Landslides along National Roads in Central Highland in Sri Lanka: Review on Current Situation and Suggestion for Further Development of Landslide Mitigation along Highways in Sri Lanka Yoshinori KAWAMURA, Oriental Consultants Global Co., Ltd. Based on the experiences gained throughout the Landslide Disaster Protection Project (LDPP), one of the JICA loan Projects in Sri Lanka, in which I took a responsibility as an Acting Team Leader/Senior Geologist from May 2014 to October 2015, to facilitate introduction and adaptation of Japanese technology against Landslide in Sri Lanka. I would share my knowledge of landslides and common mitigation methods in Sri Lanka and discuss future development of landslide countermeasures for cut slopes alongside the national highways. In addition, I made suggestion on landslide countermeasures to be incorporated designing of new or improved highways in Sri Lanka as much as possible. 1. Notable Features of Landslides in Sri Lanka 1.-1 Deterioration of Strength of Landslide Mass Due to Abundant Precipitation As an island nation like Japan, Sri Lanka receives abundant precipitation from the surrounding Indian Ocean and Bay of Bengal mainly through the 1 st and 2 nd monsoon seasons. In many areas in the Central Highland and southwest parts of the island, annual precipitation reaches 3,000mm/year or more. It is around 1.5 times as much as that in Japan; annual precipitation in Japan reaches 2,000mm/year or less. The tremendous amount of precipitation has substantial influence on the features of landslides in Sri Lanka. The photo on the left shows the situation of the middle part of the Koslanda Landslide, which occurred on 29 th October 2015 and claimed 37 lives. Even though the photo was taken 1 month after the disaster, a wet and marshy condition of the landslide mass is easily understood. At the occurrence of the landslide, it is likely that the landslide mass decreased its strength by excessive saturation derived from the heavy rain fall and rapidly moved downward,
possibly in a form of partial debris flow. The photo on the left gives an example of slope failure along the National Road A005 between Kandy and Nuwara Eliya. The failure occurred in December 2014 after a rainfall continued for a couple of days. Also in this case, an excessive rainfall saturated the surface of the slope and caused the slope failure involving the saturated shallower parts of the slope. The photo below illustrates an overview of the Lilisland Landslide, which occurred in September 2015 and claimed 7 lives. Thin layer of residual soil or colluvium underlain by the hard and fresh base rock was saturated by a tremendous rainfall and slid downward in a form of deris flow. As explained above, in many cases in Sri Lanka, heavy rainfall and subsequent saturation of surface soil take a major role in triggering landslides. 1.-2 Erratic Geological Condition; Sudden Change in Weathering Condition between Highly Weathered Parts and Fresh Bedrock In many cases in Japan, degree of weathering is remarkable at the surface and gradually becomes lesser
as the depth deepens. The same tendency is observed in Sri Lanka as well. At some outcrops in Sri Lanka, however, we often observe erratic variations in weathering condition of rocks. The photo above shows a condition of a cut slope near the milestone of 55 km of the National Road A026 between Kandy and Mahiyanganaya. Fragile parts of highly weathered rock exist just adjacent to grayish fresh rock. The photo on the left explains the situation of the landslide at the milestone of 42.5 km of the Southern Expressway, which occurred in November 2012. Highly weathered parts appeared at the main scarp (reddish brown part). Just behind the main scarp, outcrops of fresh rock exhibit the escarpment (grayish color). Based on the given conditions, the highly weathered parts are highly likely to lie just beside or close to the fresh parts of the rock. Other than the example above, at many cut slopes along the Southern Expressway, we often observe close proximity or neighboring distribution between highly weathered and fresh parts of rocks. Thus such sudden changes in weathering conditions between highly weathered parts and fresh bedrock are considered relatively common in Sri Lanka. 2. Landslide Countermeasures in Sri Lanka The landslide countermeasures common in Sri Lanka are explained below: 2.-1 Surface Drainage Surface drainage is common and essential landslide countermeasure in Sri Lanka. Engineers in Sri Lanka often prioritize surface drainage and consider it as the primary method to quickly drain surface water from landslide mass and prevent surface water from flowing into landslide mass from surrounding areas. Considering the vast amount of rainfall in the landslide prone Central Highland, it is rational to prioritize surface drainage as the primary method in landslide mitigation. 2.-2 Horizontal Underground Drainage In the past, foreign contractors introduced horizontal drainage to
some critical locations in the country. In recent days, local contractors are able to install horizontal drainage by themselves. Horizontal drainage is now becoming popular in Sri Lanka and even utilized as an emergency countermeasure against landslides. The photo above shows horizontal drainages installed by local contractors as an emergency countermeasure against the landslide near the milestone of 114km of the Southern Expressway, which occurred in October 2015. The photo on the left explains an end of a drainage pipe installed by a foreign contractor at Watawala Landslide. At Watawala Landslide, each end of steel drainage pipes is exposed to the surface so that entire length of the pipes is easily cleaned. Under the tropical climate in Sri Lanka, drainage pipes are often clogged by thriving microbes. Thus cleaning of drainage pipes are great concerns in Sri Lanka. In the selection process of landslide countermeasures in LDPP, maintenance methods of horizontal drainage were repeatedly discussed. 2.-3 Soil Nailing Soil nailing is the most common countermeasure against slope failure in Sri Lanka and is often installed in the aim of increasing stability of natural/artificial cut slopes. Soil nailing is similar to rock bolt in Japan but the maximum length of reinforcement bar to be installed to the ground reaches 12 meters, 2 times as long as that of rock bolt in Japan. In some cases, stabilization by soil nailing is expected to expand to wider areas of slopes with concrete beams connecting adjacent nail heads. In the photo above, the reinforcement bars for the connecting beams are observed. In addition to above, general countermeasures, such as gabion and concrete retaining wall, are commonly used in Sri Lanka. Essential capability against landslide is widely available in the Country.
3. Suggestions for Future Development of Landslide Mitigation along Highways in Sri Lanka 3.-1 Consideration to Excessive Rainfall Sri Lanka experiences the abundant precipitation, which is substantially larger than that in Japan, and such excessive rainfall may decrease strength of landslide mass or surface of slopes in some extent and cause severe erosion on the surface. In designing a set of countermeasures for a landslide in Sri Lanka, further consideration to surface drainage is advisable than usually paid in Japan. 3.-2 Slope Countermeasures Incorporated into Construction of Highways and Response to Erratic Weathering / Geological Condition Through the experience of the cut slope failures and landslides which have occurred during and after the construction of the Southern Expressway, public understanding of Sri Lanka is being prepared to accept a new idea of introducing countermeasures against landslides and slope failures while constructing or improving highways especially in mountainous regions. When a new construction or an improvement of a highway involving cut slopes of certain magnitude is planned, the following procedures and considerations are recommended if we want to minimize the risks of slope failures or landslides along a new/improved highway based on the Japanese Experience: 1) Drilling investigations are necessary not only on the centerline of the highway but at the shoulders of cut slopes on representative cross sections. A representative cross section is usually at the highest or the most critical section of a cut slope. If considered necessary, drilling investigation behind and/or middle of a cut slope is advised. Based on the drilling investigations, geological profiles on the representative cross sections are prepared and used for slope stability analyses and subsequent consideration of necessary countermeasures, 2) If aerial photos and detailed topographic maps are available, aerial photograph reading and geological field survey are advisable in order to detect faults and/or fractured zones which may imply cut slope failures or landslides to be induced by the construction, 3) If exploration and analysis with enough accuracy are available, seismic explanation may be done to supplement the geological profiles, and 4) Based on the consideration through 1) ~3), for the cut slopes where a slope failure or a landslide is anticipated, appropriate sets of countermeasures are required to be incorporated into the design of the highway. The cost of countermeasures is cheaper
and necessary construction time is shorter than those in the case where the countermeasures are constructed after cutting of the slope is completed. Even though countermeasures against slope failure/landslide are incorporated into the design of a highway, it is quite challenging to accurately predict where such sudden changes in weathering/geological conditions we often observe at outcrops in Sri Lanka exist. Thus, in general, especially in construction of cut slope, appropriate responses to the changes in weathering/geological conditions are necessary during construction stage. Technical specifications and contract agreements, therefore, need to be carefully prepared so that appropriate responses can be smoothly executed. In general, orderer in Sri Lanka likely to avoid variations or claims as much as possible which may entail significant increase in construction costs. Thus, not only from viewpoints of engineering but from those of contract management, while allowing adequate responses to sudden changes in weathering/geological conditions revealed during construction, carefully prepared arrangements are required to minimize cost increase due to such changes. Such arrangements are necessary especially in the areas with erratic weathering/geological conditions but are not yet well prepared because such countermeasures like Japanese ones are still new to the countries other than Japan or some other European nations. Continuous efforts to establish such arrangements are desirable. 3.-3 Methods of Slope Stability Analysis and Design of Countermeasures For most of the cases, in the normal design procedure for Japanese countermeasures against landslides, slope stability analysis is essential. Through the experience in LDPP, engineers in Sri Lanka have some understandings and capabilities on slope stability analysis but still remain further rooms for the improvement considering the Japanese Experiences. Further understandings on selectin of appropriate analysis method, setting parameters for analysis, and practical operation of stability analysis are essential to enhance further capacity. In addition, collection and subsequent organization of records of landslide/slope failure and data of soil/rock properties at landslide/failure sites are desirable to facilitate future developments of landslide countermeasures in the country.