Mae Moh, Lampang, a small town in northern Thailand, approximately 600 km far from Bangkok, contains a large, open-pit, lignite mine. The mine is managed and operated by the Electricity Generation Authority of Thailand (EGAT) to provide 15 million tonnes of coal per year for its power plants. In 2011, EGAT selected a pit wall 300 m wide (called Area 4.1) to conduct a large-scale experiment to better understand how pit wall displacement responded to undercutting. A monitoring system was installed to measure surface and subsurface movement. During the rainy season in 2016, Area 4.1 experienced a number of remarkable events related to precipitation. The main sequence of slope undercutting to excavate lignite and back filling of Area 4.1 was completed in summer 2017. Area 4.1 was continually monitored and studied during the large-scale experiment, including deploying a ground-based radar system. Inverse velocity technique, using measured movement data from the radar, was applied to predict slope failure and warn site operators. In the 2016 rainy season, Area 4.1 was warned the failure many times, but the undercut slope was visually stable. The undercut slope displayed stick-slip behavior each time the alarm sounded, and the area was appropriately evacuated. The following dry season, Area 4.1 was mined for lignite and backfilled. The mining and backfilling were done as planned in May 2017. The stability of the supported undercut slope Area 4.1 has been satisfied since the following rainy season.


Mae Moh mine is the largest open pit mine in Thailand. It is located in Mae Moh, Lampang, a small town in northern Thailand, approximately 600 km from Bangkok. The mine is managed and operated by the Electricity Generation Authority of Thailand (EGAT) to provide 15 million tonnes of lignite a year for EGAT's 10 coal-fired power plants.

Khosravi et al. (2011) used physical models in the laboratory to investigate the behavior of undercut slopes. The study showed that undercut slopes were stable if the undercut span width was narrower than the maximum undercut span width. The maximum undercut span width is greatly affected by the rock strength and the slope angle. In 2011, EGAT then proposed a field experiment in the mine to further study these behaviors (Mavong et al., 2013 and 2014). They selected a northeastern low wall, named Area 4.1, as the experimental site; the area was mined in two stages. In the first stage, Area 4.1 was excavated from its northern end to the center. A weak-plane interface between an underburden Gray claystone and a thin clay seam, called G1, was daylighted as expected on the pit wall approximately 150 meters each stage without backfilling. The backfill supported the potential sliding block along the weak plane. Therefore, the first stage required a series of sequential excavating and backfilling. The first stage was successfully completed in mid-2013, with the weak plane partially daylighted.

In 2016, the mine planned the second stage – to mine the remainder of the lignite in Area 4.1, this time starting from the southern end. Fig. 1a. shows the beginning of this second stage. Mining began in February 2017, in the middle of the dry season, at the southern end and proceeded along the working face (red line) in a northwesterly direction. Before mining Area 4.1 in the rainy season of 2016, the area was continually monitored for surface and subsurface movements of the undercut slope, the water pressure in the weak plane, and precipitation. The monitoring data unveiled remarkable movements of the undercut slope in response to precipitation; this paper presents some of this key monitoring data. After the lignite was depleted, the backfilling in the area was started and successfully completed at the end of May 2017 before the rains inaugurated. Fig. 1b. shows the location of the undercut low wall and backfilling (yellow shaded area) after mining was completed in Area 4.1.

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