CVE 308 SOIL MECHANICS

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

2. 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.

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

4. Soil: Engineering Materials • Soil as constructional materials • Soil as foundational materials

5. 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

6. 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.

7. 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.

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

9. Types of Soil Structures • Primary soil structure • Secondary soil structure

10. 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

11. 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

12. 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.

13. 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

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

15. Uses of Flow Nets • Seepage discharge • Hydraulic pressure • Seepage pressure and • Exit gradient

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

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

18. 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.

19. Methods of compaction • Standard Proctor compaction Test • Modified AASHTO compaction Test

20. 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.

21. 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.

22. Determination of settlement in terms of Compression index • Determine coefficient of volume 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 = vv+vs

23. 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%)

24. Typical e – p curves • Sands • Clays

25. 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

26. 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

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

28. Field Tests Methods • Vane shear test • Standard Penetration test • Dutch Cone Penetration test