Internal Erosion Stabilization of Cohesionless Soil Using Lime

Abstract

Soil embankments are valuable for the adequate reserve and supply of water to multiple industries. However, they are susceptible to internal soil erosion, which may ultimately lead to structural collapse. To counteract this issue, soil stabilization is practiced in the construction industry. This paper proposes the internal erosion stabilization of cohesionless soil using quicklime. For this research, two cohesionless soil types were investigated and treated with quicklime: poorly graded and well-graded cohesionless soils. For poorly graded soil, the lime percentage varied from 0.0% to 6.0% based on the soil’s weight, while for well-graded soil, it ranged from 0.0% to 3.0%. All the soil specimens were cured for 24 h and tested using the hole erosion test (HET) to replicate the internal erosion effortlessly. The analyzed results demonstrated the efficiency of quicklime as an internal erosion stabilizer for cohesionless soils. The optimum lime content for poorly graded cohesionless soils was 5.0%; for well-graded, the percentage was approximately 3.0%. Moreover, adding lime significantly improved the strength, critical shear stress, and erosion rate index of the soil.

1. Introduction

The adequate availability and supply of water is essential for the effective operation of industries such as construction and agriculture. Earthen embankment structures generally provide proper water sources to these industries by alternating, averting, or reserving water [1]. Thus, the structural failure of earthen embankments can lead to a catastrophe, destroying multiple lives and properties [2]. Luthi [3] argued that approximately 30–50% of embankment failures and mishaps are caused by the internal erosion of the soil. Even Hanneman [4] and Foster et al. [5] found that internal erosion is the most prominent reason for embankment failures. Internal soil erosion is triggered by soil particle erosion due to water penetration through the structure [3,6]. A continuous flow of water through the soil leads to the formation of holes in the structure, resulting in structural failure. Hence, seepage is considered a threat to earthen structures during construction’s design and operation stages. Therefore, engineers are advised to be cautious and aware of the properties of materials used in earthen structures [2]. Due to the gravitas of this issue, recent studies [7,8] have emphasized the effects of internal erosion of soil on earthen structures and their counteractive measures. Many studies [9,10,11,12] have investigated the factors affecting internal soil erosion, such as soil type, water, and material properties, and soil-piping resistance relationships. For dispersive soil, soil resistance against erosion rate increases with higher salt content in the water [9]. To measure the embankment soil erosion parameters, hole erosion test (HET) and slot erosion test (SET) methods were established by Wan and Fell [13]. Soil investigation determined that factors such as fine soil and clay content, clay mineralogy, dry density, compaction water content, dispersity, degree of saturation, plasticity, and cementing materials (such as iron oxides) affect their shear strength parameters, permeability, settlement, and erosion rate values [14,15,16]. In addition, it was concluded that higher erosion with reduced critical shear stress occurs in non-cohesive and coarse-grained soils compared to fine-grained soils.

To prevent embankment dam failure, stabilization techniques are performed on the soil, increasing the soil strength and restricting the formation of holes in the structure. Soil stabilization may be performed by physical or chemical means [17]. Many research studies [18,19,20,21,22,23,24,25,26,27] have focused on the effect of adding stabilization materials to the soil. Lemaire et al. [18] investigated the inclusion of a cement–lime mix in silty soils. The silty soil was stabilized by adding 5.0% and 1.0% of cement and lime, respectively. Moreover, a significant increase in the structure’s unconfined compressive strength and microporosity filling was noticed. In addition, stabilization with a lime and micro silica admixture of silty sand soils was investigated by Karimi et al. [19] in the presence of sulfates. The results demonstrated a significant increase in the soil’s California Bearing Ratio (CBR) value and reduced swelling properties. Thus, micro silica waste material successfully improved the silty sand soil resistance. Khemissa and Mahamedi [20] mixed cement and lime in over-consolidated clay and determined that the addition of 8.0% of cement and 4.0% of lime into the clay improved its bearing capacity, durability, and shear strength. Al-Aghbari, Mohamedzein, and Taha [21] tested the stabilization effects of Portland cement and cement kiln dust on desert sands. The results showed remarkable improvements in the shear strength, maximum dry density, and unconfined compressive strength of the soil. Hence, the study concluded that combining cement and cement kiln dust enhances desert sand’s shear strength and compressibility properties [21]. They also investigated the impact of utilizing municipal solid waste incinerator ash to strengthen desert sand soil. Incinerator ash enhanced the shear and unconfined compressive strength characteristics with lower permeability [22]. Recently, Soundarya [23] studied the effects of fly ash and ground granulated blast-furnace slag (GGBS) on lime-stabilized mud blocks. Approximately 5% of lime was added, and the fly ash and GGBS contents varied from additions of 0% to 10% (with a 2% increment). The results demonstrated a positive correlation between the amount of fly ash and GGBS and the compressive strength of the mud blocks. Wet compressive strength was significantly affected by the increase in the GGBS amount compared to the fly ash amount. Additionally, fly ash improved water absorption rates more than GGBS since fly ash is more efficient in producing cementitious materials with lime and reducing pore interconnectivity. Stabilized compressed earth blocks were also tested with lime and a combination of cement and lime by Malkanthi, Balthazaar, and Perera [24]. It was found that the cement and lime addition provided higher compressive strength than the addition of just lime. Ten percent lime gave one of the best performances, and it was concluded that lime-stabilized blocks might be used for single-story buildings. However, for Grade 2 block strength with 15% and 10% of clay and silt, respectively, a mix of 5% of cement and 5% of lime was required. For blocks with 5% clay and silt, approximately 7% cement and 3% lime were required. Studies have also used a combination of cement and rice husk ash [28] or cement and fly ash [29] to stabilize the soil. Attom and Shatnawi determined that clayey soil is strengthened with the inclusion of wheat husk [30]. Internal erosion soil stabilization using chemicals was investigated by Vinod et al. [25] and Vakili et al. [26]. Vinod et al. [25] found that adding lignosulfonate to dispersive soils results in better soil erosion coefficients and critical shear stress, ultimately providing higher soil strength and stabilization. In comparison, Vakili et al. [26] mixed lignosulfonate and reinforced polypropylene in dispersive soil to stabilize it against internal erosion. Other studies emphasizing soil stabilization include [31,32,33,34,35,36,37,38,39]. Although significant research has been performed using lime to stabilize clayey soils, a lack of research exists on using lime to stabilize cohesionless soils such as sand. One of the few studies on the effect of lime stabilization on cohesionless (sandy) soil with different curing times was conducted by the authors [40]. For this study, poorly and well-graded soil types were mixed with quicklime and tested after 1, 2, and 7 days of curing. Using the hole erosion test (HET), the soil types were determined to be stabilized to an optimum level with a minimum of 2 days of curing. Another study on the effect of lime stabilization of coarse sandy soil against internal erosion was performed by Elandaloussi et al. [41]. The erosion test results showed that the lime stabilization effect begins with a curing time of only 24 h due to the agglomeration of fine particles. Moreover, as per their reported results, longer curing times, such as 3 months, did not significantly increase the efficiency of lime treatment.

Thus, this research studies the effect of different lime percentages at a single curing time, and this emphasizes the effect of the internal erosion stabilization of cohesionless soils with different lime percentages and a short (24 h) curing time. As mentioned, the authors performed a similar investigative study on the effect of curing time on lime-stabilized cohesionless (sandy) soil in [40]. Poorly and well-graded soils were examined with quick lime in these studies. Different lime percentages were mixed with the soil based on the soil weight. The soil samples were compacted and prepared in a standard proctor mold with a relative compaction rate of 95.0%. Lime addition resulted in improvement in the soil strength and erosion rate index. The rest of the paper has been organized in the following manner: Firstly, the research significance is presented. Next, the methodology used to prepare and test the soil samples is provided. Following this, the results and their detailed discussion are presented, including the addition of lime effect on water flow path, critical shear stress, erosion rate index, and internal soil erosion. Lastly, the conclusion of the study is provided.

2. Research Significance

Soil embankments are essential for providing, diverting, and retaining water for different purposes. Since the primary component of any soil embankment is sand, it is vital to stabilize the sand being used for the effective and successful operation of the structure. Based on the reviewed literature, lime is found to be an efficient and cost-effective stabilizer for clayey soil. However, to the best of the author’s knowledge, there has been insufficient research investigating the stabilization effect of the addition of lime to cohesionless soil. Hence, this study tries to increase the knowledge base by studying the internal erosion stabilization of cohesionless soil using lime and promoting its utilization with confidence in the industry. This research studied two cohesionless soil types (poorly and well-graded), which were cured for 24 h. The poorly graded soil was mixed with lime in percentages from 0.0% to 6.0% (based on the soil’s weight), and well-graded soil was mixed with lime in percentages from 0.0% to 3.0%. The hole erosion test (HET) was adopted to replicate the internal erosion of the soil scenario. This study assists in analyzing lime-stabilized cohesionless soil properties. It also supports the raising of awareness about internal soil erosion in earth-fill dams and counteractive measures using soil stabilization. Further studies may be performed on this topic using other stabilization materials, such as solid waste and bitumen.

3. Experimental Program and Methodology

The experimental program of this study is inspired by [13] and the authors performed a similar investigation previously [40] to study the curing time effect.

3.1. Material Properties

The material properties of the soil studied are displayed in

Table 1. Soil Properties (similar to [40]).

Property	Soil A	Soil B
Specific Gravity, Gs of soil	2.60	2.67
Amount of Clay (%)	0.00	4.00
Amount of Silt (%)	0.60	8.00
Amount of Sand (%)	99.40	88.00
Coefficient of Uniformity, Cu	1.60	11.05
Coefficient of Curvature, Cc	0.90	2.26
Optimum Moisture (or Water) Content, ωop (%)	11.95	13.00
Maximum Dry Density, $\rho$dmax (kg/m3)	1690.00	1908.00
Classification	Poorly Graded	Well Graded

. Specific tests were performed to determine the initial soil properties in accordance with ASTM standard procedures. Based on the results in Table 1, it is clear that Soil A was poorly graded while Soil B was well-graded cohesionless soil. The manufacturer provided the quick lime properties (which can be found in [40]).

3.2. Specimen Preparation

Lime percentages from 0.0% to 6.0% were added for Soil A, while, for Soil B, lime percentages from 0.0% to 3.0% were used. Thus, three soil specimens were prepared and cured for each lime percentage for 24 h. The soil specimen preparation included the following steps: Firstly, the amount of soil, lime, and water (based on optimum moisture content) was measured and mixed. Then, compaction of the prepared mix in a mold was performed with a relative compaction rate of 95%. Following this, a hole was drilled with a diameter of 6 mm through the compacted soil specimen’s cross-section, as shown in Figure 1a (this step was conducted to initiate soil piping erosion). Lastly, the specimen was labeled and stored for its curing time. An average of two specimens for each lime percentage was considered for higher accuracy of the results. The final results are discussed in this paper for each lime percentage. As mentioned, this study adopted the hole erosion test, HET (by Wan and Fell [13]) to analyze the soil resistance against internal erosion. This method is preferred because of its simplicity, ease, and lower cost. Moreover, it is considered the best method to replicate piping erosion behavior [42]. The HET apparatus and schematic diagram are displayed in Figure 1b,c, respectively.

3.3. Specimen Testing Process

The upstream chamber of the HET apparatus was filled with gravel particles of 20 mm in diameter to regulate and filter the water flow. Subsequently, the prepared specimen was fitted between the inlet and outlet chambers with the help of O rings and bolts (as shown in Figure 1b). The water head ranged between 800 and 1200 mm, and the test was initiated by allowing the water to flow through the hole of the specimen. At different intervals of test run time, the rate of flow was measured using the outlet pipe. The test ran until the specimen’s failure, with a minimum of 45 min. At the end of the test, the water flow was stopped, and the specimen was extracted. Finally, the final hole diameter was determined, and the test apparatus was cleaned before reusing it for other specimens. It is advised that future researchers interested in researching this topic should ensure that the HET apparatus is built to be leakproof to avoid any water leaks during the testing process. Additionally, during the test, the flow rate (water volume and time) must be determined accurately by measuring them at multiple time intervals throughout the test. Furthermore, the final hole diameters are advised to be measured using a vernier caliper for higher accuracy.

3.4. Process for Analysing the Data

The HET test data helped determine the soil’s shear stress at the start of erosion and the erosion rate index. Erosion occurs when the water force outpaces the hydraulic shear stress [13]. The soil erosion rate index ($I$) parameter is a good indicator of the soil’s internal erosion resistance. Stronger soil internal erosion resistance provides a higher erosion rate index. The soil erosion descriptions for different erosion rate indices are provided in

Table 2. Internal Erosion Descriptions for Various Erosion Rate Indices [13,40].

Group Number	Erosion Rate	Description
1	Less than 2	Extremely Rapid Erosion
2	2 to 3	Very Rapid Erosion
3	3 to 4	Moderately Rapid Erosion
4	4 to 5	Moderately Slow Erosion
5	5 to 6	Very Slow Erosion
6	Greater than 6	Extremely Slow Erosion

.

Initially, Reynold’s number ($R$) was utilized to determine the type of water flow as per Equations (1) and (2):

$$V_t=\frac{Q_t}{\pi \left(\frac{{\phi }_t^2}{4}\right)}$$

(1)

$$R=\frac{V{\phi }_t{\rho }_w}{\nu }$$

(2)

where $V_t$ denotes the average velocity of water flowing through the hole (m/s) at time t, Q_t is the flow rate at time t (m³/s), ${\phi }_t$ is the water flow path (hole) diameter at time t (m), ${\rho }_w$ is the water density (kg/m³), and $\nu$ denotes the absolute water viscosity (Pa·s) = 1.00 × 10⁻³ [40,43]. Using the flow type, flow rate, and initial and final measured hole diameters, the friction factors ($f_{Tt}$ or $f_{Lt}$) were determined at the start and end of the test with the help of Equations (3) and (4) [13,40]:

$$Turbulent Flow:{\phi }_t=[{\frac{64Q_t^2{f}_{Tt}}{{\pi }^2{\rho }_{w}g{s}_t}]}^{1/5}$$

(3)

$$Laminar Flow:{\phi }_t=[{\frac{16Q_t{f}_{Lt}}{\pi {\rho }_{w}g{s}_t}]}^{1/3}$$

(4)

where $f_{Tt}$ or $f_{Lt}$ is the turbulent or laminar friction factors at any time, respectively, ${\rho }_w$ is the water density, $g$ is the acceleration due to gravity, and $s_t$ is the soil sample hydraulic gradient. Based on the calculated initial and final friction factors, the friction factor versus the time curve was plotted, giving the friction factor value at any time ($f_{Tt}$ or $f_{Lt}$). Then, the hole diameter at any time (${\phi }_t$) was calculated by reusing Equations (3) or (4) with the evaluated friction factors at any time ($f_{Tt}$ or $f_{Lt}$). Further, knowing the ${\phi }_t$ values, the rate of diameter change with time $\left(\frac{d{\phi }_t}{dt}\right)$ was determined. Next, using the evaluated values, soil dry density (${\rho }_d$) and Equations (5) and (6), hydraulic shear stress (${\tau }_t$) and erosion rate per unit surface area (${\epsilon }_t^{·}$) at time t were determined [13,40]:

$${\tau }_t={\rho }_{w}g{s}_t\frac{{\phi }_t}{4}$$

(5)

$${\epsilon }_t^{·}=\frac{{\rho }_d}{2}\frac{d{\phi }_t}{dt}$$

(6)

$${\epsilon }_t^{·}=C_e({\tau }_t-{\tau }_c)$$

(7)

$$I=-log\left(C_e\right)$$

(8)

Subsequently, as illustrated in Figure 2, the erosion rate (${\epsilon }_t^{·}$) was plotted against hydraulic shear stress (${\tau }_t$) to obtain the soil erosion coefficient ($C_e$) (as per Equation (7)). Lastly, utilizing the $C_e$ value and Equations (7) and (8), the critical shear stress (${\tau }_c$) and soil erosion rate index ($I$) values were respectively determined. The $I$ value was then compared with the details in Table 2 to describe the soil erosion type [13,40].

4. Results and Discussion

4.1. Addition of Lime Effect on Water Flow Path

The effect of lime percentage on the water flow path (hole) diameter over their test run time (in minutes) for Soils A and B with 24 h curing time is illustrated in Figure 3. For Soil A, with an initial hole diameter of 6.0 mm, an increase in hole diameter of approximately 27.0 mm (33.0–6.0 mm) was observed at 1.0% of lime, with a test run time of approximately 7 min. However, at 2.0% of lime, the increase in the hole diameter was approximately 15.5 mm (21.5–6.0 mm), with a test run time of 10 min. Therefore, a 42.6% reduction in hole diameter occurred with the increase in lime content from 1.0% to 2.0%, along with a longer test run time. This demonstrates that the internal erosion resistance of the soil improved with the increase in the addition of lime. Moreover, it is evident from Figure 3a that with a further increase in lime percentage, the change in water flow path (hole) diameter reduces, with the lowest change and longest test run time occurring at 6.0% lime for Soil A. At 6.0% lime, only a 4.3 mm increase (10.3–6.0 mm) in the hole diameter was observed with a test run time of approximately an hour (60 min). Thus, an 84.1% reduction in the change in diameter was obtained with the increase in lime from 1.0% to 6.0%. Hence, the soil strength improved significantly with a higher lime content and reduced the internal erosion rate by resisting the change in the water flow path (hole) diameter. The results for 0.0% of lime in Soil A are not provided since the soil specimen collapsed within a few seconds of the test run time; thus, data could not be derived for this specimen. Therefore, it was noted as a failure point and was deemed insignificant. Similarly, Soil B portrayed that a higher lime percentage decreased the change in water flow path diameter (as shown in Figure 3b). An increase of 9.0 mm (15.0–6.0 mm) was noted at 0.0% lime with a test run time of 6 min. At 1.0% of lime, the hole diameter change was approximately 2.2 mm (8.2–6.0 mm), with a test run time of 51 min. Thus, the water flow path diameter change was decreased by 75.6% with a longer test run time. A minimalistic change in water flow path diameter of 2.0 mm (8.0–6.0 mm) was noticed at 3.0% lime with a test run time of more than 2 h (approximately 135 min). Hence, the inclusion of 3.0% lime in Soil B and 24 h of curing time decreased the diameter change by 77.8% compared to the Soil B specimen without any lime. Therefore, a higher amount of lime strengthened the cohesionless soil and significantly improved its internal erosion resistance ability.

Additionally, Figure 4 illustrates the water flow path (hole) diameter (mm) at the end of the test against their lime percentages (%) for Soils A and B. It displays that the final hole diameter reduced with more lime. Furthermore, it demonstrates that Soil B specimens at 24 h curing time performed considerably better than Soil A specimens. Since 0.0% of lime in Soil B did not fail instantly, unlike Soil A, it resisted internal erosion briefly. Additionally, the final diameter of the hole for all lime percentages was lower in Soil B when compared to Soil A. This occurred due to the soil’s physical properties. Soil A was a poorly graded cohesionless soil with lower strength due to the absence of clayey and silty soil elements, resulting in lower stability. On the other hand, Soil B (well-graded cohesionless soil) showed a significant amount of silt and clay (which react well with lime) and, therefore, had slightly higher strength and stability, and thus it showed better resistance against internal erosion. In addition, Soil A required double the amount of lime (approximately 6.0% of lime) compared to Soil B (which required only approximately 3.0% lime) to significantly reduce the change in diameter. Hence, Soil B reacted and stabilized better with lime in comparison with Soil A. Similar findings were deduced in [40].

4.2. Addition of Lime Effect on Critical Shear Stress

Figure 5 displays the critical shear stress values for Soils A and B with their respective lime contents (%) and 24 h curing time. Higher lime percentages resulted in an increase in critical shear stress values for both Soil A and Soil B, as illustrated in Figure 5. Although an increase in lime from 1.0% to 2.0% in Soil A did not improve the critical shear stress value (=85 N/m²), a further increase in lime to 3.0% slightly improved the value to 97 N/m². Thus, an approximately 14.1% increase in critical shear stress occurred at 3.0% lime, compared to 1.0% lime in Soil A. According to the results, the highest critical shear stress of 115 N/m² for Soil A was obtained at 5.0% lime. Thus, increasing the lime from 1.0% to 5.0% resulted in 35.3% higher critical shear stress. Moreover, a further increase in lime did not increase the shear stress value, it can thus be deduced that Soil A was optimally stabilized at 5.0% lime and 24 h of curing time. In contrast, the critical shear stress of Soil B rose from 100.5 to 105 N/m² with an increase in lime from 0.0% to 1.0%, respectively. Therefore, for Soil B, the stress increased by 4.5% with the addition of 1.0% lime in comparison to the Soil B specimen without any lime. Moreover, a further increase in lime to 3.0% improved the critical shear stress value to 140 N/m², which was approximately 39.3% higher compared to the critical shear value of 0.0% lime. A lime percentage greater than 3.0% was not performed since the soil was observed to be optimally stabilized at 3.0% lime and 24 h curing time. Hence, higher lime percentages resulted in improving the critical shear stress of the soil. Previous researchers also deduced these findings [34,40,44,45]. In addition, the critical shear stress values of Soil B were determined to be higher in comparison with Soil A. As mentioned, this is attributed to the fact that Soil A was a poorly graded cohesionless soil with lower strength and stability, while Soil B was a well-graded cohesionless soil with slightly higher strength and stability due to the presence of clay and silt. Due to this, Soil B required only approximately 3.0% of lime to reach a shear stress value of 140 N/m², while Soil A required approximately 5.0% lime to reach a shear stress value of 115 N/m². Hence, it is evident that Soil B reacted well with lime, providing higher critical shear stress and strength to resist internal erosion compared to Soil A, similar to the reported results in [40].

4.3. Addition of Lime Effect on Erosion Rate Index (I_HET)

To adequately represent the effectiveness of the internal erosion stabilization of cohesionless soil using lime, the erosion rate index (I_HET) value was evaluated for the two soil types. Figure 6 provides the erosion rate indices for different lime percentages in Soils A and B within 24 h of curing. This figure demonstrates that higher lime percentages result in improving the I_HET value until the effective lime stabilization of the soil occurs. For Soil A with 1.0% lime and 24 h curing, the I_HET was 3.69, representing a moderately rapid erosion type, as per Table 2. Increasing the amount of lime to 2.0% increased the I_HET to 4.00, a moderately slow erosion type, as per Table 2. Therefore, increasing the lime percentage increased the I_HET value (by 8.4%) and the soil’s internal erosion resistance ability. With higher lime percentages such as 5.0% (or 6.0%) and 24 h of curing, the I_HET of Soil A improved to a value of 4.69, which is considered to be a moderately slow erosion type, as per Table 2. Therefore, increasing lime from 1.0% to 5.0% led to a 27.1% improvement in the I_HET value (as shown in Figure 6). This validates that an increase in Soil A’s lime content caused the internal erosion stabilization of the soil. For Soil B at 0.0% lime and 24 h curing time, the I_HET was determined to be 3.69, a moderately rapid erosion type, as per Table 2. With the addition of 1.0% lime, the I_HET increased by 27.1%, changing the soil erosion resistance from a moderately rapid to a moderately slow erosion type. A further increase in lime to 3.0% increased the I_HET to a value of 5.301, representing a very slow erosion type, as per Table 2. Therefore, increasing the lime from 0.0% to 3.0% resulted in improving the I_HET value by 43.7%. These results also validate that increasing the lime content improves the internal erosion resistance of soil, similar to the deductions reported in [34,40,44]. Moreover, Soil B again demonstrated better results, with comparatively higher erosion rate indices with respect to Soil A (refer to Figure 6). Soil B reached a 13.0% higher I_HET value with a lime content of only 3.0%, in comparison to Soil A with a higher lime content of 5.0%, demonstrating the significance of the initial soil properties.

4.4. Internal Erosion Types of Soil A and Soil B

The erosion parameters for Soils A and B at different lime percentages are displayed in

Table 3. Final hole erosion test results of Soils A and B.

Lime Percentage	Erosion Parameters	Soil A	Soil B
0.0%	Final Hole Diameter, φf (mm)	-	15
	Critical Shear Stress, τc (N/m2)	-	100.5
	Erosion Rate Index, IHET	-	3.69
	Soil Erosion Type	Failed Instantly	Moderately Rapid
1.0%	Final Hole Diameter, φf (mm)	33	8.2
	Critical Shear Stress, τc (N/m2)	85	105
	Erosion Rate Index, IHET	3.69	4.69
	Soil Erosion Type	Moderately Rapid	Moderately Slow
2.0%	Final Hole Diameter, φf (mm)	21.5	8
	Critical Shear Stress, τc (N/m2)	85	133.33
	Erosion Rate Index, IHET	4	5.22
	Soil Erosion Type	Moderately Slow	Very Slow
3.0%	Final Hole Diameter, φf (mm)	19	8
	Critical Shear Stress, τc (N/m2)	97	140
	Erosion Rate Index, IHET	4	5.301
	Soil Erosion Type	Moderately Slow	Very Slow
4.0%	Final Hole Diameter, φf (mm)	12.5	-
	Critical Shear Stress, τc (N/m2)	100	-
	Erosion Rate Index, IHET	4.22	-
	Soil Erosion Type	Moderately Slow	-
5.0%	Final Hole Diameter, φf (mm)	10.5	-
	Critical Shear Stress, τc (N/m2)	115	-
	Erosion Rate Index, IHET	4.69	-
	Soil Erosion Type	Moderately Slow	-
6.0%	Final Hole Diameter, φf (mm)	10.3	-
	Critical Shear Stress, τc (N/m2)	115	-
	Erosion Rate Index, IHET	4.69	-
	Soil Erosion Type	Moderately Slow	-

. Increasing the lime percentage caused a reduction in the final water flow path diameter and an increase in the critical shear stress (τ_c) and erosion rate index (I_HET) values. Thus, higher I_HET means stronger soil, higher shear strength, and lower internal erosion hole diameter values. Based on the results in Table 3, Soil A required approximately 5.0% of lime to effectively stabilize against internal erosion. As Soil A achieved the highest I_HET and τ_c values of 4.69 and 115 N/m², respectively, at 5.0% lime, and as per Table 2, this soil demonstrated a moderately slow erosion type. Moreover, Soil A showed a significantly small rise in water flow path diameter at 5.0% lime with a test run time of 60 min (1 h). Table 3 shows that a further increase in lime does not improve the strength and internal erosion resistance of the soil. Thus, Soil A was optimally stabilized with the addition of 5.0% lime and curing of 24 h. For Soil B, approximately 3.0% of lime was required for its optimum stabilization with curing of 24 h. At 3.0% lime in Soil B, the I_HET and τ_c were equal to 5.301 and 140 N/m², respectively. This I_HET value represents a very slow erosion type, as per Table 2. In addition, Soil B displayed a minimal increase in the water flow path of 2.0 mm with a constant pressure head of 1200 mm and a test run time of 135 min (more than 2 h). Thus, it can be deduced that the soil was stabilized. Hence, Soil B was stabilized with 3.0% lime with curing of 24 h. Similar conclusions were drawn by [40]. For an illustration of some soil specimens’ final water flow path holes, refer to Figure 7.

Figure 8 demonstrates the relationship between the obtained final hole diameter (φ_f) and τ_c against the I_HET values of Soils A and B with different lime percentages and 24 h of curing. It displays that a higher erosion rate index lowered the final internal erosion hole diameter and increased the critical shear stress (τ_c) values for both soil types. For Soil A, adding 5.0% lime resulted in a 68.2% lower final hole diameter, 35.3% higher τ_c value, and 27.1% higher I_HET value. For Soil B, the addition of only 3.0% lime resulted in a 46.7% lower final hole diameter, 39.3% higher τ_c value, and 43.7% higher I_HET value. Thus, Soil B reacted significantly better with lime than Soil A. Again, this was because Soil B contained clay and silt (refer to Table 1). Since clay and silt are binding materials and lime reacts much better with soils with binding materials, Soil B showed better results. In contrast, Soil A was poorly graded with an absence of clay and silt; consequently, it showed lower shear strength and a relatively weaker reaction with lime. Hence, Soil A required a higher lime content (5.0%) for stabilization than Soil B. Nevertheless, even with a higher lime percentage, Soil A (due to its weak properties) resulted in lower I_HET and τ_c values by 11.5% and 17.9%, respectively, compared to Soil B. Additionally, the internal erosion type of Soil A with 5.0% lime (moderately slow) was inferior than Soil B with only 3.0% lime (very slow). Therefore, it was determined that the initial soil type, properties, and gradation played a vital role in the internal erosion stabilization effect of the soil using lime.

5. Conclusions

The internal erosion stabilization of cohesionless soil using lime was investigated in this paper. The hole erosion test (HET) was adopted and conducted to execute the experimental testing of the study, as it best replicated the soil’s internal erosion. Two cohesionless soil types, poorly graded and well-graded, were tested with various lime percentages and 24 h of curing time. HET test results were analyzed, and different erosion parameters, including water flow path diameter, critical shear stress (τ_c), and erosion rate index (I_HET), were evaluated. Based on the findings, the following conclusions were drawn:

• Quick lime is an adequate internal erosion stabilizer for cohesionless soils;
• The water flow path diameter can be reduced and controlled with a higher amount of lime for both types of cohesionless soil;
• An increase in lime content leads to stronger critical shear stress (τ_c) and higher erosion rate index (I_HET) values for cohesionless soil;
• The inclusion of 5.0% lime in poorly graded soil resulted in a 68.2% lower final hole diameter, 35.3% higher critical shear stress (τ_c), and 27.1% higher erosion rate index (I_HET) value. In contrast, for Soil B, the addition of only 3.0% lime resulted in a 46.7% lower final hole diameter, 39.3% higher τ_c, and 43.7% higher I_HET value;
• For poorly graded cohesionless soil, with 5.0% optimum lime content, the internal erosion improved from a moderately rapid erosion type to a moderately slow erosion type, and for well-graded soil, with only 3.0% of lime, the internal soil erosion improved from a moderately rapid to a very slow erosion type;
• Lower lime is required to stabilize well-graded cohesionless soil as opposed to poorly graded soil. Moreover, stabilized well-graded soil demonstrated comparatively better erosion parameters than stabilized poorly graded soil. This is related to the clay and silt (binding materials) present in well-graded soil, which gives higher strength and better supports the lime reaction. Hence, well-graded soil is preferred in construction compared to poorly graded cohesionless soil.

6. Recommendations for Future Work

This study was performed at the American University of Sharjah (AUS) as part of their research program to investigate the internal erosion stabilization of cohesionless soil using lime. Based on the experience, it is highly recommended to ensure that the testing apparatus is built to be leakproof and that multiple flow rates are noted at various time intervals to increase the accuracy of the results. Moreover, due to the limited scope of this study, not all factors could be investigated. Thus, further research may be performed to investigate internal soil erosion with different (1) soil properties such as clay, silt, and sand contents; cohesions; angles of internal friction; optimum moisture contents; maximum dry densities; and gradations; (2) hole erosion test factors such as the pressure head and test run time; and (3) internal erosion testing methods such as the slot erosion test (SET), rotating cylinder test, jet erosion test (JET), and soil dispersity tests (such as pinhole, Emerson crumb, and double hydrometer tests). Additionally, studies related to other soil stabilizers (bitumen, oil shale, fly ash) can be conducted on this topic.