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Rehabilitation of leakage and seismic damaged problem of Mae Suai Earth zone composited RCC Dam

Suttisak Soralump1*, Chinoros Thongthamchart1, Montri Jinagoolwipat1 and Apisit Boonpo1

Dam Safety Research Unit, Geotechnical Engineering Research and development center, Civil Engineering Department, Kasetsart University, Thailand

ABSTRACT

Mea Suai dam is a zone earth dam with RCC spillway at the center. Differential settlement at the joint between RCC spillway and the earth filled caused leakage problem near the crest of the dam. The RCC blocks at the differential settlement area were crack and its life joint were opened. Repair work has been done, the leakage flow was reduced but not totally disappeared. Furthermore, the dam experienced the strong earthquake and more cracks have been seen. The downstream communities requested that the dam shall be decommissioned. Extensive studies have been done to understand the behavior of this dam during both static and seismic condition. Geosynthetics Reinforced Soil together with steel sheet piles will be used to replace the RCC crest blocks.

KEYWORDS: Dam, Roller Compacted Concrete Dam, Dam leakage, Earthquake and dams

1. INTRODUCTION

These are instruction for the preparation of full papers to be published in the Proceedings of the 19SEAGC & 2AGSSEA. Please follow the instructions carefully.

Mae Suai dam is the earth zone dam with the RCC section in the center of downstream slope. The RCC section is used as an overflow spillway. The RCC material type is a low phase RCC and covered its surface with the conventional concrete (CVC). The dam is 59 m high with 400 m crest length. The storage capacity is 73 Million cubic meter. The spillway was designed for 500 years return period of flood. The dam located in Mea Suai district, Chiang Rai province and has been in service since 2003.

Figure 1 shows the longitudinal and transverse section of the dam. The foundation of the RCC was excavated into the sound rock. Sandy material in the river bed has been removed to prevent the under seepage and potential of soil liquefaction. Grouting has been done as necessary. The RCC section consists of the main part uses as an overflow spillway and the gravity retaining walls at both sides uses to create flow channels and retains the earth dam at both sides (Figure 2). The RCC section is surrounded with the zone earth dam. Upstream part next to the RCC center section is the impervious core with embedded internal filter to reduce the water pressure and discharge the seepage water into the RCC gallery. Shell section is made of semi-impervious coarse grain earth filled with horizontal drain to drain the water during the reservoir drawdown period in order to maintain the slope stability of the shell zone. The earth zone extends to both sides of the abutments. Core trench has been excavated to the foundation rock in the river bed and abutments.

The 6 m high RCC retaining blocks have been constructed over the earth filled material at the downstream side of the dam crest. This is to reduce the earth fill work on the downstream slope and hence lower the construction cost (Figure 3). Near the joint between spillway and the earth dam, the transition trapezoidal RCC block named block D has been constructed.

Furthermore, to prevent the erosion at the crest of the earth dam while the spillway is being overtopped, the RCC block named A, B and C have been constructed as a water guide wall (wing wall) shown in Figure 4. Theses blocks is placed directly over the earth filled material.

(a) Longitudinal section

* ผู้เขียนผู้รับผิดชอบบทความ (Corresponding author) E-mailaddress: soralump_s@yahoo.com ** ตีพิมพ์ครั้งแรกใน 19th Southeast Asian Geotechnical Conference & 2nd AGSSEA Conference (19SEAGC & 2AGSSEA)Kuala Lumpur 31 May – 3 June 2016

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(b) Cross section

Figure 1 Dam sections (RID, 1999)

Figure 2 RCC spillway and gravity retaining wall

Figure 3 RCC crest block (RID, 1999)

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Figure 4 Spillway wing wall (Photo by RID)

Figure 4 Cont.

2. LEAKAGE PROBLEM AND INITIAL REPAIRED

In 2004, after 1 year of service and the water overflowed the spillway, the leakage has been observed at the downstream crest in the contact area between the earth fill dam and the RCC spillway structure. The water flows out behind the RCC block and appeared to be clear. The leakage could be observed if the reservoir elevation reaches a certain elevation near the crest of the dam. The maximum flow rate was 1.5 cubic meters per minute with the approximated head water of 1.2 m. Fig 5 shows the photos of the leakage.

(a) Leakage occurred near the joint between RCC and earth fill,

the differential settlement is clearly seen

B

A

Pipes were formed

(b) Leakage found behind RCC crest block D

Figure 5 Leakages in 2004(Photo by RID) Visually, it can be seen that there is a differential

settlement between the earth filled section and the RCC spillway section. The settlement of the RCC section is approximately 0.18% of its height and the earth filled section is nearly 1% (Figure 6). Even though the differential settlement has been expected and the original design has the construction details to cope with but the leakage is still unplanned. The investigation done by Royal Irrigation Department (RID) found the diagonal cracks and opening of

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RCC lift joints in the RCC block D (Figure 7). Theses openings may contributed to the leakage. Another possibility is the seepage of the water under the RCC block. Figure 8 show the possibility of flow paths of RCC block D and the RCC crest blocks. The repair work has been done in 2005 by installing the impervious membrane over the surface of RCC block D as shown in Figure 9. The repair work seems to be effective in reduction the rate of leakage flow but some leakage still persisted. Sub-drain system for filtering and collecting the leakage water then installed at the downstream toe of RCC crest block and RCC block D to prevent the unnecessary seepage (Figure 10). However, it is found that, at the same water elevation in the preceding years (+507.00 m.MSL), the rate of leakage increased significantly every year which indicated suffused flow that may lead to piping failure (Figure 11).

Figure 6 The settlement of the RCC section and earth fill

Figure 7 Cracks in the RCC block and opening of RCC lift joints

during the repair work in 2005 (Photo by RID)

Figure 8 Possible leakage paths

Figure 9 The repair work by installing the impervious

membrane over the surface of RCC blocks (Photo by RID)

Sub-drain

Side-drain Ditch

RPD3RPD2

Figure 10 The water filtering and collecting system(Photo by

RID)

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ervo

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RDP3

RDP2

Figure 11 The discharge from intercept drainin 2010

3. 2014’s CHAING RAI EARTHQUAKE

Despite the leakage problem that has not been completely solved, on the 5th of May 2014, the 6.3 magnitude earthquake occurred in Chiang Rai province and strong felt at the dam. This earthquake is the largest earthquake ever recorded by instrument in Thailand. The epicentre reported by Thai Meteorological Department (TMD) was located at 19.685oN and 99.687oE in Phan District and about 18 km from Mae Suai dam. More than 10,369 houses were damaged in which 475 of them were destroyed. Department of Mineral Resources (DMR) reported that the earthquake was generated by Mae Lao fault segment, a branch of the active Phayao fault zone (Figure 12). Seven months after the main shock there were more than eight hundred aftershocks

Figure 12 M 6.3 Earthquake and aftershocks surrounded Mae

Lao fault segment

4. Effect to the surrounded dams

There are about 50 dams located within 150 km from the earthquake epicentre (Figure 13). All of them located in the 2A to 2B zone, according to UBC classification. Those dams were designed using pseudostatic analysis since it is a method that commonly used for the dam design in seismic prone area in Thailand. The seismic coefficient of 0.1 is used in the 2B zone. With this seismic coefficient value used, damage to the dam may have been seen if PGA is more than 0.2g. Based on the attenuation equation proposed by Sadigh et al. (1997) which fitted well with the previous earthquakes data occurred in the northern Thailand, the dam located about 5-15 kilometer from epicentre may subjected to the estimated PGA in the range of 0.20g to 0.40g, respectively. Figure 14 also shows this attenuation relationship compared with the actual records from 23 recording stations in Thailand during M 6.3 Chiang Rai earthquake (Soralump et al., 2014).Therefore, there is a large possibility that the dam located within 15 km (PGA>0.2g) from the earthquake epicentre, may have seen some damage. With this estimation, excluding Mea Suai dam, there were two small homogeneous dam, Huai San To and Huai Mae Mon dam, located in the radius of 15 km from earthquake epicentre (Figure 15). Consequently, the visual inspection was done to theses dams. As expected, longitudinal and transverse cracks with crack width of less than 3 cm have been observed at the crest of the dam (Figure 16). After 72 hours of closely monitored those dams had no evident of failure progression.

Figure 13 Location of dams within 150 km radius of earthquake epicentre

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Figure 14 PGA Attenuation curve plotted from actual data from M 6.3, Chiangrai Earthquake comparing with Sadigh’s

attenuation relationship (Soralump et al., 2014)

Figure 15 Location of 2 small earth dams and Mea Suai dam

comparing with earthquake epicentre.

Figure 16 Some minor cracks observed on the crest and

abutment of earth dam

Figure 16 Cont.

Table 1 shows the PGA values recorded at various

dam sites during this earthquake event. As for Mea Suai dam, an accelerometer located at the right abutment of the dam recorded the PGA value of 0.33g (Figure 17). This PGA was quite high comparing to the seismic coefficient used in the design. Therefore, some damage at the dam crest can be expected.

Table 1 Summary of PGA values recorded at 6 large dams during the 5th May 2014 earthquake (RID, 2014 and EGAT, 2014)

Name of the dam

Distance from

epicentre (km)

Peak ground acceleration (g) Dam

Foundation Dam crest

Abutment

Bhumibol 281 0.0014 0.0199 - Sirikit 231 0.0009 0.0025 0.0015 Mae Chang 156 0.0073 - - Mae Suai 20 - - 0.33 Huai Mae Mon*

10 0.19 - -

Huai San To *

5 0.42 - -

*Estimation from Sadigh et al. (1997).

Figure 17 Acceleration-time history recorded at the right

abutment of Mai Suai Dam from the 5th May 2014 earthquake. (RID, 2014)

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5. RESPONSE OF MAE SUAI DAM

At the time of the earthquake, the water storage in the dam was about 60% of its capacity. Therefore the water level was much lower than the spillway crest. Generally, there was no serious damage to the dam but some minor damage raised serious concern to the downstream communities.

5.1 Cracks

New cracks on the RCC block A, B, C and D have been seen after the main earthquake and series of aftershock. Those blocks are not the main safety component of the overall stability of the dam. Beside, all cracks appeared to be at the location above maximum water level. Basically, it is typical that a strong earthquake could create longitudinal or transverse crack on the dam crest. But since the crest of Mae Suai dam is partly an RCC material, therefore cracks were seen more obvious.

Generally there were two types of cracks that have been observed (Figure 18), shallow crack, mostly damage to the concrete facing. Deep cracks, located at the joints between blocks. Most of them are vertical joints. There was no crack or damage to the spillway section or none of the cracks on the earth zone has been seen.

(a) Crack of concrete facing

(b) Joint crack

Figure 18 Cracks from M6.3 earthquake

5.2 Instrumentation data

According to the surface monuments on the dam crest, the settlements caused by the shaking have not been observed (Figure19). However, the piezometers installed at the foundation of RCC spillway and foundation and upstream slope of earth dam, responded sharply to the main shock and to some strong aftershocks (Figure 20). However, the pressure decreased to the value prior to earthquake within 24 hours after the shock. Figure 21 show the location of the piezometer that pore pressure increased. Similarity, the observation wells installed in the downstream area also responded with the seismic forces. It means that the ground water system also responded and fluctuated with the movement of the ground.

It is understandable that the increasing of pore water pressure in the foundation of dam and spillway and downstream area may be related with the fluctuation of the groundwater table during the earthquake. However, the increasing of pore water pressure in the upstream shell and core zone of earth dam may need more explanation. Excess pore water pressure may be generated from cyclic loading. According to Soralump and Prasomsri (2015), for compacted soil used for core and shell zone in Thailand, certain number of loading cycles and cyclic shear strain amplitude needs to be applied in order to create a positive permanent excess pore water pressure. Number of cycle of at least 10 cycles and volumetric threshold shear strain of more than 0.02% is required for permanent excess pore water pressure. Judging that the number of loading cycle of M6.3 will be less than 10 cycles, therefore, the permanent excess pore water pressure may not be able to be generated. Therefore, the generation of excess pore water pressure might be related with the fluctuation of ground water system.

Figure 19 Settlement data since the end of construction till

the time of M6.3 earthquake

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Figure 20 Respond of pore water pressure during the main and

aftershocks.

a) Earth dam section

b) Spillway section

Figure 21 Location of piezometers that responded to the seismic forces

5.3 Resonance behavior

To confirm the resonant phenomena, the resonant frequency was measured using a seismometer on the crest of the spillway. The dam excitation was caused by water flow through the outlet works. Resonance tests were performed with seismometer oriented on the longitudinal and the transverse directions. Figure 6 shows the resonance plots from these tests. The resonant frequencies are nearly 21.14 Hz for the longitudinal direction and 21.20 Hz for the transverse direction (Figure 22). Because the frequency of the earthquake is about 10 Hz which does not conformed to the natural frequency of

the dam, resonance of the structure from the excited seismic force should have not occurred, at least at the spillway section.

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Figure 22 Resonance tests of Longitudinal and Transverse on Mae Suai dam

5.4 Community response

Even though the dam is performed well according to the engineering evaluation, however people who lives downstream of the dam were still very much concerned about the safety of the dam. Since the dam has experienced the leakage before the earthquake and has not been completely solved. With the new cracks opened from the earthquake, the concerns became a panic and serious since the community required that the dam shall be decommissioned. Several meetings were set up to discuss about the practical solution and the consensus were to keep the water storage lower than 60% of storage capacity until the dam has been fully repaired.

6. DAM REHABILITATION

Royal irrigation department decided to rehabilitate Mae Suai dam. The first author is the head of the design team.

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6.1 Design concept

Repair works shall be able to solve the leakage problem and make the dam to perform well with the future earthquake. The design concepts are as followed:

1. RCC blocks A, B, C, D and blocks on the crest of the earth dams will be removed (Figure 23). Those blocks are rigid and partly damaged from the previous differential settlement. Even though they are safe but their brittleness may cause future cracks from the seismic load. It will raise the concern to the community again.

2. RCC block on the downstream crest will be replaced by more flexible structure. In this case, Geosynthetics Reinforced Soil (GRS) wall will be used. There shall not be part that rigid and brittle that crack can be generated when subjected to seismic force or differential settlement.

3. There shall be no further significant displacement both vertically and horizontally from the load of a new GRS wall.

4. The new crest shall be able to control the normal seepage and prevent leakage along the joints.

5. Seismic analysis will be based on Safety Evaluation Earthquake (SEE) criteria.

Figure 23 Removing of the RCC block and dam crest

6.2 New dam components

The new components replacing the old one based on the above design concept are:

1. GRS dam crest (Figure 24), consists of wire mesh GRS wall. The GRS wall is design based on Tatsuoka et al (1997). The gabion wall is anchored to the gabion in the opposite side in order to provide additional confining pressure near the slope face and hence increasing the strength of compacted sand during the earthquake. Hot dipped galvanized steel sheet pile will be placed on both side of the GRS wall. Sheet piles will work as a water tightness barrier. In addition, it designed to minimize lateral movement that may cause from the weight of new GRS wall to the exiting soil below the cut level. Join sealant will be used at sheet pile joints. Upstream sheet

pile wall will be surrounded by compacted clay. If there will be leakage through the GRS wall, leakage will be drained to the colleting drain and to the dam filter system.

Figure 24 GRS crest wall

2. Spillway wing wall (Figure 25), made of reinforced

concrete invert T shape. However, it will sit on the micro piles which tipped down to the sound rock. This is to ensure that there will be no further settlement. Retaining wall is at the right angle of GRS wall. Therefore, the sheet piles continue from the GRS wall will turn at the right angle to install underneath the retaining wall (Figure 26). Finally, sheet pile row will ended at the RCC spillway. Special drill need to be used to embed sheet piles into RCC spillway.

Figure 25 Spillway wing wall

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(a) Top view

(b) Section view

Figure 26 Detail of connection between GRS wall, sheet piles and wing wall

7. CONCLUSION

1. It is not avoidable to have a large differential settlement between the high earth fill zone and rigid RCC or concrete zone, therefore we should avoid having this whenever possible.

2. The RCC transition block D placed between the earth fill zone and RCC zone, therefore it must be rotated since the earth fill settled about 1% but the RCC settled about 0.18%. Differential settlement created the bending in the RCC block D and caused internal cracks and opened the RCC lift joints.

3. Even though the leakage occurred but the dam shall be safe since it flows through the RCC material in which suffusion flow might not be able to occur and the leak occurred near the dam crest where hydraulic head is low. However, the rate of the leakage still increased, indicating that the cracks or lift joints may have some progression, this may be because further differential settlement.

4. Cracks in the RCC crest block due to the earthquake may not be harmful in the engineering point of view. However, for the community who lives downstream, unnecessary worries posted to them. Therefore, the rigid and brittle RCC crest block will be replaced with the more flexible GRS wall.

5. More investigation need to be done to conclude the behavior of piezometer during the earthquake. 8. ACKNOWLEDGEMENT

The author would like to thank Dr. Sompop Sujarit and Mr. Surasit Intarapracha from Royal Irrigation department for providing information.

Thanks to Dr. James A. Bay and Dr. Susit Chayprakaykeaw for the resonance test. 9. REFERENCES

[1] Royal Irrigation Department (RID). (1999) Final design report.

[2] Royal Irrigation Department (RID). (2014) “Peak ground acceleration, Peak ground velocity, Duration of motion, frequency content of Main shock M6.3 at Phan district, Chiangrai provinces”, Bangkok, Thailand.

[3] Electricity Generating Authority of Thailand (EGAT). (2014) “Measurements of peak ground acceleration of EGAT dam.”

[4] Sadigh, K., Chang, C.-Y., Egan, J.A., Makdisi, F., and Youngs, R.R. (1997) “Attenuation relationships for shallow crustal earthquakes based on Califirnia strong motion data”, Seismological Research Letters., v. 68, p. 180-189.

[5] Soralump, S., Feungaugsorn, J., Yangsanphu, S., Jinagoolwipat, M., Thongthamchart, C. and Isaroranit, R. (2014) “Impacts of 2014 Chiangrai Earthquake from Geotechnical Perspectives”, E.I.T. Conference of lesson learn from Mae Lao Earthquake, Chaing Rai province.(Thai langue).

[6] Soralump, S. and Prasomsri, J. (2015). "Cyclic Pore Water Pressure Generation and Stiffness Degradation in Compacted Clays. " J. Geotech. Geoenviron.Eng.,10.1061/(ASCE)GT.1943-5606.

[7] Tatsuoka, F., Koseki, J., Tateyama, M. (1997). “Performance of Earth Reinforcement Structures during the Great Hanshin Earthquake.” Special Lecture In: Proceedings of the International Symposiumon Earth Reinforcement, IS Kyushu ‘96, Balkema, vol. 2, pp. 973–1008.