Effect of intermittent wetting on the UCS of lime ...

Effect of intermittent wetting on the UCS of lime-treated expansive soils Tamer ELKADYa,b 1 Abdullah SHAKER a, Arif A. BAIG a, Mosleh AL-SHAMRANI a a Bugshan Research Chair in Expansive soils, Civil Engineering Department, College of Engineering, King Saud University, Riyadh b Soil Mechanics and Foundations Research Laboratory, Faculty of Engineering, Cairo University, Giza, Egypt

Abstract. Laboratory determination of design lime content to be applied in the field involve preparing soil-lime specimens with different lime contents to evaluate their swelling potential and unconfined compressive strength (UCS) after curing for 7 or 28 days. Curing of specimens is typically conducted under the conditions of ambient temperature and relative humidity greater than 95%. During field application, lime-treated expansive soils may be subjected to cases of increase moisture. This paper investigates the effect of wetting during curing period on the UCS of lime-treated expansive clay. An experimental program was devised where lime-treated specimens were subjected to different wetting schemes (whether a single wetting event or multiple wetting events) and the UCS at the end of the curing period is evaluated. Other parameters investigated included lime content (2%, 4% and 6% on dry weight basis) and curing period (7, 14 and 28 days). Results of the tests indicated that wetting schemes had a detrimental effect on the UCS of lime-treated specimens; especially at low lime content (i.e., 2%). Furthermore, as the lime content increased, the percentage reduction in UCS decreased. Keywords. Wetting schemes, Expansive soil, Lime.

1. Introduction The volume change induced by moisture sensitive expansive soils cause serious damages to pavements and light structures. One of the techniques used to arrest the swelling behavior of expansive subgrade is lime treatment. The improvement due to lime addition is derived from two physico-chemical reactions: cation exchange and pozzolanic reactions. Cation exchange is considered to be a short-term reaction that tend to modify the parent soil to increase the workability of the soil. In contrast, the pozzolanic reaction is a long-term process. During this reaction, cementitious compounds (calcium silicate hydrate and calcium aluminate hydrate) are formed that bond clay particles together (Nalbantoglu and Tuncer 2001; Al-Mukhtar et al. 2010). The main stream of research related to lime-treated expansive soils focuses on the plasticity, swelling potential, swelling pressure and unconfined compression strength and stiffness of lime treated expansive soils (Basma and Tuncer 1991; Afès and Didier 2000; Nalbantoglu and Tuncer 2001; Al-Rawas 2005). In ideal laboratory conditions, 1

Corresponding Author.

lime-treated samples are typically cured under controlled laboratory conditions of constant temperature and relative humidity greater than 95%. In the field, lime-treated expansive subgrades may experience changes in moisture condition due sudden rainfall, or rise in water table Further research is required to investigate the effect of wetting on the UCS of limetreated expansive soils. This study investigates the UCS of lime treated specimens subjected to different wetting schemes during curing. Two types of wetting schemes were considered; namely, single wetting event and multiple wetting events.

2. Materials and Methods This section provides a description of materials and experimental techniques used in this study. 2.1. Material Used Expansive soils are encountered at different locations in Kingdom of Saudi Arabia (KSA) which vary in geological origin and swelling characteristics (Abduljauwad and Al-Sulaimani 1993, Azam et al. 1998, Al-Shayea 2001, Azam 2003). The expansive clay used obtained from the city of Al-Qatif which located on the western shoreline of the Arabian Gulf in the eastern province of Saudi Arabia (26° 56′ 0″ N, 50° 1′ 0″ E). Soil samples were obtained from the field using open pits excavated to a depth of 2.5 3.0 m below ground surface. Ground water table was not encountered during sampling. Samples were bagged and transferred to King Saud University laboratory for full characterization. A summary of geotechnical characterization results are provided in Table 1. Mineralogical characterization of Al-Qatif soil performed using X-Ray diffraction analysis concluded that the predominant clay minerals are montmorillonite and palygorsite which are swelling minerals. Lime used in this study was commercial grade calcium hydroxide (assay 90%), supplied by Saudi Lime, Riyadh, KSA. Estimation of the optimum lime content value for lime-clay mixtures was evaluated using ASTM D 6276 (1999) originally proposed by Eades and Grim (1996). The optimum lime content defines the lime content beyond which further increase in lime content will have negligible effect on improvement of soil. According to ASTM standard, the lowest percentage of lime that results in a soillime pH of 12.4 is considered the optimum lime content. Results of test performed on soil indicated that the optimum lime content was 4% by dry weight. Additional lime contents equivalent to ±2% of optimum lime content were considered in this study. In other words, the lime contents considered in this study was 2%, 4% and 6% by dry weight of clay. 2.2. Sample preparation and mixture characterization Soil samples obtained from the field were air dried, pulverized and sieved using sieve No. 40. Expansive clay and lime were mixed thoroughly at target water content and stored in plastic bags for 24 hours to allow for the mix to homogenize. Samples were then statically compacted to target dry unit weight with final dimensions of 50 mm in diameter and 100 mm high. Target water content and dry density were corresponding

to the optimum moisture content and maximum dry unit weight evaluated in accordance with ASTM D698. Figure 1 shows the compaction curves of lime-treated clay as a function of lime content. Table 1. Geotechnical Characterization Data for Al-Qatif Soil Test Specific Gravity, Gs Liquid Limit, w L (%) Plastic Limit, w P (%) Shrinkage Limit, w sh (%) % passing Sieve No. 200 Unified soil classification Swelling Characteristics ASTM D 4546 Swelling potential (%) Swelling pressiure (kPa)

Value 2.77 130 45 17.5 97.5 CH 18-20 400 -550

Dry unit weight (kN/m2)

14 0% Lime 2% Lime 4% Lime 6% Lime Zero Air Voids

13

12

11

10 30

35

40

45 50 Water content (%)

55

60

Figure 1. Compaction curves of lime treated expansive soils.

2.3. Unconfined compression strength and soaking schemes The unconfined compressive strength (UCS) for lime-treated specimens was evaluated following test procedure in accordance with ASTM D5102. The main purpose of this test is to determine the change in strength of lime-treated expansive clays subjected to different wetting schemes during the designated curing periods (i.e., 7, 14 and 28 days). The wetting schemes considered in this study are illustrated in Figure 2. This figure shows the number of wetting events and the day at which wetting was conducted during the curing period (represented by a black square). For instance, W2-28 indicates lime treated specimens that underwent 2 wetting event during the 28 days curing period.

At each wetting event, wetting was conducted using capillary soaking which represents a reasonable moisture state under typical pavement conditions. In this procedure, lime-treated specimens was wrapped with an absorptive cloth around the circumference of the sample and placed on a porous stone that is contact with the water as shown in Figure 3. The porous stone and the absorptive cloth remain in contact with water throughout the capillary soak process which is 24 hours. At the end of capillary soaking period, the sample was removed and re-sealed using plastic wrap and stored in humid curing environment till the end of curing or next wetting event. At the end of the curing period, the lime-treated specimens were tested for unconfined compression test. Specimens were loaded to failure at a constant strain rate of 0.5%/minute in a compression machine. Axial load and axial deformation were measured using a load cell and displacement transducer; respectively. The water content and volume change of specimen at the beginning and end of curing periods were determined.

3. Results and Discussions 3.1. Effect of single wetting event As shown in Figure 2, specimens subjected to a single wetting event are designated as W1-28a, W1-28b, W1-14a, …etc). W1-28a refer to a wetting scheme where wetting took place for at the last day of curing before being tested for unconfined compression strength; while W1-28b represent the wetting taking place at a day in the middle of the curing period. UCS was taken as the peak stress from the stress-strain curve. The UCS of lime-treated specimens with wetting are compared with that without wetting as shown in Figure 4. The UCS reduction factor shown in Figure 4 represents the reduction in shear strength due to a single wetting event. The unconfined compression strength reduction factor (UCS-RF) defined as: UCS-RF =

𝑈𝑈𝑈 (𝑤𝑤𝑤ℎ𝑜𝑜𝑜 𝑤𝑤𝑤𝑤𝑤𝑤𝑤)−𝑈𝑈𝑈 (𝑤𝑤𝑤ℎ 𝑤𝑤𝑤𝑤𝑤𝑤𝑤) 𝑈𝑈𝑈 (𝑤𝑤𝑤ℎ𝑜𝑜𝑜 𝑤𝑤𝑤𝑤𝑤𝑤𝑤)

× 100 %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 W1-28a W1-28b W3-28 W5-28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 W1-14a W1-14b W2-14 1 2 3 4 5 6 7 W1-7a W1-7a

Figure 2. Wetting schemes considered in this study

Figure 3. Capillary soaking procedure of lime-treated Specimens

UCS reduciton faqctor (%)

100

80

60

W1-28D-a W1-28D-b W1-14D-a W1-14D-b W1-7D-a W1-7D-b

40

20

0

0

2

4 Lime content (%)

6

Figure 4. UCS reduction factor for lime-treated expansive clay due to single wetting events for different curing periods

From Figure 4, it is apparent that lime-treated specimens experienced reduction in UCS as a result of single event wetting. The maximum reduction in UCS was observed

8

for lime-treated specimens with lime content 2%. As the lime content increases, the UCS-RF was observed to decrease indicating lime-treated samples are more durable to the wetting event. Reduction in UCS is attributed to collapse of cementation bonds developed by lime treatment due to the swelling of clay particles. On the other hand, the wetting day had a notable effect on the UCS-RF which is more pronounced for lime-treated samples with 4% and 6% lime content. In other words, lime-treated specimens wetted in the middle of the curing period (i.e., W1-7b, W1-14b, W1-28b) experienced higher reduction in UCS than lime-treated specimens wetted on the last day of the curing period (i.e., W1-7a, W1-14a, W1-28a). This can be attributed to lime-samples wetted on the last day of curing had the opportunity to develop strong cementation bonds due to development in pozzolanic reactions making the specimens more resistance to wetting. In addition, for the same single wetting event, as the curing period increases the UCS-RF decreases. 3.2. Effect of multiple wetting events The variation in UCS of lime-treated specimens at the end of curing period as a function of number of wetting events during the 28 days curing period is shown in Figure 5. At low lime content of 2%, it is observed that there is a little or no reduction in UCS of lime treated specimens especially after the first wetting event. However, at 4% and 6% lime content, the reduction in UCS is considered more visible and increases as the wetting events increases. The reason for continual reduction in UCS with wetting events can be linked to the lime or cementation bonds developed within the sample being leached or washed out during each wetting event. Furthermore, it is observed that lime-treated specimens with 6% lime content show higher UCS than that for 4% and 2%. This is attributed to the premise that lime-treated specimens with 6% have strongly developed cementation bonds and consequently are more durability to wetting events. 1400 2% lime - 28 days

1200

4% lime - 28 days

UCS (kPa)

1000

6% lime - 28 days

W1-28a

800 W1-28b

600 400 200 0 0

1

2 3 Number of wetting events

4

Figure 5. Variation in UCS for lime-treated expansive clay with number of wetting events

5

4. Summary and Conclusions In order to better understand the in-situ behavior of lime-treated expansive clays, limetreated specimens subjected to various moisture conditions need to be considered. The unconfined compressive strength (UCS) is one of the main design parameters for flexible pavement. In this study, the effect of different wetting schemes, lime content, and curing period on the UCS of lime-treated specimens were investigated and the following results were obtained: 1. Lime-treated specimens subjcted to a single wetting event experienced notable reduction in UCS. The reduction in UCS expressed as UCS reduction factor (UCS-RF) ranged between 80% to 52% for samples cured for 28 days. The highest UCS-RF was observed for specimens with lime content of 2%. 2. For single wetting event, the reduction in UCS for lime treated samples depended on the day at which the wetting took place. Lime-treated specimens wetted on the last day of curing experienced lower reduction in UCS as compared to specimens wetted in the middle of the curing period. 3. It is was observed that as the number of wetting events increases, the UCS of lime treated specimen decreases. This conclusion was more obvious for specimens with lime content of 4% and 6%. Furthermore, the rate of UCS reduction was observed to decrease as the number of wetting event increase. Acknowledgement This paper is a part of a project funded by the National Plan for Science, Technology and Innovation (MAARIFAH), King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia, Award Number (11-BUI-1901-02). References [1]

A.A. Al-Rawas, A.W. Hago and H. Al-Sarmi, Effect of lime, cement and saroj on the swelling potential of an expansive soil from Oman, Building Environ., 40 (2005), 681-687. [2] A.A. Basma, and E.R. Tuncer, Effect of lime on volume change and compressibility of expansive clays, Transp. Res. Record, 1295 (1991)., 52-61. [3] ASTM D 5102, Standard Test Method for Unconfined Compressive Strength of Cohesive Soil, Vol. 4.08, D-18 Committee on Soils and Rocks, West Conshohocken, PA (2013). [4] ASTM D 6276, Standard Test Method for Using pH to Estimate the Soil-Lime Proportion Requirement for Soil Stabilization, Vol. 4.09, D-18 Committee on Soils and Rocks, West Conshohocken, PA. (1999) [5] ASTM D 698, Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort (12 400 ft-lbf/ft3 (600 kN-m/m3), Vol. 4.08, D-18 Committee on Soils and Rocks, West Conshohocken, PA, (1998). [6] M. Afès, M. and G. Didier, Stabilization of expansive soils: the case of clay in the area of Mila (Algeria), Bulletin of Engineering Geological Environment, 59(1) (2000), 75-83. [7] M. Al Mukhtar, A. Lasledj and J.F. Alcover Behavior and mineralogy changes in lime-treated expansive soils at 50oC, Applied Clay Science, 50 (2010), 199-203. [8] N. Al-Shayea, The combined effect of clay and moisture content on the behavior of remolded unsaturated soils, Engineering Geology, 62 (2001), 319-342. [9] S. Azam, Influence of mineralogy on swelling and consolidation of soils in eastern Saudi Arabia, Can. Geotech. J., 40 (2003), 964-975. [10] S. Azam, S.N. Abduljauwad, N.A. Al-Shayea, and O.S.B. Al-Amoudi, Expansive characteristics of gypsiferous/anhydritic formations, Eng. Geol., 51 (1998), 89-107. [11] S.N. Abduljauwad and G.J. Al-Sulaimani.. Determination of swell potential of Al-Qatif clay, ASTM Geotech. Test. J., 16(4) (1993), 469–484.

[12] Z. Nalbantoglu, and E.R. Tuncer, Compressibility and hydraulic conductivity of chemically treated expansive clay, Can. Geotech. J., 38 (2001), 154-160.