**4. Causes of landslides**

A change in hillside equilibrium is mostly attributed to the landslide occurrences. The three factors that influence stability of hill slopes are [11] as follows:

#### **4.1. Internal properties of earth materials**

Landslides have mostly occurred along North–South orientation in the eastern part of the city. Around Chittagong City, Dupi Tila, Tipam and Boka Bil Formations are exposed. However, landslides occurred mostly in the sandstone of Dupi Tila and Upper Tipam Formations. The sandstone of these formations consists of 52–74% medium to coarse sand, 28–45% fine sand and only 2–4% silt. Field investigation and laboratory tests show that landslides happened in sandstone which consists of very little or no silt and clay particles. Sands are primarily medium to coarse grained, poorly sorted, moderately weathered, loose with no cementing materials, highly porous and permeable and easily powdered between the fingers, indicating little bond strength between individual particles. It is massive, joints and fractures are not common but occasionally with inter-bedded iron bands. Unconfined compressive strength, cohesion and angle of internal friction of sandstones range from 1.2 to 1.8 kgf/cm2 , 0.0459 to 0.0801 kg/cm2 and 32.3° to 34.3°, respectively. Hezen's uniformity coefficient is less than 2. The specific gravity ranges from 2.39 to 2.57 and the permeability ranges from 0.2116 to 0.5184 cm/s.

#### **4.2. Geomorphic setting and environment**

**3. Description of landslides**

138 Engineering and Mathematical Topics in Rainfall

**4. Causes of landslides**

**4.1. Internal properties of earth materials**

tion but subsequent landslide buried even the rescuers.

Locations of landslides in Chittagong City indicate that most of them occurred in sandstone of Dupi Tila and Upper Tipam Formations. The hills of Dupi Tila and Upper Tipam Formations exhibit rugged topography with numerous valleys and ridges. With an average elevation of about 30 m from the ground, the hills are dome-shaped with a flat top. The lengths of the flanks range from 15 to 50 m, and most of their gradient varies from 10° to 40°. The profile of the slopes is primarily convex to concave and finally fairly regular at the end. Two types of valleys are prominent here. They are V-shaped and U-shaped valleys. V-shaped valleys mainly occur along the strike and have a higher gradient in the eastern side than that in the western. U-shaped valleys are formed across the strike and they are shallow, open and smooth. The hills are rolling type with sub-dendritic medium-dense drainage system [10].

Most of the landslides are found small in size but there are a number of slides at each location. A high death toll at some locations can be attributed to repeated landslides one after another in the same area. In some cases, when a landslide took place, local people start rescue opera-

Most of the landslides initiated as a slide type but eventually terminated as dry flow, with exceptions in some rock fall areas. Due to modification of hills around the city, slopes are observed with a gradient of almost 90° in many places. Each landslide shows well-developed main scarp and surface of rupture. The slope materials are characterized by loose sandstone with little or no silty shale or shale. High permeability of sandstone allows rainwater to infiltrate easily. Except small bushes and grass cover sparsely, most of the hill slopes are barren. The main scarps have heights of about 1 m or less. In some places, lunar cracks were developed above the main scarps, in the crown of the landslides. Rainwater penetrates more easily through the lunar cracks, and it was sufficient to initiate the surface rapture. The main bodies of the landslides are generally free from debris and minor scarp. Under the influence of grav-

A change in hillside equilibrium is mostly attributed to the landslide occurrences. The three

Landslides have mostly occurred along North–South orientation in the eastern part of the city. Around Chittagong City, Dupi Tila, Tipam and Boka Bil Formations are exposed. However, landslides occurred mostly in the sandstone of Dupi Tila and Upper Tipam Formations. The sandstone of these formations consists of 52–74% medium to coarse sand, 28–45% fine sand and only 2–4% silt. Field investigation and laboratory tests show that landslides happened in sandstone which consists of very little or no silt and clay particles. Sands are primarily medium to coarse grained, poorly sorted, moderately weathered, loose with no cementing materials,

ity, sediments slide and started to flow rapidly over the gentle part of the slopes.

factors that influence stability of hill slopes are [11] as follows:

The area is characterized by dome-like low hills with a flat top. The flanks of the hills are very short in length (15–50 m). The natural slope angle of landslide areas varies from 34 to 84° but most of the slopes are more than 40° which is greater than the average values of the angle of internal friction of slope materials (26–34°). Hence, slopes are unstable under natural condition. The profile of the slopes is convex at the top, at the middle it is concave and it is fairly regular at the bottom. The valleys and gullies, which follow the east-west direction, are open, shallow, U-shaped and smooth. The valleys trending north-south are V-shaped, relatively deeper, elongated and parallel to the strike of the bedding. The hill slopes lack large trees and covered mostly with grass- and shrub-type vegetation.

#### **4.3. Independent external factors**

External factors are triggering factors that cause landslides. Excessive precipitation, human activities and earthquakes are the three most common triggering mechanisms of landslides. On or close to the day of landslides, there is no record of earthquakes. Human activities such as deforestation and hill-slope modification through cutting are common phenomena. Chittagong City is characterized by excessive precipitation within a short period of time, the important triggering factor for landslides.

The magnitude, intensity and duration of rainstorm play vital roles in determining whether a hill slope will fail or not. Rainfall lubricates and increases the weight of slope materials. Excessive rainfall weakens earth materials by displacing air and increasing the pore water pressure along shear surfaces. The probability of ultimate failure enhances when surface materials are porous and permeable and are underlain by sediments of low permeability. Inadequate drainage facility of rainwater also causes more infiltration into the ground.
