The limestone deposit of Mt. Buko, located in the Chichibu region of the Saitama Prefecture, is being mined by Chichibu Taiheiyo Cement, Ryoko Lime Industry, and Buko Mining companies limited. In 1974, the three mining companies made agreement on the cooperative mining operations by bench-cut from the top of Mt. Buko and organized “The Task Group for Research on Rock Slope in Chichibu Area” or shortly “Chichibu Zanken” in Japanese. Since the scale of the final rock slope at the end of operations will be 800 m high and 5,000 m wide, the most important and critical issue of Mt. Buko operations is the stability management of the huge rock slope. The cooperative production started in April of 1981 and at the end of March 2017, the 36 years long cooperative operations mined out about 248 million tonnes of limestone. The mechanical behavior of the Mt. Buko's rock slope has been monitored by several kinds of devices and methods since November 1994. In September 2007, a heavy rainfall of a typhoon caused unprecedentedly distinct but short-term movements in some part of the rock slope, which made the three mining companies intensify monitoring and prompt groundwater drainage from the rock slope. In order to reduce permeation of rain into limestone, paving works of roads on the rock slope are being carried out. Moreover, twenty three steel pipes (80 m long × 600 mm in diameter) were piled into the rock slope and retaining rock piles of total weight of some 2.4 million tonnes are being constructed to reinforce the sher resistance of invisible planes of weakness.

This study examines reliable methods to monitor and manage the stability of a limestone rock slope. The surface displacement of the rock slope of Mt. Buko has been measured over the past 20 years, employing several tools and methods including automated polar system (APS), global positioning system (GPS), and leveling. The rock slope of Mt. Buko will eventually reach gigantic proportions; it will be 800 m high × 5 km long at the end of mining operations. Monitoring work was often associated with data that oscillated irregularly due to negative influences from environmental factors such as temperature, atmospheric pressure, and humidity. However, APS measurements have provided the less fluctuation data by calculating differences in slope distance. The differences in slope distance are obtained by subtracting the slope distance at a reference point in the least displaced area from those at other points. GPS measurements have provided baseline analysis data by setting the reference point in the least area. The GPS data can be converted into slope distance and compared with APS data, and the result of this has shown there are no discrepancies between two types of data. Employing APS and GPS has enabled highly accurate monitoring that produced reliable data due to cross comparisons of each data source.

Mt. Buko's rock slope has been formed by mining activity since 1973 and it will be 800 m high and 5 km long at the end of the mining operations. To manage the rock slope in a stable condition over the long term, mining companies operating at Mt. Buko have conducted numerical analysis of the rock mass and installation of various monitoring devices, such as APS (Automated Polar System) and GPS (Global Positioning System), on the slope surface. APS measurements have shown a sudden and obvious displacement of a section of the rock slope of Une Mine that likely occurred during heavy rainfall in September 2007. This was a different behavior pattern from the expected elastic deformation. It was clarified that displacement measurements in the rock slope, in addition to slope surface measurements, were important for understanding the overall behavior of the rock mass and for determining effective countermeasures for the displacement. In 2008, an observation drift was excavated in the center of the displacement. Measurements of the rock mass were taken along a 307m long observation drift using several crack gages and a total station. Analysis of the long-term monitoring data in the drift has shown that the displacement inside the rock mass corresponded to the data obtained by APS and GPS. Moreover, measurements in the drift were much more stable (showing high accuracy and reliability) than those on the surface, which tended to be affected by meteorological conditions such as lightning, heavy rain and dense fog.

In the present study, the relation between groundwater fluctuations on a rock slope and the dis-placement of the Mt. Buko rock slope is analyzed. In general, the strength of a rock mass behavior is subject to the influence of water. Rock slope stability consequently declines due to rainfall, especially in limestone terrains, where the rain permeates quickly into the rock mass. The Une Mine has therefore drilled a large number of drainage holes, and installed monitoring devices to measure the groundwater level, in order to analyze the impact of groundwater level fluctuations on the displacement of the rock slope. In addition, the understanding of groundwater fluctuations within a rock slope is important for the purpose of obtaining appropriate locations to drill holes, promoting a more effective drainage. Analysis of a long-term trend in groundwater level fluctuation, especially during and after rainfall, showed that areas with higher groundwater levels were linked to the geological setting of the rock slope, and were probably related to the pre-mining topography of Mt. Buko. This study also revealed that groundwater levels have clear correlations with the displacement of crack gages measured in the observation drift, where the lowering in groundwater levels correspondingly leads to a reduction in the rock slope displacement.

The Une Mine is located in the Chichibu region of Saitama Prefecture, Japan, and excavates limestone from the top of the Mt. Buko using the bench cut method. The scale of the final rock slope when mining operations ends will be 800 m high and 5 km wide; therefore monitoring devices such as the Automated Polar System and Global Positioning System have been used to monitor the movement of the slope surface. In addition to these slope surface monitoring, detailed monitoring devices were installed inside the rock slope to clarify the cause of displacements of the Une Mine's rock slope that have occurred since September 2007. Long-term monitoring data such as groundwater level and crack gages inside an observation drift have shown that groundwater fluctuation due to rainfalls could adversely effect on the displacements of the rock slope. Therefore, the drainage of groundwater from the rock mass and the prevention of infiltration of rainfall into the rock mass were considered to be effective countermeasures. Since 2012, the Une Mine has implemented various countermeasures, including drilling drainage holes from the slope surface and observation drifts, well pumping, and paving work on the mine road to reduce risks resulting from groundwater rise. Moreover, deep steel pipes and retaining rock piles were constructed to reinforce the shear resistance of possible planes of weakness in the lower part of the final slope. As a result of the series of countermeasures for the displacements caused by rainfall and groundwater fluctuations, the rock slope of the Une Mine is now in a stable condition. In addition, a simulation study on the prediction of groundwater behavior by multiple regression analysis suggests that the groundwater level will be well-controlled, even at heavier rainfalls than those recorded in the past.

Forward and downward displacements were measured in a section of a limestone slope in the Une mine, Ryoko Lime Industry Co. Ltd. Although the causes of rock slope behaviors at Une site have not been clarified, but rainfall unambiguously impacts on the rock slope. Une mine has taken countermeasures that are generally considered to be effective to stabilize the rock slope without delay. In this study, displacements were monitored using a crack gage installed over the fracture plane in an observation crosscut excavated in the rock slope on 1,010 mL. And it is found, the measured fracture displacements can be classified into two types:“ Type 1 displacement” not directly related to rainfall and“ Type 2 displacement” due to rainfall. All the displacements observed on the monitoring points located on the rock slope and by the crack gages installed other than 1,010 mL were analyzed and found to be composed of the Type 1 and Type 2 displacements. The areas with a large impact of rainfall were identified and the geological/geotechnical characteristics of the areas were investigated to prepare new countermeasures. In order to evaluate the effects of the countermeasures, the author formulated a statistical equation predicting the rock slope displacements from the rainfall history before applying any countermeasures for stabilization. It is confirmed the statistical equation gives a fairly fine agreement between actual and predicted displacements before commencing the countermeasures reducing the rainfall impacts. In addition, the statistical equation also makes possible the quantitative evaluation of the effects of the countermeasures, namely difference between predicted displacement without any countermeasures and observed displacement after activating the countermeasures. As a result, the statistical equation will be used for planning comprehensive and strategic stabilization projects in future for the Une rock slope.