the availability of material. 5- Identification, supervision and analysis of additional 12 investigations.
FEASIBILITY STUDY OF DAM RAISING The Feasibility Study looked at raising the dams by 6, 9 and 12 m and concluded that that the optimum choice was to raise them by 9m. This configuration presented the lowest cost/benefit ratio. Resettlement cost for the residents and businesses which had become established around the reservoir rim since original construction was a significant factor.
Later, WAPDA retained the same group of Consultants, under the same name of MJV to prepare the Tender Documents Preparation and perform the Construction Supervision of Mangla Raising Project.
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MAIN FEATURES OF RAISED MANGLA DAM • Raising the crest of the embankment dams by 9 m and construction of a 600 mm high parapet along both sides of the crest. • Raising of the embankments require widening of the downstream shoulders from the toe to crest. The upstream shoulder of the Sukian Dyke was also widened to accommodate its vertical impervious core. Total embankment length is 14 km and additional fill volume required 30,000,000 m3. • One additional Saddle Dam at a low point in the reservoir rim • Underwater placement of 1 500 000 m3 of additional material to the upstream toe weight berm on the Intake Embankment 14
MAIN FEATURES OF RAISED MANGLA DAM • Additional foundation pressure relief wells on the downstream toe of the Main Dam. • Impervious blankets on the floor of the reservoir in selected locations to reduce seepage through the foundations. • Raising the spillway headwalls
• A new 250 m long x 18 m high RCC weir to control flows to the Emergency Spillway with design capacity of 6 500 m3/s. 15
Comparison Between the original and raised Mangla Dam
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INSTRUMENTATION The dams at Mangla was well instrumented during the original construction. About 600 piezometers of various types were installed to monitor the behavior of earth fill embankments, their abutments and foundations. Settlement gauges, slope indicators and survey markers were installed to monitor the foundations and fill settlements.
The equipment worked satisfactorily and a large number of instruments were still in service. However, with the passage of time some of the installations were blocked by foreign material. 19
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EMBANKMENT DESIGN
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RAISED EMBANKMENT DESIGN Embankment Material: The dam raising work required about 30 million m3 of various fill materials. The structure of the various embankments varied according to foundations and the materials available locally.
SEEPAGE ANALYSIS: The hydraulic gradients and quantities of seepage through the zones of the embankments were estimated using the finite element software SEEP/W. Filters and drains were provided in the raised sections of all the embankments to prevent piping. 25
EMBANKMENT DESIGN SLOPE STABILITY ANALYSIS: The Morgenstern and Price method, programmed in SLOPE/W (GEO-SLOPE 2002), was used for two dimensional slope stability analysis of the embankments. In the two dimensional analysis, the calculated factors of safety sometimes varied considerably from section to section and in some cases fell below acceptable values.
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SLOPE STABILITY ANALYSIS Three dimensional restraint was invoked where the two dimensional factor of safety was less than the target factor of safety. Three cases were analyzed for each section of embankment. 1- Normal drawdown from EI 1242 ft to EI 1040 ft, with taget factor of safety was 1.5, while slip cirlce passing under intake embankment was 1.6. 2- Steady seepage with reservoir full: In this case the downstream slope analysis was critical. The target factor of safety was 1.5, while under intake it was 1.6
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SLOPE STABILITY ANALYSIS 3- Emergency drawdown from EI 1242 ft to EI 1040 ft, and target FOS was not less than unity. The design parameters for both foundation and fill were developed from testing carried out at Mangla and Kalabagh. These parameters were used in the analysis and are given in below tables
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Table#1, Bedrock Design Parameters
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Table#2
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Earthquake Induced Deformation Deformation due to earthquake was calculated using a method based on Newmark’s method (NEWMARK 1965). The criteria for stability of slopes and deformation caused by earthquakes were met for all the raised embankments. Design features for various embankments of the project are shown below.
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Mangla Dam Water Flow
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CONCLUSION The construction for the Dam Raising Works was carried out under a single contract. Nine firms/joint ventures of foreign and local contractors purchased Tender Documents. Six submitted tenders. The tenders were opened on 20th March 2004. WAPDA awarded the Contract to M/s CWE-JV, the lowest tendered and a Letter of Acceptance was issued on 14th June 2004 for an amount of Rs 13.8 billion, 36% lower than the Engineer's Estimate. The successful Contractor was a joint venture of one Chinese firm CWE as a lead, and four Pakistani contractors. DESCON and SACHAL remained to project completion but SMADB and Inter Construct left in the early stages. 38
CONCLUSION Cont. The Contractor started mobilization in July 2004. The construction period was 39 months which was scheduled to be completed on 19th September 2007. In the event there were 27 months of delay in total and the Taking over Certificate (TOC) was issued by WAPDA only on 26th December 2009. The total construction cost of the Project is Rs 15.8 billion. Mangla Dam was the largest embankment dam project of its time when completed in 1967. The raising project that was completed in December 2010 places the project once again at the top of the list of largest dam raises. The total length of dam embankment after raising is 14 km with maximum height of 148 m. The work required 30 million m3 of fill materials. 39
REFERENCES • ICE (1967), Proc. of the Institution of Civil Engineers, UK, 1967, 38 (November), pp. 337-576, and 1968, 41 (September), pp. 119-203 reprinted under the title of 'Mangla" by Binnie & Partners. • MANGLA JOINT VENTURE (2001). feasibility study report, MANGLA DAM RAISING PROJECT. • MANGLA JOINT VENTURE (2003). Project Planning Report, Volume IV, MANGLA DAM RAISING PROJECT • HTTP://WWW.SCRIBD.COM/ (The World’s Largest Online Library)
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