Two common problems soils encountered in construction work in Nigeria are collapsible soils and expansive soils. These soils are called problem soils because they have properties that cannot be safely and economically used for the construction of civil engineering structures without adopting some stabilization measures. With regard to major zones of Nigeria, the common problem soils in Nigeria are structurally unstable residual lateritic soils, black cotton soils (BCS) of north-eastern Nigeria, soft clay shale (attapulgite) of north-western Nigeria, peaty clays/organic clays/soils of Lagos, and clayey peat of Niger Delta region of Nigeria (Bolariwa and Ola, 2015). These soils are unique in different parts of Nigeria. Problem soil generally causes foundation cracks, severe structural damage, heaving, and cracking of sidewalks, roads, etc. Therefore, adequate management of expansive and collapsible soils is very important in the development of civil engineering structures.
Prior to the actual construction of any structure, it is required that site/soil investigations are carried out to know the type of soil present at the site in other to recommend a suitable foundation for the site. This requirement is often not done especially by private clients because of the cost involved. Thus, Engineers are often pushed to make uneconomical assumptions and designs for such foundations probably out of fear.
Bolarinwa and Ola (2015) carried out a review of major problem soils in Nigeria and at the end of the research work, they produced contour maps showing the isopleth of classification and compaction characteristics of problem soils. The research is necessary because problem soils possess similar classification and compaction characteristics. With the knowledge of this, the Engineer would be guided to understand the type of soil present at his proposed site of construction and can roughly estimate the soil-bearing capacity from index properties. This would guide him in making the right and economical decisions and designing the foundation of each type of structure that he is about to construct. This contour map does not in any way invalidate soil tests as soil test is the surest way to confirm in situ soil properties. This map can only be consulted in the situation where it is impossible to carry out soil tests due to logistics and financial constraints and the interpretation should preferably be done by a geotechnical engineer who has a good understanding of soil properties and behaviour.
The factors considered in the classification are classification characteristics of the soils which include: Atterberg limit tests, sieve analysis tests, etc, compaction characteristics, and strength characteristics that include Maximum Dry Density (MDD), Optimum Moisture Content (OMC), and California bearing ratio (CBR), etc. These values are important because they are indicators of the type of soil present at a site. Stable soil and problem soil would definitely not have the same properties. The table below shows the summary of some soil properties around the six (6) geopolitical zones of Nigeria.
| Author | Location | Type of Soil | Property | Range |
| Ola (1987) | North-Eastern Nigeria | Black Cotton Soil | Soil passing No 200 BS sieve | 92% |
| Sand Content | 0-39% | |||
| Silt | 0-47% | |||
| Clay | 13-100% | |||
| Liquid Limit | 23-78% | |||
| Plastic Limit | 12-36% | |||
| Plasticity Index | 6-36% | |||
| Oluyemi-Ayibiowu and Ola (2015) | Free Swell | 30 – 140% | ||
| Maximum Dry Density (MDD) | 1530 – 2070 kg/m3 | |||
| Optimum Moisture Content (OMC) | 25% (1530) – 7% (2070) | |||
| California Bearing Ratio – CBR (4-day soaked) | 2 – 5% | |||
| CBR (unsoaked) | 8 – 13% | |||
| AASHTO class | A-7-5 or A-7-6 | |||
| USCS class | CL or CH | |||
| (Ola, 1983b and 1988) | North-western Nigeria (Sokoto) | Soft Shale (attapulgite) | Soil passing No 200 BS sieve | 98% |
| Clay Fraction | 58% | |||
| MDD | 1137 kg/m3 | |||
| OMC | 41% | |||
| Unconfined Compressive Strength (UCS) | 100 – 124.1 kN/m2 | |||
| Omange et al (1988) | South-South (Rivers and Bayelsa State) | Silt and Clay Fraction | 23% to 73% | |
| Liquid Limit | 0 – 71% with a mean value of 40% | |||
| Plasticity Index | 0 – 39% with a mean value of 24% | |||
| MDD | 1430 – 2400 kg/m3 with a mean of 1430 kg/m3 | |||
| OMC | 8.2 to 22.8% with a mean of 14% | |||
| Omange and Aitsebaomo (1989) | South-South (Delta and Edo State) | Liquid Limit | 0 – 89.4% | |
| Plastic Limit | 0 – 36.4% | |||
| Plasticity Index | 0 – 53% with a mean value of 17.92% | |||
| MDD | 1320 – 2290 kg/m3 with a mean of 1940 kg/m3 | |||
| OMC | 6.6 – 24.1% with a mean of 12.7% | |||
| CBR (96-hour soaked) | 2 – 92.5% with a mean of 34.88% | |||
| Sadiku (1985) | South-South (Cross River) | Liquid Limit | 44 – 79% | |
| Plastic Limit | 19 – 40% | |||
| AASHTO Class | A-7; A-2-7 | |||
| MDD | 1210 kg/m3 | |||
| OMC | 10.6 – 46.6% | |||
| Maximum free swell after 24 hours of soaking | 0 – 4.3% | |||
| Farrington (1983) | South-West (Lagos) | Peat | Liquid Limit | 75 – 250% |
| Plastic Limit | 30 – 175% | |||
| Plasticity Index | 20 – 120% | |||
| Organic Content | 10 – 40% | |||
| Specific Gravity | 2.20 – 2.68 | |||
| MDD | 800 – 1600 kg/m3 | |||
| Shear Strength | 5 – 12 kN/m2 | |||
| Okunade (2007) | South-East (Abia and Imo) | Liquid Limit | 20 – 70% | |
| Plastic Limit | 10 – 50% | |||
| Plasticity Index | 5 – 55% | |||
| MDD | 1700 – 2100 kg/m3 | |||
| OMC | 9 – 24% | |||
| CBR (96-hour soaked) | 5 – 55% | |||
| CBR (unsoaked) | 10 – 80% | |||
| Omange et al (1988) | North-Central (Federal Capital Territory, Abuja) | Clay Fraction | 3 – 54% | |
| Silt Fraction | 20 – 85% | |||
| Sand Fraction | 35 – 95% | |||
| Liquid Limit | 19.5 – 80% | |||
| Plastic Limit | 11 – 45% | |||
| Plasticity Index | 6 – 47% | |||
| Activity Ratio | 0.56 – 3.68 | |||
| MDD | 1670 – 2220 kg/m3 | |||
| OMC | 8.5 – 28% | |||
| CBR (4-day soaked) | 6 – 46% | |||
| CBR (unsoaked) | 9 – 70% |
The original copy of the paper can be downloaded HERE. Any further correspondence concerning the paper can be directed to the author.
