By Vijay Pal Singh Chauhan
Construction of the 86-mw Malana project in Himachal Pradesh, India, was completed in just 30 months. The plant went on line on July 5, 2001. However, less than three months later, a failure caused the entire reservoir to empty in less than 20 minutes.
Project owner LNJ Bhilwara Group determined that this failure was caused by settlement of the reservoir floor. The failure damaged a portion of the reservoir floor and undermined four of the 23 blocks that make up the foundation of the gravity dam.
To repair the damage, LNJ Bhilwara Group backfilled the void below the four blocks with concrete, performed contact grouting through the drainage gallery of the dam, grouted the dam body in the area of the undermined blocks, and treated the peripheral and contraction joints to prevent seepage. Work performed to prevent a similar failure in the future included constructing a box drain around the reservoir perimeter, completing unfinished balance of protection work in the diversion channel, and lengthening the seepage path between the reservoir and dam.
Design of the project
Malana is a run-of-river facility that uses water from Malana Stream, a tributary of the Parvati River. Construction began in January 1999, and the project was commissioned in July 2001.
Main features of the project are:
- Diversion barrage at Elevation 1900 meters, designed to pass a probable maximum flood (PMF) of 600 cumecs;
- Head regulator with four gates for regulating discharge up to 26.25 cumecs;
- Four desilting basin tanks to exclude particles down to 0.2 millimeter;
- Gravity dam made of 23 blocks with an average height of 20 meters;
- Reservoir to store about 250,000 cumecs of water for peaking power;
- Intake structure to draw water into the headrace tunnel;
- 3.3-kilometer-long, 3.65-meter-diameter headrace tunnel;
- 915-meter-long, 2.2-meter-diameter penstock;
- Surface powerhouse; and
- 22-kilometer-long 132-kilovolt transmission line.
The main civil works were founded on the river-borne material, which is a matrix of large boulders, cobbles, pebbles, and fine sand. During site investigation, RITES Ltd. of India found that there was no rock up to 30 meters deep in this area. Boulders as large as 3 to 4 meters in diameter were encountered during geotechnical investigations.
The reservoir floor is a doubly reinforced 300-millimeter-thick reinforced concrete slab underlain by geotextile and geomembrane. To make the reservoir floor impermeable, the joints were treated with polysulphide compound. The reservoir floor level is at Elevation 1878 meters, and the top of the gravity dam is at Elevation 1894 meters.
Failure of the reservoir floor
At 9:40 p.m. on September 20, 2001, a portion of the reservoir floor in front of blocks 13, 14, 15, and 16 of the dam settled. On that day, the reservoir contained about 220,000 cubic meters of water. In less than 20 minutes, the entire contents of the reservoir escaped from below the foundations of blocks 13 through 16. This meant a flow on the order of 160 cumecs was generated immediately after settlement of the reservoir floor.
Figure 1 on page 26 shows a plan view and Figure 2 shows a section of the area affected by the failure of the reservoir floor. The foundations of blocks 14 and 15 were completely undermined, and the foundations of blocks 13 and 16 were partly undermined. Later that day, investigators at the project found that the reservoir floor had settled by 5 to 6 meters in an area measuring about 500 square meters, immediately adjacent to the affected dam blocks.
Figure 1: Settlement of the reservoir floor at the 86-MW Malana project occurred behind and under foundation blocks 13 through 16 of the gravity dam.
Even after removal of their complete foundation, blocks 14 and 15 remained in position. However, the failure subjected these blocks to considerable stress and strain that resulted in opening of several lift joints. In addition, several cracks developed horizontally and diagonally in the body of the dam blocks, both upstream and downstream. Finally, the contraction joints of the dam blocks opened up by 5 millimeters to as much as 75 millimeters.
Figure 2: The area under the foundation blocks undermined by water exiting the reservoir was as deep as 6 meters and extended from upstream to downstream of the dam.
The openings of the contraction joints were recorded daily after the failure. For about a month after the failure, measurements indicated the openings of the contraction joints continued to increase. This continued opening was attributed to settlement.
Probable causes of the failure
Consultants and the project team analyzed the reasons for failure of the storage reservoir. The most probable cause was considered to be piping action caused by sub-surface flow under the reservoir floor. This led to erosion of the foundation material. As a result, the reservoir floor slab settled and caused a rupture of the underlying geomembrane and its connection with the dam.
Two other pre-existing factors that could have been responsible for the failure were: non-completion of spreading of the geotextile and geomembrane, and incomplete filling of boulders near one end of the diversion channel. The diversion channel began carrying water in June 2000. The water flowed alongside the toe of the dam from blocks 12 to 16. This could have caused undermining of the foundation strata below blocks 13 through 16.
In early June 2001, a flood caused back-flow of river water through this pipe into the drainage gallery of the dam. The gallery was filled with water and was unapproachable for inspection until mid-August 2001.
In the second week of September 2001, the project team observed that there was considerable seepage into the drainage gallery from the drainage holes of blocks 17 through 22 of the dam. The static head of water in the drainage holes in blocks 21 and 22 was as high as 2.5 meters above the floor level of the drainage gallery. This meant there must have been a considerable amount of seepage from below the reservoir.
Reservoir levels were maintained around Elevation 1888 meters from July to September 2001. Because the reservoir level remained high, the piping action continued and the groundwater levels could not be observed in the piezometers in the drainage gallery.
One other potential reason for failure of the reservoir floor could be improper compaction of the fill material below the floor, adjacent to blocks 13 through 21.
Making repairs and improvements
Several measures were undertaken to rehabilitate the project and prevent a similar situation in the future. This work, which began in November 2001, was the result of meetings held with consulting firm RSW International Inc. in Montreal, Quebec, Canada. The work was performed by P&R Engineering Services in Chandigarh, India.
Concreting below the dam foundation
As a result of undermining of the foundations of blocks 13 through 16, horizontal and diagonal cracks appeared in the body of the dam. Immediately after emptying of the reservoir, backfilling of concrete in the undermined portion of the dam blocks was organized. A pool of water had formed under and in front of these dam blocks. Dewatering pumps were installed to lower the level of the pool below the foundations of the dam blocks. Under-seepage from the founding strata was still 15 to 20 liters per second until about the middle of October 2001.
Using concrete to fill the undermined portion was organized from both the upstream and downstream sides of the dam blocks. The area was not fully dewatered for fear that the foundation material below blocks 13 and 16 would be eroded under the influence of pore water pressure. This could result in collapse of blocks 13 through 16, which would have been catastrophic. As a result, concreting was carried out under water both through pumping and the tremie method of placement.
Over a period of about 25 days, about 2,500 cumecs of concrete was poured into the undermined portion up to the foundation level of the dam blocks. Pipes were embedded to fill the voids between the fresh concrete and the undermined and exposed base of the dam blocks with grout. A total of 189 metric tons of cement was consumed in void grouting through these pipes.
Contact grouting through the drainage gallery
The project team also decided to carry out contact grouting through the drainage gallery to fill all the gaps below the gravity dam through the drainage holes. The drainage holes were provided all along the floor of the drain in the drainage gallery at a spacing of 3 meters center-to-center. A total of about 100 metric tons of cement was consumed to complete this grouting.
Grouting of dam body of blocks 12 through 16
The scheme for grouting the dam body envisaged drilling holes 32 millimeters in diameter and 1.5 meters deep in the horizontal cracks wherever visual inspection indicated that the crack was quite deep. Along the diagonal cracks, 12-millimeter-diameter holes were drilled 300 millimeters deep and at a spacing of 300 millimeters center-to-center. A non-ferrous, non-gaseous, expanding type of cementitious grout consisting of dry premixed blend of ultra fine special cement was injected into the cracks under pressure. It did not contain chloride.
Two rows of 56-millimeter-diameter holes also were drilled from the top of the dam up to a level 1 meter above the roof of the drainage gallery, for carrying out cement grouting from the top of the dam. The grouting was carried out in stages of 3 meters. Cement grout consumption in these blocks was on the order of 61 metric tons.
Treatment of peripheral joints in the reservoir
The lower and upper joints of the sloping portion of the reservoir opened to a width of 40 millimeters after the reservoir failure. The project team decided to provide double protection to these joints. The opening was filled with bitumen, then graded filter material was placed over the joint. This was covered with 2-millimeter-thick neoprene sheets. After securing the sheets with the 100-millimeter-wide aluminum strip on both sides, another layer of graded filter material was laid. This was covered again with 1.5-millimeter-thick geomembrane. The geomembrane was secured on one side with the newly constructed toe wall and on the other side with the semi-gravity wall. A toe wall was constructed along the perimeter of the low level channel and also along the gravity wall. The purpose of this toe wall was to fix the geomembrane and to seal the joint between the reservoir floor and the vertical walls.
Treatment of contraction joints to prevent seepage from the reservoir
To prevent seepage through the contraction joints that opened up after the reservoir failure, the project team decided to cover each contraction joint with an 8-millimeter-thick, 300-millimeter-wide polyvinyl chloride (PVC) strip fixed to the gravity dam with galvanized angles. The mastic filler was filled between the PVC strip and the face of the dam.
Constructing a box drain around the reservoir perimeter
As explained earlier, the most probable cause of settlement of the reservoir floor was piping action due to sub-surface flow. To properly channel this flow, a box drain was constructed 1.2 meters below the reservoir floor at the foundation level of the dam, all along the perimeter of the reservoir. (See Figure 3.) To provide access for building this drain, the existing reservoir floor had to be dismantled. While dismantling the reservoir floor, care was required to protect the existing geomembrane so that it could be joined with the new geomembrane. About 4,000 square meters of area was dismantled over a period of about two months.
The box drain was constructed in concrete with several hundred perforations. The drain was wrapped with 20-40 size aggregate and geotextile to prevent clogging of the drain spouts over time. The existing collector drains in the reservoir were connected to the box drain. On top of this, another layer of geotextile and geomembrane was provided, in addition to the existing layer. The layers of 300-millimeter-thick doubly reinforced floor in M-20 concrete were laid after.
Completing balance of protection works in the diversion channel
Construction of the diversion channel was to be finished in two non-monsoon seasons with a gap of one monsoon season. In the first non-monsoon season, placement of large boulders over the geotextile and geomembrane to act as protection works in part of the diversion channel was completed. In addition, construction of the gravity dam from blocks 1 through 12 was completed. In the second non-monsoon season, construction of the de-silting basin and reservoir and construction of the gravity dam from blocks 13 to 23 and the related intake works were completed. Once this work was completed, the project was commissioned. It was planned to execute the remaining protection works in the third non-monsoon season.
After this failure, LNJ Bhilwara Group undertook a review of the protection measures in the diversion channel. The measures previously planned to be installed in the diversion channel were sufficient only to protect the channel up to block 17 of the dam. Considering the under-seepage from the founding stratum, LNJ Bhilwara Group determined it would be prudent to extend the protection in the diversion channel up to the end of the dam structure, up to block 23.
The protection work in the diversion channel was taken up again in October 2001 and was completed by the middle of April 2002.
Lengthening the seepage path between the reservoir and dam
During the first construction of the reservoir floor, a geomembrane was laid under the reservoir floor and connected with the reservoir wall using galvanized iron strips and anchors. At the time of the failure, there must not have been any scope for even minor flexibility and adjustment for the geomembrane to avoid breaking its connection with the face of the dam. As soon as the reservoir floor settled, the geomembrane snapped at many points. In fact, the complete connection of the geomembrane with the gravity wall from blocks 10 to 22 was snapped.
Establishing a program to monitor under-seepage
Once the remediation work was complete, it was necessary to monitor the under-seepage below the reservoir floor, as well as in other areas. A surveillance program prepared to monitor the groundwater level consisted of:
- Installation of ordinary and pressure gauge piezometers; and
- Measurement of seepage in the drainage gallery by installing V-notches and rectangular notches.
The piezometers were installed in the drainage gallery of the gravity dam and in other areas, such as around the desilting tank and dam blocks and at the toe of the slope on the semi-gravity wall side. The maximum depth of the borehole for installation of the piezometers in open areas was 23 meters. The depth of boreholes in the drainage gallery was 7 meters.
In June and July of 2002, LNJ Bhilwara Group established a stage-wise filling and emptying program for the storage reservoir. As part of this work, the seepages were observed in the drainage gallery and piezometer readings were recorded outside and inside the drainage gallery. Results indicate that all of the seepage water is being collected in the drainage gallery. Maximum seepage observed in the drainage gallery at full reservoir level of Elevation 1893 meters was 105 liters per second. This is acceptable under the present circumstances.
In the end, LNJ Bhilwara Group determined that the reservoir floor settled due to unsafe exit gradients created by piping action and also due to leftover unfinished and unprotected portions of the diversion channel. s
Mr. Chauhan may be reached at Lanco Green Power, A-59, Sector 61, Noida, Uttar Pradesh 201301 India; (91) 11-23733333; E-mail: chauhanvpsc@ rediffmail.com.
Vijay Chauhan, a civil engineer, is executive director with Lanco Green Power Pvt. Ltd. Formerly, he was vice president of projects for LNJ Bhilwara Group in India and was in charge of construction and commissioning of the 86-mw Malana project.