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THE UNIVERSITY OF QUEENSLAND

[Investigation of shear stress behaviour of compacted Kaolin clay mixed with salt solution]

Student Name: [Chuangcheng, Lin] Course Code: [ENGG7240] Supervisor: [Alexander Scheuermann] Submission date: [27th October 2017]

Thesis starts here

Abstract Shear stress performance has been considered as a significant feature of the soil properties, for it is a major cause of construction failure. The collapse of the building occurs when the stress applied on the soil exceed the shear strength (shear stress performance) of the soil. As a key component of the Brisbane soil, which has been utilised widely as the base soil material in a construction project located in Port of Brisbane, the kaolin clay, however, has attracted special attention due to its high proportion in the Brisbane soil. This thesis intends to mitigate the possible construction failure risk due to shear strength by proffering a legitimate method to improve the shear strength of the kaolin clay. The study investigates the shear stress performance of the kaolin clay mixed with salt solution processed by compaction method, the result of which, will be further compared with the shear strength outcomes of the kaolin clay possessed by other three methods (the shrinkage, the consolidation, and the compaction with soil mixed with fresh water) which were generated by Direct Shear Test. Results suggest that the soil mixed with fresh water processed by the compaction method generates the most satisfying outcome in shear stress performance, which may can be further applied to enhance the construction stability in the Port of Brisbane.

Contents 1 Introduction .................................................................................................................... 5

1.1 Background and Motivation .................................................................................... 5

1.2 Aim of the Thesis .................................................................................................... 6

1.3 Thesis Organization ................................................................................................. 7

2 Literature Review ........................................................................................................... 7

2.1 Introduction ............................................................................................................. 7

2.2 Shear stress and Micro-structure ............................................................................. 7

2.2.1 Introduction of shear stress and micro-structure .............................................. 7

2.2.2 Literature Review of shear stress and micro-structure ..................................... 7

2.3 Shear stress and consolidation ................................................................................. 9

2.3.1 Introduction of shear stress and consolidation ................................................. 9

2.3.2 Literature Review of shear stress and consolidation ........................................ 9

2.3 Shear stress and shrinkage ..................................................................................... 10

2.3.1 Introduction of shear stress and shrinkage ..................................................... 10

2.3.2 Literature Review of shear stress and shrinkage ............................................ 10

2.3 Shear stress and Compaction ................................................................................. 11

2.3.1 Introduction of shear stress and compaction .................................................. 11

2.3.2 Literature Review of shear stress and compaction ......................................... 11

3 Thesis Scope ................................................................................................................. 12

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4 Methodology ................................................................................................................. 13

4.1 Introduction ........................................................................................................... 13

4.2 Overall Design ....................................................................................................... 13

4.2.1 Experiment Plan ............................................................................................. 13

4.2.2 Soil initial condition obtaining ....................................................................... 15

4.2.3 Soil sample preparation .................................................................................. 16

4.2.4 Result obtaining .............................................................................................. 17

4.3 Fall Cone ............................................................................................................... 17

4.3.1 Introduction of the fall cone test ..................................................................... 17

4.3.2 Experimental setup and operational process of the fall cone test ................... 17

4.4 Proctors Compaction test ....................................................................................... 18

4.4.1 Introduction of the Proctors Compaction test ................................................. 18

4.4.2 Underlying theory and experiment setup of Proctors Compaction test .......... 19

4.5 Direct Shear Test ................................................................................................... 20

4.5.1 Introduction of the Direct Shear Test ............................................................. 20

4.5.2 Underlying theory and experimental setup of Direct Shear Test ................... 20

5 Results .......................................................................................................................... 21

5.1 Results of liquid limit and initial condition ....................................................... 21

5.2 Results of soil mixed with salt solution with 34%, 37%, and 40% water content ..................................................................................................................... 23

6 Discussions ................................................................................................................... 26

6.1 Comparison between results from different water content .................................... 26

6.2 Comparison between results from compaction approach with water and salt solution ........................................................................................................................ 27

6.3 Comparison between results from four approaches .............................................. 29

7 Conclusions & Future Work ......................................................................................... 32

7.1 Conclusions ........................................................................................................... 32

7.2 Future Work ........................................................................................................... 33

8 Reference ...................................................................................................................... 33

Appendix ........................................................................................................................... i

Appendix A: Example of experimental data ................................................................. i

Figure 1 Port of Brisbane [1] ............................................................................................. 5 Figure 2 Transcosna Grain Elevator Failure & Niigata foundation failure ....................... 6 Figure 3 Example of soil microstructure (a) Before compaction (b) After compaction [7] ...................................................................................................................................... 6 Figure 4 Relationship between plane porosity, shear strength parameters and moisture .. 8

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Figure 5 Relationship between peak strength and residual shear strength ........................ 9 Figure 6 Relationship between normalised shear stress and consolidation stress ........... 10 Figure 7 Relationship between shear stress and dry bulk density. o Unnamed alluvial gley, ●Wicken, □ Fladbury .............................................................................................. 11 Figure 8 Pre-compaction and shear strength relationship ............................................... 12 Figure 9 Shear stress and normal stress at peak for different compactions ..................... 12 Figure 10 Overall experiment design .............................................................................. 13 Figure 11 Relationship between vertical stress (effective normal stress) and shear stress (performance) [25] ........................................................................................................... 14 Figure 12 Water content soil density relationship example [24] ..................................... 15 Figure 13 LL, PL, and SL [25] ........................................................................................ 16 Figure 14 (a) utilised salt (b) salt weighing (3) salt solution obtaining ........................... 17 Figure 15 Oven & Balance .............................................................................................. 18 Figure 16 Cone penetrometer &cup ................................................................................ 18 Figure 17 Water content and density curve example [30] ............................................... 19 Figure 18 Mohr’s circle failure envelope [32]................................................................. 20 Figure 19 Shear test apparatus ......................................................................................... 21 Figure 20 Liquid Limit of kaolin clay ............................................................................. 22 Figure 21 Water content and soil density relationship of kaolin clay ............................. 22 Figure 22 Results of soil mixed salt solution with 34% water content ........................... 23 Figure 23 Results of soil mixed salt solution with 37% water content ........................... 24 Figure 24 Results of soil mixed salt solution with 40% water content ........................... 24 Figure 25 Shear stress performance result of soil mixed salt solution with 34% water content ............................................................................................................................. 25 Figure 26Shear stress performance result of soil mixed salt solution with 37% water content ............................................................................................................................. 25 Figure 27 Shear stress performance result of soil mixed salt solution with 40% water content ............................................................................................................................. 25 Figure 28 Cohesion performance in different water content ........................................... 26 Figure 29Result from compaction method with salt solution.......................................... 27 Figure 30 Result from compaction method with water ................................................... 28 Figure 31 Result from Shrinkage method ....................................................................... 29 Figure 32 Result from consolidation method .................................................................. 30 Figure 33 Comparison of shear stress performance of soil with 34% water content among four methods ........................................................................................................ 31 Figure 34 Comparison of shear stress performance of soil with 37% water content among four methods ........................................................................................................ 31 Figure 35Comparison of shear stress performance of soil with 40% water content among four methods ........................................................................................................ 32

Table 1Results of direct shear ........................................................................................... 8 Table 2 Experiment design table-vertical stress, τ -maximum shear stress .................... 15 Table 3 Results from fall cone test .................................................................................. 21 Table 4 Results of maximum shear stress, σ-vertical stress, τ -maximum shear stress ... 24 Table 5 Summary of shear stress feature in soil mixed with salt solution in different water content.................................................................................................................... 26 Table 6 shear stress coefficient of in two methods .......................................................... 28

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Table 7 Maximum shear stress in each experiment, σ-vertical stress, τ -maximum shear stress ................................................................................................................................ 28 Table 8 Experimental results of shrinkage method, σ-vertical stress, τ- maximum shear stress ................................................................................................................................ 29 Table 9Experimental results of consolidation method, σ-vertical stress, τ -maximum shear stress ....................................................................................................................... 29 Table 10 Shear stress performance ranking of four methods, σ-vertical stress, τ -maximum shear stress ...................................................................................................... 30

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1 Introduction This introductory chapter illustrates the fundamental background and underlying motivation of this research for shear strength investigation on compacted Brisbane soil mixed with the salt solution. A description of compaction process and the relevant merits and demerits are presented after the briefing section, together with the research aims and the thesis chapter organization.

1.1 Background and Motivation Port of Brisbane (POB), the main container of the Queensland marine transport, is located at the mouth of the Brisbane River with a reclamation expansion program being processed (see in Figure 1), where the soil from the adjacent rive has been utilised as the maintenance dredged materials [2]. However, as the utilised soil originates in the coastal area, the shear strength of the sample is normally with a low value which can pose serious geotechnical problems due to the lateral movements.

Figure 1 Port of Brisbane [1]

Hence, the motivation of this thesis is to eliminate the threats from those lateral movements which may result in the failure of the construction above the Brisbane soil. Among all the soil components of Brisbane soil, the clay, however, counts for the most dominating material as it underlines the reclamation site up to 30 meters [3]. Thus, a specific soil material of kaolin clay has been selected as the investigated experimental subject in this research.

Of all the features within the soil properties, the shear strength behaviour, without any doubt, is of great significance, for it governs the lateral movement performance which may induce catastrophic construction failure. Disastrous soil failures can be initiated by both the unanticipated extra loading on the soil surface and the infirm shear strength endurance, where the Transcosna Grain Elevator Failure [4] and Niigata foundation failure [5] can be cited as examples (see in Figure2).

Hence, considering the importance of the shear strength from the aspect of the safety of construction, an improvement strategy is required to be proposed. Previous studies suggest that the shear stress performance (shear strength) of the soil can be altered by changing the micro-structure of the soil, where several methods, including soil

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consolidation, soil shrinkage, and soil compaction, can be utilised as promotion approaches (see in Figure 3)[6].

Figure 2 Transcosna Grain Elevator Failure & Niigata foundation failure

Figure 3 Example of soil microstructure (a) Before compaction (b) After compaction [7]

To identify the associated effects in these methods comprehensively, the shear strength investigations towards kaolin clay are required to be conducted respectively. Consequently, the results of the above investigation and the comparison among them are able to present a comprehensive vision and a legitimate improvement strategy.

This thesis addresses the effect of the one of the above improvement approach: compaction to the Brisbane soil mixed with the salt solution, with comparison among results from other approaches to be analysed.

1.2 Aim of the Thesis The proposed investigation is aimed at addressing the following issues:

Aim1: To obtain a comprehensive understanding of shear stress performance and relevant cognition, by both theoretical and practical approaches.

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Aim2: To investigate the water content versus soil density correlation graph of the Brisbane soil that lays the groundwork for the further research process.

Aim3: To identify the soil shear strength property in marine-soaked Brisbane soil by compaction method, identify the relationship between shear stress and normal stress of the improved soil, by both numerical and experimental methods.

Aim4: To compare the results in four methods and develop a legitimate approach to promote the soil shear strength behaviour.

1.3 Thesis Organization The thesis is organised as follows:

Chapter 1 illustrates the fundamental background and underlying motivation of the research, together with the aims and thesis structure.

Chapter 2 overviews the previous studies of the relevant topic, proffering an academic background and historical review.

Chapter 3 provides a scope of this thesis, which entails topic definition and exclusion determination.

Chapter 4 presents the operational experiments entailed in this research and the underlying mechanism and rationale.

Chapter 5 introduces the outcome of the research, laying the ground for further analysis.

Chapter 6 discusses the previous results and carries out the analysis.

Chapter 7 summarise the thesis together with suggestions for future works.

2 Literature Review 2.1 Introduction This chapter presents a comprehensive coverage of the overviews of the previous studies that investigated the soil shear strength performance. Several aspects associated with this area with will be introduced in this literature study, which comprised of soil micro-structure, consolidation, shrinkage, and compaction, for they form the fundamental cognition over this research.

2.2 Shear stress and Micro-structure 2.2.1 Introduction of shear stress and micro-structure The micro-structure governs the fundamental component formation of the soil, therefore, it contributes significantly to the mechanical behaviour (such as shear stress) of the soil. This section overviews the past researches that focus on the relationship between micro-structure and shear stress from different aspects.

2.2.2 Literature Review of shear stress and micro-structure An investigation towards a volcanic soil on its shear behaviour has been completed by Avsar et al. in 2005 [6], together with the impacts from the micro-structure on this

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behaviour. Results suggest that two different types of shear strength behaviour had emerged in the direct shear box test, representing that the parameters of shear strength in this soil with a comparatively lower normal stress range surpass those from high normal stress. Then a comparison between natural and tested samples has been conducted, which uncovered that the bonding material within the soil governs this behaviour significantly.

Wei et al. carried out a research to demonstrate the relationship between shear strength and microstructure in the Tuff soil in 2012 [8], where the influence posed by moisture content and micro-structure characteristics (including plane porosity, average pore size, circularity, orientation degree, and plane pore distribution fractal dimension) on soil shear strength performance has been identified (example see in Figure 4).

Figure 4 Relationship between plane porosity, shear strength parameters and moisture

A similar study has been conducted by Zuo et al. after few years, where a more comprehensive investigation on micro-characteristics and shear strength was carried out, proffering more information from the micro-structure aspect [9].

Salt content (%)

Optimum moisture content (%)

Maximum dry density (g/cm3)

Apparent cohesion (kPa)

Internal friction angle (◦)

Unconfined compressive strength (MPa)

0.57 16.70 1.86 11.70 9.40 0.32 6.62 16.10 1.82 25.50 17.10 0.59 12.36 15.60 1.74 30.00 24.30 0.84 22.12 13.20 1.76 35.60 28.90 1.76 37.32 12.70 1.70 52.10 32.80 1.97 45.74 12.50 1.75 66.40 35.60 4.41 54.01 12.90 1.78 81.30 39.70 4.91 72.81 11.80 1.72 109.20 41.50 6.89 86.31 10.30 1.69 116.80 45.20 7.43

Table 1Results of direct shear

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In 2014, Liu et al. studied the micro-structure characters of a saline soil and its related mechanics behaviour [10], where a series of direct shear tests and compress tests with different salt content have been conducted (see in Table 1).

As such, it can be seen that the micro-structure of the soil has impacts on the mechanical behaviour of the soil then, the soil chemistry (salt content), however, influences the microstructure of the soil. Hence, conclusion can be proposed as that the salt content may govern the mechanical behaviour (shear stress performance) of the soil.

2.3 Shear stress and consolidation 2.3.1 Introduction of shear stress and consolidation Consolidation refers to an approach that decreases the total volume of the soil, experiments have proven that the shear stress performance varies with the consolidated level of soil [11], for it both alters the macro-structure and micro-structure. Thus, this section reviews the results in the past few decades with a specific theme of consolidation.

2.3.2 Literature Review of shear stress and consolidation Benjamin et al. carried out a research where the shear stress properties of a mud at low consolidation stress levels has been investigated with a specific designed set-up [12], results show that a ratio between undrained peak shear strength and vertical consolidation stress can be identified to address the linear relationship (Figure 5).

Figure 5 Relationship between peak strength and residual shear strength

Similarly, Naeem et al. performed a program of tests to address the effects of consolidation stress level on the shear behaviour of a blue clay in 2012 [13], where a linear correlation has also been proved (see in Figure 6).

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Figure 6 Relationship between normalised shear stress and consolidation stress

The shear stress performance investigations toward consolidated soil are also available in the analogous fields, such as powders [14], rock joints [15], and lactose powders [16]. Thus, there is no doubt that the consolidation method can alter the shear stress performance to some level.

2.3 Shear stress and shrinkage 2.3.1 Introduction of shear stress and shrinkage Shrinkage is a dewatering process of soil, unlike the consolidation, it alters the soil structure by eliminating the water within the soil and then induces the shrunken volume [17]. As it also can be utilised as an approach to adjust the soil properties, this section illustrates the correlations between shrinkage and shear stress that have been found in the previous studies.

2.3.2 Literature Review of shear stress and shrinkage In 2010, Andrew et al. investigated the shrinkage effects on a series soil characteristic, in which they found that shrinkage is of greater value in sheared soils compared with compressed soils [18].

Spoor et al. identified that the shrinkage amount governs the change in shear strength, where a schematic figure has been generated to illustrate the relationship [19].

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Figure 7 Relationship between shear stress and dry bulk density. o Unnamed alluvial gley, ●Wicken, □ Fladbury

According to the figure, it can be seen that with the increasing level of shrinkage, the dry bulk density increased as a result, which lead to the soar of the shear stress performance. Thus, similarly, the shrinkage method also improves the shear strength of the soil in a way.

2.3 Shear stress and Compaction 2.3.1 Introduction of shear stress and compaction Compaction induces the deformation of soil structure by applying the external force as well but with a different method [20], considering the effects it has on the soil properties adjustment, this section reviews the relevant discoveries in this field.

2.3.2 Literature Review of shear stress and compaction In 2009 Hemmat et al. conducted a research to address the relationship between pre-compaction stress and shear strength of a sandy soil [21], which has proven to be linear as a result (see in Figure 8).

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Figure 8 Pre-compaction and shear strength relationship

Later in 2015, Wang et al. also performed a series of direct shear box tests with five different compactions [22], which reinforces the theory that compaction facilitate the increasing of shear strength (see in Figure 9).

Figure 9 Shear stress and normal stress at peak for different compactions

Hence, considering the impact from compaction method on soil shear strength, apparently, this approach also contributes to the promotion of shear stress performance.

3 Thesis Scope Considering the great significance of soil shear stress performance in the construction of the building, it is beneficial to promote this feature of soil by utilising the methods identified in the literature review, which can be summarised as follows:

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• Changing the chemistry of the soil (salt content in this case) • Soil consolidation • Soil shrinkage • Soil compaction

In this thesis, a specific scope has been determined as to identify the shear strength behaviour of kaolin clay mixed with salt solution, which will be further compared with the results from other three methods (compaction with soil mixed with fresh water, consolidation, and shrinkage).

4 Methodology 4.1 Introduction Detailed illustration of the experimental processes of this thesis will be presented in this chapter, together with underlying theories and related equipment.

4.2 Overall Design 4.2.1 Experiment Plan The ultimate aim of this thesis is to compare the shear stress performance of the soil processed by four different approaches. A schematic graph has been developed to illustrate the procedure (see in Figure 10).

Figure 10 Overall experiment design

The steps of experiment can be summarised as follows:

1. Mix the soil sample (kaolin clay) with fresh water or salt solution. 2. Utilise consolidation, shrinkage, and compaction to process the soil mixed with

fresh water and use compaction method again to process the soil mixed with the salt solution. To make the results comparable, the prepared soil processed by all the methods should reach the same initial condition (details see in Section 4.2.2).

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3. Conduct the shear test with direct shear test apparatus in the laboratory. 4. Collect and process the data, analyse and compare the results.

The outcome of direct shear test is mainly governed by two factors: the soil condition and the soil micro-structure. The micro-structure of the soil, as it is mentioned in the literature review, can be altered by the methods that used to process the soil. As for the soil condition, it is suggested that the shear stress performance varies with the soil condition (such as water content and soil density), therefore, a diverse investigation of with different water content is required in this research, for it proffers the results with a more comprehensive coverage. Hence, three initial soil conditions in total have been obtained in this thesis to investigate the soil shear strength under different water content. The determination of these conditions is illustrated in Section 4.2.2.

Theory of shear stress analysis indicates that the obtaining of the shear stress performance is achieved by carrying out the direct shear test on the same soil with three different vertical stresses applied on the top of the soil respectively (details see in Section 4.5). Generally, the selection of vertical stress can be random as the relationship between shear stress performance and vertical stress is linear (see in Figure 11).

Figure 11 Relationship between vertical stress (effective normal stress) and shear stress (performance) [25]

Considering that the soil initial condition obtained in the consolidation method, however, is governed by the vertical stress applied on the top of the soil as the consolidation will squeeze the air and water out, the vertical stresses utilised in all the methods should in line with those in the consolidation test to reach the same initial condition. Thus, by calculating the properties of the soil, the vertical stresses have been selected as 50kPa, 67.4kPa, and 88kPa [23].

Then by combing all the influencing factors, 9 sets of direct shear experiments are required in this thesis. A table has been obtained to present the experiment plan as follows (Table 2):

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34% Water Content 37% Water content 40% Water content

σ(kPa) τ(kPa) σ(kPa) τ(kPa) σ(kPa) τ(kPa)

50 NA 50 NA 50 NA

67.4 NA 67.4 NA 67.4 NA

88 NA 88 NA 88 NA

Table 2 Experiment design table-vertical stress, τ -maximum shear stress

4.2.2 Soil initial condition obtaining The premise of the comparison of the results from four approaches is that the initial condition of the soil sample which represents the macroscopic condition of the soil, should be same. This initial condition depends on the water content and the soil density of the soil. To be more specific, the samples prepared by different methods (compaction with water, compaction with salt solution, consolidation, and shrinkage) should reach the same water content and soil density condition for making the results from these approaches comparable.

To obtain this condition, a schematic graph that illustrates the relationship between the water content and soil density of the kaolin clay should be developed (example see in Figure12).

Figure 12 Water content soil density relationship example [24]

The dotted line in the graph (the saturation line) represents that the soil is fully saturated and there is nearly no air within the soil. Considering that the methods such as compaction will compress the soil and eliminate the air, the selected initial condition, however, should all be located near the saturation line.

The experiments, for making the results diverse and reliable, have been designed with three different initial conditions, the selecting of which involves the obtaining of the liquid limit, plastic limit, and shrinkage limit of the soil (see in Figure13).

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Figure 13 LL, PL, and SL [25]

Where the liquid limit (LL) stands for the boundary between liquid and plastic states, the plastic limit (PL) stands for the boundary between plastic and semi-solid states, the shrinkage limit stands for the semi-solid and solid states.

This research will focus on the liquid limit determination obtained by fall cone test (see in Chapter 4.3 in detail), the result of which will be combined with the results of the plastic limit, shrinkage limit from other research [23, 26, and 27] and generate the initial condition all together.

4.2.3 Soil sample preparation As this research is carried out with salt solution, the special preparation of salt water is required before the soil-solution-mixing, where 58.44g sodium chloride has been added into 1000g fresh water and generates a salt solution with a concentration of 5.52% (Figure 14).

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(a) (b) (c)

Figure 14 (a) utilised salt (b) salt weighing (3) salt solution obtaining

After the obtaining of salt solution, it will be mixed with kaolin clay to reach the specific initial condition, the formula for water content calculation is:

𝑤𝑤 =𝑚𝑚𝑤𝑤

𝑚𝑚𝑠𝑠

Where:

W stands for water content

𝑚𝑚𝑤𝑤 stands for the mass of the water

𝑚𝑚𝑠𝑠 stands for the mass of the soil

It is noticeable that in order to obtain the same soil water content, the preparation of soil sample only consider the weight of water, which means the weight of salt has not been considered when calculating the water content.

4.2.4 Result obtaining Then the soil sample is compacted by utilising the Proctors Compaction test (see Chapter 4.4 in detail) to change the microstructure of the soil, after which a direct shear test (see Chapter 4.5 in detail)will be conducted to address the shear stress performance of the compacted soil.

4.3 Fall Cone 4.3.1 Introduction of the fall cone test The fall cone tests generates the measurement results of the liquid limit of the kaolin clay, which is utilised, together with plastic limit and shrinkage limit, to obtain the soil initial condition of the soil samples.

4.3.2 Experimental setup and operational process of the fall cone test The completion of fall cone tests entails the following apparatus: (1) oven, (2) balance, (3) Cone penetrometer, and (4) cup (see in Figure 15-Figure 16).

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Figure 15 Oven & Balance

Figure 16 Cone penetrometer &cup

The test were carried out on four samples that vary in water content, the samples were filled into the cup separately without and air void, then the penetrometer was adjusted so that the cone joint would touch the surface of sample exactly.

After the preparation of all the processes above, the vertical clamp was released for 5 seconds to obtain the penetration levels that can be utilised to determine the liquid limit of the kaolin clay by plotting a water content penetration graph and determining the specific water content value when the penetrate depth is 20mm [28].

4.4 Proctors Compaction test 4.4.1 Introduction of the Proctors Compaction test The Proctors compaction test processes the soil sample by compacting, resulting in the modification of the micro-structure of the soil and altering the shear stress performance of the soil.

Thus, to identify the underlying relationship between compaction and soil shear stress behaviour, the investigation of soil obtained by Proctors Compaction test, however, is of great significance.

The purpose of the Proctors Compaction test is twofold: to identify the correlation between soil density and water content of compacted soil so that the range of testing

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water content can be obtained and to find out the initial condition of the test soil so that the possessed soil can reach the expected condition.

4.4.2 Underlying theory and experiment setup of Proctors Compaction test The obtaining of the water content and density curve entails the process of Proctors Compaction test, where the soil material is compacted with hammers to reduce the air voids within. The dry density of soil is governed by:

𝜌𝜌 = 𝑀𝑀/𝑉𝑉

1 + 𝑤𝑤

Equation 1Equation for dry density of soil

Where:

M stands for the total mass of the soil

V stands for the volume of the soil

W stands for the water content of the soil

During the conduction process of the compaction, a 1000ml soil mould has been compacted by a rammer weighs 2.6 kg on a detachable base plate through a collar. The utilisation of oven and balance has been adopted to calculate the weight of wet soil and dry soil [29].

Then by summarising several test results that vary in soil water content, a schematic relationship between water content and density can be obtained. In Figure 17 a previous curve has been presented to exemplify the expected result.

Figure 17 Water content and density curve example [30]

On the other hand, as the soil water content reduces as a result of compaction, several compaction tests should also be carried out to achieve the idea processed sample condition for the final comparison.

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4.5 Direct Shear Test 4.5.1 Introduction of the Direct Shear Test Shear strength of the soil refers to the maximum resistance to the shearing stress [31]. The Direct Shear Test generates the performance of the soil shear stress. Thus, by comparing the results from Direct Shear Test, the result evaluation from four methods (compaction with the soil mixed with water, compaction with the soil mixed with salt solution, shrinkage, and consolidation) can be obtained, which may generates the recommended approach to improve the soil shear stress performance.

4.5.2 Underlying theory and experimental setup of Direct Shear Test The shear strength is governed by the following equation:

𝜏𝜏𝑓𝑓 = 𝑐𝑐′ + σ′ tan Ø′

Where:

𝜏𝜏𝑓𝑓 stands for shear stress

𝑐𝑐′ stands for cohesion(inter-particle friction)

σ′ stands for effective stress

Ø′ stands for effective angle of friction

The value of shear stress in the formula will be given by the Direct Shear Test and the value of effective normal stress has been determined pre-emptively (detail see in Section 4.2).

The following schematic illustration presents the visualised relationship between these stresses:

Figure 18 Mohr’s circle failure envelope [32]

According to the graph, a linear relationship is presented between shear stress and normal stress, which suggests that, by plotting several shear stress behaviours under different shear stress conditions, this relationship can be obtained and utilised to predict the shear stress under other normal stresses. Moreover, the effective angle of friction and cohesion of the soil (Ø' and c’), which act as the main contributors of the soil shear strength performance, can also be obtained [32]. Through the generating of the information above, the result can be analysed and be developed into further comparison.

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The Direct Shear Test is conducted by a fully automatic electromechanical shear apparatus (see in Figure 19). This static machine generates the normal stress and shear stress automatically and inputs the outcome to the connected computer.

Figure 19 Shear test apparatus

As it is illustrated in the Section 4.2, overall 9 sets of direct shear test will be carried out to identify the shear stress performance of the compacted soil (mixed with salt solution) by utilising this machine.

5 Results 5.1 Results of liquid limit and initial condition The results from fall cone test can be summarised into the following table:

Number of tests

Data from attempt 1 (mm)

Data from attempt 2 (mm)

Averaged value

Container weight

Soil weight +container

Dry weight + container

Water weight

Water content (%)

1 18.9 17.1 18.0 23.8 68.0 49.2 18.8 50.7 2 18.7 19.4 19.1 12.1 50.1 38.6 11.5 77.4 3 20.1 19.9 20.0 13.4 37.5 26.8 10.7 80.5 4 21.5 20.5 21.0 17.1 40.5 29.9 10.7 83.5 5 24.5 22.0 23.3 15.5 40.4 28.2 12.3 96.5

Table 3 Results from fall cone test

By plotting the averaged penetration value and water content, a graph can be obtained to illustrate the underlying relationship:

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Figure 20 Liquid Limit of kaolin clay

Based on the theory in Section 4.3.2, the corresponding water content of 20mm penetration represents the liquid limit of kaolin clay, then by tracing the curve in Figure 20, a value of 80% can be obtained as liquid limit. Hence, the liquid limit of kaolin clay obtained from fall cone test has been determined as 80%.

Figure 21 Water content and soil density relationship of kaolin clay

0102030405060708090

100

17.5 18 18.5 19 19.5 20 20.5 21 21.5

Wat

er co

nten

t (%

)

Penetration depth (mm)

Liquid Limit

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Meanwhile, the value of shrinkage limit has been determined as 25% [26] and that for plastic limit is 35% [27].

Besides, the relationship between water content and dry density of the kaolin clay has been obtained by measuring the soil density under different water content, a schematic graph (Figure 21) has been obtained to illustrate this correlation.

According to the figure, it can be seen that the range of curve that almost reaches saturation line is from 32% to 42%. As the plastic limit has been determined as 35%, below which the soil will just begin to crumble, the range of selected water content initial condition of the soil should varies from 34% to 40%. Thus, the three initial conditions have been determined as 34%, 37%, and 40%.

5.2 Results of soil mixed with salt solution with 34%, 37%, and 40% water content The statistic of direct shear test is generated automatically by the software in the connected computer (example see in Appendix). The analysis of shear stress performance is built on the relationship between shear displacements and shear stress, suggesting that the statistic of shear displacement and effective tangential stress (the shear stress in the file) should be collected. Hence, by developing the relationship graph about these two factors, three figures that illustrate the shear stress performance of the soil mixed with salt solution with 34%, 37%, and 40% water content respectively can be obtained, where:

Figure 22 Results of soil mixed salt solution with 34% water content

-10

0

10

20

30

40

50

0 2 4 6 8 10 12

Shea

r Str

ess (

Kpa)

Shear Displacement(mm)

34% Water Content

50KPa 67.4Kpa 88Kpa

24



Based on the theory from Section 4.5.2, the shear stress performance is governed by the peak value of shear stress under different vertical stress. Thus, a table has been obtained to summarise the highest figure in each line in the graph.

34% Water Content 37% Water content 40% Water content σ(kPa) τ(kPa) σ(kPa) τ(kPa) σ(kPa) τ(kPa) 50 29.8 50 24.7 50 22.8 67.4 36.9 67.4 28.9 67.4 30.6 88 44.4 88 49.9 88 33.7

Table 4 Results of maximum shear stress, σ-vertical stress, τ -maximum shear stress

The obtaining of shear stress strength is then achieved by plotting the data in the table, where:

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Figure 25 Shear stress performance result of soil mixed salt solution with 34% water content

Figure 26Shear stress performance result of soil mixed salt solution with 37% water content

Figure 27 Shear stress performance result of soil mixed salt solution with 40% water content

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The two dominating factors that control the feature of shear stress performance of kaolin clay, the cohesion and effective angle of friction, according to the general theory, however, can be determined by measuring the intersection of the relationship curve and Y axis of the graph and by calculating the inverse tangent of the slope of the curve respectively, which gives:

34% Water Content 37% Water content 40% Water content c ф (°) c ф (°) c ф (°) 10.8 21 11.6 18.9 9.8 15.7

Table 5 Summary of shear stress feature in soil mixed with salt solution in different water content

6 Discussions 6.1 Comparison between results from different water content The cohesion value (c in Table 4) represents the inter-particle friction of the angle, suggesting that the higher value can produce the higher shear stress performance. With the increasing of water content, apparently, the cohesion remains stable with minor variation (10.8 for 34%, 11.6 for 37%, and 9.8 for 40%, see in Figure 28), which indicates that the altering of water content nearly has no impact on this feature. Possible reason could be interpreted as that the difference of water content is not significant enough (3% in this case) to result in the interaction force between the soil particles. To identify this hypothesis, an extending research can be carried out to investigate the cohesion difference in soil (mixed with salt solution) with a series of water content.

Figure 28 Cohesion performance in different water content

The other property that governs the shear stress strength is the soil friction angle (ф in Table 4)). As ф defines the relationship between friction shear resistances of soils together with the normal effective stress, a larger value of friction angle, however, will lead to a more satisfying performance of the shear stress. Past studies suggest that the soil friction angle has almost no correlation with the normal stress in the direct shear

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stress test but it varies with changing soil density [33]. Hence, considering that the density of kaolin clay is 2.65g/cm^3 [34], far heavier than water (1g/cm^3), it can be deduced that lower water content will produce higher bulk density. Since the figures in the Table 4 indicates that the friction angle declines with the increasing of water content (21° for 34%, 18.9° for 37%, and 15.47° for 40%), by combining the relationship between water content and bulk density, a conclusion can be proposed as that the friction angle of kaolin clay has positive correlation with bulk density of the kaolin clay., which can also be reinforced by the previous studies [35].

6.2 Comparison between results from compaction approach with water and salt solution In this chapter a comparison of the shear stress performance results from compaction approach (the one with water and the one with salt solution) will be conducted. The data of the compaction method with soil mixed with fresh water was obtained from another thesis[26].Two comprehensive results figures of the relationship between vertical stress and maximum stress have been obtained (Figure 29-30), which can be further developed into a table that illustrates the shear stress properties of the tested soil (Table 6).

Figure 29Result from compaction method with salt solution

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Figure 30 Result from compaction method with water

Water content 34% 37% 40% Shear stress coefficient c ф (°) c ф (°) c ф (°) Compaction with salted soil

10.8 21.0 11.6 18.9 9.8 15.7

Compaction with watered soil

6.8 46.4 17.1 21.7 12.2 15.9

Table 6 shear stress coefficient of in two methods

According to the table, it can be seen that the difference among the cohesion value of soil mixed with water is still not significant enough (6.8 for 34%, 17.1 for 37%, and 12.2 for 40%), which reinforces the results from the soil mixed with salt solution, suggesting that the involvement of salt may hardly alter the variation of cohesion of the kaolin clay. Meanwhile, the friction angle in the soil mixed with water, which in line with the tendency of results from soil mixed with salt solution, also presents a declining trend with the increasing of water content. Hence, by summarising the information above, it can be concluded that the participation of salt will not change the relationship feature between water content and soil shear stress properties.

On the other hand, to compare the shear stress performance in a macro-scope vision, the data of maximum shear stress in each experiment has been obtained (Table 7).

34% Water Content 37% Water content 40% Water content σ (kPa)

τ (kPa) of soil mixed with water

τ (kPa) of soil mixed with salt solution

τ (kPa) soil mixed with water

τ (kPa) of soil mixed with salt solution

τ (kPa) soil mixed with water

τ (kPa of soil mixed with salt solution

50 56.5 29.8 30.0 28.9 25.8 22.8 67.4 82.6 36.9 56.9 34.2 32.8 30.6 88 96.8 44.4 46.2 41.8 36.7 33.7

Table 7 Maximum shear stress in each experiment, σ-vertical stress, τ -maximum shear stress

The data in the table suggests that in all of the experiments, the shear stress performance results of watered soil, however, are greater than those of salted soil,

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indicating that addition of salt impair the shear stress strength of the soil to some degree. Thus, from the aspect of compaction test only, the one with water has a better performance than the one with salt solution.

6.3 Comparison between results from four approaches The results of shear stress experiments from shrinkage and consolidation methods, which are collected from another two thesis [23 and 27], have been summarised below, together with the schematic figures that present the relationship between vertical stress and maximum stress.

34% Water Content

37% Water content 40% Water content

σ (kPa) τ (kPa) of shrunken soil

τ (kPa) of shrunken soil

τ (kPa) of shrunken soil

50 32.4 23.5 21.5 67.4 34.8 29.2 24.6 88 40.7 36.0 31.4

Table 8 Experimental results of shrinkage method, σ-vertical stress, τ- maximum shear stress

34% Water Content

37% Water content 40% Water content

σ (kPa) τ (kPa) of consolidated soil

τ (kPa) of consolidated soil

τ (kPa) of consolidated soil

50 26.2 25.5 23.8 67.4 28.8 28.5 29.4 88 34.3 37.0 37.8

Table 9Experimental results of consolidation method, σ-vertical stress, τ -maximum shear stress

Figure 31 Result from Shrinkage method

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Figure 32 Result from consolidation method

Then, by summarising the performance of shear stress in each method, a table can be obtained to analysis the data with shear stress performance ranking (Table 10).

Based on the ranking, apparently, the shear stress performance of the method utilised the compaction technique with water rank the first over all the condition combinations (except once). For further analysis, the figures that visualise the data have been developed below (Figure 33-35).

34% Water Content 37% Water content 40% Water content σ (kPa)

τ (kPa) Rank τ (kPa) Rank τ (kPa) Rank

Shrinkage 32.4 2 23.5 4 21.5 4 50 Consolidation 26.2 4 25.5 3 23.8 2

Compaction with salted soil

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Shrinkage 34.8 3 29.2 3 24.6 4 67.4 Consolidation 28.8 4 28.5 4 29.4 3


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82.6 1 56.9 1 32.8 1

Shrinkage 40.7 3 36.0 4 31.4 4 88 Consolidation 34.3 4 37.0 3 37.8 1


44.4 2 41.8 2 33.7 3


96.8 1 46.2 1 36.7 2

Table 10 Shear stress performance ranking of four methods, σ-vertical stress, τ -maximum shear stress

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Figure 33 Comparison of shear stress performance of soil with 34% water content among four methods

Under a condition of 34% water content, the experiment results are distinctive for compacted soil mixed with water when the vertical stress applied on the top of the soil is greater than 19kPa. The differences among other three are not significant as the maximum shear stress of them concentrate in the similar area.

Figure 34 Comparison of shear stress performance of soil with 37% water content among four methods

When water content of the soil reaches 37%, the friction angles from four methods are noticeably similar as the slope of each lines are nearly the same. However, with the slightly different increasing trend and the different cohesion coefficient, the differences

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of shear stress can still be identified, where the compaction with watered soil method rank the first over all the vertical stress, followed by compaction with salted soil method, consolidation method, and shrinkage method.

Figure 35Comparison of shear stress performance of soil with 40% water content among four methods

As for the water content of 40%, the shear stress performance from compaction (watered soil) method still surpasses the other three. However, since the consolidation is with a greater friction angle under this condition, it can be expected that the shear stress strength of consolidation is expected to exceed the compaction method if the vertical stress is greater than 88kPa.

7 Conclusions & Future Work 7.1 Conclusions This thesis investigated the shear stress performance of the kaolin clay soil that was mixed with salt solution and processed by the compaction method. Through experimental and numerical procedures, the cohesion and friction angle of the soil with different water content (34%, 37%, and 40%), which governs the performance of shear stress, have been successfully identified. Detailed analysis of these features and further comparison with the similar results from other experiments (shrinkage, consolidation, and compaction with water) were then carried out to address the relationship among them. The conclusions of this thesis are summarised as follows.

• For the soil processed by the compaction approach, on one hand, the altering of water content has been proved to have no impact on the cohesion of the kaolin clay. The comparison among experiment results from different water content indicates that the cohesions of each experiment are nearly identical. On the other hand, the analysis on friction angle demonstrates that this feature decreases with the increasing of water content and thus has positive correlation with bulk density of the kaolin clay.

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• The difference between compacted soil with salt solution and compacted soil with fresh water has been investigated, the cohesion of the soil mixed water still with minor variation in different water content, suggesting that the involvement of salt may hardly alter the variation of cohesion of the soil. Meanwhile, it has also been proved that the participation of salt will not change the relationship between water content and the friction angle of kaolin clay.

• Through the comparisons among four approaches (compaction with soil mixed with salt solution, compaction with soil mixed with fresh water, shrinkage, and consolidation), it has been identified that: for soil with 34% water content, when vertical stress exceed 19kPa, the compaction method (the one with water) ranks the first on shear stress performance; for soil with 37% water content, the shear stress performance of the compaction method (the one with water) ranks first over all the vertical stress; for soil with 40% the compaction method (the one with water) has the most satisfying shear stress at first but it is expected to be surpassed by the consolidation method when vertical stress is greater than 88kPa.

7.2 Future Work The observations and conclusions that were made in this thesis can be extended to the follows:

• The results in this thesis are all generated in one experiment, which may induce the inaccuracy of the data. Hence, the reproducing of data can be carried out in the future to eliminate the contingency.

• The water content utilised in this thesis is limited (only 3 sets), to identify the in-depth relationship between shear stress performance and water content of the kaolin clay, a more comprehensive research can be conducted with a greater coverage of different water content.

• As the shear stress performance investigation was only carried out with direct shear test, considering that there is other shear test method available (such as Triaxial Shear Test), further exploration of shear stress can be conducted by adopting different shear test approaches.

• Previous studies have identified that the zeta potential determines the stability of the chemical particle as it characterizes the motion of this chemical particle in the solution [36]. Taking that into consideration, further investigation can be proposed as to identify the shear stress performance of the soil mixed with salt solution with different zeta potential.

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Appendix Appendix A: Example of experimental data

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