Problems Associated with Pliocene Clay in New Sohag Urban Area, Sohag, Egypt

Marine Pliocene clay deposits occupy part of the low desert zone in the New Sohag City, located Southwest of Sohag city by 10km. The clay deposits are a significant geologic hazard and cause extensive damage worldwide each year. They pose serious problems to engineering structures such as pipelines [1] and roads (rutting) [2]. Expansive soils are characterized by swelling clay minerals (e.g., smectite group). When wetted, the clay minerals absorb water and expand, causing a swelling pressure. Conversely, when it dries, it shrinks, leaving large voids in the soil. Swelling clays can control the behavior of any soil if the clay is more than 5% by weight. Many simple and easy tests could give good results for determining swelling hazards. These simple tests, as some of the most representative properties in the estimation of swelling potential, include the free swell test, Cation Exchangeable Capacity (CEC), Saturation Paste (SP), and Plasticity Index (PI). Snethen et al. [3] concluded that the Liquid Limit (LL) and Plastic Index (PI) are the best indicators of potential swell. Youssef [4] concluded that the Pliocene clay deposits in the low desert zone of Sohag governorate have a swelling characteristic.

Figure 1:Shows a typical topographic section from south to north moving through the Sohag New City and the new Sohag University sites. Note the distribution of clay deposits along the cross-section.

In the southwest Sohag area (New Sohag City), the Pliocene clay can be found in four different situations (Figure 1). The first case is the presence of the Pliocene clay under the Issawia limestone. The second case is where the Pliocene clay is under the Pleistocene sands. The third case is where the Pliocene clay is located under gravel deposits. The fourth case is where the Pliocene clay is located on the surface. Interpretation of satellite images, geo-radar data (recorded in the New Sohag City), and boreholes (drilled in the new site of Sohag University) indicates that the depth of the Pliocene clay ranges from 0m and increases toward the east to reach 20m along the eastern margin of the New Sohag City and the new site of Sohag University. Free swell tests were performed on some samples in the study area. The results indicate that the free swell of these deposits ranges from 75 to 120%. These free swell values indicate the swelling characteristics of these deposits.

Many tension cracks have been recorded in Quaternary deposits and the new buildings’ compacted sand layer (2m thick) (Figure 2). The field investigation indicated that the seepage water from the agricultural farms located about 60m north of this site mainly causes the expansion of Pliocene clay. This clay layer is about 3 to 4m under the Quaternary and compacted soils. As these clays absorb water, they produce uplift pressure. This uplift pressure caused these tension cracks with an opening at the surface and depleted downwards.

Figure 2:Shows the effect of the presence of Pliocene clay in the development area. a) Cracks in the compacted soil at one urban site. b) tension cracks in the quaternary deposits.

1. Azam S (2006) Large-scale odometer for assessing swelling and consolidation behaviour of Al-Qatif clay. In: Al-Rawas AA, Goosen MFA (Eds), Expansive soils: recent advances in characterization and treatment. Balkema Publishers-Taylor and Francis, The Netherlands, pp. 85-99.
2. Aiban SA, Al-Abdul Wahhab HI, Al-Amoudi OSB, Ahmed HR (1998) Performance of a stabilized marl base: A case study. Construct Build Mater 12(6-7): 329-340.
3. Snethen DR, Johnson LD, Patrick DM (1977) An evaluation of expedient methodology for identification of potentially expansive soils. Soil and Pavements Laboratory, U S Army Eng Waterway Exp Sta, Vicksburg, MS, Rep. No. FHWA-RE-77-94, NTIS PB-pp. 289-164.
4. Youssef AM (2008) Mapping the pliocene clay deposits using remote sensing and its impact on the urbanization developments in Egypt: Case study, East Sohag area. Geotech Geol Eng 26: 579-591.