Central Thailand Landslides triggering by drought

Chanchai Srisutam, Chayapol Techatitinan, Worawoot UttasahapanichSoil engineering investigation division, Office of tophographical and geotechnical surveys,

Royal Irrigation department, Thailand.

AbstractThe central part of Thailand is made of sediments from the higher level areas. The deltaic

sediment covers the lower region of central Thailand, which is the outlet of Chao Phaya River in Bangkok and surrounding area. Since the deltaic sediment is mainly soft clay, channel embankment landslides are always triggered by drought. The Central Thailand Landslides cause property damages such roadways and irrigation structures. Ground morphology and geological structure influence landslides in central Thailand. Geotechnical information is needed for landslide analysis to find a cause of landslide, remedial measure and protection of channel embankment in the central part of Thailand. This paper discusses the geotechnical investigation and the use of investigation data in the slope failure analysis. The back analysis method is useful to evaluate soil engineering properties. Also, an adequate data helps engineer to have a confident determination for remedial measure and slope stability management.

Keywords landslide, drought, central part of Thailand, geotechnical investigation

IntroductionIn January 1999, there was a drought in central Thailand. Water in irrigation channel is

reduced from full supply level to dead storage by agricultural pumping. This trigged landslides in central Thailand. Figure 1 shows locations of landslides. They were the channel embankment landslides at Hok-Wa channel, Rapeepat Yaek-Tok channel and Preamprachakorn channel (Figure 2).

Figure 1. Areas of 1999 landslides in central Thailand

Rapeephat Yaek-Tok

Landslide

Hok-Wa

Landslide

Pleamprachakorn

Landslide

When the landslide occurred, it damaged roadway and irrigation structure. The foundations of sliding embankments are sediments. Since the sediment is mainly soft clay and the water in channel was reduced, the foundation of embankment had not enough shear strength to resist the load overlay it. Those landslides came with the unique problems but created great understanding for engineers who work in Irrigation Department.

Rapeephat Yak-Tok Landslidekm.24+660 - km.24+725km.26+125 - km.26+195km.26+550 - km.26+615km.27+500 - km.26+555

Hok-Wa Landslidekm.13+380 - km.13+430

Preamprachakorn Landslidekm.12+695 - km.12+730km.14+980 - km.15+040km.16+570 - km.16+610

Figure 2. Physical appearance of central Thailand landslide

Geology of central ThailandThe central Thailand is the flat area, which made of the sediment from the higher

surrounding area. The sediment soil is divided into two types. One is the fresh-water sediment, which covers the upper part of the central Thailand, while the other, the deltaic sediment, covers the outlet of Chao Phaya River in the lower region. The deltaic sediment is mainly soft clay (Bangkok clay) with the thickness of about 5-15 meters. The typical soil profile of the lower region of central Thailand is shown in Figure 3. The thicker soft clay can be found at the area closer to the Gulf of Thailand. The ground water table is level at about 1-2 meters below the ground surface. The soft

clay is overlain by 1-2 meters crust, and is underlain by medium to stiff clay layer. The medium to stiff clay layer is underlain by sand layer. Beneath the sand layer are the series of the stiff clay layer followed by another sand layer and so on. The bed rock is estimated at the depth of 500-1,000 meters from ground surface.

Figure 3. General soil profile of the lower region of central Thailand

The investigation of landslide areasStandard Penetration Test (SPT, ASTM D 1586) and Field Vane Shear Test (ASTM D

2573) were operated at the landslide areas. It showed the foundation soil layers. Soil mechanical properties of medium to hard clay and sand can be evaluated from SPT, N values. The undrained shear strengths of soft clay were obtained from Field Vane Shear test. They vary between 0.48-4.715 t/m2. Undisturbed samples of soft clay were acquired by Shelby tube soil samplers (ASTM D 1587). The undisturbed samples were test in a laboratory for gradation, Atterberg limits, water content and unconfined compressive strength. The summarized foundation soil profile is shown in Figure 4. It shows an uneven thickness of soft clay. Liquid limit and Plastic limit of soft clay are 51.50-83.40% and 23.40-43.70%, respectively. 30.10-104.92% are water content of soft clay. Unconfined compressive strengths vary between 0.41-16.21 t/m2.

Figure 4. Summarized foundation soil profile of land slide areasAnalysis of landslide

The remedial measure and slope stability management

Conclusion

References