**5. Mechanism of landslides**

The slopes are almost barren and hence rainwater easily infiltrated into the slope materials that eventually increases the pore water pressure. As a result, crown cracks developed on top of hills. Those cracks allow much more water to penetrate the materials. During heavy rainfall, pore water pressure surpasses the shear resistance of the slope materials. Landslides initiate when this shear resistance threshold is exceeded.

The pore water pressure generally develops in the convex part of the slope. As the pore water pressure exceeds the resistance of the materials to shear, the slope materials start to move. Initially, the materials with a huge volume of water in the convex part of the slope begin to slide and roll over the concave part and finally on a fairly regular part (settlement areas). The extraordinary rapid movement of sediment and water destroys and buries people and houses, and finally due to the non-cohesive nature of slope materials, the slide terminates as dry flow. The steps involving the development of landslides (**Figure 5**) can be summarized as follows:

**6. Landslides prevention and control**

The choice of landslides prevention and control measures depend upon the geological condition, physiography, available technology and above all the socioeconomic condition of the

Landslide Susceptibility of Chittagong City, Bangladesh, and Development of Landslides…

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Avoid method: If slope is extremely hazardous and landslides control measures are expen-

Water control method: Usually, rainwater infiltrates into the slope materials and makes them exposed to landslides. Proper management of rainwater through drainage from the slope instead of infiltration into the soil can significantly reduce landslide vulnerability. This can be

Excavation method: A slope turns out to be vulnerable when the natural angle of the slope becomes higher than the angle of internal friction of slope materials. The main objective of excavation method is to stabilize the slope by modification of the slope angle so that the inter-

Retaining structure: Structures that can be used for slope stability are buttresses, retaining

The best prevention method for Chittagong area might be complete avoidance of hazardous slopes. A second option might be excavation method which suggests reforming the slope so that the slope angle becomes less than the angle of friction of slope materials. Thirdly, retaining structures like retaining wall, guide walls or buttresses can be constructed to protect the slopes from further landslides. However, if we could harness the modern communication and information technology to provide an in-advance warning prior to an incidence by developing real-time landslides early-warning system, then that should be one of the best options at

The hilly part of Chittagong City and its surrounding areas have been classified into four zones on the basis of locations of past landslides, rock types and their geotechnical properties (**Figure 6**). These potential zones are very closely correlated with the geology of the area. Based on these parameters, Zone-I is most dangerous and Zone-IV is relatively less dangerous for landslides hazard. In Zone-I, the rock type is mostly loose, highly weathered sandstone and Zone-IV constitutes mostly of compacted shale. Considering the age of the rock types, Zone-I is younger than Zone-IV. Zone-I in the map represents Dupi Tila and Upper Tipam Formations, zone-II corresponds to Middle Tipam, zone-III falls into Lower Tipam and

Rainfall-induced landslides are common phenomenon in Chittagong and in adjoining areas where scores of people are being killed each year due to landslides. Under present situation, it

people. Important landslides prevention and control methods are [11] as follows:

realized by afforestation and building suitable drainage structures on slopes.

sive, complete avoidance of the area is the only sustainable solution.

nal friction angle of slope materials is higher than the slope angle.

**7. Landslides susceptibility zoning and early warning**

walls, shear keys, guide walls, and so on.

finally zone-IV to Boka Bil Formation.

least to save lives.


**Figure 5.** Stages of development of landslides.
