PRELIMINARY INVESTIGATION ON THE FAILURE OF THE BA-LING CHECK DAM IN THE WATERSHED OF SHI-MEN RESERVOIR IN TAIWAN

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1 PRELIMINARY INVESTIGATION ON THE FAILURE OF THE BA-LING CHECK DAM IN THE WATERSHED OF SHI-MEN RESERVOIR IN TAIWAN Su-Chin Chen. 1 *, Yu-Hsan Chang 2, Ben-Kang Chen 3, Yi-Chuan Liu 4, Zheng-Yi Feng, 5*, Lai, Bor-Hsun Lai 6 ABSTRACT The Ba-ling check dam built in 1960s collapsed during Typhoon Wayne in September, To understand the causes of the failure, this study investigates the geological and hydrological conditions near the dam site. In addition, concrete core samples and rebars were collected from the dam remains to examine the strengths of the materials. It is shown that both the strengths of concrete and rebars are actually not severe degraded. The foundation of the dam was excavated and drilling work was carried out to better understand the situation of riverbed and the foundation base. Electrical resistivity image profile was executed for 3 sections to estimate the erosion of the dam foundation and bearing strata. Preliminary investigation results are obtained indicating that the failure could be due to erosion of the foundation base of the dam. In addition the base is not founded on a firm rock layer while it was built, which causes the foundation base to loosen during flood scouring and stress concentration in the dam body. However, the failure mechanism has to be further investigated and verified. Key Words: Dam, Resistivity image profile, Scouring, failure INTRODUCTION The Ba-ling check dam was built on the riverbed of Dahan River in Shihmen watershed near Taoyuan, Taiwan (UTM coordinate , ). It is a very large counterfort concrete dam filled with soils, including the top slab, counterfort buttresses, face wall, bottom slab and back-fillings as shown in the cross-sections of Figure 1 and 2. The dam was completed in 1977 with height of 38m and length of 80 m. It was blocking about billion m 3 of sediments. During the attack of Typhoon Wayne on September 18 of 2007, the dam finally 1 Professor and Chairman, Department of Soil and Water Conservation, National Chung-Hsing University, Taichung 402, Taiwan, R.O.C 2 Doctoral Student, Department of Soil and Water Conservation, National Chung-Hsing University, Taichung 402, Taiwan, R.O.C 3 Chairman, Kang and Associates, Inc., Taiwan 4 Section Chief, Northern Region Water Resources Office, Water Resources Agency, Ministry of Economic Affairs, Taiwan, R.O.C 5 Associate Professor, Department of Soil and Water Conservation, National Chung-Hsing University, Taichung 402, Taiwan, R.O.C (*Corresponding Author, Tel: , Fax: , tonyfeng@nehu.edu.tw) 6 Director, Northern Region Water Resources Office, Water Resources Agency, Ministry of Economic Affairs, Taiwan, R.O.C -348-

2 reached failure state after 30 years of service. Photos taken before and after failure of the dam are shown in Figure 3. In order to know the dam's structure strength, concrete strength tests and rebar tensile tests were carried out. For better understanding the failure situation of the dam, excavation, drilling and electrical resistivity imaging were performed. A B B A Fig.1 Layout of Ba-ling check dam Section A-A Fig.2 Cross-sections of Ba-ling check dam Section B-B -349-

3 Fig. 3a. The dam was fully filled and there is no more capacity for sediment. Landslide happened at the left abutment of the dam. (2002/10/24) Fig. 3b. The face wall of the dam was still normal; however, two secondary downstream dams were failed. (2005/9/7) Fig. 3c. Work was done to prevent the enlargement of landslide. (2006/8/15) Fig. 3d. The dam failed on Sep. 18, 2007 and sediment was transport to downstream. (2007/9/19) Fig. 3e. The riverbed was scoured deeper and the counterfort buttresses were exposed. (2007/9/24) Fig. 3f. Geophysical survey and drilling were conducted for investigation. (2007/10/5) Fig. 3g. The riverbed was further scoured deeper. (2008/10/20) Fig.3 Photos before and after the failure of Ba-ling check dam -350-

4 INVESTIGATIONS FOR THE FAILURE OF THE DAM Geological condition: The geology formations near the dam are slightly metamorphosed rocks, slates and sandstones. As shown in Figure 4, the geological and geomorphological condition is complex. The dip angle of rock formation is changing and overturned, because there is a syncline axis at the downstream of the dam and a fault at the upstream of the dam.. Rock layers were gained the earth stress and transformed into curved, inverse, rend and sheared. Dislocation and fracture of the rock layers can be seen at many locations. The dam was sitting at a geological area. Sediment level before collapse Overturned strata Syncline axis Fig.4 The geological condition near the dam Rainfall records: The maximum one day rainfall records of typhoons attacking the dam area during 2004 ~ 2007 were collected and listed in Fig. 5. Typhoon Aere in 2004 gave a maximum one day rainfall of 452mm and the dam did not failure. Daily Cumulative Rainfall (mm) /8/24 AERE local damages in the secondary dam and left-bank slope /9/12 HAIMA /10/25 NOCK-TEN 2005/7/18 HAITANG /8/5 MATSA /8/31 TALIM Events(Date/Typhoon name) /10/2 LONGWANG /7/13 BILIS PM 4:00 the main dam failed /8/18 SEPAT /9/18 WIPHA Fig.5 Maximum one day rainfall during typhoons in 2004 to 2007 Strength of concrete: Concrete cores were taken from two locations on face slab and three locations at the conterfort buttresses. The compressive strengths of the face slab samples are ranging from 400 to 500 kg/cm 2 and for the buttress samples are ranging from 500 to 750 kg/cm 2. All strengths of the samples are higher than their designated strength of 210 kg/cm 2. Therefore, it is suggested that the failure of dam is not resulting from concrete degradation. Strength of rebar: There are 6 locations chosen of the dam structure to sample re-bars for tests of yielding stress, tensile strength and extension rate. The test results of most re-bar -351-

5 samples fulfill the requirement of the design standard, although a few samples slightly lower than the standard. Therefore, the strength of re-bars is not largely degraded either. Investigation of the foundation of the dam: The bedrock surface of the stream channel varies a lot. Near the left bank the bedrock surface is shallow. Near the right back and central of the river, the bedrock surface is in a deeper depth. Therefore, the foundation of the dam is not all on a sound bedrock surface. Central part and right side of the dam foundation is founded on the sediments of the riverbed. Foundation excavation: Five locations were excavated to check the foundation of the dam as shown in Figure 6. The foundation caissons cannot be found at Location 1, 2 and 3 at the area of the main dam confirming that the foundation is all lost in this area. At Location 4 and 5 of the secondary dam, disintegrated remains of secondary dam concrete were found m Fig.6 Five locations of the excavation Borehole drilling: Drilling work was taken to understand the depth of the surface deposit, the condition of main dam and secondary dam, the quality of bedrock. There were 18 boreholes drilled as shown in Figure 9. The drilling depths are ranging from 17.5 to 33 m and are about 5 to 6 m into the sound bedrock. The result shows that at boreholes BH-4, BH-5, BH-6, BH-7 and BH-8, foundation concrete slabs of the main dam were found at a depth of about 6 to 8 meters, while no concrete was found at BH-9. Concrete cores belonging to the structure of the secondary dam were acquired from BH-10, BH-11 and BH-14. The cores of BH-10 and BH-11 should be parts of the foundation slab of the secondary dam. The concrete core at depth from 9 to 23 m of BH-14 should be the caisson of the secondary dam. There was no concrete sampled at BH-12, BH-13 and BH

6 L2:72m L1:72m Fig.7 Locations of the 18 boreholes and layout of RIP survey L3:186m Legend Location of boreholes Electrical resistivity image profiling: Resistivity image profiling (RIP) was used to find the rough shape of the foundation and caissons of the dam after failure. The layout of the RIP survey is shown in Figure 7. Figure 8 shows the result of L-3 Pole-pole array. The L-3 survey line, 186 m, crosses the main dam and secondary dam. The intervals between the electrodes are 3 m. It is found that the resistivity between the main dam and secondary dam is discontinued which indicates that this part of structure is already lost. Elevation(m) Top the main dam foundation slab NE L-3(Pole-Pole array) SW 470 主壩底版頂部電阻率不連續副壩底版頂部疑似鋼筋 465 水泥斷塊 BH BH-5 L-1 BH-8 L-2 BH-11 BH-14 BH-17 BH 礫石 450 主壩副壩 445 Main dam Secondary dam 岩 盤 Bedrock Distance(m) Fig.8 Results of the L-3 Pole-Pole array RIP Resistivity discontinuous area Top of the secondary dam foundation slab (ohm-m) 鋼筋水泥礫石含水乾燥岩盤 steel concret moist The possible fracture of reinforced concretes gravel dry -353-

7 SUMMARY The structure between the main and secondary dam of Ba-ling check dam was built to prevent scouring of the main dam foundation. However, it was damaged and disappears. This may be the major reason causing the foundation soils beneath the main dam to be scoured away and finally causing the main dam to deform and collapse. It is not likely that this dam is failure by material degradation or not enough strength, because both the strengths tests of concrete and re-bars still fulfill their design standard. The foundation of the dam was not built on sound bedrock or using piles. When the soft gravel materials were scoured, the dam may be tilted, deformed and finally failed. However, additional investigation should be taken to further verification of close failure mechanism. REFERENCES Northern Region Water Resources Office, Water Resources Agency, Ministry of Economic Affairs (2008). Investigation on the Causes of Ba-ling Dem Failure and its Mitigation Plan, Water Resources Agency, Ministry of Economic Affairs, Taipei, Taiwan. (in Chinese) -354-