Review of studies, mostly carried out at laboratory scale, on the utilisation of Rice Husk Ash (RHA) for improvement of deficient soils in Nigeria is presented. Although, few studies have focused on using the RHA as soil improving additive alone, most of the studies have been on its usage as additive to the conventional soil stabilizers (cement and lime). The studies generally showed improvement in the geotechnical properties of soils, either modified or stabilised with the ash, thus indicating the potentials of using this agricultural waste for improvement of geotechnical properties of deficient soils. This suggest that using this material at large scale level in geotechnical engineering practice will help in the provision of stable and durable geotechnical structures, reduce cost of soil improvement and the environmental nuisance caused by the unused waste. This will considerably add to the economic value chain of rice farmers/producers in Nigeria.
Black Cotton Soil (BCS) collected from Guyuk, Adamawa State-Northeastern Nigeria, was treated with 0 to 100% Reclaimed Asphalt Pavement (RAP) in 10% steps to evaluate the microstructure and strength of the compacted mixtures. The index property results show that the BCS and RAP are classified under clay of high plasticity (CH) and poorly graded sand (SP) respectively, according to the Unified Soil Classification System (USCS). An extraction test gave a RAP bitumen content of 5.99%, which is within the value of 5-6% recommended in the literature. An x-ray diffraction test carried out on both the BCS and RAP showed that the BCS predominantly consisted of quartz, microcline, albite and kaolinite, which is similar to the results obtained in the literature, while the RAP, however, consisted of quartz, albite, orthoclase, phylogopite and actinolite, which is slightly different to what is reported in the literature, probably due to the source of the bitumen. The results of compaction, at a modified energy level, conducted on the mixtures, show that the Maximum Dry Density (MDD) increased from 1.890 to 2.034 mg/m 3 at 30% RAP content, after which the value fell to1.925 mg/m 3 at 100% RAP content. The Optimum Moisture Content (OMC) however, decreased from 13.7% at 0% RAP, to 8.8% at between 40-60% RAP content, after which the value increased marginally to 9.5% at 90% RAP. Similar to the MDD, the California Bearing Ratio (CBR) increased from 11% at 0% RAP to a maximum of 35% at 30% RAP content, after which the value fell to 5% at 100% RAP content. 30% RAP is therefore the optimal mixture giving the highest strength and can be used as a sub-base material for roads with light traffic use, according to the Nigerian General Specification for Road and Bridge Works. Hence the 3.07% bitumen obtained for this mixture can be adopted as the fixation point for BCS-RAP mixtures. Durability was found to be far lower than the resistance to loss in strength of 80% suggested in the literature.
Effect of density on consolidation and creep parameters of a clay soil was investigated using a soil classified according to Unified Soil Classification System (USCS) as Clay of High plasticity (CH) and composing majorly of secondary minerals, including montmorillonite. The air-dried soil was compacted at five different compaction energy levels (Reduced Standard Proctor compaction energy, Standard Proctor compaction energy, West African compaction energy, Reduced Modified Proctor compaction energy, and Modified Proctor compaction energy). Specimens for consolidation tests were molded at the five different compaction energy levels (densities). The consolidation parameters (initial void ratio, compression index, and preconsolidation pressure) were observed to be empirically related to the compaction energy. The creep parameters (i.e. primary compression index, secondary compression index, and magnitude of creep) were observed to increase with increases in loading to 387kN/m2, after which the values decreased. Curves resulting from these relationships were observed to increase with increases in compaction energy level and tent towards straight line at Modified Proctor compaction energy. Maximum magnitude of creep estimated for three years was observed to reduce from 455.5 mm at Reduced Standard Proctor compaction energy through 268 mm at West African compaction energy to 247.4 mm at Modified Proctor compaction energy levels.
This study proposes a method of gradually loading plate load on-site using lever arms to squeeze out pore water from clayey soils, allowing the soil to settle. Several types of tests were conducted, including a conventional field plate load test (CFPLT), a numerical field plate load test (NFPLT) and an innovative field plate load test (IFPLT) proposed in this study. Three trial pits with soils of varied engineering properties were studied using CFPLT, which employed the use of a heavy jack for load application, the NFPLT test using PLAXIS and an IFPLT, which employed a lever arm to magnify the applied static load. Disturbed soil samples collected from these trial pits were tested for index properties while the undisturbed soil samples were tested using the undrained triaxial compression test (UTCT) and laboratory consolidation tests. The results of the index properties classified these three clay soils as silt of low plasticity (ML) for clay from site 1, and clay of low plasticity (CL) for clay from site 2 and 3. The cohesion and angle of internal friction from the UTCT recorded cohesion values were 28, 29 and 37 kN/m2 for sites 1, 2 and 3, respectively, while the angle of internal friction values were 13, 8 and 6° for sites 1, 2 and 3, respectively. The plate load testing using the three methods showed similar graph pattern except that the allowable load occurred at approximately 350 kN/m2 for the CFPLT and 150 kN/m2 for the IFPLT. The high value of bearing capacity in CFPLT is due to the short period of time taken to load from a jack, which allowed the test to be completed within a short period of time. The ultimate bearing capacities computed from the laboratory test have values of 315.0, 231.0 and 270.0 kN/m2 for sites 1, 2 and 3, respectively. These values agree closely with the bearing capacities obtained for CFPLT but higher than the values recorded for the IFPLT. This is probably due to the long period of sustained loading during testing, which allowed for dissipation of pore water during each loading. Settlements obtained using the IFPLT were close to 25 mm, which is recommended as minimum settlements for building structures BS 8004, 1986.
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