OPTIMIZATION AND MODELLING OF RICE HUSK ASH-CLAY SOIL STABILIZATION

ABSTRACT

The over dependent on the utilization of industrially manufactured soil improving additives (cement, lime etc), have dept the cost of such stabilized soils financially high.   The hitherto have continued to deter the underdeveloped  and poor nations of the world  from providing accessible  pavement  to their  rural dwellers  who  constitute  the higher  percentage  of their population and are mostly, agriculturally dependent.   A number of researchers have studied the physical and chemical properties  of rice husk ash.   The  high percentage  of siliceous material in rice husk ash indicates that it has potential pozzolonic properties.  Findings show that RHA which are waste materials can be used to upgrade expansive soil as a construction material.  Thus the use of agricultural waste (such as rice husk ash – RHA) will considerably reduce the cost of construction and as well reducing the environmental hazards they cause. However  it  appears  that  none  of  these  studies  referred  to  any  guideliens  in  the  binder selection  process.     Thus,   the   study  obtained   the  empirical  models  of  soil  material characteristics such as California Bearing Ratio (CBR), Optimum Moisture Content (OMC) and Maximum Dry Density (MDD), which are desirable for road construction.  The empirical models  which  are  based  on  Scheffe’s  optimization  theory  were  developed   from  the experimental  results  of  soils  obtained  from  three  different  locations  in  south  –  eastern Nigeria.  The results show that A – 6 soil type improved to A – 2 – 6 after stabilization with RHA.   CBR value of the soil improves to an average of 14.47% from 5.67%.  Surprisingly, the average value of the MDD reduces to 1.58kg/m3  from 1.89kg/m3.  While average OMC went up from 16.63% to 21.93%.   The empirical models show a very close result to  the experimental results.  Since RHA is much cheaper than lime and cements, use of rice husk ash in soil stabilization can result in cost reduction in construction.

CHAPTER ONE INTRODUCTION

1.1      GENERAL INTRODUCTION

According   to   Brooks   (2009),   clays  exhibit   generally   undesirable   engineering properties.  They tend to have low shear strengths and to lose shear  strength further upon wetting or other physical disturbances. They can be plastic and compressible and they expand when wetted and shrink when dried. Some types expand and shrink greatly upon wetting and drying – a very undesirable feature. Cohesive soils can creep over time under constant load, especially when the shear stress is  approaching  its shear strength,  making them prone to sliding. They develop large lateral pressures. They tend to have low resilient modulus values. For these reasons,  clays  are generally poor materials for foundations.  Such characteristics exhibit by clay soils can be harnessed through soil improvement techniques.

Geotechnically, soil improvement could either be its modification or stabilization, or both. Soil modification is the addition of a modifier (additive and conditioner) to a  soil to change its index properties, while soil stabilization is the treatment of soils to  enable their strength and durability to be improved such that they become totally suitable for construction beyond their original classification. Stabilization can be mechanical or chemical and several types of stabilizing agents have proved to be suitable under different conditions of soil and environment.

Many procedures have been developed to improve the physical behaviour of soil. The soil stabilization techniques include:

• Stabilization with lime.

• Stabilization with cement.

• Stabilization with a combination of lime and cement

The following are some of the important materials which have proved good.

• Fly Ash for the construction of the embankments and stabilization of sub-base and base-courses.

• Steel and copper slags for the construction of sub-base and base-courses.

• Marble dust in sub-grade and sub base.

Undoubtedly the most widely applied methods involve the use of inorganic cementing agents  (Jha  &  Gill,  2006).  The  effectiveness  of  such  agents  relies  on  the  formation  of cementing bonds between the particles in the soil system. Continuing,  (Jha & Gill, 2006) noted that lime as an additive, brings several beneficial changes in the engineering properties of soil such as decrease in soil plasticity and shrink  –  swell potential apart from improving strength characteristics.

Stabilization of soil by lime is achieved through cation exchange, flocculation  and agglomeration, lime carbonation and pozzolanic reaction. Cation exchange and  flocculation agglomeration reaction takes place rapidly and brings immediate change in soil properties, where  as,  pozzolanic  reactions  are  time  dependent.  These  reactions  involve  interaction between soil silica and (or) alumina and lime to form various types of cementing agents thus enhancing  the strength.  The chemical processes  modify the  soil structure  whereby larger grain aggregates are formed, leading to several advantages in the suitability of soil in road construction.

Certain natural substances, such as volcanic ash reacts with lime addition much better than do the ordinary soil types. If such materials are added to soil, the  efficiency of lime stabilization may be greatly increased. The characteristics  of  compacted soil, if improved, resulting from residue utilization like fly ash, blast  furnace  slag, rice husk ash etc mostly brings environmental and economic benefits.

Soil stabilization  has been widely  recommended  for developing  countries  for  the construction of various elements of the pavements (Ting, 1971).

The reasons usually put forward are that the use of locally available materials will lead  to lower  costs.  The  over  dependence  on  the  utilization  of  industrially  manufactured  soil improving additives (cement, lime etc), have kept the cost of such stabilized soils high. This hitherto  have continued  to deter the underdeveloped  and  poor  nations of the world from providing accessible pavement to their rural dwellers who constitute the higher percentage of their population and are mostly, agriculturally dependent. Thus the use of agricultural waste (such as rice husk ash – RHA) will considerably reduce the cost of construction and as well reduce the environmental hazards they cause.

Sear (2005), in his work showed that Portland cement, by the nature of its chemistry,

produces  large  quantities  of CO

2

for every ton of its final product.  Therefore,  replacing

proportions  of  the  Portland  cement  in  soil  stabilization  with  a  secondary  cementitious material like RHA will reduce the overall environmental impact of the stabilization process.

1.2      STATEMENT OF PROBLEM

With the increasing road making activities in the south – eastern Nigeria, and in the light of scarce desirable sub-grade materials in the region; efforts have been geared over the years towards finding economical alternative improvement techniques to soils of that locality. Just like in geotechnical  engineering,  where the underlying  principle  has  always  been “using better quality of sub-grade materials”, which decreases the thickness of pavement and thus reduces the cost of construction to cost effective road making and elongates the life span of the constructed roads. In the south eastern region, soils used for construction of sub-grade are in short supply. Further, the soil for sub-grade collected from extensive area along length of roads show deficient  engineering  properties for their road making use due to depositional

history. Numerous works have proved that these can be improved to ensure the satisfactory performance of the constructed road.

Due to rapid industrialization throughout the world, the production of huge quantity of waste materials creates not only environmental problem but also depositional hazards. Safe disposal of the same  is very vital issue and such situation can be  addressed  by the bulk utilization of these said materials mainly in the field of civil engineering applications.

As road construction benefited from the stabilization process, a number of guidelines based on soil stabilization have been developed throughout the globe. Most of the guidelines are equipped with comprehensive guide and mechanism in analyzing potential natural soils to be  used  in  the  soil  stabilization  process.  In  view  of  this  an  experimental  program  was undertaken to determine the characteristic variations on mixing the rice husk ash and clay in the   locally   available   soil,   to   attain   significant   gain   in  engineering   and   supporting characteristics for sub-grade construction.

Among such areas in this soil improvement drive which are begging for attention is the development  of  mathematical  models  that  would  encourage  wider  application  of  soil improvement  techniques.  These models are sought to provide  reference and  ratios to the construction industries on the use of available soil and the agricultural paddy waste dust RHA to  the  enhancement  of  engineering  properties  of  soils.  This  was  done  to  facilitate  the application of laboratory results and to serve as a guide in predicting relationship between variables, in addition to reducing the rigors of laboratory work; which enables estimation of results, thereby saving both time and energy.

1.3             STUDY OBJECTIVES

Need for building materials is growing at an alarming rate and in order to meet  the demand for new buildings, new ways and techniques must be evolved. The only binder used in most developing countries is Portland cement. Even though this binder is not a low cost

material, its application is very common in the low income population. Although Portland cement is required for high-strength applications, Portland cement is predominantly used in low-strength applications as foundations, plasters, mortars and soil stabilization. The wrong application of cement in this manner is not only unnecessarily costly but, more important, technically defective. It is estimated that only about 20% of the word-wide use of cement requires the strength of Portland cement.  One of the reason there is a greater demand of Portland cement in developing countries is because it has an “enormously powerful value”. Acceptance of low-cost materials by general population requires the generation of confidence that the materials being proposed can have the same behaviour as Portland cement but at much less cost.

The main objective  of stabilization  is to  improve  the performance  of a material  by increasing its strength, stiffness and durability. The performance should be at least equal to, if not better than that of a good quality natural material.

The objective of this study is to consider upgrading expansive soil as a  construction material  using  rice  husk  ash  (RHA),  which  are  waste  materials.  Thus,  encouraging  the building  of  new  roads in both urban and  rural areas  where  there  is  shortage  of quality construction materials, especially when expansive soils are encountered. Hence, the specific objectives of this study include the following:

i.     Finding  alternative,  economical  and  environmental  friendly  source  of  soil improvement techniques using RHA on the expansive soils in the south – east region.

ii.      Determining  the  required  ratios  of  Soil  to  RHA  required  for  corresponding required soil improved characteristics.

iii.      Formulating models for soil engineering properties such as California  Bearing

Ratio  CBR,  Maximum  Dried  Density MDD  and  Optimum  Moisture  Content

OMC as dependent variables, with the percentage of RHA and local soil as the independent variables.

iv.      Verifying the ability of the models to predict reasonable results; results from the models were compared with results of other tests.

1.4             GENERAL METHODOLOGY

This study is imminent in geotechnical and soil material engineering. The approach to the realization of the objectives would be by reviewing the general properties of lime, rice husk  ash  and  the  anticipated  soil  material.  Earlier  work  done  in  this  regard  would  be thoroughly looked at. This is to enable the study form its basis and relevance.

Response surface methodology techniques would be studied to select the best workable function to suite the non – linear behaviour  of soil. Hence,  the solution of  the expected function would be sought according to mixture design constraint. Scheffe (1958) suggested models for mixture designs. The study would use second degree polynomials of  Scheffe’s and shall be applied according to (Okafor & Oguaghamba, 2009).

The  results  of  the  experiments  and  the  models  shall  be  compared  to  ascertain  the conformity of the models.

1.5              SIGNIFICANCE OF STUDY

This study will hopefully be relevant in the following respects:

i.     The design and construction of pavements and foundations on expansive soils ii.      To the material engineering students and practicing engineers.

iii.      To the governmental agencies that are entrusted with the responsibility of ensuring quality control.

iv.      To civil engineering consultants; contractors and their clients.

It is also perceived that it would generate further research in the following areas:

i.     Modeling of maximum dry density and California Bearing Ratio of soil – RHA

mixture.

ii.      Modeling of optimum moisture content and Atterberg limits of soil – RHA mixture.

1.6            SCOPE OF WORK

This work is limited to soils from Eke – Obinagu borrow pit, Emene, Enugu State; Egbede  Borrow Pit, Aba, Abia State;  and Ugwuaji  Nkanu,  Enugu  State.  Laboratory and experimental study shall be carried out on soil samples from these locations to ascertain their insitu engineering properties. Literatures relating to Rice husk ash potentials and expansive soils  are  understudied  to  ascertain  their  interactive  condition.  The  soil samples  shall  be modified and stabilized using RHA as an additive   and their subsequent properties such as: the  California  Bearing  Ratio,  Maximum  dry  density  and  optimum  moisture  would  be examined through experimental and  laboratory technique. The results of the stabilized soil samples shall be modeled using  the Scheffe’s  second degree polynomials.  The predictive strength of the models shall be tested and verified using other laboratory results.

1.7             LIMITATIONS OF STUDY

The results  could  be affected  by the  inherent  limitations  associated  with  laboratory results. Though, the deviation of the models results from their experimental results will be verified to for marginal, satisfactory and acceptable results.

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