CVE 308 SOIL MECHANICS ENGR S.O ODUNFA DEPT. OF CIVIL ENGINEERING UNIVERSITY OF AGRICULTURE, ABEOKUTA

INTRODUCTION HISTORY OF SOIL MECHANICS Soil mechanics is the science which studies the physical, chemical and mechanical properties of soils to solve problems related civil engineering. Prof. Terzaghi was the founder of soil mechanics To assure the integrity of structures under impose loads is one of the reason for its establishment.

Importance and Applications Deals with problems related to soils Highway embankments and cuts Flow of water Foundation – Shear failure Compressibility of soils.etc.

Soil: Engineering Materials Soil as constructional materials Soil as foundational materials

Origin of Soil Soil is the disintegration of rocks through weathering Weathering: Biological weathering Chemical weathering Mechanical weathering Transportation and Deposition of materials. Soil composition and phase relationship

Physical and Chemical Nature of Soil Both physical and chemical characteristics of soil are the reflection of the parential materials constituents where the soil is formed.

Engineering Properties of Soil Physical properties e.g Shapes, density, etc Mechanical properties e.g Strength, Deformation, etc Mechanical properties are direct functions of its physical properties. Soil as multiphase and particulate.

Soil Structures Soil structure is geometric arrangement of the soil particles and the interparticle forces among them. Geometric arrangement Interparticle forces

Types of Soil Structures Primary soil structure Secondary soil structure

Major group of soils are Cohesionless soil e.g sand, mainly exhibit Single-grained structure and Honey-combed structure Cohesive soil e.g clay soil, exhibit Flocculated and Dispersed structures

Seepage, Capillary flow and Permeability Seepage is the ease at which water flows through the soil and this is possible due to the presence of voids within the soil particles. Permeability is the property that allows the flow of water/fluid through the soil. The flow of water through the soils is assumed to follow Darcy’s law: Q/t = KAH/l

Where Q = Quantity of water flowing t = time for quantity Q to flow; K = coefficient of permeability for the soil A = area of cross section through which the water flows; H = hydraulic head across soil l = length of flow path through soil.

Seepage through the soil Seepage can be effectively study by the use of flow nets Flow net is a pictorial representation of the path of flow through a soil. Flow net comprises of flow lines and equipotential lines Construction of flow net

Control of Seepage Through embankment:- Rock Toe/filter and Horizontal blanket methods Through foundation:- Impervious cut offs and Chimmy drain methods

Uses of Flow Nets Seepage discharge Hydraulic pressure Seepage pressure and Exit gradient

Determination of the coefficient of Permeability Coarse grained soils- Constant – head permeameter method Fine –grained soils – Variable head permeameter method

Compaction of Soils Compaction is the method of densifying soil mass through mechanical means. Moisture content and Dry density relationship

Objectives of Compacting the soils compaction To reduce the void ratio and thus the permeability of the soil To increase the shear strength and therefore the bearing capacity of the soil To make the soil less susceptible to subsequent volume changes and therefore the tendency to settlement under load or under the influence of vibration.

Methods of compaction Standard Proctor compaction Test Modified AASHTO compaction Test

Consolidation and settlement Consolidation is the removal of water from soil mass to enable the soil particles become wedged closer together which results to change in volume due to reduction in void ratio of the soil mass. Settlement is the vertical downward displacement brought about by such a volume change. It can takes years for the complition of the process.

Types of Consolidation Normal consolidation – Takes place under normal condition Over consolidation –When the overburden materials which caused the soil mass to be consolidated is removed due to erosion and in these circumstances the soil may have been subjected to a pressure greatly in excess of its present overburden.

Determination of settlement in terms of Compression index Determine coefficient of volum e of compressibility, Mv first Mv = av/1=eo av = coeficient of compressibility eo = original void ratio of the soil mass av = ∆e/∆p ∆ e = Reduction in the void ratio after full consolidation has taken place due to load ∆p ∆p = Additional stress V = v v +v s

Vs = v/1+ eo ∆v = ∆H.A/HO.A ∆e/1+ eo = ∆H/HO = ∆n = MV∆p ∆H = MV∆Pho ∆H = [cc log(PO + ∆P)/PO HO = original height ∆H = change in height A = cross sectional area ∆n change in porosity Cc = 0.009(LL – 10%)

Typical e – p curves Sands Clays

Stress Distribution in soil mass To determine stresses, Boussinesqs formula δz = 3P/2πZ²[1/(1+{r/z}²) 5/2 Δz vertical normal compressive stress P = vertical concentrated load Z vertical distance below P r = horizontal distance from P

Shear Strength Shear strength of most soils is made up of a combination of Friction and Cohesion Soil mass fails by shear and occurs along a definite plane

Laboratory Determination of shear strength Methods are Unconfined compression test Triaxial Direct shear test

Field Tests Methods Vane shear test Standard Penetration test Dutch Cone Penetration test

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