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Procedia Engineering 189 ( 2017 ) 25 – 32
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1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the scientific committee of the International conference on Transportation Geotechnics and Geoecologydoi: 10.1016/j.proeng.2017.05.005
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Transportation Geotechnics and Geoecology, TGG 2017, 17-19 May 2017, Saint Petersburg, Russia
Polymers for stabilization of soft clay soils
M. Mirzababaeia,*, A. Arulrajahb, M. Oustonc aCentral Queensland University, Melbourne, Victoria 3000, Australia
bSwinburne University of Technology, Hawthorn, Victoria 3122, Australia cCentral Queensland University, Rockhampton, Queensland 4702, Australia
Abstract
In this study, the influence of two chemical additives, (i.e., poly(vinyl alcohol), PVA and 1,2,3,4 Butane Tetra Carboxylic Acid, BTCA on the engineering properties of an expansive clay soil is investigated. The effect of polymers on the unconfined compression strength of soil samples prepared at maximum dry unit weight (i.e., 16.2 kN/m3 and 17% water content) and a lower dry unit weight (i.e. 10.8 kN/m3 and 48% water content) was evaluated. PVA and BTCA added at dosages of 0.1% to 1.5% and 0.1% to 0.5% respectively to both compacted soil samples and cured for 1 and 14 days. The results of unconfined compression tests on clay soil samples stabilized with different PVA and BTCA contents cured for 1 and 14 days indicated that such hydrophilic polymers improve the compression strength of both dense and soft clay soils significantly and their strength even increases with curing time. However, the efficiency of the additives is highly dependent on the unit weight of the soil. Furthermore, the durability of stabilized samples was also examined using soaking tests and results revealed that these polymers improve the durability of clay soils once they are soaked under water.
© 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International conference on Transportation Geotechnics and Geoecology.
© 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International conference on Transportation Geotechnics and Geoecology.
Keywords: Soft clay soils; Unconfined compression strength test; Strain energy; Poly(vinyl alcohol); 1,2,3,4 Butane Tetra Carboxylic Acid
© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the scientific committee of the International conference on Transportation Geotechnics and Geoecology
26 M. Mirzababaei et al. / Procedia Engineering 189 ( 2017 ) 25 – 32
1. Introduction
The stability of the embankments is highly vulnerable to the presence of weak water-stable clay aggregates that may result in erosion during wet season [1]. Therefore, highways, railways and other engineering structures founded on soft soils may suffer an excessive settlement in wet season if adequate improvement measures are not sought. Fortunately there are a number of reported and well documented soil stabilization/reinforcement techniques that can mitigate the adverse effect of problematic soils (especially expansive clays and soft clay soils). Lime, fly ash and cement are amongst the traditional soil stabilizing agents that are widely popular between practicing geo-engineers to suppress the swelling properties and increase the compressive strength of expansive soils [2,3,4]. Such additives generally contain calcium that flocculate clay particles by balancing the electrostatic charges of the clay soil particles and by reducing the intra-particle’s repulsive electrochemical forces. This leads to adhesion of clay particles together and forming flocculated particles with improved engineering properties including higher strength, lower plasticity, increased workability and alleviated swelling pressure [5,6,7]. The abovementioned chemical reactions occur in two phases, with both immediate and long-term (i.e., pozzolanic reaction) improvement in soil strength [8].
Nontraditional stabilizers have also become increasingly available for civil and military applications. These include salts, acids, enzymes, lignosulfonates, petroleum emulsions, polymers, and resins. These chemicals improve the engineering behavior of soil by forming physical/chemical bonds with clay minerals within the soil. Polymers have been reported to develop a substantial strength improvement effect in clay soils due to formation of bond between clay minerals and polar end groups of polymer. There are also several commercial chemical stabilizers that are claimed to increase the strength with less curing time and higher durability compared to traditional stabilizing additives. However, the constituents of these chemicals are typically neither disclosed by the supplier nor documented elsewhere.
Ajayi-Majebi et al. [9] reported the improving effect of an epoxy resin (bisphenol A/epichlorohydrin) and a polyamide hardener on clay-silt soils. In their study, 3-day cured stabilized soil samples with 4% additives showed significant increase in unsoaked California Bearing Ratio (CBR). Tingle and Santori [10] reported successful application of lignosulfonate and synthetic polymers for improving the unconfined compression strength of both lean and fat clay soils. Page [11] reported significant benefit of spraying poly(vinyl alcohol) to prevent crust formation and enhance the stability of clay soils subjected to simulated heavy rain. Mirzababaei et al. [12] also investigated the effect of two polymers including 3 to 10% poly(methyl methacrylate) and 1 to 3% poly(vinyl acetate), respectively on the free swell potential of 3 different fat clay soils. They reported significant reduction in free swell potential and formation of aggregated clay-granular matrices with the addition of polymers. This article discusses the application of poly(vinyl alcohol) (PVA) as a polymeric binder for stabilizing soft clay soils. PVA is the largest water-soluble biodegradable polymer chain that has excellent film forming and adhesive properties [13]. It is also resistant to grease, oil, and solvent. It is highly hydrophilic and PVA solutions can be prepared easily by dissolving PVA in water [14]. On the other hand, water based polymers like polyvinyl alcohol (PVA) are known to be eco-friendly (i.e., nontoxic and odorless) and are widely used in cosmetics. PVA is a non-ionic water-soluble polymer possessing high ability of aggregating clay soils [15]. However, PVA-encapsulated clay particles are still vulnerable to water and may tend to disperse with increase in moisture content of the soil. Cay and Miraftab suggested to make PVA, water insoluble using a crosslinking method such as freezing/thawing, methanol treatment, chemical crosslinking, or irradiation. In their study, 1,2,3,4 butanetetracarboxylic acid (BTCA) was used to crosslink PVA to form hydrogel structures that are three-dimensional hydrophilic polymer networks capable of absorbing large amounts of water. The results of their research indicated that crosslinking with BTCA improve the water stability of PVA membranes and make them resistant after water treatment.
In this study PVA was used as the main stabilizing additive along with 1,2,3,4-Butanetetracarboxylic acid (BTCA) as crosslinking agent. A series of unconfined compression strength tests was carried out on highly plastic clay soil samples treated with proportionate quantities of PVA and BTCA. A number of samples also cured for 14 days to investigate the effect of stabilizing additives on the unconfined compression strength of the clay soils in the long-term. Samples with optimum PVA/BTCA contents also soaked in water to investigate the stability of stabilized samples subjected to excessive wetting.
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2. Materials
The soil sample used in this study was collected from Sarina Township in Central Queensland of Australia. The sample is classified as highly plastic clay, according to the Unified Soil Classification System (USCS) with an activity ratio of 0.50 and specific gravity of 2.71. The grain size analysis of this soil indicated that it contains 93.3% clay particles with liquid limit and plastic limit of 74% and 27%, respectively. The maximum dry unit weight and optimum moisture content of the soil were determined using the modified Proctor compaction test. The engineering properties of the clay soil used in this study have been presented in Table 1. Table 2 shows the properties of the chemical additives used in this study.
Table 1. Soil properties.
Soil Classification CH
Specific gravity Gs 2.71
Activity 0.50
Grain size analysis Sand (%) 3.60
Silt (%) 3.10
Clay (%) 93.30
Atterberg limits Liquid limit WL (%) 74.00
Plasticity index IP (%) 47.00
Compaction characteristics
Maximum dry unit weight (kN/m3) 16.20
Optimum moisture content (%) 16.80
Swelling properties at
Swelling pressure (kPa) Free swell (%)
218.50 11.50
Table 2. Properties of chemical additives.
Poly(vinyl alcohol) ([-CH2CHOH-]n)
Molecular weight (g/mol) 130,000
Specific gravity Gs (at 25 °C) 1.26
Viscosity 4% in H2O (20 °C)(mPa.s) 16~20
1,2,3,4-Butanetetracarboxylic acid (C8H10O8)
Molecular weight (g/mol) 234.16
3. Experimental program
To investigate the efficiency of the chemical additives used in this study on the compression strength of the clay soils with different initial void ratios, soil samples compacted to two different dry unit weights (i.e., 16.2 kN/m3 and 10.8 kN/m3). Figure 1 shows the selected unit weight/water content pairs on the modified Proctor compaction curve of the soil. A set of UCS tests designed respectively to determine the optimized amount of PVA and BTCA for stabilizing the soil samples at pre-determined unit weights.
In the first stage, the unconfined compression strengths of non-treated soil samples prepared at dry unit weights of 16.2 kN/m3 and 10.8 kN/m3 with no curing and also 14-day curing were determined. In the next stage, to optimize the PVA content, a series of unconfined compression strength tests was carried out on soil samples treated with 0.1%~1.5% PVA with no curing and also 14-day curing for both compacted samples at dry unit weight of 16.2 kN/m3 and 10.8 kN/m3. In the final stage, to investigate the effect of BTCA on the unconfined compression strength of PVA-treated soil samples, proportionate amount of BTCA was added to the optimized treated samples based on the results of the previous stage. In this study, the amount of PVA and BTCA to reach the desired contents were determined based on the solution in a liter of distilled water and the solution used to prepare soil samples with pre-determined moisture content.
28 M. Mirzababaei et al. / Procedia Engineering 189 ( 2017 ) 25 – 32
Figure 1. Modified Proctor compacion compaction curve of soil.
4. Results and discussion
4.1. Effect of additives on the compression strength of dense samples
Figure 2 shows the stress-strain behavior, strain energy and UCS values of non-treated and treated clay soil samples with dry unit weight of 16.2 kN/m3 (i.e., initial void ratio of 0.64 and =16.8%). The strain energy at peak is defined as the required energy to deform the specimen to the strain equivalent to peak stress and is calculated from the area under the stress-strain curve up to the peak value [16,17]. The UCS and peak strain energy of non-treated soil increased slightly with 14-day curing period (Figures 2a&b). Figures 2c&d show the stress-strain behavior and peak strain energy of treated soils with 0.1%, 0.3%, 0.5%, 1% and 1.5% PVA without curing and 14-day curing period. Treatment of soil samples with dry unit weight of 16.2 kN/m3 with PVA without curing decreased the UCS of the treated soil samples. This is due to excessive increase in lubrication effect of soil particles without necessarily let the additive create strong bonds between adjacent particles. However, the UCS of samples subjected to 14 days of curing increased by 29% from 549.5 kPa (i.e., non-treated sample) to 710.1 kPa (i.e., treated sample with 1% PVA). The development of strain energy with addition of PVA to 1% in cured samples also showed the increase in ductility of treated samples. Therefore, it was decided to maintain the PVA content to 1% for samples with dry unit weight of 16.2 kN/m3 and optimize the BTCA content in the next stage. Figures 2e&f show the stress-strain behavior and strain energy of soil samples treated with 1% PVA and 0.1%, 0.3% and 0.5% BTCA cured for 14 days. The addition of BTCA in this study was only for crosslinking PVA polymer to make it insoluble in water when the treated soil is exposed to soaking condition. Therefore, the strength gain was not meant for addition of BTCA to the treated soil samples. As it is depicted in Figures 2e&f, both UCS and peak strain energy of PVA treated samples increased with increase in BTCA content to 0.5%. Therefore, for samples with dry unit weight of 16.2 kN/m3 the optimum PVA and BTCA contents were found to be 1% and 0.5%, respectively.
4.2. Effect of additives on the loose samples
Figure 3 shows the stress-strain behavior, strain energy and UCS values of non-treated and treated samples with dry unit weight of 10.8 kN/m3 (i.e., initial void ratio of 1.46 and =48%). The UCS and strain energy of non-treated soil were insignificant and did not vary with curing time (Figures 3a&b). Increase in PVA content to 1.5% without curing, increased the UCS of the soil by 6.1 times (Figures 3c&d). Curing for 14 days further improved the UCS and peak strain energy of treated soil with 1.5% PVA. The UCS of 14-day cured sample with 1.5% PVA increased from 6.2 kPa to 116.8 kPa (i.e., equivalent to 17.8 times). The peak strain energy of this sample also increased by 6.5 times. Therefore, to investigate the effect of BTCA addition on the UCS of samples treated with 1.5% PVA, further UCS tests carried out on samples treated with 0.1%, 0.3% and 0.5% (Figures 3e&f). However, addition of BTCA decreased the UCS of PVA treated samples significantly.
1011121314151617
0 10 20 30 40 50Dry
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ght (
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Moisture content (%)
(max)=16.2 kN/m3, e0=0.64, opt= 16.8%
=10.8 kN/m3, e0=1.46, =48%
29 M. Mirzababaei et al. / Procedia Engineering 189 ( 2017 ) 25 – 32
(a)
(b)
(c)
(d)
(e)
(f)
Figure 2. Stress-strain and UCS relationships for stabilised clay soil with unit weight of 16.2 kN/m3 (symbol format: e.g. 1P:1% PVA, 1BTCA:
0.1% BTCA, 1C: 1 curing day).
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0.1P 0BTCA 1C0.3P 0BTCA 1C0.5P 0BTCA 1C1.0P 0BTCA 1C1.5P 0BTCA 1C0.1P 0BTCA 14C0.3P 0BTCA 14C0.5P 0BTCA 14C1.0P 0BTCA 14C
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1P 1BTCA14C1P 3BTCA 14C1P 5BTCA 14C
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UCS (kN/m3)
30 M. Mirzababaei et al. / Procedia Engineering 189 ( 2017 ) 25 – 32
(a)
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Figure 3. Stress-strain and UCS relationships for stabilised clay soil with unit weight of 10.8 kN/m3 (symbol format: e.g. 1P:1% PVA, 1BTCA: 0.1% BTCA, 1C: 1 curing day).
4.3. Effect of soaking on the treated samples
For evaluating the stability of stabilized samples against water soaking, four cured soil samples including non-treated soil samples and treated soil samples with optimum content of PVA and BTCA with dry unit weights of 16.2
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0.1P 0BTCA 1C0.3P 0BTCA 1C0.5P 0BTCA 1C1.0P 0BTCA 1C1.5P 0BTCA 1C0.1P 0BTCA 14C0.3P 0BTCA 14C0.5P 0BTCA 14C1.0P 0BTCA 14C1.5P 0BTCA 14C
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1.5P 1BTCA 14C1.5P 3BTCA 14C1.5P 5BTCA 14C
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31 M. Mirzababaei et al. / Procedia Engineering 189 ( 2017 ) 25 – 32
kN/m3 and 10.8 kN/m3, respectively were soaked under the water. The stability of samples monitored visually with progress of time. Non-treated soil sample with higher dry unit weight disintegrated within 30 minutes of soaking followed by non-treated soil sample with lower dry unit weight after one hour of soaking. The treated samples last for four days without breaking down. At the end of fourth day, treated samples were removed from water and their UCS were determined accordingly. Figure 4 shows the condition of soaked samples at the time of soaking, 5 minutes and 4 days after soaking. Figure 5 shows the stress-strain behavior of the treated soaked samples after 4 days. It is evident that even after four days of soaking both mixes are still stable and they maintained some compression strength. However, the UCS of both mixes approached the same value irrespective of their initial dry unit weights.
(a)
(b) (c)
Figure 4. Stability of soaked samples a) soaking time: none b) soaking time: 5 minutes c) after 4 days.
Figure 5. Stress-strain behavior of soaked samples (after 4 days).
5. Conclusions
In this study a series of unconfined compression strength tests was carried out to investigate the effect of two chemical additives namely poly(vinyl alcohol) (i.e., PVA) and 1,2,3,4-Butanetetracarboxylic Acid (i.e., BTCA) on the compression strength of clay soils with relatively stiff state (i.e., dry unit weight of 16.2 kN/m3) and relatively soft state (i.e., dry unit weight of 10.8 kN/m3). The results showed that PVA and BTCA could improve the UCS and ductility of clay soils significantly. The optimum PVA content for soil stabilization was dependent on the dry unit
0
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1P 5BTCA 16.2D 1.5P 5BTCA 10.8D
Non-treated Dry unit weight: 10.8 kN/m3
1.5% PVA, 0.5% BTCA, Dry unit weight: 10.8 kN/m3
1% PVA, 0.5% BTCA Dry unit weight: 16.2 kN/m3
Non-treated Dry unit weight: 16.2 kN/m3
Non-treated Dry unit weight: 10.8 kN/m3
1.5% PVA, 0.5% BTCA Dry unit weight: 10.8 kN/m3
1% PVA, 0.5% BTCA, Dry unit weight: 16.2 kN/m3
Non-treated Dry unit weight: 16.2 kN/m3
Non-treated samples 1.5% PVA, 0.5% BTCA, Dry unit weight: 10.8 kN/m3
1% PVA, 0.5% BTCA, Dry unit weight: 16.2 kN/m3
32 M. Mirzababaei et al. / Procedia Engineering 189 ( 2017 ) 25 – 32
weight, initial void ratio and water content of the soil sample. Addition of 1% PVA led into moderate increase in the UCS of soil sample with dry unit weight of 16.2 kN/m3 (i.e., e0:0.64, ω=16.8%). Curing of soil samples for 14 days also improved the UCS and peak strain energy of treated samples. For samples with dry unit weight of 10.8 kN/m3 (i.e., e0:1.46, ω=48%), the UCS increased with PVA content to 1.5%. The UCS of these samples increased significantly from below 10 kPa to just over 116 kPa with addition of 1.5% PVA and 14 days curing period. Non-treated and treated samples with optimum additive contents were soaked in water for four days. Non-treated samples prepared at both initial void ratios broke down quickly within the first 30 minutes of soaking. However, treated samples could last for four days with a sound shape suitable for carrying out UCS tests. The results of this study showed that PVA as a nontoxic and organic copolymer can be used effectively for improving the strength of soft soils with high water contents. The mechanism of soil improvement with addition of PVA involves absorbing extra water in the soil by PVA polymers to form aggregated soil particles and hydrogel layers between particles instead of pore-water. BTCA used in this study was mainly for making PVA hydrogels insoluble in water and they did not participate in strength gaining process. This study is still in progress and the current article is based on the partial results at this stage.
References
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