Investigation of Pindan Soil Modified with Polymer Stablisers for Road Pavement

Road failures are often caused by structural weaknesses, and particularly unsealed roads are vulnerable to water as water easily flows into road structures. Moisture susceptibility of materials is an important aspect when pavements are designed as moisture can weaken bonds between aggregates. Pindan soil is a red soil, known as a soft and moisture sensitive soil. Polymer stabilisers have been proved that they can improve soil mechanical properties by providing an internal waterproofing. Studies of the polymer-Pindan soil stabilisation have been focused on engineering performances, but literature shows little information on the fundamental information of Pindan soil. This project focuses on fundamental information of Pindan soil and its improved performances using polymer stabilisers. Plastic index, specific gravity and particle size distribution were tested to obtain the basic properties. Compaction, Unconfined Compressive Strength and California Bearing Ratio tests were performed to determine the mechanical properties. The chemical property was examined using X-ray diffraction. Furthermore, the waterproof effect of the polymers on the stabilised Pindan soil was investigated from capillary rise tests. In addition, the mechanical properties of individual soil grains were investigated using nanoindentation tests. The materials used for this investigation primarily consisted of Pindan soil collected in Broome, Western Australia, and three polymer products manufactured in Australia. Based on the results, it is evident that the failure behaviour, strain and strength as well as the basic properties of the soils are affected and changed by the Polymer stabilisers. The type of polymer influenced the optimum moisture contents and strengths rather than the amount of polymer. Similarly, Nanoindentation technology provided various information such as elastic modulus, hardness, packing density, stiffness, cohesion and fracture toughness of soils at nano-scales. Polymers can reduce water ingress and minimise moisture in the pavement structures. Thus, the structures can maintain its strength and prevent deformation, which will increase the lifetime of unsealed pavements.


Introduction
under dry moisture conditions, which can be used as the material of the pavement layers but it is a challenge to select 10 the suitable Pindan soil for a pavement material due to its variability and difficulties in quality control. It is difficult to 11 detect the suitability based on a visual inspection and simple laboratory tests to use Pindan soil as a pavement material 12 [4]. 13 Using the fundamental characteristics of Pindan soil, which can provide good strength when subjected to relatively dry 14 conditions (i.e., moisture contents lower than its optimum moisture content), this could lead to a viable option on how 15 to improve Pindan soil fundamentally. Pindan soil has self-cementation capabilities in a dry condition, and when Pindan 16 soil can maintain its dry condition by not allowing them to be wetted, it can provide sufficient strength to use for 17 construction purposes [3,4,6]. With the advancement in soil stabilising technology nowadays, a preferred drying 18 condition of soils can be maintained by using stabilising agents such as a polymer, so-called, hydrophobic (water-19 repellence) properties [7]. Therefore, stabilisation of moisture-sensitive soils such as Pindan soils with polymers has 20 been developed and expanded. Based on recent studies, polymers for soil stabilisation have a high resistance to water 21 and excellent physical properties. 22 With the development of polymer technology in which waterproofing properties can be created, wetting problems of 23 Pindan soil could be fundamentally resolved by using polymer stabilising binders as an internal waterproofing [7]. Since 24 polymers act as a method of coating the aggregate with a polymer film, each ability of the polymers have an important 25 effect on strength improvement and physical bonding [8]. However, most of the Polymer-Pindan soil stabilisation 26 studies have been focused on engineering performances of the stabilised soil, so information on Pindan soil properties 27 for road pavement is still limited. Correctly identifying Pindan soil properties, and the chemical and physical bonding 28 mechanisms associated with polymer stabilisers are significantly important to improve the performance of Pindan soil 29 and road conditions. The Pindan soil has been used as a pavement material in Western Australia, although limited 30 information exists with Pindan soil properties for road pavement. Therefore, this study aims to explore an opportunity 31 in stabilising Pindan soils with potential polymers in order to fundamentally prevent a wetting condition by creating a 32 hydrophobic property through Pindan soils-polymer mixtures. If the problems of Pindan soils are identified and solved 33 with polymers, it can be applied to other moisture-sensitive soils. This study examines the properties of Pindan soils to 34 determine mechanical properties and evaluates the stabilisation to improve the performance of the Pindan soil using 35 polymer stabilisers. 36

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The experiment consisted of producing a number of Pindan soils-polymer mixture batches with varying polymer 38 contents and curing times. There are several important processes for sample testing; selection of suitable polymers, an 39 appropriate amount of polymer and water, the mixing and curing process, and appropriate methods for testing of 40 stabilised materials. The laboratory experiment consisted of producing a number of Pindan soil-polymer mixture 41 batches with varying polymer contents and curing times. The materials used for this investigation primarily consisted 42 of Pindan soil collected in Broome, Western Australia, and three polymer products manufactured in Australia. The 43 Quantitative-XRD (QXRD) analysis for Pindan soil showed the following results as shown in Table 1 The information of the polymers is provided by suppliers in Table 2. Polymer A consists of hydrated lime and cationic 51 polymer. Polymer B and C are a polyacrylamide polymer and a styrene-acrylate copolymer. Polymer A and B have 52 been used in the field since they are manufactured to use for soil stabilisations. Polymer C does not have a protocol for 53 soil stabilisation as it is used for a raw material binder and has not been used as a soil stabiliser. The activity range zone 54 of the polymer C was selected in comparison with the maximum dry density of Pindan soil of 18.74 kN/m 3 [9]. 55 The Pindan soil was treated by adding polymer A by weights of 1%, 2%, and 3% of the soil. For polymer B and C, 60 solutions of a mixture of polymer and water were created in required polymer concentrations and then the soil was 61 mixed with the solutions. Three different proportions of polymer B and C were added to the soil. The polymer B was 62 added using ratio by weights of 0.001%, 0.002% and 0.003% of the soil. The waterproofing effect of polymers is one 63 of the important factors to improve the performance of the Broome-Pindan soil in wet conditions. Capillary rise test on 64 the compacted soil is a simple method to assess the waterproofing effect of the polymers. Thereby, the Pindan soil was 65 treated with polymers A, B and C at the rate of 2%, 0.002% and 0.7% by weight, respectively. The samples were 66 compacted to 98% of the OMC using the modified proctor compaction method and cured for 16 days in a humidity 67 cabinet in the temperature range of 21°C to 25°C at 90% humidity. The compacted samples were placed in 10 mm deep 68 of water at room temperature for 72 hours. 69 To obtain the mechanical properties of polymer-modified Pindan soils, unconfined compression strength tests (UCS) 70 and California bearing ratio (CBR) tests were performed using the modified compaction method based on Standards 71 Australia [10][11][12]. In order to evaluate the mechanical behaviour and to determine the mechanical properties of 72 individual soil grains, nanoindentation is considered one of the best techniques that can be used to obtain the mechanical evaluate physical properties on a small scale. Indentation testing is performed essentially by touching the material 75 whose mechanical properties are not known, such as hardness and elastic modulus, by using other materials whose 76 properties are known [13]. The nanoindentation instrument is recently accepted as a standard test process for the 77 characterisation of the physical properties of materials [14]. An advantage of the indentation test is that the material 78 can be characterised based on the indentation load and depth of the material during loading and unloading. Thus, 79 nanoindentation tests using Berkovich indenter tip was conducted on individual Pindan red soils from Broom, Kimberley 80 Region of Western Australia. A sample of soil was cast with an epoxy matrix then was ground and polished to reduce 81 the surface roughness [15]. For the measurement of hardness and elastic modulus of soil grain, nanoindentation test 82 was carried out with XP system with Poisson's ratio of 0.25 was assumed [16]. An application of the nanoindentation 83 technique was successfully made for the microporomechanics of Pindan soil, and a comprehensive variation behaviour 84 in the Pindan soil was observed. A number of researchers [2,4,6] have written about the behaviour of Pindan soils 85 when their moisture contents rise, and there have been several conflicting results. 86 ASTM Standards [17][18][19][20] as shown in Table 3. 92

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The CBR values for the unsoaked and soaked samples and the swelling percentage of the soaked samples were tested. 96 The relative compaction was to be 95% to 98% of maximum dry density for all CBR tests. The CBR tests on the 97 unsoaked Pindan soil samples result in the range of CBR 11.01 -12.72% for G.P samples and CBR 11.82 -12. The information on the Pindan soils used in the nanoindentation tests is shown in Table 4. According to the 106 deconvolution technique from the literature [16], the elastic modulus and hardness values for Pindan soil were observed 107 as 68.1 ± 12.7 GPa and 10.6 ± 0.9 GPa, respectively, as presented in Figure 1. The elastic modulus of Pindan soil, 108 however, is lower than the elastic modulus of quartz, which has an elastic modulus of around 124 GPa based on an 109 indentation test. 110 Similarly, the stiffness and hardness-packing density scaling were tested on Pindan soil [16]. The results of soil particle 114 properties show that stiffness and cohesion were 92.7 GPa, 4.1 GPa, respectively. The packing density of the sample 115 was determined as 0.863 ± 0.032, as shown in Figure 2. From the results of the packing density distribution, Pindan soil 116 yields total porosity ξ = 0.137. This way of determining the porosity using a statistical technique provides a new non-117 invasive approach which, otherwise, would be difficult to estimate the porosity of ground materials using a classical 118 method [21]. The indentation fracture toughness was also investigated, the median value of the fracture toughness of 119 Pindan soil was obtained as 3.7 ± 0.5 MPa m 1/2 as shown in Figure 3. The results of energy transferred fracture toughness 120 of Pindan soil was obtained as 3.1 ± 0.8 MPa m 1/2 . The approach of using energy transferred fracture toughness was 121 able to extract the fracture toughness of indentation test results. the compacted mixtures with Polymer A, B and C, respectively. Polymer A decreased capillary rise rate and reduced 131 moisture sensitivity significantly. When sample B was placed in water, it seemed to have some water effect on the 132 surface, such as a slight collapse, but after that, the surface of sample B was no longer affected by water. The untreated 133 sample and sample C were risen the water similarly and fully saturated after around 2.5 hours from the start. The 134 untreated sample was completely collapsed after the fully saturated point. All treated samples remained unchanged and 135 seemed to maintain some strength as well as shape. Swelling (S) does not appear on the treated samples after immersion, 136 and the untreated sample could not be measured as the sample shape was collapsed after saturated. It has been found 137 that the polymer could provide sufficient water-resistance to the soil during long-term exposure to water. From the 138 results, it clearly shows the capillary rise rate of samples B and C was higher than the untreated sample and the sample 139 A, as shown in Figure 4. Polymers B and C filled the void spaces and thereby reduced the void size of the treated 140 samples. Therefore, since the size of the voids became thinner, and the rate of the capillary rise was increased. On the 141 other hand, the capillary rise did not occur very much, and it went down to the original water position again for sample 142 A. Sample A almost did not change at all and maintained the sample in a dry condition. Lacey [7] explained that polymer 143 A reacts with water and soils to create a hydrophobic soil matrix between the soil particles, limiting water penetration. 144 It could be because the hydrated lime, Ca(OH)2 was reacted with water. The pozzolanic reaction occurs between the 145 free calcium (Ca 2+ ) and dissolved silica and alumina from the clay. When the reaction happens slowly at a high pH of 146 around 12. ratio tests were performed to evaluate and compare material properties and characteristics with respect to polymer soil particles by applying forces. The reduction of pore space is accompanied by an increase in soil density, and 157 in laboratory conditions, an increase in soil density is generally seen as an increase in soil strength due to adding 158 more soil. Figure 5 illustrates the compaction curves within the same scale to compare the untreated and treated 159 samples. Table 5 provides the optimum moisture contents (OMC) and maximum dry density (MDD) values obtained 160 from the moisture-density relationship curve of the compacted samples. As can be seen in Figure 5, the effect of adding 161 the polymers on the moisture-density relationship for the Pindan soil can be assessed through the compaction test results. 162 Comparing the OMC and MDD between the untreated sample and all mixtures, all polymers reduced OMC and 163 increased MDD values. The mixture can reduce the required water content to achieve the MDD, which is desirable for 164 the construction of the Kimberley region. However, the dry unit weight was getting rapidly reduced after each OMC 165 and the dry density decreased to or below the dry unit weight of the untreated sample. Each of the polymers maintained 166 a graph of the same pattern at different doses. The increase in density is probably because the polymers filled the pore 167 space and reduced the porosity of the treated samples. The untreated Pindan soil recorded a maximum dry unit weight 168 at a moisture content of 9.4%. When the moisture content is below 9.4%, the compaction is interrupted because of the 169 high friction between the soil particles and the moisture content interferes with the compaction due to the high pore 170 water pressure after 9. MDD for the treated samples after the OMC, which was equal or more reduced than the untreated samples decreased. 179 After the OMC, the reaction of water and polymer occurs, but the polymer might leak out with the water during or after 180 the compaction process. Another reason is that after the polymers fill the void spaces between the soil particles, these 181 materials, which have smaller densities than the density of the soil, could replace the small particles of the sand or clay 182 to reduce the density rapidly. 183   Table 6 for each sample.

California bearing ratio 218
The CBR test is generally used to evaluate the subgrade strength of roads, and its value can be used to determine the 219 thickness of pavement layers. The unsealed pavement design should use the lowest CBR values, mostly from soaked 220 samples. The results of the CBR test for unsoaked and soaked conditions are presented in Table 7, and the typical UCS 221 graphs for samples with and without polymer stabilisers in unsoaked and soaked conditions are presented in Figure 7, 222 respectively. In the CBR test, when mixing the polymer C with Pindan soil, the moisture content should not exceed the 223 optimum moisture content. When the amount of water greater than the optimum moisture content was added, the density and soaked conditions. The 1% content of the polymer A did not affect the CBR value, which may increase over time, 231 as shown in the UCS results. The reaction of the polymer might not be started and the reaction would gradually take 232 place over time, which also applies to all polymers. The polymers might need more time to react to increase the CBR 233 value. Polymer A showed higher CBR value as the amount of polymer increased, whereas the CBR value decreased as 234 the polymer amount of polymer C increased. For polymer B, it had the highest CBR values when the polymer ratio was 235 0.002% in both unsoaked and soaked conditions. 236 stabilised base-course maintains its strength and prevents the deformation of subgrade structures, which increases 246 the lifetime of unsealed pavements. Polymer B also has economic benefits in road design by stabilising the soil 247 and increasing its strengths. 248

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This paper presented the fundamental properties of Pindan soils and stabilisation to improve its performance using 250 polymer stabilisers. Pindan soils were tested according to classification categories based on basic physical 251 characteristics such as index properties and particle size distribution, and they were classified as silty sand (SM) and the 252 plasticity index was non-plastic. The Pindan soils also showed similar mechanical property values according to the 253 compaction and CBR tests. Similarly, nanoindentation technology provided various information such as elastic 254 modulus, hardness, packing density, stiffness, cohesion and fracture toughness of soils at nano-scales. In addition, in 255 UCS test, when compared to the performance of soil with no stabilisers in one hour curing condition, it appears 256 that all treated samples provide higher strength and strain. Based on Compaction and CBR test results, all the tested 257 Pindan samples did not exhibit any moisture-sensitivity behaviour, thereby the Broome-Pindan soil can be used as 258 a road material for the base, subbase and subgrade pavement structures in both dry and wet conditions. The 259 capillary rise test proved that polymer stabilisers have a high resistance to water and can play the role of 260 waterproofing. Each of the polymers showed different mechanical properties and material failure modes. It is 261 recommended that polymer C is not to be used as a road stabiliser because the bonding is weak to water and 262 significantly reduces the CBR values. Polymer A, on the other hand, is resistant to water and reduces water ingress, 263 thus stabilising the pavement structures to maintain strength, increasing the life of the unsealed pavement. Polymer 264 B also stabilises the soil well and increases its strength even when the amount of polymer is changed. 265

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