Assessment of Durability of Chemically Stabilized Soils Using Different Moisture-Susceptible Methods

Abstract

The durability of chemically treated subgrade soils is important for pavement design life, and it is a major concern for transportation and defense agencies. Stabilized soil layers should retain strength and stiffness properties under wetting–drying-related durability cycles and moisture exposure. Many durability test methods, including wetting and drying, tube suction, and Engineer Research and Development Center’s wet-test methods, have been developed and used in several stabilization studies in the past. It is important to study and determine how these methods address the long-term performance of chemically stabilized soils with a focus on stabilizer permanency. Therefore, this research study conducted a comprehensive laboratory investigation using all three methods to assess the durability of stabilized soils as per the US Department of Defense protocols. Three types of soils, silty sand, clayey sand, and high-plasticity clay, and three types of stabilizers, vinyl acetate-ethylene (VAE) copolymer, Portland cement type I, and hydrated lime, were considered in this study. Silty sand, clayey sand, and high-plasticity clay soils were treated with VAE copolymer, cement, and lime, respectively. Results of cement-stabilized soil showed that it is unsusceptible to moisture fluctuations, whereas VAE-stabilized soil indicated its higher susceptibility to moisture variations. Both the ASTM wet–dry (W-D) test and tube suction test methods revealed that lime-treated clayey soil is moisture-susceptible. All three methods showed similar findings, with good agreement existing between tube suction and ASTM W-D tests for all combinations of soils and stabilizers considered in this study.

Keywords

durability moisture susceptibility polymer stabilization cementitious stabilization chemical stabilization subgrade

Sometimes, natural subgrade soils might be poor and may not have sufficient strength and stiffness properties to support the pavement structures. To enhance these properties, the chemical soil stabilization technique is commonly used by transportation and defense agencies to stabilize poor natural subgrade soils. Chemical stabilization with cement, lime, and fly ash, is often considered an economic solution for improving the strength and stiffness properties of weak natural soils (1 –9). Alternative to traditional stabilizers, non-traditional stabilizers such as polymers, enzymes, petroleum emulsions, lignosulfonates, and resin have also been utilized for improving material properties in the past few decades (10 –15). Although chemical stabilization methods improve strength and stiffness properties in normal conditions, they may or may not pass the durability tests. The durability of chemically stabilized soils is an important assessment for the mix design of chemical stabilization and is a concern for highway and defense agencies as this assessment is linked with the permanency of stabilizer treatment (6, 8, 17 –20).

Dempsey and Thompson defined durability as “the ability of materials to retain their stability, integrity and maintain an adequate amount of long-term residual strength to provide sufficient resistance to changes in climatic conditions” ( 16 ). Several researchers have studied the durability of chemical stabilizers such as cement, lime, fly ash, and polymer under different conditions like wetting–drying, freezing–thawing, moisture susceptibility, and leaching ( 1 , 6 , 12 , 19 –22). Stabilized materials should be able to withstand different environmental conditions caused by seasonal changes, such as wetting–drying or freezing–thawing, that affect their strength and stiffness properties ( 6 ).

The permanency of chemical stabilization methods depends on the ability of stabilizers to bind soil particles together and hold them together for the long term. The long-term performance of stabilizers is affected by several factors such as climatic conditions (wet–dry or freeze–thaw), water intrusion, and leaching. Several researchers reported that the long-term performance of chemical stabilization of soils is primarily affected by moisture ( 1 , 6 ).

Wet–dry cycle tests determine how well or how stabilized soil maintains its engineering properties and performs in the field when exposed to extreme environmental conditions. They can also simulate field climatic conditions in the laboratory within a short time ( 6 ). Laboratory procedure “ASTM D559,” developed originally for cement-stabilized soils, is a standardized method for replicating field environmental conditions through wetting–drying cycles. In this procedure, compacted cement-treated soil specimens are cured in a humid room for 7 days and then subjected to a maximum of twelve wet–dry cycles, and weight loss from wire brushing is determined after twelve cycles. Many researchers observed that the wire brushing process varies because of the human element; therefore, several research studies have replaced it with unconfined compressive strength (UCS) testing after the completion of twelve cycles ( 6 , 19 , 21 , 23 ).

Solanki et al. conducted cyclic wetting–drying tests on five soils commonly encountered in Oklahoma, USA ( 21 ). They stabilized soils with hydrated lime, Class C fly ash, and cement kiln dust, then specimens were subjected to wet–dry cycles. After one cycle, the specimens showed an increase in UCS values; however, all specimens failed after two cycles. Chittoori et al. conducted extensive durability studies on lime- and cement-treated soils ( 19 ). Eight soils with varying plasticity and mineralogical characteristics from different parts of Texas, USA were selected. Wetting–drying cycle tests were conducted as per ASTM D559, but instead of brushing the specimens with a wire brush after each cycle, they conducted UCS tests after zero, three, seven, fourteen, and twenty-one wet–dry cycles. They found that the presence of montmorillonite in expansive soil adversely affects the durability of stabilized soils, as the soils with a high percentage of montmorillonite failed quickly during the wet–dry cycles, but those with a low percentage survived all twenty-one cycles. They also learned that the type of additive used affects the durability of stabilized soils and reported that cement-treated soil specimens performed better and remained intact for more wet–dry cycles than soil treated with lime at the same dosage.

A joint research project was conducted by the Finnish National Road Administration and the Texas Transportation Institute, and developed a tube suction test to determine the moisture susceptibility of aggregate base materials ( 24 ). The tube suction test uses a percometer to measure the dielectric value of materials. Past research studies reported that the presence of water significantly affects the dielectric constant ( 22 , 24 , 25 ). Scullion and Saarenketo suggested that the tube suction test can be a suitable alternative to the conventional wetting–drying cycle test method, as water affinity is a good indicator of both the moisture sensitivity and durability of materials ( 24 ). Syed studied four unbound aggregates and observed that the presence of expansive clay minerals in the clay fraction of aggregate showed high surface dielectric constants ( 26 ). It was concluded that the tube suction test is suitable for measuring the moisture susceptibility of chemically stabilized materials. Yeo et al. demonstrated that the tube suction test is an effective method for assessing the moisture susceptibility of cement-treated base materials ( 25 ). Zhang and Tao conducted a detailed durability study on cement-treated low-plasticity soils and compared the results of tube suction tests, 7-day UCS, and wetting–drying durability tests ( 22 ). Test results revealed that the molding water content affects dielectric value. They concluded that the results from the tube suction test, 7-day UCS test, and wetting–drying durability tests are comparable for predicting the durability of cement-treated soils. Solanki et al. conducted tube suction tests on untreated and treated soils and reported that the dielectric values of all the soil specimens were higher than 16 ( 21 ). The tests were conducted on different sizes of specimens to determine whether the specimen size affects the dielectric value, and it was concluded that size did not influence dielectric value. The tube suction test method was originally developed for aggregate base materials; however, a few researchers have used this method for assessing the durability of treated soils.

The U.S. Army Engineer Research and Development Center (ERDC) researchers developed a simple wet-test procedure to measure the moisture susceptibility and strength loss of stabilized soil ( 10 , 27 , 28 ), as they deemed the procedure traditionally used to be unrepresentative of field conditions, too complicated for large numbers of repetitions, and/or too harsh to permit effective specimen evaluations ( 10 , 27 , 28 ). Santoni et al. conducted a detailed study on the stabilization of silty sand with non-traditional stabilizers, including polymer ( 27 ). Twelve non-traditional stabilizers were selected in addition to asphalt emulsion, cement, and lime. The authors reported a reduction of the UCS of all the specimens except those treated with 9% cement- and 9% emulsified asphalt-treated, which they attributed to the deterioration caused by moisture exposure. Other studies conducted by Tingle and Santoni ( 10 ) and Santoni et al. ( 28 ) also used this wet-test procedure for soils stabilized with non-traditional stabilizers and observed a reduction in strength after moisture exposure.

Several moisture-based durability tests have been developed, but no consensus has been reached on which method best simulates field durability-related environmental conditions. A rational durability test method for stabilized geomaterials needs to be recommended after analyzing all the existing moisture-susceptible durability test methods. Therefore, this research study considered these research gaps and conducted an experimental study to compare the outcome of three moisture-susceptible durability test methods.

The objective of the present research study was to assess the performance of stabilized soils by all three moisture-susceptible durability test methods, including the tube suction test, ASTM wet–dry test, and ERDC wet test. Three soil types and three stabilizer types were considered in this study and soil–stabilizer combinations were designed as per the Department of Defense (DOD) stabilization protocols. One stabilizer type (vinyl acetate-ethylene copolymer or cement or lime) for each soil at three different dosages was considered to assess the durability of treated soils. The scope of the research investigation was limited to one stabilizer per each soil type, and therefore the present study and results do not provide individual assessments of stabilizer effectiveness for a given soil type. A summary of the results of all three moisture-susceptible tests and discussions were provided for a better understanding of the durability screening of each method. Future research directions were also presented.

Experimental Program

Materials

Three types of natural soils were considered in the present research. Two coarse-grained soils collected locally from Texas and one fine-grained soil from the U.S. Army ERDC, Vicksburg, Mississippi were used. Basic soil characterization tests including wet sieve analysis, hydrometer analysis, Atterberg limits, and specific gravity tests were conducted as per respective ASTM standards ( 29 ). Moreover, the sulfate content of soils was measured according to Tex-145-E ( 30 ). The grain size distribution curves of all three soils are shown in Figure 1. All three soils were classified as silty sand (SM), clayey sand (SC), and high-plasticity clay (CH) as per the Unified Soil Classification System (USCS). The physical and chemical properties of the soils are provided in Table 1.

Figure 1.

Grain size distribution of all three soils considered in this study.

Table 1.

Basic Soil Characterization Test Results for All Three Soils

Parameters	Soil-1	Soil-2	Soil-3
Sand (%)	82.4	53.5	0.0
Silt (%)	16.2	5.5	43.0
Clay (%)	1.4	41.0	57.0
Liquid limit (%)	NP	21	83
Plastic limit (%)	NP	14	31
Plasticity index (%)	NP	7	52
USCS classification	SM	SC	CH
Specific gravity, G_s	2.67	2.62	2.74
Sulfate content, mg/l	< 25	< 25	< 25

Note: USCS = Unified Soil Classification System; SM = silty sand; SC = clayey sand; CH = high-plasticity clay; NP = non-plastic.

Three types of stabilizers: VAE copolymer, Portland cement type I, and hydrated lime were selected for improving the engineering properties of soils in this study. Cement and lime are calcium-based stabilizers and are commonly used by transportation and defense agencies ( 5 , 32 ); the VAE copolymer is a non-traditional stabilizer and was selected to assess its effectiveness in improving engineering properties.

In this study, each soil was stabilized with only one stabilizer. VAE copolymer was considered for soil-1, cement was considered for soil-2, and lime was used for soil-3. In this study, the stabilizer and dosage were selected as per the US DOD protocol, UFC 3-250-11 ( 31 , 32 ). The initial copolymer dosage of 5% was considered for VAE-treated soil-1 (SM) as recommended by the ERDC, Vicksburg, MS. An initial cement dosage of 7% was determined for soil-2 (SC) based on the soil type recommended by UFC 3-250-11 ( 31 , 32 ). Initial lime dosage of 5% was determined for soil-3 (CH) using the Eades and Grim pH test as per UFC 3-250-11 ( 31 , 32 ).

The maximum dry unit weight (MDUW) and optimum moisture content (OMC) of untreated and treated soils were determined by conducting a modified Proctor compaction test according to ASTM D1557 ( 29 ). For treated soils, compaction tests were conducted at the initial stabilizer dosage determined, as per UFC 3-250-11 ( 31 , 32 ), and provided in the previous paragraph. Table 2 presents the results of compaction tests for all three control soils and mixture of stabilizer and soil.

Table 2.

Modified Proctor Compaction Test Results for All Three Soils

Soil type	Stabilizer type (dosage)	Maximum dry unit weight, pcf (kN/m³)	Optimum moisture content (%)
Soil-1 (SM)	Control (none)	111.8 (17.6)	12.0
Soil-1 (SM)	VAE copolymer (5%)	113.0 (17.8)	11.0
Soil-2 (SC)	Control (none)	124.0 (19.5)	10.4
Soil-2 (SC)	Cement (7%)	125.4 (19.7)	10.4
Soil-3 (CH)	Control (none)	103.0 (16.2)	19.6
Soil-3 (CH)	Lime (5%)	94.3 (14.8)	21.8

Note: VAE = vinyl acetate-ethylene; SM = silty sand; SC = clayey sand; CH = high-plasticity clay; pcf = pounds per cubic foot.

In the case of SM soil stabilized with VAE copolymer, an increase in the MDUW was observed. This could be explained by the lubrication effect of VAE copolymer on soil particles. The OMC was reduced after VAE mixing with soil. Compaction test results showed a considerable increase in the MDUW of SC soil after cement mixing with soil. This could be attributed to the higher specific gravity of cement (G_s = 3.15) relative to the soil. Another possible reason for this trend could be the size of the cement particles. Cement consists of very fine-grained particles that fill the voids between sand particles and increase the dry unit weight of sand–cement mixture ( 33 ). The OMC remained the same after the cement was mixed with the SC soil. The MDUW of CH soil was reduced after lime was mixed with soil. This reduction was because of flocculated structures resist compaction after mixing, which means that they occupy larger spaces in the soil matrix, which leads to a reduction in the MDUW ( 34 ). An increase in the OMC was observed after lime mixing with CH soil, as lime causes dissociation to occur, increasing the amount of water needed and thereby increasing the OMC ( 34 ).

Specimen Preparation

For the VAE copolymer-treated specimens, a homogeneous mixture of dry soil and water was prepared, and a precalculated dose of the copolymer was added and thoroughly mixed until a uniform color was obtained. The amount of water already present in the VAE copolymer was accounted for in preparing the treated soil mixtures. A predetermined amount of cement or lime was homogeneously mixed with dry soil until a uniform color was obtained, then water was added to the mixture. All the mixtures were then statically compacted into a cylindrical mold having a height of 142 mm and a diameter of 71 mm at respective OMCs and MDUWs. All cement-treated specimens were compacted within 30 min of mixing the soil and cement to prevent their initial set. A similar time was also followed for lime- and VAE-treated soil specimens. Specimens were de-molded and then placed according to their required curing conditions for targeted curing time. Triplicate specimens were prepared for durability tests. For the treated specimens’ preparation, the same MDUW and OMC were used for all three dosages of each stabilizer, which makes the same initial strength at all three dosages.

Table 3 presents stabilizer dosage for all three soils and stabilizers. Treated soil specimens were prepared at VAE copolymer dosages of 4% (P4%), 5% (P5%), and 6% (P6%) by weight of dry soil. Cement-treated specimens were prepared at cement dosages of 5% (C5%), 7% (C7%), and 9% (C9%) following ASTM D1632-17 ( 29 ). Lime dosages of 5% (L5%), 7% (L7%), and 9% (L9%) were used for preparing lime-treated specimens.

Table 3.

Stabilizer Dosage and Curing Periods for Soils

Soil Types	VAE copolymer dosage (%)	Cement dosage (%)	Lime dosage (%)	Curing period (days)
Soil-1 (SM)	4, 5, & 6	na	na	7
Soil-2 (SC)	na	5, 7, & 9	na	7
Soil-3 (CH)	na	na	5, 7, & 9	28

Note: VAE = vinyl acetate-ethylene; SM = silty sand; SC = clayey sand; CH = high-plasticity clay; na = not applicable.

VAE polymer-treated specimens were cured at 22 ± 2°C and 50% relative humidity for 7 days, which can be considered an air-drying curing method and is usually preferable to the moist curing method for polymer treatment ( 27 ). It is also recommended by the supplier, as it simulates actual field conditions during military construction operations in the laboratory ( 10 , 27 , 35 ). Cement-treated specimens were cured in a humid room (22 ± 2°C and 100% relative humidity) for 7 days and lime-treated specimens were cured in a sealed chamber at 22 ± 2°C and 100% relative humidity for 28 days.

Moisture-Susceptible Durability Tests

The current study investigated the performance of stabilization methods by determining the strength loss during wetting–drying procedures or moisture exposure. Three moisture-based durability test methods, namely tube suction test, ASTM wet–dry test, and ERDC wet-test methods, were considered in this study and are discussed in detail in the sections below.

Tube Suction Test Method

The tube suction test method was originally developed for evaluating the moisture sensitivity of granular base materials; however, a few research studies have used this method for assessing moisture susceptibility of treated soils ( 21 , 22 , 25 ). The current study conducted this test on untreated and treated subgrade soils following the Tex-144-E ( 36 ) standard. The tube suction test setup is shown in Figure 2. A surcharge of 2.26 kg (5 lbs.) was applied, as shown in Figure 2a, to ensure adequate contact between the top surface of the specimen and the dielectric probe. Dielectric value (DV) was measured every day at five locations and an average of five readings was reported as the dielectric value for that day. After completion of the tube suction tests, specimens were oven-dried to determine their moisture content.

Figure 2.

Tube suction test method: (a) dielectric probe with surcharge attached placed on specimen and (b) Percometer (© Prince Kumar).

ASTM Wet–Dry Test Method

Conventional wet–dry tests were performed according to ASTM D559-15 ( 37 ). In each wet–dry cycle, cured specimens were placed in a water bath at room temperature for 5 h, followed by oven drying at a temperature of 71°C for 42 h. In this study, UCS tests were conducted after the completion of six and twelve cycles, and the results were compared with those that were zero wet–dry cycles. All the UCS tests were performed using a universal testing machine at a constant rate of strain of 0.90%/min. Figure 3 shows the wetting–drying process for cement-treated SC soil specimens.

Figure 3.

Wet–dry cycle test method: (a) wetting and (b) drying (© Prince Kumar).

ERDC Wet-Test Method

The ERDC wet tests were performed according to the procedure outlined by Santoni et al. ( 27 ). Specimens were placed on their sides in a 25.4 mm (1 in.) deep water bath for 15 min, as shown in Figure 4a. After specimens were removed from the water bath, they were allowed to drain for 5 min as shown in Figure 4b. UCS and RLT tests were conducted on the specimens and the results were compared with those for specimens that were not subjected to soaking.

Figure 4.

U.S. Army Engineer Research and Development Center (ERDC) wet-test method: (a) specimen during the 15 min soaking period and (b) specimen after soaking (© Prince Kumar).

Analysis of Test Results and Discussion

This section presents the results of all three moisture-susceptible durability test methods. A detailed discussion of the test results is presented in the following sections.

Tube Suction Test Results

VAE Copolymer Stabilization

The variation in the DV with time obtained from tube suction tests conducted on control and VAE-treated SM soil specimens is shown in Figure 5. An increase in DV is directly related to an increase in water content. It can be observed that the water reached the top of the control soil specimens quickly but took a few days to reach the top of the VAE copolymer-treated soil specimens. This indicates that the VAE copolymer filled some pores of the soil matrix, increasing the amount of time required for the water to reach the top of the specimen. The DV of the control soil specimens reached its maximum value of 24.4 in 4 days and became almost constant. The maximum DVs for VAE-treated soil were 21.5, 20.6, and 19.7 for VAE dosages of 4%, 5%, and 6%, respectively. The VAE copolymer treatment reduced the DV of SM soil, and larger dosages further reduced it. Reduction in the maximum DV attributed to VAE treatment varied between 12% and 19%, which shows an improvement in moisture susceptibility at all VAE copolymer dosages compared with untreated SM soil. In Figure 5, DVs of 10 and 16 are denoted with blue and orange dotted lines, respectively.

Figure 5.

Variation in dielectric values with time for control and vinyl acetate-ethylene (VAE) copolymer-treated silty sand (SM) soil.

The Tex-144-E ( 36 ) standard recommends a criterion for determining the moisture susceptibility of granular base materials. When DV is less than 10 then it is considered as “not moisture susceptible”; if DV is between 10 and 16 then it is “marginally moisture susceptible”; and granular base materials are “moisture susceptible” if DV is greater than 16. These criteria were developed for granular base materials and no criterion is available for sands and fine-grained soils. Therefore, the above criteria were used in this study to comment on the moisture susceptibility of untreated and treated soils.

The DVs of control and VAE copolymer-treated SM soil were greater than 16, therefore the soil was considered “moisture susceptible.” Even though the VAE treatment reduced DVs, it did not provide sufficient resistance against water intrusion; therefore, the VAE copolymer treatment is considered moisture susceptible.

Cement Stabilization

Variations of the DVs of untreated and cement-treated SC soil specimens with time are shown in Figure 6. For untreated soil, the DV began increasing after the fifth day and achieved a maximum value of 20.9 on the fifteenth day, where it remained almost constant. From this observation, it can be seen that the water resulting from capillary absorption did not reach the top of the control soil specimens in 5 days. The DVs of the cement-treated specimens did not change until the ninth day, then they began slowly increasing and after a few days reached the maximum value. The maximum DVs for the cement-treated soil were 11.4, 10.1, and 9.0 for cement dosages of 5%, 7%, and 9%, respectively. The reduction in maximum DV resulting from cement treatments compared with control soil varied between 45% and 57%. The test results showed that the DVs significantly reduced after cement treatment and reduced even further with an increase in the cement dosage. The incremental rate of the DV was reduced with an increase in the cement dosage. The cement stabilization forms cementitious compounds that bind soil particles and form a compact soil matrix ( 38 ). Also, the amount of cementitious compounds increases with an increase in cement dosage. This resulted in slow water movement owing to capillary action.

Figure 6.

Variation in dielectric values with time for control and cement-treated clayey sand (SC) soil.

The final DV of the untreated soil specimens was greater than 16, so it was considered “moisture susceptible”. Both 5% and 7% cement dosages used for stabilization exhibited a DV between 10 and 16 indicated by blue and orange dotted lines, respectively; therefore, they were considered “marginally moisture susceptible.” The cement-treated soil specimen with 9% cement dosage showed a DV of less than 10 and was considered “not moisture susceptible.” Based on the above results, it is concluded that the cement-stabilized SC soil is not susceptible to moisture.

Lime Stabilization

Figure 7 shows variation in the DVs of control and lime-treated CH soil with time. It was observed that the untreated CH soil took the longest time among all three soils to complete the tube suction test and it was almost a month. The time required to reach water on the top surface of the soil specimen was long because of the high content of fines in CH soil and capillary action. Therefore, the DV of the control CH soil did not increase for a long time and then slowly increased with time. The maximum DV value for the untreated CH soil was 52.2, and after reaching it, it became almost constant. It is worth mentioning that visual observation of untreated CH soil showed swelling of the specimens and exhibited an increase in diameter and height during testing. The lime stabilization significantly reduced the DV values, which were constant for up to 3 days, and then quickly increased at all three lime dosages. The lime stabilization induces flocculation and agglomeration of clay particles, which increases the effective grain size of soil particles ( 38 , 39 ), which potentially affects capillary action and reduces the time requirement to reach water at the top surface of the lime-stabilized specimen compared with untreated soil. The total testing time was 10 days for lime-treated soil specimens but 34 days for untreated CH soil, which demonstrates the effect of the difference in the size of soil particles before and after lime stabilization on the DV. The maximum DV of the CH soil reduced by 33% to 44% after lime stabilization, but both the untreated and lime-treated CH soil specimens showed maximum DV higher than 16, which indicates “moisture susceptible” and leads to the conclusion that both control and lime-treated CH soil specimens are moisture susceptible.

Figure 7.

Variation in dielectric values with time for control and lime-treated high-plasticity clay (CH) soil.

ASTM Wet–Dry Test Results

VAE Copolymer Stabilization

Wet–dry cycle tests were conducted on VAE copolymer-stabilized SM soil, and the results are presented in Figure 8. Both the untreated and VAE-stabilized SM soil specimens collapsed completely during the wetting of the first wet–dry cycle; therefore, their retained UCS values were considered zero. The control SM soil specimens collapsed quickly after being immersed in a water bath. The soil specimens treated with all dosages of VAE collapsed after a few hours of wetting, indicating that VAE copolymer bonds were not resistive against water ingress.

Figure 8.

Strength retention after wet–dry cycles for control and vinyl acetate-ethylene (VAE) copolymer-treated silty sand (SM) soil.

Cement Stabilization

Figure 9 shows variations of the retained UCS with the number of wet–dry cycles for untreated and cement-stabilized SC soil. The control soil specimens collapsed immediately after being immersed in water, which indicates a 100% loss of strength after immersion. After completion of six and twelve wet–dry cycles, one set of the cement-treated specimens was oven-dried and tested in dry condition, and another set of specimens was immersed in water for 5 h and then tested in wet condition. The UCS test results presented in Figure 9 show the treated specimens exhibited a significant increase in strength after six wet–dry cycles in both dry and wet conditions, and an additional increase was observed after twelve wet–dry cycles. This increase in the UCS of dried specimens was attributable to the enhanced hydro-chemical reactions caused by an increase in temperature and a reduction in the water content. Here, this increase in strength can be explained by matric suction theory. The relationship between matric suction and changes in water content is well established by previous researchers, who observed that the matric suction increases with a decrease in the water content (40 –43). Therefore, the reduction in water content induced matric suction, which increased the UCS. Strength was reduced in the wet-condition specimens because of the loss of matric suction as a result of water immersion. In both conditions, strength was increased with an increase in the cement dosage. In summary, test results showed that cement-stabilized SC soil is durable against wet–dry cycles at all cement dosages.

Figure 9.

Strength retention after wet–dry cycles for control and cement-treated clayey sand (SC) soil.

Lime Stabilization

Pictures of lime-treated CH specimens during wet–dry cycles are shown in Figure 10. Wet–dry cycle test results for control and lime-stabilized CH soil are shown in Figure 11. Untreated CH specimens collapsed completely during the first wetting phase of the wet–dry cycle, demonstrating a 100% reduction in strength. The lime-treated soil specimens showed some improvement, but they also failed after a few wet–dry cycles and were not able to survive the complete wet–dry test. The test results showed that 5%, 7%, and 9% lime-treated soil specimens failed after three, four, and six wet–dry cycles, respectively. It was observed that the first drying cycle of lime-treated specimens induced small cracks on the surface of the specimens, and in subsequent wet–dry cycles, the size of the cracks increased until they became open cracks, as shown in Figure 10a. During wetting, water penetrated easily through the open cracks and degraded the soil specimens, which resulted in the complete failure of the lime-treated specimens, as presented in Figure 10b. With an increase in lime dosage, the specimens were more resistant to developing open cracks, but none of them survived to complete six wet–dry cycles; therefore, the UCS tests could not be conducted on lime-treated CH soil specimens. In conclusion, the results of the wet–dry cycle tests showed that lime-treated CH soil is not durable against wet–dry cycles.

Figure 10.

Lime-treated high-plasticity clay (CH) soil specimen (dosage = 7%): (a) after the second cycle of drying and (b) after the fourth cycle of wetting (© Prince Kumar).

Figure 11.

Strength retention of control and lime-treated high-plasticity clay (CH) soil after wet–dry cycles.

ERDC Wet-Test Results

VAE Copolymer Stabilization

Figure 12 depicts the control SM soil specimen during 15 min of soaking and shows that it almost completely collapsed after 5 min, demonstrating a 100% reduction in UCS values. The VAE-stabilized SM soil specimen presented in Figure 13 did not collapse during the 15 min soaking period but failed while it was being removed from the water. From Figure 13, it can be observed that the specimen absorbed water almost to the point of saturation, which resulted in a loss of strength. Figure 14 presents UCS values of untreated and VAE-treated SM soil before and after soaking. All the VAE-treated soil specimens showed 100% strength loss after soaking, indicating that the VAE copolymer does not provide resistance against moisture susceptibility.

Figure 12.

Control silty sand (SM) soil specimen: (a) at the start of soaking and (b) after 5 min of soaking (© Prince Kumar).

Figure 13.

Vinyl acetate-ethylene (VAE)-treated (dosage = 4%) silty sand (SM) soil specimen: (a) at the start of soaking and (b) after 15 min of soaking (© Prince Kumar).

Figure 14.

Unconfined compressive strength (UCS) test results of U.S. Army Engineer Research and Development Center (ERDC) wet-test method for control and vinyl acetate-ethylene (VAE) copolymer-treated silty sand (SM) soil.

Cement Stabilization

UCS test results of untreated and cement-stabilized SC soil specimens before and after soaking are depicted in Figure 15. The weight of the control soil specimens increased by about 5.5 g after 15 min of soaking, which resulted in a significant 75% reduction in strength. Cement-treated soil specimens at all cement dosages showed a negligible reduction in UCS after soaking. It is important to mention that the weight of cement-treated specimens only increased by about 0.5 g after soaking, which shows negligible absorption of water. This observation shows that the cement stabilizer provided good bonding between soil particles.

Figure 15.

Unconfined compressive strength (UCS) test results of U.S. Army Engineer Research and Development Center (ERDC) wet-test method for control and cement-treated clayey sand (SC) soil.

Lime Stabilization

Figure 16 shows the UCS test results of untreated and lime-stabilized CH soil before and after soaking for 15 min. The untreated soil specimens absorbed about 16.5 g of water during 15 min of soaking, which resulted in a 51% reduction in the UCS. The lime-treated specimens absorbed about 4 g of water during 15 min of soaking, and their strength was only slightly affected. This shows that the lime-treated CH soil specimens were less moisture susceptible than the control CH specimens.

Figure 16.

Unconfined compressive strength (UCS) test results of U.S. Army Engineer Research and Development Center (ERDC) wet-test method for control and lime-treated high-plasticity clay (CH) soil.

Comments on Moisture-Susceptible Durability Test Results

All three moisture-susceptible durability test methods were conducted on all three soils considered in this study before and after stabilization. Detailed discussions of the test results are presented in the above sections, and final comments on durability test methods and results are presented below.

Results of tube suction tests showed untreated SM, SC, and CH soils were “moisture susceptible.” All control soil specimens failed during the first wetting cycle of the wet–dry test. The SM soil specimens failed during the ERDC wet test; however, SC and CH soil specimens showed a significant reduction in strength after 15 min of soaking. In conclusion, all three untreated soils are susceptible to moisture and showed complete strength loss after wet–dry cycles.

Table 4 summarizes the outcome of all three moisture-susceptible durability tests on three different stabilized soils. All three test methods showed that cement-stabilized SC soil specimens are unsusceptible to moisture. Cement-treated SC soil specimens exhibited an increase in strength after six and twelve wet–dry cycles. On the other hand, lime-stabilized CH and VAE-stabilized SM soil specimens not complete six wet–dry cycles and they failed, which showed a complete loss of strength. This difference in results of wet–dry tests is potentially because of the variations in soil and stabilizer types. Results of tube suction and ERDC wet tests showed that the VAE-stabilized SM soil specimens are moisture susceptible. The tube suction test results revealed that lime-stabilized CH soil was susceptible to moisture. However, the ERDC wet-test method showed that lime-treated CH soil was not susceptible to moisture and exhibited only a slight reduction in strength. This discrepancy is attributed to the brief soaking period considered by the ERDC wet-test method. It is important to note that the ERDC wet-test method allows the specimens to be soaked in water for only 15 min, which is less than other methods. Consequently, assessments of the moisture susceptibility of treated soil specimens using the ERDC wet method may not accurately reflect its long-term durability.

Table 4.

Summary of Moisture-Susceptible Durability Test Results

Test methods	Soil + stabilizer
Test methods	SM + VAE	SC + cement	CH + lime
TST	Moisture susceptible	Not moisture susceptible	Moisture susceptible
WDT	Moisture susceptible	Not moisture susceptible	Moisture susceptible
EWT	Moisture susceptible	Not moisture susceptible	Not moisture susceptible

Note: TST = Tube suction test; WDT = ASTM Wet–dry test; EWT = ERDC wet test; VAE = vinyl acetate-ethylene; SM = silty sand; SC = clayey sand; CH = high-plasticity clay.

The cement stabilization forms cementitious compounds, which bind soil particles strongly and are not influenced by moisture intrusion. On the other hand, VAE copolymer stabilization does not provide resistance to moisture susceptibility. The lime stabilization method performed better than the VAE copolymer against moisture intrusion, but it is also susceptible to moisture. Only the ERDC wet-test results exhibited that the lime stabilization method is not moisture susceptible. Overall, the cement stabilization method performed well against moisture susceptibility. The moisture susceptibility is key in predicting loss of strength and stiffness, which affects the long-term performance of chemical stabilization methods and can potentially anticipate the durability of stabilization methods.

All the above concluding remarks are based on the test results obtained for soils and stabilizers considered in this study. It is important to mention that each stabilizer was considered for different soil types, therefore direct comparison of results of different stabilization methods will have limitations. Future studies are needed to strengthen this outcome by performing durability studies on other soil and stabilizer types.

Summary and Concluding Remarks

The durability of chemically treated subgrade soils is a major concern for transportation agencies as it is important for pavement design life. The long-term performance of chemical stabilization of soils is primarily affected by moisture. Therefore, the present study assessed the performance of stabilized soils by three moisture-susceptible durability test methods, including the tube suction test, ASTM wet–dry test, and ERDC wet test. One stabilizer type (either VAE copolymer or cement or lime) for each soil at three different dosages was considered to assess the durability of treated soils. The results of all three durability tests were compared and a detailed discussion of the findings was presented. This paper presented interesting outcomes about the durability of stabilized soils based on three moisture-susceptible test methods.

All three untreated soils are susceptible to moisture and showed complete strength loss after wet–dry cycles. Results of all three moisture-susceptible durability tests indicated that VAE copolymer-stabilized SM soil is susceptible to moisture and cement-stabilized SC soil is unsusceptible to moisture. In the case of lime-stabilized CH soil, conventional ASTM wet–dry test and tube suction test methods revealed that lime-treated soil is moisture-susceptible; however, the ERDC wet-test method reported it not susceptible to moisture. This discrepancy is attributed to the brief soaking period considered by the ERDC wet-test method compared with the other two methods. All three methods—tube suction, ASTM wet–dry, and ERDC wet tests—provided similar moisture susceptibility for VAE copolymer-stabilized SM and cement-stabilized SC soils. However, only tube suction and ASTM wet–dry tests predicted equivalent durability results for lime-stabilized CH soil. Overall, good equivalency exists between tube suction and ASTM wet–dry tests for all combinations of soil and stabilizers considered in this study. The ERDC wet-test provides a quick screening among all three methods as this method can provide results in a relatively short time frame. This method’s results matched well with other methods for stabilized sandy soils, but not on lime-stabilized soils where chemical reaction kinetics may play a major role in strength and stiffness property enhancements. More future durability studies are needed to validate present outcomes, including the need to perform all three studies on the same soils with different chemical stabilizers.

Footnotes

Acknowledgements

The authors would like to acknowledge the U.S. Army Engineer Research and Development Center, Vicksburg, MS for granting the funds for the research. Also, the authors would like to acknowledge Geomechanics/Geotechnical Research Group members at the Texas A&M University for their help.

Author Contributions

The authors confirm contribution to the paper as follows: study conception and design: Prince Kumar, Anand J. Puppala, Surya S.C. Congress, Jeb S. Tingle; data collection: Prince Kumar; analysis and interpretation of results: Prince Kumar, Anand J. Puppala, Nripojyoti Biswas, Surya S.C. Congress, Jeb S. Tingle, Dallas N. Little; draft manuscript preparation: Prince Kumar, Anand Puppala, Nripojyoti Biswas, Surya S.C. Congress. All authors reviewed the results and approved the final version of the manuscript.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was funded by U.S. Army Engineer Research and Development Center, Vicksburg, MS Award #W912HZ-21-BAA-01.

ORCID iDs

Prince Kumar

Anand J. Puppala

Nripojyoti Biswas

Surya S. C. Congress

Jeb S. Tingle

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