1. Introduction
The process of urbanization results in a lack of suitable building areas for construction activities due to poor ground conditions. Therefore, sites that were abandoned in the past are now being reconsidered for construction. Saline soil is a well-known problematic soil. They are typically distinguished from other surficial soils by a higher salt content, higher compressibility and lower shear strength. Structures supported on saline soils might experience significant stability issues. Saline soil is widespread across the Middle East, North Africa, the United States, and Australia [
1]. The critical percentage of salt that significantly alternates the performance of this soil is 5% [
2]. The physical and chemical properties of the saline soils were improved via chemical treatment with lime, cement, cement kiln dust, fly ash, waste gypsum, and microsilica [
3,
4,
5,
6,
7]. The studies listed above employed traditional stabilization techniques which are appropriate for shallow mixing conditions and comparatively thin and shallow saline soil deposits. For soil layers that are thicker and deeper, the deep soil mixing (DSM) technique can be utilized. The DSM method has a number of benefits, including speed, low vibration and noise levels, the ability to stabilize soil at greater depths, and the production of little spoil soil. This method was successfully used to stabilize soft soils for supporting structures and embankments, slopes stabilization, and reduction in liquefaction [
8].
Very few studies had investigated the application of DSM for soil stabilization of saline soils. Jung et al. [
9] presented the first and the only study which employed a wet DSM technique for saline soil in the UAE to reduce settlement and improve the bearing capacity of a shallow foundation as an alternative to traditional piling. The employed binder in DSM is typically cement; however, it may also occasionally be fly ash, geopolymers, microsilica, or glass powder [
9,
10,
11,
12,
13,
14]. The environmental impact of geotechnical engineering applications, such as grouting and soil stabilization using cement, is very harmful due to high CO
2 emissions [
15]. Approximately 2% of cement’s overall CO
2 emissions are attributable to its use in soil improvement works [
16]. Thus, substituting another material for 10% of cement in geotechnical engineering applications results in a 6.1 Mt CO
2 reduction [
17]. Therefore, the use of waste materials in ground improvement is an area of study that is developing very fast. Utilizing waste materials could benefit the ecosystem and solve the issue of waste disposal. Marble powder is one of the waste materials that can be used to partially replace cement in DSM. Marble powder and dust are waste materials that results from polishing of marble blocks. Several studies reported that during marble block production, 25% of the volume of the block will consist of marble powder and dust, which will cause environmental pollution [
18,
19,
20,
21]. Therefore, reusing this material in DSM will have a positive eco-friendly effect on the environment.
Bhadriraju et al. [
22] reported that several factors influence the treatment of soil using the DSM technique, specifically the sample preparation process and the soil/binder mixing process. Thus, the methodology of laboratory samples preparation plays a significant role in the strength of the treated soil. Many studies on DSM techniques reported a traditional mechanical mixing of the soil, binder and water that are not consistent with the field methods where the soil and binder mixtures are poured into molding cylinders [
11,
12,
13]. Very few studies, such as Esmaeili et al. [
10] and Frikha et al. [
23], presented a scaled-laboratory apparatus similar to the field deep soil mixing techniques where a vertical and rotating auger mixed the binder and the soil.
Esmaeili et al. [
10] presented a study treating a loose salty dry sand subgrade (artificial subgrade) with different salt percentages (0, 5, 10 and 20% salt) using microsilica additives. Three percentages micro-silica additives, 10, 15 and 20%, were added to a cement solution with a w/b ratio equal to 1. The samples were subjected to UCS tests after curing periods of 7, 14 and 28 days. The results indicated that addition of 15% microsilica resulted in a 21% improvement in UCS after 28 days curing period. The shortcomings of the applied DSM method are that the slurry delivery system depended on a gravitation method employing a tank placed at a height of 2.5 m from the ground floor rather than utilizing a slurry pump injection to apply a pressure while injecting the slurry into the soil. Such a method may have a significant effect on the mixing quality. Also, this research involved a w/b ratio equal to 1.0, and hence, the effect of w/b ratio was not addressed in this study.
Frikha et al. [
23] investigated the treatment of soft clay high organic matter percentages equal to 22% using only cement. The conducted testing program involved the examination of five different parameters. These parameters included curing time at 7, 14, 21, and 28 days. Additionally, the injection pressure was tested at 50, 100, and 150 kPa, while the w/c ratio was equal to 0.7, 1, and 1.2. The rotation rate parameter was assessed at 15, 30, and 40 rpm. The findings indicated that higher w/c ratios resulted in decreased shear strength of the soil. Conversely, increasing injection pressure improved soil shear characteristics. The curing period had a notable impact on the treated soil’s shear strength. When the rotation rate of drilling tools increased, there was a significant reduction in soil shear strength. Additionally, injecting grout cement at high rotation rates hindered the formation of a cohesive column and resulted in unsatisfactory mixing. This study focused on the operational parameters and the w/c ratio effect on the soil improvement degree while ignoring the amount of cement. Also, only cement was employed in this study as a stabilizer rather than other additives or wastage material. In addition, the author used only the Triaxial testing method; meanwhile, there were no microstructures analysis nor non-destructive testing on the samples to verify the results of the Triaxial test.
Both studies by Esmaeili et al. [
10] and Frikha et al. [
23] applied a DSM technique in the laboratory for artificial saline dry sand and organic soft clay with 163% water content, respectively. The physical and chemical properties of the soil, including its grain size distribution, water content, Atterberg limits, clay mineral type, cation exchange capacity, soluble silica and alumina content, pore water pH, and organic matter content, first have an impact on the properties of the treatment applicability and then on the treated soil properties if successful [
24]. The treatment of saline sandy silt using a developed DSM apparatus in the laboratory has not been addressed elsewhere.
Accordingly, the current research aimed to present a detailed study on the treatment of saline silt soils using the deep soil mixing technique. A scaled-laboratory DSM setup that simulates the in-situ DSM method was developed in the laboratory. Furthermore, this study investigates the potential use of marble powder (MP) for the purpose of partial cement replacement in DSM.
2. Materials Properties
A large quantity of soil samples was collected. The soil was dried overnight in the oven (1 full day). Afterwards, the soil was grinded to break the cemented particles due to the presence of salt. To characterize the soil, grain size distribution ASTM D6913-04 [
25], Atterberg limits ASTM D4318 [
26], and standard proctor test ASTM D698-12 [
27] were performed. These tests were repeatedly conducted on identical samples. The particle size gradation of the soil is presented in
Figure 1. Both, the chemical composition and physical properties of the soil are listed in
Table 1 and
Table 2, respectively. This soil can be classified as a low plastic organic soil (OL) based on the Unified Soil Classification System (USCS). The soil field density and water content were determined at three (3) different locations using the sand cone test method as per ASTM D1556 [
28].
Ordinary Portland cement (OPC) and marble powder (MP) are employed as binders for soil stabilization. OPC that conforms to ASTM C150 [
29] was employed. The binders’ physical properties were determined. Particle gradation analysis, Atterberg limits and specific gravity ASTM D854-02 [
30] were carried out.
Figure 1 shows the particles gradation of OPC and MP. Atterberg limits, specific gravity and chemical analysis of MP and OPC are presented in
Table 1 and
Table 2. The MP has been imported from a marble industry factory as a byproduct in the form of a powder.
4. Stabilization Mixture Ratios
Recent studies revealed that binder proportions of 10, 20 and 30% on the basis of the dry soil mass were recommended for DSM [
32,
33,
34,
35]. In cases of soils with a significant amount of organic matter and soils with an excess of salt, particularly sulphates, which may delay cement hydration, special consideration is necessary. Smith [
36] stated that saline soil treatment has been mentioned to present some challenges, but these can be overcome by adding more cement.
Based on the above review, a binder content of 30% is considered in this study to reduce the effect of salts in saline soils. Unlike the previous studies where binder consists of pure cement, marble powder will be used to partially replace the cement. In order to establish the optimum binder design, different MP replacement ratios are selected to replace the cement in the total binder using the following OPC/MP ratios: 100/0, 90/10, 80/20, 70/30, 50/50, and 30/70. The content of OPC and MP are provided in terms of the mass of dry soil. In addition, water/binder (w/b) proportion is a significant parameter that have an impact on the strength of the stabilized soil. As referenced in many literatures, when the w/b proportion increases, the stabilized strength decreases. To determine the optimum water/binder (w/b) ratio, different w/b ratios samples were prepared.
Table 3 and
Table 4 present different mixes with different binder % and w/b ratios, respectively.
5. Specimens Preparation
The soil sample was sieved using sieve No. 4 (size of 4.75 mm) to remove any gravels and large size particles. Steel molds with dimensions of 150 mm diameter and 300 mm height have been selected to prepare the soil according to the determined field density and water content. Using a mechanical mixer, the soil was completely mixed with water. Then, the soil was filled into the mold and compacted into five layers up to 250 mm height. The mold weight before and after filling was recorded together with soil water content determination to control the target soil density. The molds were covered and sealed to maintain the soil water content until mixing time.
5.1. Binder Slurry Preparation
The binder has been prepared in a slurry form. First, the cement was mixed properly with the marble powder before adding water. Afterwards, the required quantity of water was added to the binder materials (Cement and MP). The binder was mixed with water using a 250 rpm mechanical mixer for at least 10 min to form a homogenous slurry.
5.2. Specimens Treatment Procedures
The procedures for the DM treatment are as follows: (a) the slurry was poured into the injection graduated container; (b) while the volume of the required injected slurry was pre-determined, the slurry volume was recorded before and after the injection process for quantity control requirements taking into account the amount of the slurry that fills the injection hoses and the auger stem; (c) the auger rotation speed was adjusted at 80 rpm; (d) the mixing auger was located at the center of soil top surface, and the verticality of the mixing auger must be checked prior to the mixing process; (e) mixing and injection processes were commenced; and (f) after completion of the mixing process, the volume of the remaining slurry was recorded together with the weight of the mold after treatment. The molds were covered with sealed plastic bags. The specimens were extracted from the mold after curing duration (7 days). The diameter of the specimen was measured precisely at different locations to ensure the quality of the mixing process. The samples were trimmed to a length/diameter ratio of 2.0. The samples were cut with a miter saw at both ends to get planar areas parallel to the sample height. After trimming, a griding tool was used to adjust a slight irregularity of the sample surfaces.
To ensure adequate quality of the treated soil, some measurements were recorded before, during and after samples preparation. The binder amount was pre-determined as 30% from the dry soil mass. The soil density and water content before treatment were recorded followed by the amount and volume of the injected slurry considering the losses. The treated soil weight was recorded. Water content of the treated soil was determined just after treatment. In terms of installation characteristics, the penetration and withdrawal rate (mm/min) were controlled using a stopwatch and a constant rate of penetration. Also, the rotation speed (rpm) was calibrated. Such measurements can control not only the quantity of the binder but also the quality of the mixing consistency.