Using Nanomaterials in Stabilization of Soil for Oil Infrastructures

The design foundations of storage tanks for oil industry experiences significant problems due to the widespread occurrence of weak and compressible soil which resulted in foundation failure. In this study, soft soils were taken from two locations and mixed with three types of nanoparticles which were nano-alumina (nano Al2O3), nano-copper (nano CuO), and nano-magnesium (nano MgO). Nanomaterials were incorporated in small percentage (less than 1%) by dry weight of soil. The tested geotechnical characteristics included the water content, dry density, and the unconfined compressive strength. The results showed significant enhancements in the maximum dry density and unconfined compressive strength. The level of enhancement depended on the type of nanomaterials and the contents. Improved strength and hardening properties were shown with the utilization of nano CuO material in comparison to the soil samples with the other nanomaterials additions, with its optimum addition of 0.7% provided an increment rate of 662.7% while the optimum nano CuO which is about 1% showed a 532% increasing rate in the compressive strength of S1 soil. It was noted that the maximum dry density and unconfined compressive strength enhanced with the increase in the nanoparticles content until reaching a percentage in which the strength decreased. The optimum content of the nano MgO was 0.3% while the optimum nano Al2O3 content was about 0.3% for soil S1 and was about No.28(9) 2020 Journal of Petroleum Research & Studies (JPRS) E37 0.1% for soil S2. The presence of nanomaterials in excessive contents caused agglomeration of particles which had negative influences on mechanical characteristics of the soils. Generally, the incorporation of finer particles like nanoparticles even with low amount would improve the geotechnical characteristics of soils with the consideration of the potential environmental benefits, these combined admixtures are intended to lower the cost and become a more sustainable and environmental alternative for soil stabilization.


Introduction
Storage tanks are widely spread where the oil industry represented by producing and refining crude oil . Nevertheless, large storage tanks with diameter larger than 76 m are rare and need soil with considerably high bearing capacity to lower settlement to sustain the rigid skid system. For such sensitive structures, careful investigations and design are required, most importantly where the underneath soils have seasonal variations in engineering characteristics [1,2].
Ideally, sensitivity structures like large tanks are built on pile as foundation, particularly in sites consisted of thick compressible soils having problems of settlement and seasonal variations. Nevertheless, the utilization of pile can be uneconomic when the thickness of clay soil is large [3]. For example, the pile foundation in many sections of the mangrove swamp is considered uneconomic for large tanks because of the thick compressible clay layers. The issues associated with pre-loading is that consolidation of the subsurface layers is considerably low due to the unfavourable situations occurred in the site represented by clay sediments that have low permeability and thickness of compressible layers [1]. Figure (1) showed photographs of storage tank problems in petroleum sector. Soft soils presented conventionally in areas that have high moisture content. When the Moisture content reached liquid limits, the soils would exhibit low shear strength and high level of settlement [1]. Hence, the construction on these soils required selecting firm and No.28-(9) 2020 Journal of Petroleum Research & Studies (JPRS) E39 strong foundations in order to meet the criteria of settlement and bearing capacity both before and after construction. Soil stabilization is the method utilized to improve soil properties in order to meet specifications of various projects [4,5]. Many authors have studied the stabilization of various types of soft soils utilizing various materials. Lime, cement and by-product like silica fume, rice husk ash, and fly ash are the conventional materials utilized for enhancing soils [6,7,8,9,10].
Nanotechnology is the technology working on creating various types of nanomaterials (NM) and their applications in many sectors. This typically can be donated as NP i.e. nanoparticles. NM is referred to material with size of 100 nm or lower in term of the dimensions [11,12]. In geotechnical engineering, the major nanotechnology strategy is the enhancement of soil poor characteristics with the addition of nanomaterials. The chemical and physical properties of soil can significantly be influenced with the presence of only a small quantity of nanomaterials in the soil because of the morphological characteristics as well as the high surface area and charges of such substances.
Recently, significant attentions have been paid to the utilization of nanoparticles due to the possibilities of using them in various applications and their low cost, high reactivity and eco-friendly green aspects [13,14,15]. Biosynthesized nanomaterials have beneficial features represented by using them in biomedical, agriculture and engineering industry [16,17].
In soil stabilisation, various studies were conducted for investigation of effect of using nanomaterials on the properties of weak soils. [18] Investigated the strength and plasticity characteristics of a soft soul treated with a nanomaterial. Through unconfined compressive strength and Atterberg limits tests, they reposted that the mixture of Modified Montmorillonite Nano clay with the soil considerably enhanced the unconfined compressive strength and increase the plasticity index of the soil. [19] Studied the effect of four kinds of nanomaterials (nano-alumina, nano-silicanano-silica, nano-copper, and nanoclay) on the stabilisation of a collapsible soil. They found that the physical and chemical properties of soil could be enhanced by the use of appropriate amount of nanomaterials. [20] found that the use of recycled polyester fiber and nano-silicon dioxide increased the unconfined compressive strength, shear strength and modulus of elasticity of soft clay [21] indicated that the inclusion of nanoparticles in kaolin clay led to an increase in the No.28-(9) 2020 Journal of Petroleum Research & Studies (JPRS) E40 shrinkage limit and a reduction in the total volume of the treated clay. [22] In their research indicated a considerable reduction in swelling and enhanced workability of an expansive soil stabilized with nano-alumina and nano-magnesium oxide. [23] Conducted a detailed study for evaluating the influence of using nano-copper, nano-clay, and nano-alumina on The unconfined compressive strength was used to indicate the mechanical characteristics of the soil samples while the physical properties were treated using the standard proctor test.

Soils:
Two locations in Malaysia were selected to extract two types of soft soils used in this study. Soil (S1) was taken from Transkrian, in the State of Penang while soil (S2) was extracted from Banting, in Selangor, Malaysia. The depth of excavation ranged from 0.5 to 1.0 m and all the soil samples were distributed. They were excavated by hand shovels and taken from bottom of the borrow pit. Table (1) gives the index characteristics represented by plasticity index (PI), Unified soil classification (USCS), and grain size fraction of the selected soils. Soil S1 had a relatively low specific gravity (Gs) because of incorporating high amount of organic substances in compared to soil S2. Hence, soil S1 was indicated to No.28-(9) 2020 Journal of Petroleum Research & Studies (JPRS) E41 be organic (OL). Also, the soils were kind of acidic which is the conventional type of soil presented in tropical areas.

Nanomaterials:
The main characteristics of the utilized nanomaterials were illustrated in the  Compacted samples were taken by placing tubes with 38 mm in diameter in the soil utilizing a compression machine. Samples were extruded from the tube using an extruder, after that they were cut into 89 mm long samples. Immediately then, all samples were tested utilizing a test carried out in accordance with the ASTM (D2166-65). result was in agreement with those of the [24] who stated that compaction is influenced by some factors including specific gravity and the particle size of the stabilizer and the soil.

Results and Discussion:
The optimum moisture increase was related to the excess absorbed water that held in the flocculent structure of the soil due to its porous nature. This additional water in the soil can be justified by its content of organic materials.
However, for soil S2, the use of all types of nanomaterials reduced the optimum water content and increased the maximum dry density. The optimum water content reduction was attributed to the nanomaterials tendency of absorbing the moisture from the soil as previously explained. Also, the high surface area of the nanomaterials can reduce the optimum water content [25] which in turn can result in a decrease in the soil voids' number [26]. The dry density increase can be related to the high density of the nanomaterials in comparison with that of the soil. Moreover, the nanoparticles filled the pores between the soil particles and thereby reducing the porosity and bonding the particles together. compressive strength improvement of about 5.6% for soil S1 and a considerable strength improvement of about 341.8% for S2. Whereas, the optimum nano-Al 2 O 3 content was about 0.3% for soil S1 with compressive strength improvement of 111% and was about 0.1% for so S2 with compressive strength improvement of 400%. The results were in a good agreement with those of the [28,23].
It was noted from below figures that for all types of nanomaterials used, the improvements in compressive strength of the treated soil S2 were considerably higher than those of the soil S1. This could be as a result of the organic content of the soil S1. [29] Reported that the presence of high organic content can reduce the formation of stabilisation products since it influences the chemical reactions of the nanomaterials with soil. However, in general, the strength of soil depends on its type and the numerology of clay. The increment in the amount of nanomaterials beyond their optimum might led to the agglomeration of nanoparticles which caused a void ratio increase, density reduction, and water content increase. The agglomeration of the nanoparticles was evident as adversely impacted the soils mechanical characteristics. The agglomeration represents a natural phenomenon involving particles sticking to solid surfaces and to each other which causes a size enlargement. According to [30] for nano-scaled particles, the agglomeration phenomena increase the necks amount between particles and thereby reducing the soil density.
The phenomena were illustrated in Figure ( E48 the nano CuO increased beyond its optimal limit of 0.7%. It should be noted that the same magnification of 25000 KX were used for taking all of the SEM photos.