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ABSTRACT

This project addresses the effects of increase in fines contents on the geotechnical properties of sandy soils and the roles the fines play in slope instability in Nsukka,  Enugu  state in Nigeria. Field investigation revealed that the major landslides and gully erosion sites in south eastern Nigeria are localized within areas where these sandy soils are present. Geotechnical investigation of these sandy soils was carried out to assess their importance in the numerous geohazards in the area. Particle size analysis show that the soil is uniform-medium grained sandy soil; with large amount of fines matrix .The  value of the sandy soil’s coefficient of uniformity (Cu) is 6.The sandy soil sample collected was sieved and the fine and coarse grain matrix separated. Particle size  analysis,  compaction test and shear tests was conducted on

samples with mix ratios of 10%, 20%, 30% fines content. The coefficient of permeability (K)

for the mixed soil sample was calculated using the Allen Hazen’s empirical method. It was found that as the fines content increased from 8.85% fines content to 30% fines content the values of K decreased from 4.26 X 10-4-5.0 X 10-6 (cm/s). The reduction in permeability values shows that the soils drainage properties reduced with increase in fines content of the soil. The compaction test result shows that, as the fines content increased from 8.85%-30%,

the maximum dry density decreased from 1.84Kg/m3  to 1.494Kg/m3. However the optimum

moisture content increased from 10% to 18%.The soils were tested under a 300 KN/m2     total normal stress in unconsolidated undrained (UU) Triaxial test and 517 KN/m2 normal stress in direct shear testing conditions. It was found that; the internal friction angle  decreased with increase  in  normal  stress  and  fines  content  while  the  value  of  cohesion  increased  with increase in fines content and normal stress under triaxial and direct shear testing conditions. From the results of the study it was concluded that the susceptibility of sandy soils within the region to landslides is enhanced by the large amount of fine grain matrix.

CHAPTER ONE

1.0 INTRODUCTION

The development of soil related problems such as gully erosion, and other mass movement processes has caused extensive damage to the environment and has driven many people away from their homes and farmlands in Nsukka town. Earlier studies have attributed the genesis and growth of these gullies to the influence of human activities on geomorphologic processes.   Qualitative and semi-quantitative methods have been employed to produce suggestions for solving the problem. However, investigations carried out by workers on these soils have shown that the primary causes of the gully erosion lie in the hydrogeological and geotechnical properties of the soils in the area.

Most natural soils are composite soils i.e. a mixture of different grain sizes. The range of particle sizes in soils is tremendous and it can range from boulders or

cobbles of several centimetres in diameter down to ultrafine-grained colloidal

materials.   Knowledge of the distribution of these grain sizes in a soil gives useful information about the engineering behavior of such soil.

Sandy soil is the most common soil type in Nsukka town (study area). It is one of the most common soil types on the planet, it can be found in some capacity in every country, geographical region and continent around the globe.  The Sandy soils in Nsukka are soils with a composite matrix of coarse and fine grain

particles. The presence of fines in sandy soil is recognized as a problem in geotechnical engineering. It is believed that fines in a soil increases soil compressibility and tendency to creep.

Geotechnical studies on sandy soils have shown that the soil texture, especially of  coarse-grained  sandy  soils,  has some  relationship  with  their  engineering

behavior.

Important  information  soil  engineers  need,  for  example  for  the  design  and

construction  of  an  engineering  structure  can  be  gotten  from  a  detailed

geotechnical study of the area; it’s from the result of such study that some

design parameters e.g. shear strength of the soil can be derived. Site related

problems  such  as  gully  erosion,  landslides,   and   collapse   or  failure  of

engineering structures can then also be predicted.

Hudec et al (2006) while describing the result of an investigation of the gully erosion prone sandy soils in southeastern Nigeria; stated that gully processes are localized in the fine- to medium-grained Coastal Plain Sands (Pliocene-Recent) and Nanka Sands (Eocene) and the medium- coarse-grained Nsukka Sandstone and  Ajali  Sandstone  (Cretaceous).  The  most  affected  deposits  are unconsolidated to poorly consolidated, cleaner, more porous and weakly cemented sands of the Nsukka formation. He also stated that the textural properties of these deposits to a large extent determine their responses to the erosive action of surface runoff because gradation analysis of the soil shows that

they are all strikingly uniform in size and that the gully dimensions (length,

depth and breadth) changes as the proportion of sand particle sizes became smaller than 1 mm.

Bopp  and  Lade (2005) stated that the mechanical behavior of a sandy soil changes depending on its grain size distribution e.g. Due to particle crushing during loading, unloading and reloading during tectonic activities, the soil could behave as either cohesive or cohesionless because of changes in the coarse/fine grains content ratio.

The   geotechnical   properties   of   sandy   soils   has   been   investigated   and documented but there is still limited understanding, as to how an increase in the smaller fines particles affect the overall geotechnical behaviour of sandy soils. Given that during mass movement processes fines content of the soils can be increased, it is important to properly investigate and understand how increase in the  fines content of  the  sandy  soil  influences  the  mechanical/  geotechnical behaviour of the sandy soil. In order to mitigate these gullies there is increased need for soil investigations to obtain information and adequate understanding of the geotechnical properties of the Nsukka Formation sandy soil deposit. The results of such investigation or study will be relevant and up to date when there is constant advancement in the experimental research concepts.

1.1 BACKGROUND OF THE PRESENT STUDY

Sandy soils in Nsukka town, Enugu state are often susceptible to various mass movement processes such as gully erosion. Field studies show that engineering construction   works   such   as   erosion   control   and   drainage   structures, embankments, and roads e.t.c done on these soils fail or collapse within a short

period of time. Various studies and investigations have shown that there are

many gully erosion sites within Nsukka town. The major ones are; the Obollo-

Eke, Obollo-Etiti gully(plate 1), and the Onuiyi gully site (plate 2). Though

some of the gullies were studied and found to be induced by rainfall and human

activity; results of these of investigations have shown that the soils on which the gully erosion occurred have a uniform grain size, and they are medium to fine grained sandy soils.

Chang N (1982) stated that textural properties play an important role in the mechanical response of soils, especially when the soils are subjected to loading. Yeh S and Kaufman L (1984) studied the liquefaction potential of clean and silty sands. They suggested that by increasing fines content of the soils the coefficient of secondary consolidation (creep) increase, while the friction angle and permeability decrease. Thevanayagam et al (1997) studied the effect of fines content and confining stress on undrained shear strength of sandy soils. The experimental data they got   indicated that the shear strength increased initially as the fines content was increased after which the shear strength value

decreased gradually with increase in fines content and confining pressure.  They noted that for silty sands the range of fines content at which the shear strength value gradually reduced is between 20-30% and they called this the threshold fines content or Transition fines content (FCt).

Chien and Oh (2002) used the resonant column tests to study the influence of fines content and initial shear stress on dynamic properties of reclaimed soil. The experimental results showed that the maximum shear modulus decreases as the fines content increases. Hudec et al. (1998) while describing the results of geotechnical studies carried out on the sandy soils in southeastern Nigeria noted that the textural properties of these soil deposits to a large extent determine their mechanical response to the erosive action of surface runoff and that gully processes are  localized  in  the  fine  to  medium  grained  Coastal  Plain  Sands (Pliocene to Recent) and Nanka Sands (Eocene) and the medium to coarse- grained Nsukka Sandstone and Ajali Sandstone (Cretaceous) of the Anambra- Imo basin region.

Plate 1: picture of gully erosion at Onuiyi haven Nsukka L.G.A

Plate 2: Picture of a section of the Obollo Eke gully site.

1.2LITERATURE REVIEW

Georgiannou et al 1990; Fear and Mc Roberts, 1995; Yamamuro and Covert,

2001, Xenaki and Athanasopoulos, 2003 investigated the effects of fines content on the various geotechnical properties of sandy soils. Georgiannou et al (1990) studied the effects of fines content on the liquefaction potential of clayey sands and they found that up to a fraction of 20% fines content does not significantly reduce the angle of shearing resistance of the coarse grain component of the soil. Georgiannou et al (1990) concluded that until a threshold fines content called the transition fines content (FCt.) is reached the soils geotechnical properties is controlled by the  coarse grain  matrix.  Covert  and  Yamamuro, (2001) investigated the effect of fines content on shear strength and dynamic settlement  of  reclaimed  soils.  They  concluded  that  the  shear  strength  of reclaimed soil decreased as the fines content increased under constant relative density.

Terzaghi, 1925; Gilboy, 1928; McCarthy and Leonard, 1963; Studied the effects of particle size distribution on the settlement behaviour of clayey sands. They concluded based on the results of their investigation that the particle size distribution is an important geotechnical parameter to consider in foundation design and construction because the shear strength of the soil is dependent on the internal friction between the grains of the soil. Gilboy (1928) studied the influence of fines content on the compressibility of sandy soils, and found that

an increase in fines content resulted in an increase in the void ratio of the uncompressed material as well as an increase in compressibility.

Olson and Mesri (1970) studied the influence of increase in fines content on mechanical properties of reconstituted clayey sand. They concluded   that the

mechanical properties of the soil   was controlled by the coarse matrix until a

certain threshold fines content (FCt.) was reached, after which the soil΄s mechanical property is significantly controlled by the fines content. Kenny, (1977) postulated that residual strength of soils with a mixed composition of clay and sands depends on the relative volumes of the clay mineral matrix (e.g. kaolinite, illite, montmorillonite e.t.c) and massive minerals like quartz, feldspar calcite.). He also stated that when the volume of the massive grains exceeds

50% of the total volume of the mixtures the residual strength of the soil is equal to that of the massive minerals.

Research  works  have  been  carried  out  by  Ofomata  (1965),  Floyd  (1965), Nwajide &Hoque (1979), Egboka & Okpoko (1985), Okagbue & Uma (1987), Onuoha & Uma (I988) and Okagbue &Ezechi (1988) on the sandy soils in southeastern Nigeria. Okagbue & Uma (1987) while investigating the performance of gully erosion control structures constructed on the sandy soils and found that these soils have high fines content. They stated that all previous workers on the soils seem to agree that soil/gully erosion is more severe in areas

of rugged terrain underlain by friable sandy soils with high fines content and unconsolidated sandy bedrock.

Naeini and Baziar (2004) studied the effect of fines content on steady-state strength of sand by a series of undrained monotonic and dynamic triaxial compression tests. They found that, as the silt content increased up to 35%, the steady- state line moved downward and the shear strength decreased. Although much work  has been  done on  soils in Nsukka,  Enugu state,  the effects of increase in fines content on the geotechnical property of the sandy soil deposit in the study area has not been investigated.

This study is thus carried out to give adequate information and knowledge of the effects increase in fines content will have on the sandy soil’s geotechnical property. This knowledge may be useful in controlling gully erosion and landslides in the area.

1.3. OBJECTIVE OF STUDY

The purpose of this work is to

      Investigate   the   effects   of   increase   in   fines   content   on   the geotechnical  properties  of  sandy  soil  taken  from  a  sandy  soil deposit in Nsukka, Enugu state.

      Study     the     influence     of     the     fines     content     on     the mechanical/geotechnical behaviour of the sandy soil.

      Determine   the   threshold   fines   content   (Fct)   at   which   the geotechnical properties of the soil are controlled by the fine grain matrix of the soil.

1.4. LOCATION OF STUDY AREA

Nsukka is located in Nsukka   Local Government Area and lies between longitudes 7°013’E and 7°035’30”W and latitudes 6°43’30”N and 7°003’50”S  of Enugu State, Southeastern Nigeria as shown in Fig.1.

It covers an area of about 480 km2 and is located within the scarp slope of the

extension of the 250km long north – south trending Arochukwu-Ohafia-Awgu-

Udi-Nsukka cuesta (Fig.2). Locally the cuesta consists of several ridges and valleys, with the underlying geologic formations exposed along the scarp-face and eventually flattening eastwards into the Cross River Plain. The study area has two major relief types. The high relief zone with undulating residual hills and the lowland or valley areas .These residual hills are the remnants of the Nsukka formation which constitute the surface layers, these layers are highly weathered and eroded and overly the Ajali Sandstone. The hills are flanked by the rolling lowlands of Oji River, Adada and Anambra Basins to the west, and the Ebonyi (Abonyi) River Basin to the east. These areas serve as the collecting centre of the run offs during the rainy season.   The valleys and the lowland areas lie  within the humid  tropical rainforest belt of Nigeria and evidences savannah-type vegetation.

Figure.1 :  Map of Nigeria Showing Study Area

21

Figure.2: Block diagram showing the Awgu-Nsukka cuesta (Modified from Okogbue et al. (1990).


This material content is developed to serve as a GUIDE for students to conduct academic research



THE INFLUENCE OF INCREASE IN FINES CONTENT ON THE ENGINEERING PROPERTIES A SANDY SOIL DEPOSIT IN NSUKKA ENUGU STATE NIGERIA.

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