An experimental study on the effect of particle size and non-homogeneity on the shear strength of soil

Soil deposits available in nature are often present as homogeneous mixtures of particles of different sizes and base minerals or as a combination of soils in different layers forming a non-homogeneous configuration. This variability in soil has great influence on the shear strength behaviour. In this study, different naturally occurring soil particle sizes and shapes (angular and flaky) have been utilized as reinforcements in the form of layers to determine their influence on the soil shear strength. Therefore, triaxial tests have been conducted on homogeneous sand (where particles are mixed proportionately), and on non-homogeneous sand (different layer combinations of different particle sizes as well as the inclusion of angular and flaky particles). It has been observed that there was a substantial increase in the shear strength for non-homogeneous arrangement compared to homogeneous sand sample. Furthermore, the addition of flaky particles from 10% to 30% by weight of fine grained sand increased the angle of internal friction (ϕ) and cohesion (c) values thereby increasing the shear strength. However, for angular-fine sand layer combination, the addition of 10-20% of angular particles by weight shows a significant increase in shear strength in comparison to 30-100% of angular particles.

General overview Natural soil deposits largely exist as mixtures of soils which consists of particles of various sizes, shapes and minerals due to which it cannot be classified as a particular soil type i.e., clay, silt or sand but rather into a combination of soils i.e., they are available in non-homogeneous forms.It is very difficult to select a relatively homogeneous soil sample which specifically represents the three classes of soil.Generally even within a certain depth of 60cm, the soil make-up could change considerably which ranges from silty-clay to silts or sandy silts [5].Soils are found in composite form due to their formation process.Mechanical behaviour of soil such as shear strength, compressibility, erodibility, permeability are dependent on composition of soil [2].Many investigations were conducted to acquire a comprehensive insight on the behaviour of soil mixtures [1][2] [3][4] [11] [14].Normally, studies were divided into two primary combinations such as gravel-sand mixtures and sand-clay mixtures.Several studies were also made to understand the shear behaviour of silt-clay or sand-clay soil mixtures [1] [11].A study investigated the influences on the dilation and shear strength of sand due to addition of gravel content.[8].The findings of the study indicated that when gravel is added to sand, the dilation and shear strength increases due to the resistance offered by gravel particles towards rotation.[12] found that the fine content in coarse-fine sand mixture greatly influences the load bearing capacity.When fine sand is added, an increase in the angle of friction and ultimately the shear strength of coarse-fine sand mixture is observed due to strong interlocking between the particles [2].When subgrade soil is mixed with gravel, it was seen that the stiffness and internal friction angle were enhanced by irregular gravel but less impact on cohesion was observed [16].[15] found that the large size gravels have a good inter-locking and re-arrangement capacity, which increases the shear strength.Direct shear and triaxial test on soil mixture of granular material with oversized particles were conducted over the years [9]; [6]; [10]; [7]; [13] found that shear strength of the soil mixture rely 1282 (2023) 012021 IOP Publishing doi:10.1088/1757-899X/1282/1/012021 2 on the relative concentration of oversized particles.In a mixture containing 48% to 78% larger particles (by weight), friction between larger particles controlled the shear strength.Smaller particles dominates the shear strength if the concentration of larger particles is less than 48% by weight.Investigation into the impact of gravel content on the shear behaviour of soil-rock mixtures revealed that the gravel content is a vital element altering the mechanical traits of soil-rock mixture in the course of sample preparation [17].When less than 25% of gravel is present, the shear parameters of the pure soil will mostly define the overall strength of soil-rock mixture as the gravel content will not be in close contact with each other and are in a state of suspension.Above 70% of gravel content, the interlocking action of gravel particles is what is responsible for the change in shear strength as they are in close contact with each other.Therefore, the gravel content of the tested soil-rock mixture samples should range from 25% to 70%.It was observed that most of the conducted researches and studies only considered the homogeneous behaviour of soil i.e., behaviour of only soil mixtures but not the non-homogeneous behaviour that is when soil is a combination of layers.Very few studies have been made regarding non-homogeneous soil [11] but these studies are limited.Thus, it becomes necessary to understand the stress-strain behaviour of non-homogeneous soil in geotechnical engineering.
Objective of the 1.2.study This study aims to understand the behaviour of non-homogeneous sand sample.The following objectives are outlined for the study: (a) To find the effect on the shear strength when soil is arranged as a non-homogeneous layered soil sample.(b) To find the variation of shear strength with different particle size.(c) To understand the stress-strain behaviour of fine sand sample when reinforced with angular and flaky particles Necessity of the 1.

study
For earth construction projects, it is crucial to assess the soil's shear strength and other properties.Soil of a particular type may not have adequate geotechnical or mechanical properties which have to be used for road embankment, foundation and construction material, etc. Innovative techniques have been used to stabilize and modify deficient soils by means of local accessible industrial or environmental waste materials, so as to minimize the construction costs.In the present study, utilization of different soil particle sizes and shape (angular and flaky) as reinforcements in the form of layers and studying the behaviour of these type of soils in finding out desired shear strength.

Methodology
This section includes the detailed discussion of the materials used, sample preparation and test procedure of the Consolidated Undrained triaxial test.A flowchart of the study is given in Fig. 1. Materials Used In the current study, the soil sample is collected near Jiabharali River (26⁰45'11.5''N 92⁰51'57.1''E) of Tezpur, Assam.The general properties of the soil were studied in the laboratory.The soil is tested for moisture content, specific gravity, relative density, etc.The results obtained from the preliminary tests on the sand sample are tabulated in Table 1.The triaxial test is then conducted to understand the shear behaviour of layered sand samples.The sieve analysis of the soil is performed.From the gradation curve shown in Fig. 2

Sample Preparation
The sample is firstly sieved using sieve sizes ranging from 0.075 mm to 2 mm.Each sample retained in these sieves is arranged in layers according to the required combinations as shown in Table 2.The sample retained in each sieve is collected as shown in Fig. 3 with angular and flaky samples also sorted out from samples retaining in 2mm sieve.The sample is then compacted layer by layer in different combinations according to the particle sizes by mild tamping.The required mass of the sample is determined by knowing the dry density of sand and the volume of the mould and is found out to be 145 gm.Therefore the mass of soil for each layer is taken as 23.5 gm.Coarse sand (1 mm <= d < 2 mm) having flaky and angular particles was used to make another phase of soil combinations along with fine sand (0.075 mm <= d < 0.425 mm).The coarse sand is added in proportions of 10 % -30 % by weight of fine sand.The whole sample is divided as per required fractions and a layer-by-layer sample is made.The first layer is filled by pouring the first fraction and compacting it with a tamping rod.The remaining layers are then filled in the same manner.

Consolidated undrained (CU) Triaxial Test
The CU triaxial test is carried out on the test sample.As the drainage is not allowed in the shearing stage, so pore pressure develops in this stage.The triaxial test gives the measure of both the total and effective stresses under different confining pressures.In this study, all the soil specimens are tested under three confining pressures i.e., 50kN/m 2 , 100kN/m 2 and 150kN/m 2 .The consolidated drained triaxial test is conducted as per IS 2720 Part XII (1981).In this test, the failure plane's stress distribution is relatively uniform and the specimen can fail on any weak plane or can simply bulge.
The shear strength at failure of the sample is calculated using the Mohr's-Coulomb equation as shown in the equation below: where, τ f is the shear stress at failure plane of the soil sample.C is the cohesion.is the normal stress.
ϕ is the angle of internal friction.
The CU triaxial test was performed for the following samples with different arrangements:

Homogeneous and non-homogeneous sand sample
The arrangement of soil including both homogeneous and non-homogeneous sand in the CU triaxial apparatus is shown in Fig. 4. For homogeneous sand sample the soil particles ranging from sizes passing 2mm and retaining 1mm to those retaining in 75 micron are homogeneously mixed as shown in Fig. 4(a).Non-homogeneous sand sample is arranged in layers in the following combinations:

Angular-fine sand arrangement
The angular and fine sand particles are arranged in the following combinations: (i) 10% Angular and 90% Fine Soil Sample : 10 % by weight of angular particles (retaining on 2mm) and 90 % by weight of fine sand (passing 0.425mm sieve).(ii) 20% Angular and 80% Fine Soil Sample: 20 % by weight of angular particles and 80 % by weight of fine sand.(iii) 30% Angular and 70% Fine Soil Sample: 30 % by weight of angular particles and 70 % by weight of fine sand.(iv) 100% Angular: Angular particles without addition of finer sand layer.Each of the above combinations are taken and arranged in two layers, the angular particles in the bottom layer and then fine soil sample is poured on the top and compacted by mild tamping.

Different particle sizes The different particle particles sizes used are as follows: (i)
1 mm-2 mm: Soil particles of sizes passing through 2 mm and retaining on 1mm (ii) 0.6mm-1 mm: Soil particles of sizes passing through 1mm and retaining on 0.6mm (iii) 0.425 mm-0.6mm:Soil particles of sizes passing through 0.6mm and retaining on 0.425 mm (iv) 0.3mm-0.425mm: Soil particles of sizes passing through 0.425 mm and retaining on 0.3 mm (v) 0.212mm-0.3mm:Soil particles of sizes passing through 0.3 mm and retaining on 0.212 mm (vi) 0.075-0.212mm:Soil particles of sizes passing through 0.212mm

3.1. strength
It is observed that when the soil sample is arranged in non-homogeneous layer the shear strength   Effect of flaky-fine sand arrangement on the soil shear 3.2.strength It is observed that when flaky particles are added to fine sand the shear strength of soil is enhanced.The variation of the strength with addition of flaky particles in different percentages by weight is shown in Fig. 7(a).The deviator stresses and c and ϕ values obtained are given in Table 4.The Mohr circles are drawn as shown in Fig. 8.  Effect of angular particles on the shear 3.3.strength It is observed that upto 20% addition of angular particles, the soil shear strength improved after which the strength decreases.The variation of the strength with addition of angular particles in different percentages by weight is shown in Fig. 7(b).The deviator stresses at failure, and c and ϕ values obtained are tabulated in Table 5.The Mohr circles are drawn as shown in Fig. 9.
Table 5. Deviator stresses at failure, c and ϕ values for Angular-Fine particles arrangement  strength It is observed that the soil's shear strength varies with the particle size.This variation is as shown in Fig. 10.It is observed that with decreasing particle sizes there is a gradual decrease in c and ϕ values.The impact of the size of particles on the friction angle and cohesion are also shown in Fig. 11.The Mohr circles for different particle sizes that are tested are also drawn as shown in Fig. 12 and c and ϕ values are obtained.The deviator stresses and c and ϕ values obtained are tabulated in Table 6 respectively.

Discussion
The following discussions can be made from the study: ▪ In the study, it was found that the c and ϕ values increased for non-homogeneous soil sample in comparison to homogeneous sample and ultimately the shear strength.▪ However, when the soil was arranged such that the finer particles are placed in between two coarser layers the c and ϕ values came out lesser as compared to other arrangements.This can be because in triaxial test, the shear planes go through the middle and on the top and bottom there will be restraints from the caps.The fine layer is at the middle so the shear zone will mainly go to the portion having lesser ϕ value.When the coarse layer is in the middle the shear plane will go to the middle and a higher ϕ value is obtained.▪ The shear strength tends to increase when flaky particles are included.An increase in the ϕ values was observed when the flaky particles was increased from 10% to 30% by weight.The maximum angle of internal friction was observed when 100% flaky particles was taken.The increase in cohesion is also observed as flaky content increases.The maximum cohesion is shown when flaky content is 100%.The increase in ϕ values as flaky content increases could be due to the shape and arrangement of flaky particles.Flaky particles have lesser void percentage as compared to that of the angular particles which has more void percent, thus providing a denser arrangement in the flaky sample.▪ In case of angular and fine particle arrangement, the cohesion was observed to decrease with the increase in angular particle content from 10% to 20%.On the other hand, the internal friction angle increased by about 4% from the previous value containing 10% angular content.With the further increase in angular content to 30% by weight, both c and ϕ values decreased to some amount.Fine sand particles with concentration of angular particles between 10% -20% dominate the shear strength.As the % of angular particles is increased the density becomes lesser as the angular particles concentration by weight increases.▪ On comparing following combinations: (a) 10% flaky-90% fines and 10% angular-90% fines (b) 20% flaky-80% fines and 20% angular-80% fines and (c) 30% flaky-90% fines and 30% angular-70% fines, it was observed that the angular arrangement upto 20% showed a higher value of shear stress as compared to that of 20% flaky arrangement.But with further increase in the angular content with upto 30% by weight the shear stress decreased for all the confining pressures, compared to flaky arrangement.▪ Out of all the flaky-fine sand combination and angular-fine sand combinations, the highest deviator stress was observed in the sample having 100% flaky soil content, whereas in the sample having 100% angular content the lowest deviator stress was observed.▪ The shear behaviour of the soil as well as its contact surface and rolling and sliding resistance are influenced by the corresponding voids among the grain mass as a result of the change in the particle size .The maximum shear strength as well as angle of internal friction increases when the particle size increases.A gradual decrease in c and ϕ value was observed with decreasing particle sizes.This could probably be due to the reason that with more fine content although the soil seems to be densely packed, the volume of voids is more in the fine sand content than that of the coarse sand.This could probably have resulted a decrease in the friction angle.

5.
Based on the data observed from the results and discussions, these conclusions can be made as

Conclusion
The shear strength of the soil improved when the soil was arranged in layers as compared to when the soil was homogeneous.It was found that the c and ϕ values increased the most in fine-coarsefine layer arrangement.The c value increased by 3 kN/m 2 and ϕ increased from 40.04º to 43.27º respectively.Sometimes layers found in the soil may not be very thick but can only be few centimeters thin.However, this might affect the overall shear strength of the soil.Thus, this study is an effort to find out the shear behaviour of such layered soils.In the field, samples are not cylindrical and there are no restraints, therefore we cannot explainthe behaviour yet.If there are alternate layers, the combined shear strength of the layers will be probably be governed by the layer which is weaker.The non-homogeneity of the soil therefore affects the overall shear strength of the soil Particle shape and size plays a significant part in affectiong the mechanical behaviour of soil.In this study it is seen that the change in particle size influences the angle of internal friction and in turn the shear strength.The shear strength was seen to improve in case of angular-fine arrangement when compared with flaky-fine soil arrangement.But this increase was only upto when the angular content was about 20% by weight.With angular content of 30% or more the shear strength decreases probably due increase in void content.Thus, angular particles could serve as a good reinforcement to soils.But if the angular content is in excess, as compared to the fines then a decrease in strength could be observed.
In this study the soil particles were limited to certain sizes and they can further be increased to obtain test results.The constraints imposed by the smaller size of the sample provided some limitations for preparation of a large sized samples .A further research using a larger size triaxial test can be carried out for increased particle sizes.The angular and flaky particles were prepared in layers with the fine sand.A further researchon mixing them homogeneously in different proportions could help in understanding the strength behaviour.
Rough or smooth textured angular and flaky sand can also be used for understanding the shear parameters.

Combination Particle size 1 Finer 3 Coarse
to Coarser (0.150 mm to 1 mm) 2 Coarser to Finer (1 mm to 0.150 mm) Conf.Series: Materials Science and Engineering 1282 (2023) 012021 IOP Publishing doi:10.1088/1757-899X/1282/1/0120216 τ f = c+ tan ϕ (1) (i) Coarser to finer-The sand specimen is arranged in layers with particles ranging from those retaining in 1mm sieve to those passing 150 micron sieve as shown in Fig.4(b) ;(ii) Finer to coarser-The sand specimen is arranged in layers from particles ranging from those passing 150 micron to those retaining in 1mm sieve as shown in Fig.4(c);(iii) Coarse-Fine-Coarse-The sand specimen is arranged in layers such that finer layers are sandwiched between coarser layers.The sample is prepared in this arrangement is as shown in Fig.4(d);(iv) Fine-Coarse-Fine-The sand specimen is arranged in layers such that finer layers are sandwiched between coarser layers as shown in Fig.4(e).

Fig. 5 .
Fig.5.Flaky-fine particle arrangement before fine sand is poured improved in comparison to homogeneous sand sample.The c and ϕ values increased for nonhomogeneous samples in comparison to non-homogeneous sample.It was found that the c and ϕ values increased the most in 'fine-coarse-fine' layer arrangement The maximum deviator stresses, c and ϕ values obtained are tabulated in Table3.The Mohr circles are drawn as shown in Fig.6

Table 1 .
General properties of Soil

Table 2 .
Different combination of non-homogeneous soil sample

Table 3 .
Deviator stresses at failure, c and ϕ for homogeneous and non-homogeneous soil sample

Table 4 .
Deviator stresses at failure, c and ϕ values for Flaky-Fine particles arrangement

Table 6 .
Deviator stresses at failure, c and ϕ values for different particle sizes