EEG 312 Course Contents Engineering Soils

Week no

Description

1

Introduction

1. Review: physical and mechanical properties of soils. Physical properties:

i. Color ii. Density

iii. Porosity iv. Void ratio v. LL, PL, and moisture content,

vi. Grain size. vii. Permeability.

All these properties will lead to the classification of soils. Phase diagram may be used to illustrate the physical relationships. Mechanical properties: Any property that is obtained by use of force is a mechanical property.

• Compressive strength • Modulus of elasticity • Poisson’ ratio • Compression and recompression (swelling) indexes • Angle of internal friction, and • Cohesion or shear strength of soil.

The most important soil parameters are cohesion and shear angle.

2

3

Angle of internal friction: General discussions on ;

• High angle of friction indicates dense conditions in granular soils.

• Angle of internal friction may be used in the estimation of the modulus of elasticity of granular soils.

• Angle of friction may be used to find the bearing capacity factors when using Terzaghi’s equations.

Cohesion: General discussions, on based on the knowledge of cohesion

we could obtain the following;

• Unconfined compressive strength. • Compression and recompression indexes using

consolidation tests. • O.C.R. or history of the clay.

How to obtain the internal angle of friction (φ) and the cohesion of soils( c ):

Tests to obtain the internal angle of friction and cohesion of soils.

Soil type Laboratory tests Field tests φ

1. Direct shear box 2. Triaxial compression test

1. Standard Penetration test. 2. Mackintosh probe. 3. Pressure meter. 4.”Cone Penetration test”

C

1. Triaxial compression test.

2. Unconfined compression test.

1. Vane shear test 2. Cone penetration

test.

** Discussion of the above and explaining that the laboratory tests should be taught in EEG 312, while the field tests will be taught in the next semester (EEG 441).

4

Definitions and discussion related to:

• Standard Penetration Test (SPT) • Cone Penetration Test (CPT).

Standard Penetration Test N-values in Granular soils SPT- N Relative conditions Relative density Dr% 0 to 4 4 to 10 10 to 30 30 to 50 > 50

Very loose

Loose Medium Dense

Very dense

0 to 15

15 to 35 35 to 65 65 to 85 85 to 100

Standard Penetration Test N-values in Cohesive soils SPT- N Relative conditions

of soils. Approximate compressive strength (cohesion) in KN/m2

2 to 4 4 to 8 8 to 15 15 to 30 > 30

Soft

Medium Stiff

Very stiff Hard

12 to 24 24 to 48 48 to 96 96 to 190

>190

5

I Foundation Foundation types depend on soil types and type of structure.

A) Shallow foundation: • Footings and • Raft foundation.

• Raft found to bridge between soils of different characteristics to

minimize differential settlements.

B) Deep foundation: Df >> 5 B D= depth B= width • Piles are:

* Friction * End bearing, and * Composite piles.

• Piers and caissons.

Any type of foundation must satisfy two very important conditions: 1. To be safe against shear failure, and 2. To be safe against settlement whether general or differential.

C Ultimate shear strength of soils;

Definition: Shear strength is the “maximum applied load at which soils to fail in shear”. Sometimes it is known as the ultimate bearing capacity qult.

How to find the maximum or ultimate shear strength or the ultimate bearing capacity of soils?

1. Bearing capacity equations based on penetration tests:

a) Standard Penetration Test (SPT).

Terzaghi’ equations: qult = C Nc + γ Df Nγ + 0.5 γ B Nq Nc , Nγ and Nq are bearing capacity factors obtained from the following

charts q designe = q allowable

q all = qult / (F.S) , factor of safety ranges from 1 to 5.

Soil types fall in three categories:

• Cohesive soils and then the qult will be in the following form. qult = C Nc , and Nc = 5.7

• Granular soils (noncohesive) and then the qult will be in the following form.

qult = γ Df Nγ + 0.5 γ B Nq

• Mixed soils and then the qult will be in the following form. qult = C Nc + γ Df Nγ + 0.5 γ B Nq How to obtain the bearing capacity factors?

The above chart is used for cohesive and mixed soils (C + φ).

This chart is used for granular soils (Non cohesive soils) only.

*Typical exercise: Find the allowable bearing capacity of non-cohesive soil having SPT-N value 15 and density of 17 KN/m3.

b) Cone Penetration Test (CPT).

1. soft clays qult = 10 qc Nc qc (KN/m2)

2. Normally consolidated clays qult = 100/15 qc Nc

3. Over consolidated clays qult = 100/22 qc Nc

To be on the safe side the Nc may be disregarded and the above equations in 1,

2, and 3 will be for the allowable bearing capacity for clays if factor of safety is taken as follows. (F.S = 5) A reasonable equation qult = 3 qc KN/m2 for most clays. It should be noted that the (CPT) equations are not in the final forms and should be put for more discussion. 2. Plate bearing Test.

Ultimate bearing capacity may be obtained from the plate bearing test as

shown in the following three Figures;

Plate bearing test and determination of ultimate bearing capacity of soils.

3. Building codes: These codes are built based on codes of practice. Most countries have charts and tables illustrating the allowable bearing capacity of the ground for important cities. As an example, the people in Al-Safa district in Jeddah know that a well built square (1m by 1m) footing will be more than enough for 4 floors on soils in Al-Safa.

D) Settlement:

Non-cohesive: These soils will have quick (immediate) settlement because of the high permeability and hence the easy escape of water under loads.

Immediate settlement (elastic settlement, Se) in non-cohesive (granular soils) may be obtained based on (the Standard Penetration Test N- values) as follows: Se = 8 Δq /N for B < 4 ft …… Meyerhof (1965). B = width of footing Se = 12 [Δq /N] {B/(B+1)} for B> 4 ft Se = Uo U1 (qB/E) ….. see Janbu et al (1956) charts below.

Cohesive soils: The following items have to be known in order to estimate the settlement.

• It is important to know the load increase (Δσ) below the foundation. • Whether the foundation is stiff or flexible foundation type, and • Type and history of soils (Clays).

It is better to estimate the increase of stress (Δσ) below foundation as if the foundation is flexible. qapplied = unit load imposed by each column. Δσ = Ic * qapplied (a) For a circular flexible foundation use the following table.

Loading on soils will be different for different foundation whether the foundation is rigid or flexible.

Rigid foundations: such as concrete footing. ; Flexible foundation such as steel structures such as steel tanks.

However, the above tables may be good enough for most foundations.

Therefore, the settlement may be calculated form the following equation: S = {Ho/(1+eo )} Cc Log [σvf - σvo ]

S = settlements (m), Ho = thickness of the clay layer (m), eo = natural void ratio, Cc = compression index, σvf = effective vertical overburden stress (σvo) + Δσ Δσ = increase of stress due structure at middle of the layer (KN/m2). σvo = soil effective vertical overburden before building stress (KN/m2). Therefore, the total settlement (ST ) may be calculated as follows: ST = Se + Sp + Sc Se = immediate settlement (elastic settlement) in granular soils. Sp = settlement due to primary consolidation, where water is expelled from

the clay but not from the clay structure as in saturated inorganic silts and clays. Sc = settlement due to secondary consolidation, where the water is expelled from the clay structure as in highly organic silts, clays, and peat.

Available references: Foundation analysis and design by Joseph E. Bowles (1977). McGraw Hill, NY. P, *** Foundation Engineering by Ralph Peck, Walter Hansen, and Thomas H. Thornburn (1974). John Wiley & sons, NY. P *** McCarthy, David. (1988). Essential Soil Mechanics and Foundation 3rd ed. Prentice Hall. 632 p. Foundation Design and Construction. 5th ed. Longman Scientific and Technical. 846 p.

II Lateral Earth Pressures and retaining walls

Bridges and Jeddah sea port are examples of retaining structures that stand against lateral earth pressures. Three types of lateral earth pressures will be discussed below. 2.1 Earth pressure at rest. This pressure is equal to the vertical soil pressure multiplied by the coefficient of lateral earth pressure at rest (Ko). Vertical pressure being the density of soil (γ) in KN/m3 multiplied by the soil column H (m).

Ko = ν / (1- ν) Ko = 1 – sin Ø in case of coarse grained soils where ν = the Poisson’ ratio Soil type Ko Loose granular 0.5 to 0.6 Dense granular 0.3 to 0.5 Soft clay (undrained) 0.9 to 1.1 Hard clay (drained) 0.8 to 0.9 σv = γ H he = Ko σv σh = Ko γ H Therefore horizontal pressure will increase with increasing depth. The total lateral earth pressure (Po) on the wall will be the area of the triangle (H Ko γ H) divided by two. Po = 0.5 Ko H2

γ The total pressure will be acting against the wall at depth of H/3 of the

wall. If soil is saturated or submerged the total earth pressure at rest will be:

Po = 0.5 Ko H2 γsub and γsub is the submerged density of the soil.