**2. Geomorphology and geology of the area**

**1. Introduction**

regions of Southeast Asia [1–3].

132 Engineering and Mathematical Topics in Rainfall

to build a reasonably safer hilly city.

**Figure 1.** Location map of Chittagong area.

In the southeast of the country, Chittagong City has developed partly in the hilly area and the rest in the coastal plain. The area is bounded by longitudes 91° 46′–91° 50′ E and latitudes 22° 22′–22° 29′ N (**Figure 1**) that cover the Survey of Bangladesh topographic sheet no. 79 N/15, scale 1:50,000. Hills are composed mostly of weathered and loose sedimentary rocks of Tertiary (65–1.8 Ma) age which are prone to landslides. More than 200 people were killed in and around Chittagong City due to landslides between the years 2006 and 2013. Rainfallinduced landslides are common in this part of country like in other tropical mountainous

The scope of investigation is to realize the actual causes of landslides in order to recommend prevention and controlling measures against the catastrophe, zoning for landslides hazard and to establish an early-warning system for landslides to save lives and properties. A landslide-zoning map of Chittagong City has been prepared, and the investigation also includes establishment of a network of automatic rain gauges for landslides early warning. Moreover, the purpose of the study is to help the engineers, town planners, policy makers and geologists

> Chittagong area lies along the western margin of tectonically active Chittagong-Tripurafolded belt [5, 6] (**Figure 2**). The folds are mostly tight, plunging and faulted in nature with NNW-SSE alignment. On the basis of landforms, its genesis, evolution and morphodynamics, Chittagong City can be divided into three broad distinct geomorphological divisions: (1) hilly area, (2) fluvio-tidal plain and (3) tidal plain (**Figure 3**). The hilly part of the city is characterized by different types of erosional processes, and therefore landforms have distinctive erosional features, whereas Fluvio-tidal and tidal landforms are depositional landforms that has distinctive accretion features. Each type of landform is again divided into the number of geomorphic units.

> Each geomorphic unit on the map has been identified from the oldest available (years 1953–1954) aerial photograph interpretation and field checking of those units. Every small feature on the photograph was considered, and interpretation was made with the help of

**Figure 2.** Generalized tectonic map of Bangladesh and adjoining areas.

their specific characteristics. Then, a field survey was done accordingly to confirm each geomorphic unit on the map.

Only hilly areas have been taken into account for landslides investigation. Chittagong City lies in the southern plunging part of the Sitakund Anticline. The anticline is asymmetrical and is characterized by a steeper western (faulted) flank and a gently sloping eastern flank. In the plunge area around the city, the folded sediments are highly twisted and distorted [7, 8].

Moreover, since its deposition, the rocks have undergone different climatic conditions as well as tectonic activities which eventually influenced the geotechnical properties of the rocks. The presence of ripple marks and the frequent alternation of sand and silt of the Boka Bil Formation reveal that the sedimentation took place in strong current at times [9]. The change of facies from argillaceous to arenaceous points out that the environmental condition gradually changed. The presence of clay galls and lignite suggests that the rocks of Tipam Formation display arenaceous in the northern part and argillaceous in the south [9]. This change of lithofacies in the formation reveals that the gradual regression of sea started in the northern part earlier than the south. The environmental condition was later reduced to estuarine [9]. The sediments of Dupitila Formation were deposited in this

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

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135

**Figure 3.** Geomorphological map of Chittagong City and its environs.

The investigated area forms a part of the Chittagong hills constituted of rocks of the Boka Bil to Dihing Formation. Around Chittagong City, from east to west, exposed geological formations are Dihing Formation, Dupi Tila Formation, Tipam Sandstone and Boka Bil Formation [6]. Under varying environments, these formations (**Figure 4**) were deposited during Mio-Pliocene time (25–2 Ma). As a consequence, their rock types as well as their geotechnical properties are non-identical.

Landslide Susceptibility of Chittagong City, Bangladesh, and Development of Landslides… http://dx.doi.org/10.5772/intechopen.74743 135

**Figure 3.** Geomorphological map of Chittagong City and its environs.

their specific characteristics. Then, a field survey was done accordingly to confirm each

Only hilly areas have been taken into account for landslides investigation. Chittagong City lies in the southern plunging part of the Sitakund Anticline. The anticline is asymmetrical and is characterized by a steeper western (faulted) flank and a gently sloping eastern flank. In the plunge area around the city, the folded sediments are highly twisted

The investigated area forms a part of the Chittagong hills constituted of rocks of the Boka Bil to Dihing Formation. Around Chittagong City, from east to west, exposed geological formations are Dihing Formation, Dupi Tila Formation, Tipam Sandstone and Boka Bil Formation [6]. Under varying environments, these formations (**Figure 4**) were deposited during Mio-Pliocene time (25–2 Ma). As a consequence, their rock types as well as their geotechnical prop-

geomorphic unit on the map.

134 Engineering and Mathematical Topics in Rainfall

**Figure 2.** Generalized tectonic map of Bangladesh and adjoining areas.

and distorted [7, 8].

erties are non-identical.

Moreover, since its deposition, the rocks have undergone different climatic conditions as well as tectonic activities which eventually influenced the geotechnical properties of the rocks. The presence of ripple marks and the frequent alternation of sand and silt of the Boka Bil Formation reveal that the sedimentation took place in strong current at times [9]. The change of facies from argillaceous to arenaceous points out that the environmental condition gradually changed. The presence of clay galls and lignite suggests that the rocks of Tipam Formation display arenaceous in the northern part and argillaceous in the south [9]. This change of lithofacies in the formation reveals that the gradual regression of sea started in the northern part earlier than the south. The environmental condition was later reduced to estuarine [9]. The sediments of Dupitila Formation were deposited in this

**Name of the formation/age Rock type Geotechnical characters**

residual soils.

Upper Tipam Sandstone, siltstone and

Middle Tipam Silty shale and shale, bedded,

calcareous.

Lower Tipam Massive sandstone, yellowish

cross-bedded.

**Table 1.** Stratigraphic succession of Chittagong City and its surrounding areas [10].

Reddish brown to brick red, massive, highly ferruginous, weathered sandy to clayey silt, clay and pebbly sandstone at places, oxidized iron incrustation. On top weathered

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

Sandstone and alternation of silty sand and silty shale. Sandstone massive and medium to fine grained, silty sand beds are grayish to yellowish brown, thickly laminated to bedded. Silty shale is light gray to gray, very thinly laminated, fissile. Presence of iron incrustation.

occasional shale, Sandstone cross-bedded and local unconformity at the base

shale relatively hard, at places

brown to brown, medium to coarse grained, loose to dense,

Silty shale, siltstone, sandstone and alternation of sand and siltstone. Cross bedding, cross lamination, ripple marks and load casts are present.

Very soft (30–10 kg/cm2

medium relative strength, uniaxial compressive strength of 1–2 kg/cm2

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Longitudinal joints are present dipping almost parallel to the bedding, spacing varies from closed to 1.5 cm, filled with ferruginous band with coarse sand. Soft (100–30 kg/cm2

in hardness. Low-to-medium relative

Soft in hardness (100–30 kg/cm2

moderately weathered, faulted, conjugate (planar) joints are present with vertical and dipping orientation, spacing <1 cm, medium relative strength, uniaxial compressive strength >550 kg/cm2

Moderate (250–100 kg/cm2

faulted, laminated, medium to high relative strength, uniaxial compressive strength varies from 550

.

slightly to highly weathered, faulted, planar and conjugate joints are seen with vertical and dipping orientation, spacing <1 cm, ferruginous and argillaceous filling, medium to low relative strength, uniaxial compressive strength varies from 275

Moderate (250–100 kg/cm2

.

Moderate (250–100 kg/cm2

.

faulted, planar diagonal to conjugate joints present, closed spacing, filled with mainly parent material, medium to high relative strength, uniaxial compressive strength varies from 250

hard (700–250 kg/cm2

hard (700–250 kg/cm2

to 1100 kg/cm2

to 750 kg/cm2

(700–250 kg/cm2

to 700 kg/cm2

(100–30 kg/cm2

strength.

) to soft

.

137

)

),

) to

) in hardness,

), at places

) to hard

) in hardness, fresh,

) in hardness,

) in hardness, low-to-

Dihing Formation/Pliocene

Dupi Tila Formation/Mio-Pliocene

Boka Bil Formation/Early Miocene

(34–25 Ma)

(13–1 Ma)

(15–5 Ma)

Tipam Sandstone Formation/Mid Miocene

(25–13 Ma)

**Figure 4.** Geological map of Chittagong City and its environs.

environment and continued to occur towards the close of Pliocene. The occurrence of shale and sandstone pebbles, iron incrustations and silicified wood establishes the fact that the sediments of Dihing Formation were derived mostly from older Formations in a fluviatile environment [9].

The generalized stratigraphic succession and geotechnical properties of each formation of the city area are given in **Table 1**.


**Table 1.** Stratigraphic succession of Chittagong City and its surrounding areas [10].

environment and continued to occur towards the close of Pliocene. The occurrence of shale and sandstone pebbles, iron incrustations and silicified wood establishes the fact that the sediments of Dihing Formation were derived mostly from older Formations in a fluviatile

The generalized stratigraphic succession and geotechnical properties of each formation of the

environment [9].

city area are given in **Table 1**.

136 Engineering and Mathematical Topics in Rainfall

**Figure 4.** Geological map of Chittagong City and its environs.
