Reappraisal of Non-Plastic Soils

Advanced Engineering Solutions Journal Vol 2/2022 REAPPRAISAL OF NON-PLASTIC SOILS Gerald Maregesi Email: [email protected] ABSTRACT with a low plasticity index. In contrast, the fall cone underestimates the liquid limit due to reduced friction for soils with high plasticity [1]. In many cases, the liquid limit of the soil can be determined using the fall cone method, yet the soil is reported as non-plastic. This study suggests that if the liquid limit of the soil can be determined using the fall cone method, then soil possesses some plasticity which cannot be quantified using a conventional method of thread rolling. In this paper, the plasticity index of the alleged non-plastic soil is computed using a mathematical model. It is common in particular when using the fall cone method to determine the liquid limit of the low plasticity soil, yet, the soil is reported as non-plastic because the plastic limit of the soil could not be determined. Therefore an alternative procedure for determining the plasticity index of the low plasticity soil is required to avert the problem of falsely classifying the soil of low plasticity as nonplastic. This paper presents the computed plastic limit of the alleged non-plastic soil using a mathematical model. The results suggest that if the liquid limit of the soil can be determined using the fall cone method, then the soil possesses some plasticity which cannot be quantified due to the limitation of the current method of determining the plastic limit of the soil using the convention method of thread rolling. INTRODUCTION The soil is customarily classified as non-plastic when its liquid limit cannot be determined using the percussion method or its plastic limit cannot be determined using a conventional method of soil thread rolling [5,6,7]. In the percussion method of determining liquid limit, the soil with a low liquid limit tends to slide and therefore does not exhibit plastic or viscous flow. Thus, the percussion method has limitations in determining the liquid limit of low plasticity soil. The fall cone method was developed specifically to address this shortfall of the percussion method to determine the liquid limit of the low plasticity soil. Similarly, the current method of determining the plastic limit cannot be used for the determination of plastic limits for soils of low plasticity. The low plasticity soil has negligible adsorbed water, making it difficult or impossible to form the 3 mm ellipsoidal thread during rolling. LIQUID LIMIT FLOW CURVE The British fall cone moisture-penetration relationship can be modelled reasonably well using a sigmoidal mathematical function, as shown in equation (1) [2]. ℎ= 𝑥 1 + 𝐿𝐿 40 ( . ) … … (1) Where 'h' is the penetration depth at any moisture content, 'x' is the moisture content, 'm' is the average slope of the flow curve within the moisture-penetration range of 25-5 mm, and LL is the liquid limit. Furthermore, the plastic limit of the soil can be computed using parameters of the fall cone liquid limit flow curve, namely liquid limit and average slope, using equation (2). The plastic limit is the The fall cone method requires the liquid limit to be determined at 20 mm penetration, implying that even if the soil has a low liquid limit, it is possible to establish the moisture content equating to 20 mm penetration. However, the limitation of this method is that it measures both undrained cohesion and undrained friction. Therefore, the fall cone method has been found to overestimate the liquid limit due to reduced cohesion for soil 1 Advanced Engineering Solutions Journal Vol 2/2022 moisture content corresponding penetration value of 1.2 mm [2]. 𝑃𝐿 = 𝐿𝐿 𝑒 . to the The beach sand has low shear strength because it is finer than crusher sand and crushed coral rock, and its grain shape is wellrounded. … … (2) The plasticity index is the arithmetic difference between the liquid and plastic limits. Therefore, the plasticity index of the soil can be computed using equation (3). 𝑃𝐼 = 𝐿𝐿(1 − 𝑒 FALL CONE TEST COHESIONLESS SOILS . ) … … . (3) RESULTS ON The determination of the liquid limit using the fall cone method is a strength-based test. The average undrained shear strength of the soil at the liquid limit has been reported to be 1.7 kPa [3]. The plastic limit is also associated with the soil's strength being 100-fold that of the liquid limit, i.e. 170 kPa, suggesting that the plastic limit can also be treated as a strength-based test [4]. Figure 1: the moisture-penetration relationship for non-plastic soils PLASTIC LIMIT TESTING PROCEDURES The standard procedure for testing the plastic limit is based on rolling back and forward the ellipsoidal thread to 3 mm until it crumbles. For the soil to be rolled into a thread of 3 mm, the soil must contain clay minerals within the soil matrix. The plastic limit value depends mainly on the amount of adsorbed water in the soil. The amount of adsorbed water in the low plasticity soil is negligible. Therefore, the low plasticity soil cannot physically retain moisture against the applied palm force during rolling thread operations. The presence of clay minerals within the soil matrix is associated with the presence of adsorbed water. The adsorbed water around soil grains consumes a considerable amount of void space, hence reducing the permeability of the soil. Due to the reduction in effective pore space, water is not free to move through the soil, allowing the soil to be rolled into 3 mm thread. Therefore, for the soil ellipsoidal thread to remain intact during the plastic test, tensile failure of the soil must be prevented in both longitudinal and transverse directions. The failure is prevented by the cohesion of soil which is imparted to the soil with the presence of clay minerals within the soil matrix. For the soil of low plasticity, it The soil's liquid limit determination using the fall cone method can be treated as an undrained shear strength test since, during testing, the water in the soil has no time to flow out of the pores. Therefore, the fall cone measures the soil's undrained cohesion and undrained friction resistance. For cohesionless soil, the fall cone measures the frictional resistance only since there is no clay mineral within the soil matrix to impart cohesion. Figure 1 shows a moisture-penetration relationship for three non-plastic soils: beach sand, crushed coral rock flour, and crushed granitic gneiss crusher dust. Contrary to the soil exhibiting plasticity, the data indicates that the penetration of cohesionless soil decreases as the water content increases, implying that the shear strength of the cohesionless soil increases as the water content increases. The shear strength of the cohesionless soil increases because, at low moisture content, the suction is high; therefore, the shear strength of the soil increases. Based on the resistance to penetration, it can be seen that the crushed gneiss has higher shear strength than both crushed coral rock and beach sand. 2 Advanced Engineering Solutions Journal Vol 2/2022 is difficult or impossible to roll it into a thread of 3mm. ANALYSIS OF THE SOIL REPORTED AS NON-PLASTIC SAMPLES For soil with some plasticity, the shear strength of the soil decreases with an increase in water content, i.e. the penetration increases as the water content increases. Figure 2 shows a moisture-penetration relationship for the soil sample whose fall cone liquid limit is 23.4. The plastic limit could not be determined using the conventional thread rolling method; therefore, the soil was reported as non-plastic. The plastic limit of this soil was computed using equation (2), and its plastic limit was found to be 19.8, which corresponds to the plasticity index of 3.6. The flow curve slope indicates that a 7.6 mm change in penetration value is associated with a mere 1% change in moisture content. Figure 3: the relationship between the slope of the flow curve, liquid limit and computed plasticity index SUMMARY AND CONCLUSION The plastic limits of 32 soils with fall cone liquid limits of 19-31% were reported as non-plastic. The plastic limits of these soil were computed using equation (2). The summary of plastic limit and plasticity index is shown in Figure 3. The computed plasticity index for the soils reported as non-plastic was found to be in the range of 2.84-9.19%, with an average value of 6.5%. It has been demonstrated through this study that when the liquid limit for the soil is determinable using the fall cone method, then the soil possesses some plasticity which cannot be quantified using the conventional method of thread rolling. The soil with zero plasticity index, an increase in water content is not associated with an increase in penetration value, suggesting that the shear strength of the soil does not decrease with an increase in moisture content. References: 1. Prakash, K, Sridharan, A, Critical appraisal of the cone penetration method of determining the soil plasticity, Canadian Geotechnical Journal 43; 884-888, 2006 2. Maregesi, G.R, Computation of plastic limit using mathematical model, Advanced Engineering Solutions Journal Vol2/2022, Journal of Civil Engineering and Construction technology. 3. Wood, D.M. (1985), Index properties and consolidation history, Proc, 11th international conference on soil mechanics and foundation engineering, San Francisco, 703-706. Figure 2: flow curve of soil reported to be nonplastic; the model computed the plasticity index of soil to be 3.6 3 Advanced Engineering Solutions Journal Vol 2/2022 4. Sharma, B Bora, P.K (2003), plastic limit, liquid limit and undrained shear strength of soil – Reappraisal, Journal of Geotechnical and Geoenviromental engineering 5. AASHTO T90-00 (2000) Procedures for Determining Liquid Limit, American Association of State and Transportation official. 6. BS 1377: Part 2: (1990): Soil classification test, British Standard Institution 7. AASHTO T89-02 (2002) Standard Procedure for Determining Plastic limit, American Association of State and Transportation official. 4