Subscribe Now Subscribe Today
Research Article
 

Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)



David Lomeling, Juma L.L. Yieb, Modi A. Lodiong, Mandlena C. Kenyi, Moti S. Kenyi and George M. Silvestro
 
Facebook Twitter Digg Reddit Linkedin StumbleUpon E-mail
ABSTRACT

The study discusses the empirical derivation of undrained shear strength Sund from vane tests and compared this to those derived from predictions under the influence of both angular velocity (ω) and Soil Moisture Content (SMC) percentage. The study conducted on partially and fully saturated samples of sandy loam soil (Eutric leptosol) evaluated the effects of three different angular velocities and Soil Moisture Contents (SMCs) on the Sund, using a pocket vane tester. The angular velocities used in this study were 7.2, 3.6 and 1.8° sec–1, which corresponded to rotation rates of 5, 10 and 20 sec per 0.1 kg cm-2, respectively. The results showed that Sund positively correlated with time to failure (tf) irrespective of change in the angular velocity and negatively correlated with both (ω) and SMC, which both were best described using a power function: Sund = Λθ. The Sund was constant at any given SMC regardless of variations in angular velocity however, the Sund decreased with increase in SMC suggesting that Sund was largely controlled by SMC and less by w. Decoupling the torque into the cylindrical and horizontal shear components showed that the shear resistance generated by the cylindrical or vertical torque (Tv) was more or less constant around 0.1 kg cm-2 till failure, whereas the shear resistance generated by the horizontal torque (Th) was variable at about 0.3 kg cm-2. On average, the Th was three times the Tv, while the measured Sund was overestimated by about 40% higher than the predicted Sund.

Services
Related Articles in ASCI
Search in Google Scholar
View Citation
Report Citation

 
  How to cite this article:

David Lomeling, Juma L.L. Yieb, Modi A. Lodiong, Mandlena C. Kenyi, Moti S. Kenyi and George M. Silvestro, 2016. Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol). International Journal of Soil Science, 11: 49-60.

DOI: 10.3923/ijss.2016.49.60

URL: https://scialert.net/abstract/?doi=ijss.2016.49.60
 
Received: December 09, 2015; Accepted: January 30, 2016; Published: March 15, 2016



INTRODUCTION

Although, much research over the decades has focused on the undrained behavior of saturated clays, studies on the behavior of partly or unsaturated sands is now gaining interest especially within the context of liquefaction and stability of soil supporting geotechnical structures. Liquefaction phenomenon can be described as the reduction of the shear strength due to pore pressure buildup during mechanical loading. In essence, it is a reduction in soil shear strength as influenced by the degree of saturation during loading sequence (Tsukamoto et al., 2002). The undrained shear strength of desaturated sand can be influenced by the presence of air bubbles that alleviate pore pressure buildup during loading (He et al., 2014) of partially saturated sands during triaxial tests (Kamata et al., 2009) and of saturated sands during triaxial tests (Tsukamoto et al., 2004). In partly saturated agricultural soils, the shear strength can affect the performance of cultivation of implements, root growth, least limiting water range and traffic ability. In very simple terms, soil strength is the maximum shear stress it can sustain prior to failure, where a shear slip occurs along a surface. If the shear velocity is slow enough to allow the pore water in the soil to drain, this is the drained shear strength. Conversely, if the shear velocity is too fast not to allow the pore water to drain, this is the undrained shear strength. In this case, there is no change in the soil water content.

Owing to its simplicity and versatility, the vane tester has been used in the field and laboratory in the estimation of the undrained shear strength of mostly saturated clay soils. However, the interpretation of results derived from such tests has been influenced by such factors like: Non-standard shear rate, anisotrophy or rod friction effects (Schlue et al., 2007; Schlue et al., 2010; Wiesel, 1973; Lerouiel and Marques, 1996; Donald et al., 1977), delayed time between vane insertion and rotation (Terzaghi et al., 1996), disturbance due to vane insertion (Chandler, 1988), initially disturbed soil (Kulhawy et al., 1983) and stress path (Mayne et al., 2009).

The main objective of this study was to study the effects on the undrained shear strength of sandy loam soil under different soil moisture contents and shear angular velocities.

The pocket shear vane tester is a simple instrument that was used to obtain an approximate measurement of the undrained shear strength (Sund) of a semi-cohesive sandy loam soil. The measurement is obtained by applying a force (torque) needed to rotate the blades inserted into the soil to cause the failure of cylinder portion of undrained cohesive soil. Much of the shear resistance of the soil is distributed along the cylindrical surface as well as at both top and bottom ends, where the vane blades cuts the soil. The undrained shear strength is directly read out on the graduated scale in kilogram per centimeter square.

For purposes of simplicity, we assume that the components of the stress tensors of an infinitely small cube at any point where the vane blades cuts the soil are on three mutually perpendicular planes: x, y and z are equal in magnitude, with the corresponding shear forces in the opposite directions. Clearly, two things happen on the shear resistance tf upon application of torque at failure: (1) Either if the angular displacement, ω is less than 90° (ω<90°) and (2) Or if this were greater than 90° (ω>90°). Based on Sund = ±f(σn), as in case 1, the sign of Sund will be positive, while this will be negative as in case 2.

Shear resistance τf during torque application takes place along the cylindrical surface and at bottom end of the 8-blade vane. Considering that the vane blades were the equivalent to the entire diameter (D mm) of the vane foot except for a circular separation distance of the steel rod with diameter (d mm), the shear resistance generated by the torque along the cylindrical or vertical surfaces can be expressed as:

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
(1)

where, Tv (ω) is torque kg cm–2 along cylindrical or vertical surface with angular rotation (ω), (D-d) is equivalent of blade length (Fig. 1a-c), h is height of vane blade (mm). Similarly, assuming that only the bottom end of vane tester offers shear resistance by the torque applied once fully inserted into soil, this on a single vane blade may be expressed as:

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
(2)

where, 0≤r≤D-d.

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
(3)

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
(4)

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
(5)

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
(6)

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
Fig. 1(a-c): (a) Pocket vane tester with adapters, (b) Technical characteristics of pocket vane tester and (c) Differences in diameters of, D: Diameter of pedestal containing the vane blades, d: Diameter of steel rod and D-d: Length of vane blades

Dividing both sides of Eq. 6 with:

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)

gives the estimated value of τf and this is the equivalent of Sund. If h/D-d≥0.5, then this negligible, otherwise it must be multiplied by a factor-4 if h/D-d≥0.5.

Equation 1 implies that the τf especially for cylindrical surface will be uniform, whereas, at the bottom end of vane blade this varies progressively failing toward the center (Eq. 6) (De Alencar et al., 1988; Silvestri and Aubertin, 1988).

MATERIALS AND METHODS

Study site: The study was carried out over a 4 month period as from May-August, 2015 at the Research and Demonstration Farm, Department of Agricultural Sciences, College of Natural Resources and Environmental Studies (CNRES), University of Juba, South Sudan. The study area lies within the Green Belt Agrological Zone of South Sudan and is located between latitude 4°50"28 and longitude 31°35"24 with average annual rainfall of 650 mm mostly during the months of April-October. The climate of the area is tropical wet and dry climate with average temperatures ranging between 27°C during the rainy seasons to about 35°C during dry season. The soil type is a sandy loam soil, Eutric leptosol with less associated Eutric gleysol as shown in Table 1.

Forty five field vane shear measurements were conducted on each of the 9 plots with 3 (Fig. 2a, b) different treatments representing a wide spectrum of Soil Moisture Content (SMC) values that are prevalent in the field during tillage practices.

Table 1:Some physical and chemical properties of a sandy loam soil (Eutric leptosol) from the Research and Demonstration Farm, Department of Agricultural Sciences, University of Juba (CNRES, 2015)
Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
*Harmonized World Soil Data Viewer Version 1.2

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
Fig. 2(a-b):
(a) Experimental field with the nine plots with the different soil moisture contents at the Research and Demonstration Farm, Department of Agricultural Sciences, University of Juba and (b) Experimental site with the nine subdivided plots

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
Fig. 3:Eijkelkamp Theta moisture sensor attached to the penetrologger display to read out real time soil moisture content

Plots G, H and I were subjected to SMC of between 10-14% (on average 12%), plots A, C and F were subjected to SMC between 15-20% (on average 18%), while plots B, D and E were subjected to SMC of between 20-35% (on average 28%).

Prior to shearing, 5 points on each of the different plots were randomly chosen and the SMC measured using a 4-pin Theta moisture sensor (Eijkelkamp agrisearch) (Fig. 3) and read out on the Penetrologger display attached to the Theta moisture sensor. For easy experimental purposes, only the average SMC values were considered. On each of the different experimental plots, three shearing velocities of 5, 10 and 20 sec for each 0.1 kg cm–2 shearing resistance of the graduated vane tester were applied. These applied rates correspond to shearing velocities of 7.2, 3.6 and 1.8° sec–1, respectively. All field measurements and test procedures were taken using the pocket vane tester CL100 (Eijkelkamp agriSearch) with measuring range of up to 250 kPa or 2.5 kg cm–2.

For each complete revolution of the CL100 device, the value was factored by 1.0936. The vane tester was adjusted so that the reference pointer was set at zero. It was then inserted very slowly into already cleaned soil surface that was devoid of any gravel, plant roots or debris. Axial pressure was then applied ensuring that all blades were fully inserted below the soil surface, while maintaining the reference pointer still at zero. Torque was then gradually rotated clockwise to 0.1 kg cm–2 in 5 sec and subsequently released with again the reference pointer set at zero. Next, torque was again applied clockwise to 0.1 kg cm–2 in 5 sec and again released thereafter. The procedure was repeated for the predetermined shear times of 10 and 20 sec for each 0.1 kg cm–2 till failure was attained in line with ASTM D2573-01 Standard test Method for Field Vane Shear Test in cohesive soil. At failure, the peak undrained shear (peak Sund) strength was then registered in kg cm–2.

Land preparation: The piece of land was first cleaned and then divided into nine plots (Fig. 2a, b), marked from A, B, C, D, E, F, G, H and I. The size of each plot was about 150×150 cm. Soil samples from the different plots after preparation were then taken to the laboratory for both chemical and physical analysis.

RESULTS AND DISCUSSION

Figure 4 showed that soil moisture content between 18-25% had both positive loading on time to failure, tf as well as on the undrained shear strength. Conversely, the relatively low soil moisture contents below 12% had negative loading on both components. Such low SMC values less than the plastic limit (16%) would suggest that the soil grains became too brittle subsequently leading to crushing into smaller grains during shearing.

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
Fig. 4(a-c): Scatter plot with Eigenvalue scale showing the relationship between time to failure, tf and undrained shear strength Sund in a sandy loam soil with angular shear velocities ω at (a) 7.2° sec–1, (b) 3.6° sec–1 and (c) 1.8° sec–1

The significance of soil particle size on shear strength was reported by Kim and Ha (2014). Soil moisture contents close to the liquid limit range of 38% reflected more or less saturated soil conditions similar to those in soil pre-liquefaction, thereby, reducing the time to failure. It is inappropriate to account this tf in sandy soils to pore pressure buildup per sesond as in clay soils rather, this could be attributed to inter-particle soil water that enhanced a more sliding of the soil grains against each other especially at the points of contacts (Fig. 4).

Figure 5a-c showed that tf negatively correlated with SMC irrespective of the angular velocity ω and was best described using a power function. Although, at low ω = 1.8° sec–1 (r2 = 0.10) would have enhanced some dissipation of excess pore water pressure as compared to the relatively faster ω = 7.2° sec–1 (r2 = 0.43), the undrained shearing behavior under unsaturated conditions (SMC≤12%) and that under partial or full saturation (SMC≥25-36% was similar. Observations on the failure times as a function of angular velocity (tf = f(ω)), showed increased tf at low ω and vice versa for any given soil water content. It is assumed that a significant contribution to failure times was attributable to the amount of SMC encompassing the soil particles that tended to facilitate a more sliding behavior especially under partial or fully saturated conditions (Fig. 5a-c).

Figure 6a showed a negative correlation between the soil water content and the undrained shear strength Sund, whereas, Fig. 6b described this relationship under different SMCs, which both were best expressed by power function:

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
(7)

where, Λ and β are soil dependent parameters with the latter (1≥β≥2) representing the slope of the linear function between Sund and θ. The magnitude of Sund was dependent on amount of SMC. Between SMC 16-23% for example, the Sund was highest for soils with lower water contents than those with relatively higher water contents. Similar relationship was reported by Koumoto and Houlsby (2001). At some chosen SMC for example, the Sund was highest at about 0.63 kg cm–2 for the drier soil with average SMC at 12%, whereas, Sund was about 0.43 kg cm–2 for wetter soils with average SMC at 28%. There was a 46.5% decrease in Sund by a 16% increase in SMC. On average, the Sund varied between 0.63-0.18 kg cm–2, more scattered around the PL, while converging around LL, respectively. The wide ranging variability at low SMCs (15-25%) correspondingly enhanced a wide ranging Sund perhaps due to the presence of interspersed saturated pockets within the soil matrix. With SMC increase, the soil matrix tended to be more homogenized with the SMC wholly and uniformy distributed within the soil matrix, thereby prompting a small Sund variability range (Fig. 6).

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
Fig. 5(a-c): Relationship between time to failure (tf) and soil moisture content at three different angular velocities (a) 1.8° sec–1, (b) 3.6° sec–1 and (c) 7.2° sec–1 in a sandy loam soil

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
Fig. 6(a-b): (a) Relationship between SMC (%) and undrained shear strength (kg cm–2) of sandy loam soil Eutric leptosol samples and (b) Relationship between soil moisture content and the undrained shear strength of a sandy loam soil Eutric leptosol samples

The SMCs varied between 10-30%, whereas, the Sund between 02.-0.6 kg cm–2 with mean value about 0.3 kg cm–2. Similar studies on undrained shear strength of soils gave the Sund for soft soils as between 20-40 kPa and for firm soils as between 40-75 kPa. These results for the tested soil was between 20-60 kPa, which would suggest a soft to firm soil i.e., normally consolidated soil. The results of this study are in overall agreement with similar studies reported on Singapore marine clay by Robinson et al. (2003).

The entire dataset of all Sund under different soil moisture contents and angular velocities as a function of time to failure tf is represented in Fig. 7.

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
Fig. 7:Time to failure tf, as a function of the undrained shear strength Sund of a sandy loam soil

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
Fig. 8: Relationship between time to failure, tf (sec) and the undrained shear strength Sund. at three angular velocities

The results showed positive correlation between Sund and tf and was best expressed by a power function. Generally, the Sund was between 0.2-0.6 kg cm–2 with time to failure within the first 40 sec (Fig. 7).

Figure 8 shows the time to failure tf at three different angular velocities of 1.8, 3.6 and 7.2° sec–1 of each of the 45 samples. The relationship between tf and the different angular velocities were best described by a power function as:

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
(8)

where, Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol) is a constant at Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol) and α is a soil dependent parameter at (0.01≤α≤0.7). Since part of the test objective was to understand the effects of different angular velocities conducted at the same soil moisture content, the study found out that the different angular velocities; 1.8, 3.6 and 7.2° sec–1 showed different times to failure tf at 126, 56 and 35 sec, respectively. The shear strength developed on the cylindrical failure surface during shearing was equivalent to the force or torque applied and showed a positive relationship. Similar positive relationship between tf and the undrained shear strength have been reported by O'Kelly (2013). The estimated angular rotation at failure at the different angular velocities of 1.8, 3.6 and 7.2° sec–1 were about 18.12, 17.94 and 19.15°, respectively (Fig. 8).

For all tested samples, it was found out that increase in ω led to decrease in Sund. Although, no direct inferences from SMC and its influence on pore water pressure in determining the magnitude of Sund in unsaturated soils, it is argued that the ω must have been influenced by the pore water pressure. Under unsaturated soil conditions, the measured SMCs in the experimented plots ranged between 12-28% and were significantly lower than the liquid limit, so that the influence of pore water pressure at the inter-particle points of contact was less significant. This meant that further increase in the rotational shear force, there occured soil particle and soil water rearrangement that inevitably led to decreased Sund with softening of the soil skeleton.

Rate of vane rotation and the implications on Sund has been reported by Perez-Foguet et al. (1999), who highlighted that field Sund was often overestimated when compared to laboratory tests. Earlier on, Bjerrum (1973) proposed a reduction of the vane measured Sund with respect to the plasticity index of the clay. Similar studies of the rate of rotation on Sund have been reported by Wiesel (1973) and Tortensson (1977), who proposed a power function between the Sund and angular velocity as:

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
(9)

where, k1 and k2 are constants with k2 ranging between 0.02-0.07. From these studies (Fig. 9), the k2 constant of the power function closest to that reported by both authors was 0.014 at angular velocity of 1.8° sec–1 and that k2 values for all three measured angular velocities varied ten-fold between 0.01 and 0.7. The comparison of such k2 values with other results is, however, difficult as this depends on several factors such as: Vane shape, size, type and state of soil.

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
Fig. 9:
Logarithmic function representing the relationship between undrained shear strength Sund and shearing time St at the different angular velocities in a sandy loam soil (Eutric leptosol)

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
Fig. 10: Relationship between time to failure tf and predicted undrained shear strength Sund at both cylindrical and bottom end surfaces of blades of the pocket vane tester

It is worth mentioning that the experimental procedures at the different angular velocities ensured that the unsaturated soil matrix was subjected to small shear strain during each shearing sequence. However, volume compression did not lead to pore-water pressure buildup as this was significantly lower that the liquid limit (LL) of 39%. The accumulation of small radial deformations in the sandy loam soil during each cyclic shearing sequence must have led to densification and increase in particle interlocking and therefore, increase in the shearing strength and time to failure.

A better approximation of Sund with varying SMCs during shearing would be to treat the soil as a deformable porous medium, whose deformability depends on the amount and state of SMC. This would allow the application of constitutive laws relating to shear stresses and strains that incorporate time influence based on a visco-elastic-plastic theory. Saturated soils with ω≥ωL, viscous effects are considered, meanwhile under unsaturated conditions ω≤ωvp or the visco-plastic limit. An accurate estimation of Sund would be the application of constitutive laws based on visco-plastic theory and compare the measured values within the SMC range of 12-35%.

The relationship between Sund and shearing time St at the different angular velocities is shown in Fig. 10. It showed a decrease in angular shear rate with increasing Sund and was best described by a logarithmic function. For example at St = 10 sec, the Sund was lowest at about 0.16 kg cm–2 with the highest ω at 7.2° sec–1, with about 0.2 kg cm–2 for 3.6° sec–1 and about 0.45 kg cm–2 for the lowest ω at 1.8° sec–1 (Fig. 10).

However, the logarithmic function appeared to poorly describe the measured data especially at both higher Sund (≥0.5 kg cm–2) and ω (≥3.6° sec–1) than at lower values. This would suggest that the Sund of the experimented soil is anywhere between 0.1 and 0.5 kg cm–2 (10-50 kPa is the average value of normally consolidated soft sandy loam/silty or clayey silts) with optimum shear velocity of 1.8° sec–1. Higher angular velocity would lead to reduced particle rearrangement and interlocking and hence, lower Sund values. Between results are contrary to those reported by Biscontin and Pestana (1999), who found an increasing Sund with increasing peripheral velocity. Such comparisons, however, may be questionable due to the inherent nature of the experiment in terms of: Vane size (Chandler, 1988; Tortensson, 1977), type and shape (Silvestri et al., 1998; Menzies and Merrifield, 1980), field versus laboratory test (Kirkpatrick and Khan, 1984), remolded or mixed soils; thixotrophic effects (Kimura and Saitoh, 1983), strain rates, viscous mixtures and slurried materials (Keentok et al., 1985; Komamura and Huang, 1974). It can be seen that the change in Sund as a function of ω was about 25% between ω = 7.2 and 3.6° sec–1 and 125% from ω = 3.6° sec–1 and 1.8° sec–1 indicating a significant increase in Sund due to decrease in the ω.

Table 2: Relationship between Sund, angular velocity (w) and SMC of a Eutric leptosol
Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
Av: Average

Table 3: Statistical parameters showing the resultant effect of change of ω soil moisture content and Sund of a sandy loam soil. (Eutric leptosol)
Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
a, b, cNot significant at p>0.05, SD: Standard deviation, CV: Cumulative velocity

At high angular velocity (7.2° sec–1), the undrained shear strength was low (Sund = 0.309 kg cm–2), increased to 0.311 kg cm–2 at 3.6° sec–1 and finally to Sund = 0.327 kg cm–2 at angular velocity ω =1.8° sec–1 as in Table 2. This observation suggests that changes in the undrained shear stiffness are, by and large controlled by the moisture content in the soil matrix. Similarly, the slow rate of shear must have led to partial drainage leading to consolidation. This study also showed that the Sund at some given SMC, e.g., 12% was not affected by change in angular velocity which remained constant at 0.331 kg cm–2, however, the Sund was affected, if the angular velocity were kept constant e.g., at 7.2° sec–1 and the SMC varied at 12, 18 and 28% subsequently decreasing at 0.331, 0.318 and 0.303 kg cm–2, respectively (Table 2).

Generally, the effect of angular velocity is critical in determining and interpreting the real value of Sund of a given soil. Standard test sets this at 6-12° min–1 or 0.1-5° sec–1. Our test procedure at both 1.8 and 3.6° sec–1 except at 7.2° sec–1 are clearly within that range. A comparatively lower angular velocity as at 1.8° sec–1 would have resulted in partial drainage and consequently led to particle rearrangement and hardening of the soil matrix as manifested by the increase in Sund.

Examples of measured peak Sund and angle of rotation to failure at the different angular velocities ω are shown in Table 3. Generally, peak Sund was on average 0.3 kg cm–2 at the different angular velocities with relatively high angle of rotation to failure (19°) at lower angular velocity (1.8° sec–1). There was a slight decrease to 18° at 7.2° sec–1 angular velocity and then finally to17° at 3.6° sec–1.

It can be said that the angles of rotation at the different angular velocities were not significantly different at p>0.05 suggesting that the angle of rotation was a direct function of the peak Sund i.e., ω = f (Sund). These results showed that the angle of rotation was not influenced by neither the angular velocity nor the soil moisture content, but more by the peak Sund. On average, the time to failure tf was between 16 and 55 sec with peak Sund-values at 0.311 and 0.328 kg cm–2 for fastest and slowest angular velocities, respectively.

Predictions of the shear resistance generated by the torque on both vertical and horizontal surfaces are shown in Fig. 11. The predicted shear resistance was generally low and on average 0.1 kg cm–2 at the vertical or cylindrical surface, whereas, at the horizontal or bottom end of the vane blade this was comparatively higher at 0.4 kg cm–2.

For the time 0≤tf≤50 sec, the predicted Sund for the horizontal surface showed higher values than those at cylindrical surface by about 40%. The predicted torque was expressed and approximated as a function of the length of the vane blade (D-d) as:

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
(10)

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
Fig. 11:Comparison of the measured shear strength and the averages generated from both cylindrical and bottom end surfaces of the vane tester

Predicted corresponding angular rotation at failure:

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
(11)

Image for - Effects of Variable Angular Velocities and Soil Moisture Contents on Undrained Shear Strength in a Sandy Loam Soil (Eutric Leptosol)
(12)

The angle of rotation at maximum torque independent of soil moisture content was on average about 18.69°. At lowest torque of 0.17 kg cm–2, the angle of rotation was about 8.56° while at the highest torque of 0.71 kg cm–2 this was about 33.76°. The latter results with high torque and angle of rotation values suggested experimental irregularities and were therefore, an exception rather than the rule. The results showed that the horizontal shear resistance had a significant influence on the total maximum shear resistance generated at failure than the vertical or cylindrical shear resistance. Similarly, the results showed a wide range of progressive failure suggesting varying shear resistance forces between 0.05-0.2 kg cm–2 on the horizontal plane with peak failure times mostly attained within the first 50 sec. Whereas, the shear resistance in Eq. 1 at the vertical surface was more or less constant at 0.1 kg cm–2 on average, this was on average 0.3 kg cm–2 at the horizontal surface in Eq. 6. This wide ranging shear resistance values along the horizontal surface would suggest the gradual and progressive failure from the outer surface towards the center of the vane.

Figure 11 shows that the measured Sund varied between 0.2-0.6 kg cm–2 whereas, the computed value around 0.1 kg cm–2. Present results showed that the computed predictions of the Sund were overestimated by about 40% when compared to empirical values at both cylindrical and bottom end surfaces.

The computed predictions took account of the segregated shear components whereas the empirical values were simply an aggregation of both values. According to Bjerrum (1972), this could be attributable to both viscous effects and rate of rotation.

CONCLUSION

The undrained shear strength Sund was performed on a sandy loam soil (Eutric leptosol) under variable shear velocities, ω and soil moisture contents representing the different traction speeds during tillage. Angular velocities at 1.8, 3.6 and 7.2° sec–1 negatively correlated with the undrained shear strength and were best described by a logarithmic function. Similarly, the undrained shear strength negatively correlated with soil moisture content and was best described using the exponential function.

Because under unsaturated conditions the measured soil moisture contents in the experimented plots were generally lower than the soil liquid limit, the apparent buildup of pore water pressure during shearing at the different shear velocities and soil moisture contents had no influence on both the time to failure, tf and maximum undrained shear strength at failure. On the contrary, the time to failure tf showed weak positive correlation though at relatively low r2-values with increasing soil moisture content. Based on empirical and predicted results, the following conclusions can be made:

The results of applied torque at both the cylindrical or vertical (Tv) as well as the horizontal surfaces (Th) showed that the Eutric leptosol in its failure characteristics was anisotrophic. The Th was greater than Tv by factor 3
The predictions of the shear resistances using the vane tester were more reliable than empirical tests that were subject to errors and overestimation. The difference between the empirical and predicted torque ranged between 0- 40%
The magnitude of the Sund was contingent on the degree of SMC that negatively correlated. Sund increased with decrease in SMC and vice versa
The angular velocity (w) negatively correlated with Sund at constant SMC, the Sund remained constant irrespective of change in angular velocity
The time to failure, tf positively correlated with Sund independent of change in angular velocity

ACKNOWLEDGMENT

The authors are very grateful for the Norwegian-funded project NUCOOP (Norwegian Universities Cooperation Project) under the title "Post-war Livelihood and Environmental Studies" Project No. 2000/10003 hosted at College of Natural Resources and Environmental Studies (CNRES), University of Juba for the funding the purchase of penetrologger, Theta moisture sensor and pocket vane tester.

REFERENCES

1:  De Alencar, J.A., D.H. Chan and N.R. Morgenstern, 1988. Progressive Failure in the Vane Test. In: Vane Shear Strength Testing in Soils: Field and Laboratory Studies, Richards, A.F. (Ed.). ASTM International, Philadelphia, PA., ISBN-13: 9780803111882, pp: 150-165

2:  Bjerrum, L., 1972. Embankments on soft ground. Proceedings of the Specialty Conference on Performance of Earth and Earth-Supported Structures, June 11-14, 1972, Purdue University, Lafayette, IN., USA., pp: 1-54

3:  Bjerrum, L., 1973. Problems of soil mechanics and construction on soft clays. Proceedings of the 8th International Conference on Soil Mechanics and Foundation Engineering, August 2-3, 1973, Moscow, Russia, pp: 111-159

4:  O'Kelly, B.C., 2013. Undrained shear strength-water content relationship for sewage sludge. Geotech. Eng., 166: 576-588.
Direct Link  |  

5:  Chandler, R.J., 1988. The In-situ Measurement of the Undrained Shear Strength of Clays Using the Field Vane. In: Vane Shear Strength Testing in Soils: Field and Laboratory Studies, Richard, A.F. (Ed.). ASTM International, Philadelphia, PA., USA., ISBN-13: 9780803111882, pp: 13-44
Direct Link  |  

6:  Biscontin, G. and J.M. Pestana, 1999. Influence of peripheral velocity on undrained shear strength and deformability characteristics of a bentonite-kaolinite mixture. Geotechnical Engineering Report No. UCB/GT/99-19, Department of Civil and Environmental Engineering, University of California, Berkeley, CA., USA.

7:  He, J., J. Chu and H. Liu, 2014. Undrained shear strength of desaturated loose sand under monotonic shearing. Soils Found., 54: 910-916.
CrossRef  |  Direct Link  |  

8:  Kamata, T., Y. Tsukamoto and K. Ishihara, 2009. Undrained shear strength of partially saturated sand in triaxial tests. Bull. N. Z. Soc. Earthquake Eng., 42: 57-62.
Direct Link  |  

9:  Keentok, M., J.F. Milthorpe and E. O'Donovan, 1985. On the shearing zone around rotating vanes in plastic liquids: Theory and experiment. J. Non-Newtonian Fluid Mech., 17: 23-35.
CrossRef  |  Direct Link  |  

10:  Kim, D. and S. Ha, 2014. Effects of particle size on the shear behavior of coarse grained soils reinforced with geogrid. Materials, 7: 963-979.
CrossRef  |  Direct Link  |  

11:  Kimura, T. and K. Saitoh, 1983. Effect of disturbance due to insertion on vane shear strength of normally consolidated cohesive soils. Soils Found., 23: 113-124.
CrossRef  |  Direct Link  |  

12:  Kirkpatrick, W.M. and A.J. Khan, 1984. The influence of stress relief on the vane strength of clays. Geotechnique, 34: 428-432.
CrossRef  |  Direct Link  |  

13:  Komamura, F. and R.J. Huang, 1974. New rheological model for soil behavior. J. Geotech. Geoenviron. Eng., 100: 807-824.
Direct Link  |  

14:  Koumoto, T. and G.T. Houlsby, 2001. Theory and practice of the fall cone test. Geotechnique, 51: 701-712.
CrossRef  |  Direct Link  |  

15:  Kulhawy, F.H., C.H. Trautmann, J.F. Beech, T.D. O'Rourke, W. McGuire, W.A. Wood and C. Capano, 1983. Transmission line structure foundations for uplift-compression loading. Report-EL 2870, Electric Power Research Institute, Palo Alto, CA., USA.

16:  Lerouiel, S. and M.E.S. Marques, 1996. Importance of Strain Rate and Temperature Effects in Geo-Technical Engineering. In: Measuring and Modeling Time Dependent Soil Behavior, Sheahan, T.C. and V.N. Kaliakin (Eds.). American Society of Civil Engineers, New York, pp: 1-60

17:  Mayne, P., M. Coop, S. Springman, A. Huang and J. Zornberg, 2009. Geomaterial behavior and testing. Proceedings of the 17th International Conference on Soil Mechanics and Geotechnical Engineering, October 5-9, 2009, Alexandria, Egypt, pp: 2777-2872

18:  Menzies, B.K. and C.M. Merrifield, 1980. Measurements of shear stress distribution on the edges of a shear vane blade. Geotechnique, 30: 314-317.
CrossRef  |  Direct Link  |  

19:  Perez-Foguet, A., A. Ledesma and A. Huerta, 1999. Analysis of the vane test considering size and time effects. Int. J. Numer. Anal. Methods Geomech., 23: 383-412.
Direct Link  |  

20:  Robinson, R.G., T.S. Tan and F.H. Lee, 2003. A comparative study of suction-induced seepage consolidation versus centrifuge consolidation. Geotech. Testing J., 26: 92-101.
CrossRef  |  Direct Link  |  

21:  Schlue, B.F., T. Morz and S. Kreiter, 2007. Effect of rod friction on vane shear tests in very soft organic harbour mud. Acta Geotechnica, 2: 281-289.
CrossRef  |  Direct Link  |  

22:  Schlue, B., T. Moerz and S. Kreiter, 2010. Influence of shear rate on undrained vane shear strength of organic harbor mud. J. Geotech. Geoenviron. Eng., 136: 1437-1447.
CrossRef  |  Direct Link  |  

23:  Silvestri, V. and M. Aubertin, 1988. Anisotropy and In-situ Vane Tests. In: Vane Shear Strength Testing in Soil: Field and Laboratory Studies, Richards, A.F. (Ed.). ASTM International, Philadelphia, PA., USA., ISBN-13: 9780803111882, pp: 88-103

24:  Silvestri, V., M. Aubertin and R.P. Chapuis, 1993. A study of undrained shear strength using various vanes. Geotech. Testing J., 16: 228-237.
CrossRef  |  Direct Link  |  

25:  Terzaghi, K., R.B. Peck and G. Mesri, 1996. Soil Mechanics in Engineering Practice. 3rd Edn., John Wiley and Sons, Inc, New York, USA., ISBN-13: 9780471086581, Pages: 549

26:  Tortensson, B.A., 1977. Time-dependent effects in the field vane test. Proceedings of the International Symposium on Soft Clay, July 5-6, 1977, Asian Institute of Technology, Bangkok, Thailand, pp: 387-397

27:  Tsukamoto, Y., K. Ishihara, H. Nakazawa, K. Kamada and Y. Huang, 2002. Resistance of partly saturated sand to liquefaction with reference to longitudinal and shear wave velocities. Soils Found., 42: 93-104.
CrossRef  |  Direct Link  |  

28:  Tsukamoto, Y., K. Ishihara and T. Shibayama, 2004. Evaluation of undrained flow of saturated sand based on triaxial tests. Proceedings of the 3rd International Conference on Continental Erathquakes, July 12-14, 2004, Beijing, China -

29:  Wiesel, C.E., 1973. Some factors influencing in situ vane test results. Proceedings of the International Conference on Soil Mechanics and Foundation Engineering, August 2-3, 1973, Moscow, Russia, pp: 475-479

30:  Donald, I.B., D.O. Jordan, R.J. Parker and C.T. Toh, 1977. The vane test-A critical appraisal. Proceedings of the 9th International Conference on Soil Mechanics and Foundation Engineering, Volume 1, July 10-15, 1977, Tokyo, Japan, pp: 81-88

©  2021 Science Alert. All Rights Reserved