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Construction of Seepage Measurement System at Jatiluhur Dam, Indonesia
Associate Professor, MCE Dean, Military College of Engineering, Risalpur Cannt, Pakistan |
ABSTRACT
Plastic concrete diaphragm walls are generally constructed as cut-off walls across dam foundations to minimize seepage losses. These walls are mostly constructed in river alluvium. The loss of plastic concrete into voids can be predicted and controlled. On important projects two such walls are constructed in series for water tightness. Recently a seepage measurement system has been constructed at forty- year old Jatiluhur dam in Indonesia. The work involved construction of plastic concrete cut-off wall in rock-fill. This unique and difficult experience of constructing plastic concrete wall in rock-fill environment has been described in this paper.
Keywords: Keywords: Dams, seepage, measurement
INTRODUCTION
Jatiluhur dam is a rock-fill dam with a sloping clay core resting on a 200-m thick clay-stone bed. The dam is 105 m high at its deepest section, 10 m wide and 1200 m long at the crest (Figures 1 and 2). The dam is of zoned construction. The clay core has an upstream slope of 1 on 0.6, downstream slope of 1 on 0.26,6 m wide at the top and 45 m wide at the foundation level (LL 65% & PI 38%). The core has two layered filter downstream followed by an 8 m thick transition layer. On the upstream side there is 4 m thick random filter followed by a 4 m thick transition zone. The assembly of core, filters and transition zones is flanked by rockfill shells. The downstream shell is 111.5 m thick at the foundation level and slopes 1 on 1.35. The rockfill shells on the upstream side abut against the cofferdam at the section of maximum height. A zone of random fill has been added on the downstream slope from 1 on 2.5 to 1 on 4 and encloses the rockfill shell. The rockfill is made of clean andesite rock ranging in size from 200 to 2000 mm.
The dam has been experiencing problems since its construction. The first crack in dam body was noticed during construction of the main embankment. In January 1965, when the embankment construction surface reached El. 103 m, about 11.5 meters below the crest, a longitudinal crack appeared along the joint between the core and the downstream fine filter. It first appeared near the midpoint of the dam and then extended rapidly in both directions, reaching a total length of more than 500 meters or half the length of the dam. The main portion of the dam was completed to its final elevation (114.50 m) in the end of August 1965, and the downstream berm was completed soon afterwards to enhance dam safety. Soon after completion of constructions a longitudinal crack approximately 300 meters long appeared on the crest in the central portion of the dam. The maximum width of the crack ranged generally from 1.0 to 1.5 inch. Numerous horizontal cracks were observed in the upper portion of the core between 0 to 10-meter depth. A team of International experts evaluated the problems and finally concluded that the project is safe. The dam, however, experienced distress during two events drawdown in 1973 and 1987 and the crest settled by more than 10 cm in each event. A team of International Consultants carried out the safety inspection of Jatiluhur Dam in 1989. Detailed analysis of embankment slopes established that the dam has marginal factor of safety under rapid draw down conditions. Under pseudo-static earth quake loading the factor of safety was well below unity for the slopes of central section of the dam. Similarly the factor of safety for the steady state seepage condition was below international standards. Subsequently a number of remedial works were proposed and executed to enhance safety of this extremely important project. The project Consultants noted with concern that the seepage measurement devices installed at the time of dam construction were inoperable. Since increase in seepage through dam body is the first indicator of core distress, it was highly recommended that a seepage measurement system for the dam be installed. This paper describes a peculiar experience of constructing a seepage measurement flume in the body of existing dam.
Figure 1. Typical cross sections of Jatiluhur Dam, Indonesia
Figure 2. Plan view of Jatiluhur main dam
DESCRIPTION OF THE WORK COMPONENTS
In left wing of the dam seepage from clay core and foundation is diverted through filter to the lowest point at profile 60L where a 1.97 m diameter open well (designated P60L) was provided for collection of water (Figure 2). In 1992 it was established that the water in the open well P60L is connected with the tailrace water indicating that it is useless as seepage measurement tool. Accordingly a scheme was designed to prevent flow of tailrace water towards the seepage measurement well so as to measure true seepage occurring through the left wing of embankment using a v-notch flume (Figures 3 and 4).
Figure 3. Lay out plan of seepage measurement system
Figure 4. Plan and sectional view of secant bore piles filled with plastic concrete
The work comprised of following components.
i) Construction of an underground plastic concrete wall (PCW) to prevent flow of water from tail race towards P60 L seepage measurement well. Since the maximum level of tail water is 30 m, it was decided that the PCW shall be water tight up to at least this level.
ii) Construction of 80-m long, 1.5-m high RCC wall on top of buried old cofferdam to prevent flow from the river towards upstream. This wall was required to prevent seepage of tai-race water towards the up stream. Since the top of old coffer dam is at elevation 28.5 m, an additional 1.5 m high water barrier was required as shown in Figures 3 & 4.
iii) Construction of seepage measurement flume.
These construction activities are described in the follwing sections.
Under-ground Plastic Concrete Wall Construction
The construction of the secant pile plastic concrete wall was the most difficult and tricky component of the project. The wall has been constructed through random fill material comprising of shale and andesite rock-fill in varying proportions. The job involved two components: i) Design of plastic concrete to provide a flexible yet strong and relatively impervious barrier to separate the embankment seepage water from the tail race water and
ii) drilling of bore piles and their subsequent filling with plastic concrete.
Design of Plastic Concrete
The design specifications for plastic concrete required that the minimum compressive strength after 28 day’s curing is not less than 5 kg/ cm2 and its permeability shall not be more than 1X10-6 cm/s. Since large voids filled with water were anticipated in the random fill, it was required that the bentonite (potters clay) shall be non-dispersive to prevent loss of plastic concrete. A number of trial mix designs were tested and on the basis of these results the following design was selected.
| Cement | 185 kg |
| Bentonite | 100 kg |
| Sand | 700 kg |
| Gravel | 840 kg |
| Water | 230 litre |
| Retarder | 400 ml |
| Slump | 17.5 cm |
The bentonite was also tested for dispersion and it was determined to be non-dispersive.
Construction Methodology
Contractor’s local rig failed to produce the desired boring progress due to existence of boulders, as large as 2 m in size, made up of very hard andesite rock (Plate 1). Accordingly the methodology was revised and a well experienced piling rig Contractor was engaged to do this difficult assignment.
Plate 1
The job was delivered as per the following construction methodology:
Construction of a 10 m wide horizontally leveled berm at around elevation 37m.
Construction of a guide wall 0.5 m high, 1.9 m wide (Plate 2) along the centerline of the proposed Plastic concrete wall (PCW) using reinforced concrete sections ( elevation 37 m). The guide wall established the exact location and center of each borehole.
Plate 2a
Plate 2b
Hole # 1, 5, 9, etc., were designated as primary holes, # 3,7,11, etc., as secondary holes and # 2, 4, 6, etc., as tertiary holes.
An 88-cm diameter casing was initially driven up to 4m depth (Plate 2). The thickness of casing was 4 cm. The diameter of resulting borehole was 88 cm. A total of 80 holes, 78cm c/c were drilled for 70m long wall.
Subsequently auger/ buckets removed the material inside the casing (Plate 3). Similar procedure continued for lower depths by extending the casing. Drilling continued till the bedrock level was attained. In fact bore hole was extended half meter into bedrock which is shale. It was noted that the bed rock is bone dry even 30 years of reservoir operation.
Figure 3.
Plate 3
Work was done in 15 m long segments. Primary holes were bored first followed by secondary holes and finally tertiary holes. Each hole was bored and immediately concreted.
After cleaning of bedrock material the concrete tremie-pipe was lowered (Plate 4) and plastic concrete placement commenced.
Plate 4
Plate 5
The casing was withdrawn as concreting progressed (Plate 5). Oscillator equipment was used to withdraw casing.
The concrete was procured from a batching plant.
The verticality of the bore hole was checked by using a spirit level on the casing wall and the proper positioning of piles was ensured by guide wall.
Problems Experienced during Construction of Plastic Concrete Wall
Drilling operation for the construction of PCW commenced with boring of hole #49. The volume of the borehole is roughly 10 m3 and with 30% expected losses it was anticipated that 13 m3of material will fill the hole. During pouring it was observed that the plastic concrete is being drained into cavities and it required 30 m3of plastic concrete to bring the level of plastic concrete to 32.4m level, 4.4 m below the ground level (since max level of water in tail race channel is 29 m, it was required to pour plastic concrete from bed rock up to a level of 30 m. The rest of the hole was filled with lean concrete). The next hole, # 45, also consumed 20 m3of plastic concrete against estimate of 12 m3During filling of bore hole # 41, the first two concrete trucks (each 5 m3) brought the level of concrete to the ground level, but as soon as the first casing was removed (i.e., removal of bottom 3m of support) the level of concrete went down sharply and stabilized 6.2 m above the base of hole. The next two concrete trucks barely raised the level by 10 cm. It meant that all of this material was drained into cavities probably even far from the hole. The work was temporarily suspended for two hours because a fresh order was placed for more plastic concrete. When plastic concrete was poured after a lapse of two hours it was observed that the flow into cavities was reduced and the concrete level rose to the top after pouring two trucks. As the casing was removed the material went into new cavities and concrete level went down to about 8 m from base. Once again the pouring of another two trucks did not change the level by more than 10 cm and most of this material was lost in cavities. There was wait of another two hours and finally three trucks were poured down to bring concrete to a level of 33.6 m, the top of bore holes (guide wall) being 36.80 m. So in all 55 m3of material was consumed against the hole’s volume of 10 m3, an excess of more than 5 times.
The problem of excessive loss of plastic concrete into rock-fill cavities was of serious concern. To solve the problem a number of alternatives such as use of low-slump concrete, use of plastic condom to shield the hole walls were considered. It was however noted that intake of plastic concrete in the secondary piles was only 30% more than bore hole volume and in tertiary holes intake was almost the same as the hole volume. It was concluded that concrete of primary holes fills cavities around secondary and tertiary holes and therefore no change in design was made. The volume of each bore pile constructed and the actual intake or consumption of plastic concrete for each hole is depicted in Fig. 5.
Figure 5. Theoretical and actual consumption of plastic concrete in primary, secondary and tertiary bore piles
The actual average intake for each bore hole at the end of project was around double the theoretical bore pile volume. It was concluded from site experience that excessive losses took place due to following reasons:
Use of plastic concrete with design slump 17 cm, which permits flow into cavities far from source. A lower slump of 15 cm was used for primary and secondary piles but a low slump material can stiffen up quickly and may result in a through passage as experienced in case of bore pile # 48. The slump, therefore, was maintained at 17 cm for tertiary piles.
RCC Wall Construction
The Contractor excavated the existing embankment using a backhoe (Plate 6) and the excavated material was disposed-off nearby. Excavation continued up to a depth of 28.5 m, the top of buried cofferdam. The 80-m long RCC wall was subsequently constructed in 10-m long panels using 300-K concrete (Plate 8). A water stopper has been provided for every joint. The perforated PVC pipes were placed across the wall at designated location and covered with filter material (Plates 9, 10, 11, and 12). The excavated trench was back-filled with the excavated material after completion of RCC wall. The back-filling was done in 0.5-meter thick layers; each layer compacted with 8 passes of 10 T vibratory roller (Plate 13). Later sweet soil was placed and slopes grassed to restore the slopes back to their original condition.
Plate 6
Plate 7
Plate 8
Plate 9
Plate 10
Plate 11
Plate 12
V-Notch Flume Construction
The v-notch flume consisted of perforated and un-perforated 10 inch diameter pipes collecting water on the u/s of the RCC and plastic concrete wall and draining it into open channel constructed with K-225 concrete. The perforated pipes were placed on U/S of RCC wall and shrouded by filter material. On the D/S of wall un-perforated PVC pipe has been placed and shrouded with sand cushion. The channel for the v-notch was excavated using backhoe. The reinforcement bars were tied and K-225 concrete was poured for construction of the channel (Plates 11 and 12). The slopes of the channel have been protected with dressed stone in the upper reaches. A sharp-crested v-notch has been provided along with staff gauge to measure discharge (Plates 14 and 15). The invert level of PVC pipe is at 29.5 m level and water through the v-notch started to flow as soon as water level on U/S of the cut-off wall rose up to 29.5 m level (Plate 16).
Plate 13
Plate 14
Plate 15
Plate 16
TESTING OF SEEPAGE MEASUREMENT SYSTEM
To ensure that the seepage water collected at observation well P-60L is no longer connected with the tail race water and to confirm that the seepage measurement system has been installed as per design, water level observations were recorded daily in the 3 observation wells located on the u/s of the wall towards P60L well (Fig. 6).
Figure 6. Water level observations in P60L and 2 observation wells
The observations clearly indicated that as soon as the wall construction approached completion water level in the observation wells started to rise with respect to water level in the tail race channel. In about 20 days a difference in level of almost 1 meter was attained which indicated that a positive cut-off has been achieved. The specifications required isotope testing of the system to confirm its effectiveness. Accordingly a radio-isotope was introduced at 3 locations on the u/s of the wall, in well P60L and 2 observation wells. The radioactivity of tail race water was constantly monitored and even after 20 days there was no trace of isotope migration from u/s of the wall towards tail race water. This has further confirmed the effectiveness of the cut-off wall as water barrier. Seepage water started to flow out of the newly installed v-notch three months after construction of the new seepage measurement system.
CONCLUSIONS AND RECOMMENDATIONS
Seepage measurements provide a direct means to make assessment about integrity of clay core. Any distress to the core will result in increased discharge through body of the dam. In the original design of Jatiluhur on observation well P60L was used for seepage collection and its measurement. In 1992 it was established that P60L is connected to tailrace water thus making it ineffective as seepage measurement device. Accordingly a scheme was designed to isolate P60L from tail race channel water and measure dam seepage through a v-notch. The seepage water has now been isolated from the tailrace water by constructing a deep plastic concrete cut-off wall and a RCC wall on top of D/S coffer dam. The water level observations recorded in investigation bore holes and the results of isotope tests indicate that a positive cut-off has been achieved. Water level on the U/S of the cut-off gradually rose at a small but consistent rate of 2 cm/day and finally water started flowing through the v-notch. It is concluded that a seepage collection system as per project conception has been installed. It is recommended that all jobs of this peculiar nature shall be awarded only to well experienced and well equipped contractors only. Additionally for concreting in rockfill environment detailed geologic investigations shall be carried out and even then unforeseen loss of concrete into large cavities shall be accepted as a norm and not as an exception.
REFERENCES
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