An investigation on the impact of lime variations to the geotechnical properties of various soil samples of different Ph ranges
Authors:   , Bhengu Phakamani Humphrey 
Published: September 2021

When geotechnical engineers design structures, the designs are based on the assumption that specified quality levels will be achieved for each soil layer below. This is accomplished by ensuring that each layer resists shearing and avoids excessive elastic deformations (Jawad et al., 2014). When the quality of each level of soil layer is increased, the soil's ability to distribute the load over the greater area is generally increased enough to permit the reduction in the required thickness of the soil and surface layers. Many methods can be adopted in order to achieve quality levels when trying to improve the soil strength (Azadegan, Jafari and Li, 2012). One method entails the application of lime in various concentrations to soil as a soil stabilisation process. Soil stabilisation is the alteration of soil to enhance its geotechnical properties. These properties include particle size distribution, plasticity and liquid limit, linear shrinkage, Optimum Moisture Content (OMC), Maximum Dry Density (MDD) and California Bearing Ratio (CBR) and Unconfined Compressive Strength (UCS) of the soil samples. This research examines the use of lime variations to stabilise soil in order to improve the geotechnical properties of various soil samples. The soil samples had different pH ranges due to diverse mineralogical and microstructural composition. Further, this research was inspired by the paucity of published research on the topic. Over recent years, very little research on soil stabilisation has focused on the impact of lime variations on the soil geotechnical properties towards effective soil stabilisation of soil of diverse pH ranges. To conduct the research, approximately 3900 kg of soil was collected for laboratory testing purposes. A series of laboratory tests were conducted, including consistency limits, OMC, MDD, CBR and UCS. These tests were all done in compliance with the Technical Methods for Highways Part 1 (TMH1) of South Africa, with particular emphasis on methods A1, A2, A3, A4, A7, A9 and A14 (Department of Transport, 1986a), supplemented by the pavement engineering manual by the South African National Roads Agency (SANRAL) (2013). Mineralogical tests were conducted by applying x-ray diffraction (X-RD) and scanning electron microscopy (SEM) for better understanding of the microstructure of soil samples. This was to enhance the interpretation of the physical behaviour of the soil samples. Properties partially covered in this research in a form of a literature review are as follows: texture, structure, porousness, organic matter, colour, soil-depth and soil-temperature. In terms of plasticity properties (Atterberg limits), findings of the study confirmed that many of the important engineering and mineralogical properties of soil can be enhanced by the addition of lime. The application of lime led to a reduction in plasticity of both the acidic and alkaline soil samples. The reduction mentioned above occurred due to the decrease in the thickness of the layer of the soil particles that were treated. Further, from the analysis carried out, it was found that the treatment of the soil samples with lime content increased the pH of all the samples. The changes were as a result of the changes in the chemical properties (i.e. cation exchange) and the composition of the samples due to their chemical reactions with the lime additive. This is a substantial pH increase compared to the pH of natural soil (untreated soil samples). Regarding MDD and OMC, the six soil samples displayed maximum dry densities ranging from 1500 kg/m³ to 1940 kg/m³ and optimum moisture contents ranging from 14 % to 29 %. Lime content at range of 4 % to 8 % indicated the highest density of the stabilised soil samples. The densities of the soil samples showed a slight increase at the lime content of 10 %. However, for almost all samples, the results showed that further addition of lime decreases the density and increases moisture content. Based on the results, it was discovered that the MDD is achieved at 2075 kg/m³ and at 22 % of OMC for alkaline soil sample 6 when treated with lime content of 4 %. The above implies that maximum cohesion and maximum friction are achieved at pH = 8.68. For other soil samples, the MDD decreased with increase in number of days for each of the pH conditions. This indicated that the particles of the soil samples tested were being disintegrated. This reduction in the density of the soil had an impact on the strength of the soil. Typical example for the latter is alkaline soil sample 4 of pH = 9.20, with an MDD of 1591.53 kg/m³ at 18 % of OMC. The CBR for treated soil samples compacted at 25 compactive efforts recorded lower CBR values compared to the soil samples compacted at 55 compactive efforts. Alkaline soil sample 6 of pH = 8.68 indicated constant increase in CBR for respective penetration depths of 2.54 mm, 5.08 mm and 7.62 mm. The constant increase in CBR for alkaline soil sample 6 compacted at 55 compactive efforts was a direct indication of the principal chemical reactions that took place during the lime-soil stabilisation process. In order to investigate the effect of pH on shear strength of treated soil, UCS geotechnical properties of the soil samples of different pH values (i.e. pH = 4.02, 4.92, 5.55, 9.20, 8.27 and 8.68 for sample 1 to 6 respectively) were determined using the UCS machine. Based on the results of UCS strength tests, it was found that the UCS geotechnical properties increased considerably if the soil had a high pH (pH = 8.68 for alkaline soil sample soil 6 and pH = 8.27 for alkaline soil sample 5). At an alkaline pH, the increasing of ionic strength favoured face-to-face aggregation of the soil samples. The UCS geotechnical properties decreased considerably when the pH values were less than pH = 8. This was particularly evident for acidic soil samples one of pH = 4.02, soil sample 2 of pH = 4.92 and soil sample 3 of pH = 5.55. The soil pH had no significant or direct impact on the microstructural studies of the soil tested, but the microstructural properties analysis helped ascertain which elements were found in the soil. Tests used were X-RD, SEM and energy dispersive x-ray spectroscopy (EDS/EDX). The elements found were magnesium (Mg), potassium (K), calcium (Ca), titanium (Ti), iron (Fe), aluminium (Al), silicon (Si) oxygen (O) and carbon (C). Some of the soil elements found and evaluated were significantly influenced by the usage of lime. The findings of X-RD, SEM were influenced by the source of the soil. Other contributory factors that impacted negatively the geotechnical properties of the soil tested related to the loss of the cementitious elements in both the acidic (pH < 7) and alkaline (pH > 7) soil samples. Therefore, the conclusions and recommendations include the following but are not limited to: the need for a more effective approach to reviewing designs and construction procedures for soil stabilisation; the need to achieve minimisation of carbonation to suit the intended results; and the need to take into account the pH of the soil when designing structures.