Development Of Shear Strength In Soils Biology Essay

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This thesis paper focuses on the development of shear strength in soils which have altered engineering properties resulting from the artificial addition of cement and randomly distributed fibres, with respect to cement content and fibre content. Through the study of academic papers and detailed analysis of the aforesaid variables, empirical equations will be developed for describing the strength and deformation of fibre reinforced cemented soils. The equations will quantify the effects the variables have on the strength of the fibre reinforced cemented soils. In addition to this a comprehensive equation, combining considered variables is proposed. The accuracy of these empirical equations will be verified within the thesis. As means to accomplish the aims outlined above, the following must be accomplished.

Identify the physical and chemical mechanisms responsible for the development of cemented soil structures.

Analyse the interaction of cement, fibre and soil at a chemical level and thus how structures and bonds are developed.

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Evaluate data objectively assessing the quality of the source

Review previous research into the alterations in the engineering parameters and behaviour of fibre reinforced cemented soils.

Consider a range of soils for analysis providing a range of results which can then be quantified and analysed to provide information regarding variations in the strength of the soils.

Analyse previous academic research internally and externally of the University of Wollongong regarding the influence of cement content and fibre reinforcement on the development of soil strength.

Propose empirical equations to quantify the variations in strength observed through the review of experimental data.

Develop a general strength criterion to quantify the variations in soil strength in terms of cement content, fibre content and curing time.

Develop mathematical models useful for predicting variations in the strengths of soils.

Provide conclusions from results regarding the accuracy of models.

Develop a personal expertise and understanding of the topic.

Provide underpinning studies for future students to continue with in greater depth.

Rationale

Thesis Outline

This thesis has been written so that it provides the reader with relevant background information to developing an understanding of the topic prior to the discussion and analysis of the alterations of engineering behaviour which occur due to the addition of cement and fibre in a soil. It will discuss the physical and chemical mechanisms coupled with the development of fibre reinforced cemented soils. Finally, empirically consequent mathematical equations will be modelled in order to quantitatively assess the variations in soil strength associated with variable change. The comprehensive structure of this paper is as follows.

Chapter 2 - A review of academic research and discussion of engineering behaviour, physio-chemical concepts and mechanisms, interface morphologies and reinforcement of soil structures treated with cement.

Chapter 3 - An analysis of data from previous academic research into the development of strength in soils by the addition of cement. The chapter focuses on the influence of cement and curing time in respect to soil strength whilst discussing relevant theory behind observed trends. Empirical equations will be developed to model the development strength as a result of the aforementioned variables.

Chapter 4 - Development of models simulating the accuracy of the equations proposed in chapter 3 and a general strength criterion is proposed.

Chapter 5 - An analysis of data from previous academic research into the development of strength in soils by the addition of cement and fibre reinforcement. The chapter focuses on the influence of cement, fibre and curing time in respect to soil strength whilst discussing relevant theory behind observed trends. Empirical equations will be developed to model the development strength as a result of the aforementioned variables.

Chapter 6 - Development of models simulating the accuracy of the equations proposed in chapter 5 and a general strength criterion is proposed.

Chapter 7 - Summarises and makes concluding comments on the results obtained within the previous chapters of the paper providing recommendations for future study

2. LITERATURE REVIEW

2.1 Introduction

2.2. Alterations in Engineering Behaviour and Strength of Cement Stabilised Soils

2.2.1 Stress- Strain Behaviour

The stress-strain behaviour of cement-stabilised soils under triaxial conditions have been comprehensively examined (Muhunthan & Sariosseiri, 2008; F.D.Rosa, N.C.Consoli, & B.A.Baudet, 2008; Y.Wang & S.Leung, 2008; E.Ashgahari, D.G.Toll, & S.M.Haeri, 2003)

2.2.2 Shear Strength Parameters

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The two components of shear strength, angle of internal friction (Ï•') and cohesion (c') were postulated by Broms (1986) to proliferate via two processes. Formation of interlocking particles in the structure of the cement treated soil result in increases in the frictional resistance. The cohesion component of shear strength sees increase with thickness reduction of the diffused doubled-layer of adsorbed water.

The effect of soil cementation on the shear strength parameters of soil has been widely investigated (Uddin, Balasubramaniam, & Bergado, 1997; Cai, Shi, W.W. Ng, & Tang, 2006; Horpibulsuk, Miura, & D.T.Bergado, 2004; Kamruzzaman, 2002; Yin & Lai, 1998).

Uddin et al. (1997) ,Horpibulsuk et al. (2004) and Portland Cement Association (2003) reported similar observations results and conclusions in their experimental work. An increase in both the frictional resistance (Ï•') and cohesion (c') of the soil samples tested coincides with increases in cement content and curing time. Uddin et al. (1997) reported that values of cohesion and friction at cement contents of fifteen percent were asymptotic. A study conducted by Yin and Lai (1998) showed results that are controversial with those published by Uddin et al. (1997) and Horpibulsuk et al. (2004). The study was performed on a marine clay from Hong Kong and established that by increasing the cement content and decreasing the initial water content of the unmodified soil resulted in an increase in the cohesion (c') and a decrease in the internal friction angle (Ï•').

Consoli et al. (2000) found that his work showed changes in the frictional angle but not in the cohesion of tested soil samples due to increases in the curing stress.

2.2.3 Permeability

Porbaha, A., Shibuya, S. and Kishida, T. (2000). "State of Art in Deep Mixing

Technology. Part III: Geomaterial Characterization". Ground Improvement, Vol. 3,

91-110.

Kaushinger, J. L., Perry, E. B., and Hankour, R.(1992). "Jet grouting state of the practice".

Proc., Grouting, soil improvement and geosynthetics: ASCE, New York, vol. 1, 169-

181.

Broderic, G. P. and Daniel, D. E. (1990). "Stabilizing compacted clays against chemical

attack". Journal of Geotechnical Engineering, ASCE, 116(10), 1549-1567.

Broderic, G. P. and Daniel, D. E. (1990). "Stabilizing compacted clays against chemical

attack". Journal of Geotechnical Engineering, ASCE, 116(10), 1549-1567.

Porbaha, A., Shibuya, S. and Kishida, T. (2000). "State of Art in Deep Mixing

Technology. Part III: Geomaterial Characterization". Ground Improvement, Vol. 3,

91-110.

2.2.4 Compressibility

2.2.5 Modulus

Yin and Lai (1998) reported that the stiffness of modulus of elasticity (E) of a cement modified soil is a function of four variables including; initial water content of the untreated soil (Hong Kong marine clay in this circumstance), cement content, confining pressure and curing time. Determination of the modulus was performed using an external strain measurement transducer in conjunction with undrained triaxial tests. Conclusions drawn from the experimentation suggest that modulus of elasticity increases with a coinciding increase in confining pressure and cement content and a decrease in the initial water content of the untreated soil.

2.2.6 Durability

Soil stabilisation is generally used in projects where it be required that the soil have sufficient strength and durability. Cement treatment of soils has been documented to resist the damaging effects of freeze and thaw, wet and dry cycles on the bearing capacity or strength of the soil. Testing conducted by the Portland Cement Association (2003) compares the performance of raw soil, 2% and 4% cement treated soil after 60 cycles of freeze and thaw. Results have been shown in Table.......... below. Freeze and thaw cycles have actually been reported to result in increases in strength by the additional hydration of cement during the thaw cycles (Portland-Cement-Association, Properties and uses of Cement-Modified soil, 2003). The relationship between Unconfined Compressive Strength (UCS) of a soil and the durability of soil samples can be seen in the following Figure ...... From Figure.... it is obvious that with an increase in UCS resulting from cement modification that samples have an increased durability to the effects of freeze and thaw.

Table - Permanence/Durability of Bearing Value of Cement-Modified Granular Soil

Figure - Relationship between unconfined compressive strength and durability of cement treated soils based on Portland Cement Association durability criteria (ACI 230.1R-90 1990).

2.3 Basic Concepts and Mechanisms of Cement Stabilisation

2.3.1 Mechanisms Attributed to the Development of Cemented Soil Structures

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The most common cement used in cement-soil stabilisation is Ordinary Portland Cement (OPC). The average composition of Portland cement taken from various countries is shown below in Tab1e 2.1.

Early ideas regarding the influence of soil chemistry on the hardening of cemented soils to be relatively minimal or inert. Analysis was subsequently undertaken by Herzog and Mitchell (1963), Croft (1967) which suggested that the mineralogy of the soil was fundamentally accountable for the effectiveness of cement treatment of soil. Mineralogy has the capacity to retard hardening by reductions in pH of the system (J.B.Croft, The influence of soil mineralogical composition on cement stabilization, 1967).

Table 2. - Composition Averages for Portland Cement (S.Paria & P.K.Yuet, 2006)

In acquiescence with the commonly accepted notation used in cement chemistry, C symbolises CaO, S symbolises SiO2, A symbolises Al2O3 and F symbolises Fe2O3. The main constituents of OPC are tricalcium and dicalcium silicates (C3S and C2S), tricalcium aluminate (C3A) and tetracalcium alumino-ferrite (C4AF). Hydration of these compounds occurs when combined with water to form calcium silicate hydrates (CSH); calcium aluminate hydrates (CAH); and calcium aluminate silicate hydrates (CASH). The hydration process forms a cementitious paste where the aluminates are accountable for the setting of the paste and the silicates are accountable for paste hardening along with the production of lime in the form of calcium hydroxide [Ca(OH)2]. Hydration products progressively replace the water in the voids between cement grains and soil particles which provides a matrix which amalgamates the soil particulates (Huawen, 2009). Croft (1967) suggested that the exchange and cementing action of OPC and clay is similar to that of lime. The rate of cement hydration is a function of the hydration of individual components and according to (S.Paria & P.K.Yuet, 2006) may be accelerated by increasing the fineness of components, increasing the temperature of hydration or by increasing the ratio of water to solids.

There are two principal reactions that preside over the mechanisms attributed to the development of cemented soil structures: primary hydration and pozzolanic reactions which are time-dependent. Equation 2.1 illustrates the mechanism of primary hydration that occurs between water and cement. The process results in the formation of cementitiuos products, CSH, instigating expeditious strength gain and short term hardening of the cement-treated soil. The production of lime [Ca(OH)2] results in proliferation of Ca2+ and OH- ion concentrations which develops through the hydrolysis of lime [Equation 2.2] (Huawen, 2009). Hydration and Pozzolanic reactions of the main constituent of OPC, C3S [tricalcium silicates] have been shown in Equation 2.1 through to Equation 2.4.

C 3 S + H 2 O C 3 S 2 H X (hydrated gel) + Ca (OH) 2

(primary cementitious products) [Equation 2.1]

Ca (OH) 2 Ca 2+ + 2 (OH) -

(hydrolysis of lime) [Equation 2.2]

The hydrolysis of lime in the pore water as stated previously causes an increase in the concentration of OH- ions. Once the pore chemistry reaches an adequate alkaline state the process of solidification or secondary pozzolanic reaction occurs. Promotion of the dissolution of silica and alumina from clays results from the alkalinity of water in the pores of the soil. Reactions between silica and alumina take place with ions of Ca2+ forming secondary cementitious products CSH and CAH [Equations (2.3) and (2.4)]. The cementitious products harden and crystallize with time effectively improving strength characteristics of the soil (Huawen, 2009).

Ca 2+ + 2 (OH) - + SiO2 (soil silica) C-S-H [Equation 2.3]

Ca2+ + 2(OH)- + Al2O3 (soil alumina) C-A-H [Equation 2.4]

The hydration of tricalcium aluminate (C3A) and tetracalcium alumino-ferrite (C4AF) have been publicized as follows [Equation 2.5 and 2.6 respectively] (S.Paria & P.K.Yuet, 2006).

2C3A + 27H 2C3AH6 [Equation 2.5]

3C4AF + 60 H 4C3(A,F)H6 + 2(F,A)H3 + 24H [Equation 2.6]

A.Mollah, M. Yousuf, Rajan K. Vempati, T.C.Lin and D.Cocke (1995) proposed two models to explain observations associated with cement hydration and pozzolanic reactions. The two models are osmotic and crystalline models. According to the gel model, CSH is formed on individual cement particle surfaces upon hydration. The crystalline model makes the assumption that charge ions silicate and calcium form upon the contact of cement and water. The initial hydration is succeeded by the nucleation of calcium hydroxide and CSH precipitated on the surface of cement grains. The schematics of the gel and crystalline models have been shown in Figures ....... below.

Figure - Gel Model of Cement Hydration (A.Mollah, M.Yousuf, R.K.Vempati, T.C.Lin, & D.L.Cocke, 1995)

Figure - Crystalline Model of Cement Hydration (A.Mollah, M.Yousuf, R.K.Vempati, T.C.Lin, & D.L.Cocke, 1995)

Figure - Schematic representation of the arrangements of structural elements in cement stabilised soil (J.B.Croft, The Structure of Soils Stabilized with Cementitious Agents, 1967)

2.3.2 Cement Alternative Mechanisms of Soil Stabilisation

2.3.2.1 Lime Stabilisation

The process of soil stabilisation through the addition of lime has been extensively applied in civil engineering practice including foundations, roadbeds, embankments and piles (Cai, Shi, W.W. Ng, & Tang, 2006). It is most commonly associated with the stabilisation of fine grained soil. Academic studies have primarily focused on the effect lime stabilisation has on the improvements in strength in terms of the performance of clayey/expansive soils (Kumar, Walia, & Bajaj, 2007; Cai, Shi, W.W. Ng, & Tang, 2006; Narasimha Rao & Rajasekran, 1996). Fewer studies have investigated the influence of lime stabilisation on the compressibility behaviour of clayey soils. Academic research by Rajasekaran, Narasimha Rao (1996, 2002) and Sudhakar, Shivananda (2005) regarding the reduction in compressibility of soils inform of the strong cementitious bonding occurring between soil particles due to the chemical and pozzolanic reactions resulting from the addition of lime. The bonding of soil particulates effectively increase the soils yield stress and minimise strains occurring in the soil (Shivananda, 2005).

Lime addition to clayey soils alters the pH levels of the soil and allows for a process known as cation exchange. The process of cation exchange has the utmost affect on the Atterberg Limits and potential for soil swelling, the exchange reduces the clay plate's affinity for water by the reduction of electrical surface charge concentration of individual clay plates (Ma & Eggleton, 1999). A profusion of Calcium and Magnesium ions (Ca2+ and Mg2+) with greater electron affinity tend to relocate and replace other cations present in the soil including Sodium and Potassium (Na+ and K+) (Kumar, Walia, & Bajaj, 2007). According to the studies of Yong and Ouhadi (2007) the replacement of Sodium and Potassium ions (Na+ and K+) with the Calcium ions (Ca2+ ) will result in a flocculated structure. The flocculated structure consequences in a reduction in diffuse double layer thickness. The mineralogy of the soil and presence of lime affects the capacity and process of cation exchange

Cementitious reaction products produced by the lime-clay interaction harden throughout pores in the soil. There is an evident diminution in clay content and with the binding of particles in the soil matrix a resulting increase in the percentage of coarse soil particles within the final cemented soil (Kumar, Walia, & Bajaj, 2007).

2.3.2.2 Fly Ash Stabilisation

Fly ash is a by product that is generated in combustion processes. It is a residual mineral matter comprised of fine particles which are extracted from coal fired flue gases. The composition of fly ash can vary considerably with the nature of the coal source. Two variations of fly ash are recognised, classes C and F. Class C fly ash is typically produced by burning lignite and subbituminous coal. Fly ash is a pozzolanic material delineable as siliceous or siliceous and aluminous (Cokca, 2001; Kaniraj & Havangi, 2001).

Typical fly ash composition includes oxides of Silicon (SiO2), Titanium (TiO2), Aluminium (Al2O3), Iron (Fe2O3), Manganese (MnO), Magnesium (MgO), Calcium (CaO), Sodium (Na2O), Potassium (K2O), Phosphorous (P2O5), Sulphur (SO3) and unoxidised carbon (Zha, Liu, Du, & Cui, 2008; Muhunthan & Sariosseiri, 2008).

The addition of fly ash to an expansive soil reduces the soils plasticity index, activity, shear strength and swelling behaviour. Two mechanisms, physiochemical and mechanical interaction are likely to preside over the reduction in swelling of expansive clays combined with fly ash. Physiochemical interaction relates to the replacement of clay particles by non-plastic fine material from fly ash by means of cation exchange. Transferable cations present within fly ash include Ca2+, Fe3+ and Al3+ which promote the flocculation of clay particles under ionized conditions. Cation exchange consequences in agglomeration of fine clay particles to form coarse particles and better grain size distribution (Cokca, 2001; Muhunthan & Sariosseiri, 2008; Zha, Liu, Du, & Cui, 2008). As fly ash is a pozzolanic material the mechanical processes associated with a decrease in the swell potential and an increase in the shear strength result from processes of cementation which occur over time. The pozzolanic reaction results in the formation of cemented compounds typified by high shear strength and

low volume change (Zha, Liu, Du, & Cui, 2008).

2.3.2.3 Bituminous Stabilisation

The stabilisation of soils with the addition of bitumen improves the mechanical behaviour by different means to the aforementioned cement, lime and fly ash additives. The mechanisms which can be attributed to the performance improvements of bitumen treated soils include a waterproofing phenomenon and adhesion of soil particles. The method is typically applied to stabilise coarse-grained soils; however application to fine-grained soils is permissible (Department of the Army, the Navy, and the Airforce, 1994).

The bituminous coating of the soil particles impedes the incursion of water thereby reducing soil strength losses in the presence of water. The influence of water can cause reductions in soil strength and volume change. Academic study by S.M.Mirandi and P.Safapour (2009) illustrated a soil sample treated with bitumen that had significant increases in bearing capacity and improvements in durability.

The adhesion of soil particles results from the binding of soil particles to the bitumen. The bitumen acts as a cement and results in increases in soil cohesion effectively increasing the shear resistance of the soil (Department of the Army, the Navy, and the Airforce, 1994).

2.4 Basic Concepts and Mechanisms of Soil Reinforcement

2.5 Interface Morphologies and Mechanical Behaviour of Fibre-Reinforced Soil

2.5.1 Interface Morphologies of Fibre-Reinforced Uncemented Soil

2.5.2 Interface Morphologies of Fibre-Reinforced Cemented Soil

3. DEVELOPMENT OF STRENGTH IN CEMENT STABILISED SOILS

4. SIMULATION OF DATA FOR CEMENT STABILISED SOILS

5. DEVELOPMENT OF STRENGTH IN CEMENT STABILISED FIBRE-REINFORCED SOILS

6. SIMULATION OF DATA FOR CEMENT STABILISED FIBRE-REINFORCED SOILS

7. CONCLUSIONS