Over the years, Casagrande plasticity chart is mainly used to classify fine grain soils. However, the use of the plasticity chart has been questioned recently and this has led to the development of a new plasticity chart. Polidori in 2007 and 2009, respectively, developed the new plasticity and activity charts using the Atterberg's limits of pure clays (montmorillonite and kaolinite clay minerals) and their mixture with fine silica sand in different proportions. The applicability of Polidori's charts was evaluated using some residual lateritic soils from Nigeria. On the Casagrande's plasticity chart, the lateritic soils mostly plot above the A-line in the zone designated as clay and classified as either CL or CH. However, on the Polidori's plasticity chart, the lateritic soils classified as CL or CH, whereas on Casagrande's plasticity chart they are classified as ML or MH and vice versa. The classifications obtained from Polidori's plasticity chart are predominantly in agreement with the main soil fractions or component of the soils. This is different from the classification obtained from Casagrande's plasticity chart where lateritic soils with lower clay fractions than their silt/sand fractions are classified as clayey soils. Polidori's activity chart shows that lateritic soils that lie in the same plastic zone may show different behavior due to the different properties of the clay minerals in the soils. In cases where the lateritic soils lie in the zone that is not corresponding to their clay contents on the Polidori's plasticity chart, we presume that other factors apart from those stated by Polidori might also be responsible. Although the use of Polidori's plasticity chart gives a fair classification of the lateritic soils, nevertheless the peculiarity of residual soils such as the in situ structure that influenced the properties of the soils and properties developed due to weathering effects must be taken into consideration as well.
Mineralogy and geotechnical properties of residual lateritic soils derived from sandstone and migmatite–gneiss (MG) were compared. The aim was to determine the influence of the parent rock geology of sandstone and MG on their engineering properties. This was done using the statistical method of Student's t-test. Thin sections show that the MG samples are rich in feldspars and micas with up to 45 and 23% modal estimates, respectively. The dominant clay mineral in the two soils was kaolinite, while the dominant oxides were silicon dioxide, aluminium oxide and ferric oxide. The silica/sesquioxide ratio of the MG-derived soils (MGS) ranged from 1·7 to 3·2 while those of sandstone-derived soils (SS) were 2·9–6·6. SS samples which contained essentially quartz grains exhibited better compaction characteristics, higher California bearing ratio and lower plasticity than MGS. Better engineering properties exhibited by SS can be attributed to the high quartz content present in the parent rock. Feldspars and micas present in MG are weathered into plastic and hydrophilic clay minerals. These are likely to have a negative impact on the engineering properties of the derived soils. Statistical treatment of all determined engineering parameters showed significant differences in all cases except in relation to specific gravity, permeability and compressibility.
The properties of residual soils, according to literature, are sensitive to the pre-test drying method given to the sample prior to testing. Similarly, residual soils such as laterites/lateritic soils are formed under various climatic conditions, hence they show different degrees of sensitivity to pretest drying method. This work is therefore carried out to elucidate the influence of pre-test drying temperature or method on the properties of three lateritic soils that developed over three different Pre-Cambrian basement complex rocks from Ado-Ekiti, SW, Nigeria. The soils were subjected to three pre-test drying temperature before conducting laboratory tests. The pre-test drying temperature considered in this study include air-drying, oven-drying at 60° C, and oven-drying at 110° C. Pre-test drying at 60° and 110° C caused particle aggregation (which reduced the soil surface are) and loss of cohesion. Consequently, this reduced the specific gravity, optimum moisture content, clay content, consistency limits, and unconfined compressive strength of the lateritic soils. The maximum dry density and sand content increased as the pre-test drying temperature increases. The pre-test drying temperature did not significantly change the plasticity classification of the soils, however, at higher pre-test temperature the soils become less plastic. The free swell index of the lateritic soils increased with increasing pre-test drying temperature (up to 60° C) before decreasing when the temperature rose to 110° C. This study has revealed the effect pre-test drying temperature may have on the properties of lateritic soils and these may produce soil properties that may not likely indicate the actual field performance of the tested soils.
Malete is a fast growing suburb with new buildings springing up daily. There has been no documented research on the physical properties and foundation bearing capacity of the soil in this area. This research aimed at determining the suitability of this soil as infrastructure foundation. Bulk samples taken from two selected locations at varrying depths of trial pits were tested for their index and shear strength properties using standard methods. Cone penetration resistance, California bearing ratio, compaction, consolidation and permeability characteristics were also assessed. The geotechnical properties determined varied significantly with depth except for specific gravity which did not vary significantly at α 0.05 with depth. Soil samples from all pits consist mostly of poorly graded gravely sands with little fines. They contain medium to coarse grained sand fraction averagely above 85%. Penetration resistance obtained from cone pentration test ranged from 700 kN/m 2 to 950 kN/m 2. The average safe bearing capacity estimated for strip footing using a factor of safety of 3 at depth of 1 m was not less than 473 kN/m 2 anywhere in the study area. Samples from the two locations generally have good compaction parameters, medium to high permeability and low compressibilty. The highest bearing capacities were associated with the lateritized basement top. This implies that the safest depth to place infrastracture foundations in the area is the depth where lateritized basement rock is encountered.
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