Sandy Loam Soil Shear Strength Parameters and Its Colour

Abstract

The shear strength parameters—the shear cohesion and the internal friction angle—are important parameters in calculating the shear strength of the soil, which is a mechanical property reflecting the resistance of the soil to shearing. This article embeds shear cohesion and internal friction angle results of the sandy loam soil measured using the direct shear test relying on drawing the Mohr–Coulomb line. Beside the shear strength parameters, this article contains colour results of the tested soil at different moisture contents, measured using the spectrophotometric technology in the visible band (spectrum; 400–700 nm). Measuring the soil colour might be the simplest way in the field for identifying the shear strength and its parameters in case of having the colour linked to the shear strength parameters through the moisture content of the soil, and this target is achieved by laboratory empirical work. The process of correlating the soil colour and the shear strength parameters at different moisture contents is presented in this article. The measurements were carried out at the Hungarian University of Agriculture and Life Sciences (MATE—Department of Vehicle Technology Laboratory).

1. Introduction

Ending up with the shear strength and the shear strength parameters of soil is a step that is important in studying the shearing failure of the soil under a shearing load having simultaneously a consolidation normal load applied on the soil. The cohesion of the soil shows its consistency and has two connotations: in physics, soil cohesion reflects the cohesive bonding between the soil particles but, in soil mechanics, the shear cohesion is the shear strength of the soil when the applied normal stress on the surface is equal to zero [1]. The internal friction angle is a parameter reflecting the internal resistance of the soil to shearing resulting from the friction on the particle level (interparticle sliding friction and geometrical interface). The cohesion strength parameter is a stress-independent parameter, while the internal friction angle depends on the applied stress. The shear strength parameters serve in finding the shear strength of the soil by the usage of the Coulomb criterion that is of a similar idea to the Mohr failure criterion (the soil shear strength is a function of the applied consolidation stress) but combines the shear strength parameters and the applied normal stress in a linear function to calculate the shear strength [2,3].

The soil thrust which results from the shearing of the terrain by the tractive element (wheel/track) is calculated using the shear strength of the soil. In soil mechanics, the soil thrust is a property that contributes to the propelling of a vehicle [4].

The change in the soil moisture content influences its behaviour by affecting its physical and mechanical properties (load-bearing capacity and shear strength). Ahmed has shown experimentally in his article the change in load-bearing capacity of the sandy loam soil with the change in the moisture content [5]. In the case of clayey soils, the increase in the moisture content weakens the bonding between the soil particles, resulting in a decrease in its cohesion, and the decrease in the moisture increases the internal friction [6]. The soil’s physical composition, dampness (water content), density, and initial compression state affect the soil deformation resulting from the passage of a vehicle [7]. The moisture content is the most influencing soil condition on the soil strength [8].

There are different methods used for measuring the shear strength of soil, but none of these methods are standardised, so choosing the suitable method for measuring the shear strength is decided based on the case of study. For studying land locomotion cases, it is important to choose the shear test that closely emulates the interaction between the tractive element (wheel/track) and the terrain. Direct shear test resembles most closely the failure mechanism of soil beneath a tire; thus, the soil shear strength results obtained using this test can be used for studying a vehicle’s performance on the soil [8]. In the direct shear test, the soil is shearing along a predetermined plane [9], thus simplifying the case of the soil shear failure for measuring the displacement as a function of the applied shear stress, which is hard to study in the field where the failure plane is unidentified or difficult to obtain.

Soils might reflect a wide range of colours, the grey, black, white, reds, browns, yellows, and greens [10]. The resulting colour of a soil is due to different processes and conditions influencing the soil, the minerals present in the soil, and the content of the organic matter [11]. The organic matter is what mostly influences the soil colour [12], which is why scientists worked on relating the darkness of the soil colour to its organic matter content [10,13,14,15].

Different methods are used for measuring or identifying the colour of the soil, such as the Munsell charts and the spectrophotometric technology. The spectrophotometer is a precise quantitative technology that works based on tangible results (spectral reflectance). The reflected wavelength of the sent light wave (its wavelength is in the visible spectrum range; 400–700 nm) is the value that describes the colour of the soil at a specific moisture content.

The direct shear test machine will be used for measuring the soil shear strength parameters at different moisture contents; then, upon ending up with the curves fitting (cohesion and internal friction angle) the resulting shear strength parameters at different moisture contents, these parameters can be predicted from the resulting curves at any input moisture value. The soil moisture value in the field is predicted using the curve fitting the colour reflectance of the soil at different moisture contents (curve obtained from laboratory work), having a given soil colour reflectance as an input (measured by the spectrophotometer).

Finally, the shear strength and its parameters will help in studying the land locomotion of a vehicle and, if not serving in adjusting the design of the vehicle’s tractive element or its power system, at least taking the right decision (Go/no Go) before moving on a soil zone (high moisture), so the vehicle does not become immobilised. Having the strength parameters connected to the colour of the soil at different moisture contents will facilitate knowing the soil terrain strength parameters in the field (real-time terramechanics case).

2. Research Significance

The main target from the prepared article is to show the ability of predicting the shear strength parameters from the colour of the soil. The process of linking the colour to the shear cohesion and the internal friction angle is mentioned in detail. This idea is new and pioneering in the terramechanics field. Reaching convincing results (after validating field measurements) will help in predicting the performance of a vehicle on soil terrain based on the colour of the soil. This work is a tremendous step in terramechanics science (land locomotion), especially given that many fields have interest in off-road engineering projects (agriculture, military, etc.). Better vehicle performance on soil terrain leads to less fuel consumption and safe drive.

There is a gap in our work that is divided into two parts. The first part is that the measurement is restricted to the usage of a spectrophotometer measuring the colour in the visible band (spectrum; 400–700 nm), which might not be useful in measuring the colour of the soil at high moisture content. The second part is that, even though the shear strength parameters are obtained using the direct shear test, there are no cohesion and internal friction angle standardised results in the field’s database (terramechanics or soil mechanics field) to compare the obtained results to. Confirming the obtained results requires field measurement for measuring a performance parameter (such as vehicle drawbar pull) using a sensor, and the measured value is compared to the calculated result using a terramechanics model (the shear strength parameters substituted in it).

3. Measuring the Shear Cohesion and the Internal Friction Angle of Sandy Loam Soil

The measurements were carried out at the Hungarian University of Agricultural and Life Sciences using soil from the university area. The soil used is sandy loam soil (shown in Figure 1), composed of 90.50% sand (0.05–2 mm), 3.20% silt (0.002–0.05 mm), and 6.30% clay (<0.002 mm). The method used for measuring the shear strength results at different moisture contents is the direct shear test machine that is available in the laboratory (ELE 26-2112/01). The direct shear test machine is shown in Figure 2.

The soil samples were prepared in boxes and were moistured by adding water and then stirred till they became uniform. The moisture of each of the samples was measured using the moisture analyser (HE53 230 V; shown in Figure 3) that works based on the drying principle. An amount of soil is placed in it (on a tray) and dried by the analyser’s heating plates. For calculating the moisture percentage, the remaining mass is deducted from the initial wet mass and then divided by the initial wet mass (wet base moisture content calculation). The samples were placed and covered in plastic boxes to avoid the decrease in the moisture content. Starting with the first sample, the soil was placed in the direct shear box of the machine—approximately the soil amount was placed symmetrically in the two frames (upper and lower frame of the shearing box)—and then the upper frame was covered by the plate (cover plate). The screw pressing on the plate (used for applying the consolidation load by masses) was tightened but not with a high torque (smooth tight just for anchoring the plate and levelling the soil surface). A 30 kg consolidation load was applied on the plate to compact the soil, since the soil used in the test is disturbed (taken from field), while the soil in the field is undisturbed. The soil was compacted by the 30 kg mass for 5 min as a chosen compaction period (compacting the soil for a longer period influences the shear strength result). After compacting the soil for 5 min, the 30 kg mass was removed and a 10 kg mass was applied as measuring consolidation stress (applied when shearing the soil); then, the machine was operated. The shearing speed was set to be 9 mm/min (maximum speed in the machine). New soil amounts from the same sample (same moisture) were tested with the same compaction load applied for five minutes (except for the 15.03% moisture) but having 20, 30, 50, and 70 kg as measuring consolidation masses (normal stress if the load is divided by the area of the shear box).

The 15.03% moisture content soil sample was not compacted for 5 min to avoid water drainage, because the moisture is high. The tested soil saturation level (approximately liquid limit level) was around 27%, and that was obtained by adding water to dry soil (at ambient conditions) prepared in a box till it stopped the absorption of water and a small water layer appeared on the surface. The surface water (additional small amount) was removed carefully and the moisture was measured by the analyser. For checking the saturation level in another way, the hygroscopic effect of the soil was checked by filling dry soil in a tube and then inverting on a box; water was added into the box (small amount) till the soil (in the tube) stopped absorbing water and the moisture was measured by the anaylser, showing a value close to 27%.

Unsaturated soil is of a shape that is more complicated when compared to saturated soil. This complex structure has an impact on the strength, deformation, and the seepage of the soil [16,17].

The obtained shear strength result at each of the applied loads was recorded (maximum stress value in the stress–displacement diagram). The same test was repeated on the same soil type with different moisture contents.

The shear strength was plotted as a function of the consolidation stress (at the loads 10, 20, 30, 50, and 70 kg), thus ending up with a linear Mohr–Coulomb failure criterion for finding the shear cohesion and the internal friction angle of the soil at each moisture content.

The shear cohesion is the intercept of the Mohr–Coulomb line with the shear strength results axis (y-axis) and the internal friction angle is the angle that the line makes with the horizontal (acute angle).

The obtained Mohr–Coulomb diagrams are shown below (Figure 4).

4. Shear Cohesion and Internal Friction Angle of Sandy Loam Soil Results Analysis

The cohesion (physical cohesion) resulting from the bonding between similar atoms reflects how these particles are bonded to each other and, thus, is a step for understanding the behaviour of the soil.

As mentioned in the introduction, even though shear cohesion is different from soil physical cohesion, these two terms are related. The soil cohesion forms initial stress acting as compressive stress to the shear cohesion [1].

Based on the obtained results of the tested soil type, as the moisture content increases, the shear cohesion increases to a value around 21 kPa and remains approximately constant in the moisture content range from 6% to 9%, and, beyond this point, the shear cohesion increases with the increase in the moisture content, reaching a maximum point (breaking point) of a value 27.17 kPa at 11.25% moisture content. Beyond the breaking point, as the moisture increases, the shear cohesion decreases to a low value, reaching 12.72 kPa at 15.25%. The shear cohesion results are shown in Figure 5.

For loam soils, cohesion decreases with the increase in the moisture content (this can be recognised in Figure 5 at the high moisture values) and, inversely, the cohesion in sandy and clayey soils increases with increasing moisture [18].

The interaction between the soil particles leads to friction between them, and this friction acts as a resistance to the shearing load. The shear strength based on the Coulomb equation consists of two values, the first of which is the shear cohesion (C) and the second value resulting from the product of the internal friction coefficient (μ) and the applied normal stress.

$$\tau =C+\sigma \tan \Phi$$

(1)

where:

• C (kPa) is the shear cohesion;
• σ (kPa) is the normal stress applied on the soil (consolidation load divided by the area);
• ϕ (degree) is the internal friction angle (tan(ϕ), which is equal to μ; it is the friction coefficient and it indicates the slope of the line).

The increase in the applied normal load increases the compaction of the soil, thus increasing the density (density increases to a limit due to the decrease in the voids), which leads to the increase in the particle interaction. When there is not normal stress applied to the soil, the internal friction will not influence the shear strength of the soil (its influence is neglected).

The internal friction angle results of the sandy loam soil show that, at ambient conditions (no water added; moisture in the soil is due to humidity), the internal friction angle of the soil is the highest, with a value of 31.15 degrees, and, with the increase in the moisture content, the internal friction decreases to a value of 24.87 at 9.6%. Beyond the 9.6% moisture, the results remain approximately constant with the increase in the moisture content till reaching 12.65% (the results are decreasing but at a slow rate; thus, they are considered constant). Beyond 12.65% moisture content, with the increase in the moisture, the internal friction angle continues to decrease, reaching 17.04 degrees at 15.25%. The internal friction angle results are shown in Figure 6. Low moisture content means high soil friction, and the high moisture content decreases the soil friction [18].

5. Spectral Behaviour of the Tested Sandy Loam Soil at Different Moisture Contents (Soil Colour)

The description of the soil colour has been used by soil scientists in the field to classify the soil and in mapping [19,20].

The pedologists relied on the Munsell soil colour charts (1975) for more than 60 years in describing the normal range of soil colours, which is a qualitative colour estimate method [21,22]. This method relies on determining the colour visually and then comparing it to standard chips that are arranged systematically based on their Munsell’s notations.

Many scientists replaced this method by fast instrumental technologies and methods to rely on in specifying the soil colour, such as UV-VIS spectrophotometry (ultraviolet visible), leading to a precise and quantitative approach in specifying the colour (quality control) [12,20,23]. The spectrophotometer removed some of Munsell’s method limitations by using standard values and removing the human ‘judgement’ (observer viewing angle and the lighting conditions) [15,20,22,24,25,26].

The colour of the tested soil was measured using a spectrophotometer (CM-700d; shown in Figure 7). The spectrophotometer is a modern technology used for measuring the colour of a body (material); the working principle of the used type is by sending light waves in the visible band, spectrum (400–700 nm), and, based on the reflected wavelength, the colour of the soil is identified. The colour of the soil is described by the reflected wavelength, which is a digital value (quantitative). It is difficult to specify the colour of the soil based on the vision as in Munsell charts, since it is a qualitative method. Despite the errors that might result from the traditional Munsell charts method, such as human errors (vision), using this method in the case of moist soil becomes complicated. In most soil types, the increase in the moisture content of the soil makes the soil darker, but, even though not being able to see the change in the colour by the eye’s vision, the spectrophotometer is able to decode the colour of the soil to a digital value.

The sandy loam soil was moistened to be at different moisture contents. Each soil moisture value was measured using the moisture analyser (gravimetric measurement). At each moisture content value, three soil colour records were taken by the spectrophotometer, and the average of these records was taken into account. The reflected wavelength results of the soil at the same moisture content are not precisely the same (but close results), and the reason behind the difference is that the soil is a particle material. There are factors that influence the reflectance, such as the pores and the density. The reflected wavelength results of the sandy loam soil at different moisture contents in the visible band (spectrum; 400–700 nm) are shown in Figure 8.

The soil reflectance increases with the increase in the moisture content from 0.9 to 1.88%, indicating a slight brightening in the colour. Beyond the moisture 1.88 (or a value close to it), with the increase in the moisture amount, the reflected wavelength decreases, showing that, with the increase in the moisture, the soil colour becomes darker. The soil reflectance decreases with the increase in the moisture content till reaching 6.74% moisture value, and that can be recognised clearly in Figure 8 (at 700 nm). At values above 6.74% moisture, the reflectance points (clear at 700 nm) start to overlap in a reflectance range.

6. Sandy Loam Soil Colour Reflectance at 700 nm

For simplifying reading the above curve (reflectance of the spectrophotometer), the reflectance results at 700 nm (points are dispersed) were plotted as a function of the moisture content values (each at its moisture content; shown in Figure 9).

Analysing the curve in Figure 9 shows that the moisture values used in the measurement are divided into two ranges, the first section ranging from 0.9% to approximately 7.41%. Fitting a curve into the plotted points (for an accurate curve with a good regression analysis, measurement repetitions are required) will end with a useful equation for calculating the moisture based on the colour reflected wavelength at 700 nm. In this range, the reflectance is changing in a clear curve path without having overlapping in the results, the case that makes calculating the moisture possible, and, thus, ending with a single moisture result in the mentioned range upon substituting a reflectance into the curve equation (equation is not given here; measurement repetitions and equation validation are required before confirming an equation).

Even though the moisture result will not be precisely similar to the moisture value calculated (gravimetric or volumetric calculation), it should be of a value close to the precise value. The main reason behind the difference in the moisture values (colour equation calculation and gravimetric/volumetric calculation) is the reflected wavelength. Upon repeating the soil colour measurement at the same moisture content, the resulting reflected wavelength will not be precisely the same, since the material that is being studied is a particulate material. The density of the soil has influence on the wave reflectance; also, the technical usage of the spectrophotometer (the front measuring part of the instrument should be normal to the soil surface and its pressing level should be accounted, since pressing leads to soil compaction) has influence on the reflectance result.

The second moisture section is ranging from a moisture value approximately beyond 7.41% till the maximum moisture value, which is 17.65% (shown in Figure 8 and Figure 9). In this range, the reflectance is approximately constant and, thus, a horizontal straight-line fit into the points. Having the soil at any moisture value in the mentioned range, and upon measuring its colour, the colour reflectance will be the same (constant value on the horizontal line). The same reflectance indicates that either (1) beyond the moisture approximately equal to 7.41%, the soil colour is changing no more; thus, the soil absorbed an amount of water and the additional amount of water in the soil is water film occupying the space between the particles and leading to the same reflectance (water influencing the measurement).

The other reason is (2) the used spectrophotometer measurement ability. The used spectrophotometer measures the colour based on sending light waves in the visible band (spectrum 400–700), and maybe, by sending waves in this band, the spectrophotometer is unable to receive precise reflectance at high moisture values; thus, the usage of a spectrophotometer that sends waves in other ranges (such as infrared) might end up with good results at this stage.

7. Sandy Loam Soil Shear Strength Parameters Obtained Using Its Colour

As mentioned in the introduction, linking the mechanical properties of the soil to its colour facilitates knowing the infield mechanical properties of the soil from its colour. This work was achieved through laboratory work, and the results should be validated by field measurements. In the case of studying the mobility of a vehicle on a soil terrain, especially for calculating the traction (drawbar pull), some of the shear stress–displacement models require the shear strength and/or its parameters as input data for predicting the traction of the vehicle, as in Ageikin model (shear strength parameters required) shown in Equation (2) [27,28,29,30].

The shear strength of the soil is calculated using the Mohr–Coulomb model, having its parameters as both the shear cohesion and the internal friction angle. This Mohr–Coulomb model is the most used in terramechanics community (Equation (1)). Some scientists worked on modifying this model to fit specific soil types, such as clay soil [31], unsaturated soil [32], and partially saturated soil [33].

The three pioneer scientists that have settled the basics for the development of the shear stress–displacement models are Bekker, Pokrovski, and Janosi [34,35,36,37].

Oida (1975) proposed a model based on Pokrovski’s model that is only applicable on soils that are of shear stress–displacement profile with a hump and residual stress [38]. Certain types of loams are of shear stress–displacement profile having the mentioned characteristic [39].

$$\tau =\frac{1}{\frac{1}{\varsigma C+ptan\left(\Phi \right)}+\frac{t_{gr}}{E^{\prime }.│j│}}$$

(2)

where:

• τ is the shear stress;
• j is the shear displacement;
• t_gr is the grouser pitch_;
• C is the soil shear cohesion;
• P is the average ground pressure;
• Φ is the internal friction angle;
• m’ and E’ are soil shear parameters;
• ς is a model parameter.

Having the equation of the curve fitting the colour reflectance points at 700 nm (Figure 9) will help in finding the moisture content of the soil upon substituting the reflectance value (at 700 nm) in it; thus, the moisture value is obtained.

With the equation of the curve passing through the plotted shear cohesion points, the shear cohesion can be calculated at the moisture content (calculated from the colour). By using the equation of the internal friction angle curve fitting the plotted points, the internal friction angle is obtained at the moisture content calculated from the colour. The equations of the curves fitting the plotted points of both the shear cohesion and the internal friction angle are not provided in this article, considering that confirming these equations requires measurements, repetitions, and validation.

8. Conclusions

The main target behind this article is fulfilled, and it shows the ability of relating the shear strength parameters of the tested sandy loam soil to its colour. The shear strength parameters were measured using the direct shear test. The tendency of the cohesion and the internal friction angle plotted points as a function of moisture content is changing in a specified trajectory (curve can be obtained using a trendline fitting the points). With the increase in the moisture content, the colour reflectance of the soil has also shown a clear tendency (curve of specified path fitting the plotted point). By analysing the obtained results, the following tangible results about the soil behaviour (mechanical and colour) can be concluded:

• When measuring the shear strength of the soil using the direct shear test, it is important to take into consideration the effect of the consolidation load on the result and that it is recognized not just experimentally by comparing obtained results, but also in Coulomb’s equation (Equation (1)).
• The shear cohesion and the internal friction angle of soil are influenced by the moisture content and that can be recognized in the obtained results and in the soil physical behaviour.
• For the tested sandy loam soil, the shear cohesion increases with the increase in the moisture content reaching a maximum breaking point, and, beyond this point, the shear cohesion tends to have a low value. The internal friction angle of the sandy loam soil decreases with the increase in the moisture, showing the maximum value at ambient conditions (no water added) and low values at high moisture content.
• At high moisture contents, the water films existing between the particles act as lubrication, breaking the cohesive bonds between the particles and decreasing the friction resulting from the contact of the particles.
• The colour reflectance of sandy loam soil decreases with the increase in moisture till reaching a level (at a moisture value), and, beyond this moisture value, the reflectance results are approximately constant.
• Plotting the reflectance at 700 nm will result in a curve equation which is beneficial in finding the moisture if having a given colour reflectance value at 700 nm. Using the obtained moisture value, the shear cohesion and the internal friction angle can be calculated using the curves fitting the plotted points (Figure 5 and Figure 6).
• The results in this article (shear strength parameters and the colour) are of the tested soil type and cannot be generalised. Even though the tested soil is sandy loam soil, it has its mechanical composition (mentioned in the text), and this is a major reason why the results of another sandy loam soil type might be different.