Soil Properties and Foundation

SOIL PROPERTIES AND FOUNDATION

Reasat E Noor (REN)

INTRODUCTION Soil is a mixture of minerals, organic matter, gases,

liquids, and countless organisms that together support life on Earth.

Types: i) Boulders ii) Cobbles iii) Gravel iv) Sand v) Silt vi) Clay

SOIL TYPE

SAND Sand is a naturally occurring granular material

composed of finely divided rock and mineral particles.

Mineral Origin is Quartz (SiO4) Particle size range from 0.075- 2mm.

SILT Silt is granular material of a size between sand and clay,

whose mineral origin is Quartz (SiO4) and Feldspar (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8).

Silt particles range between 0.002-0.0625mm.

CLAY Clay is a fine-grained natural rock or soil material that

combines one or more clay minerals such as Kaolinite(Al2Si2O5(OH)4) with traces of metal oxides and organic matter.

Clay particles are lesser than 0.002 mm

SOIL PROPERTIESProperties Sand Silt Clay

Water holding Capacity

Low Medium to High

High

Drainage Rate

High Low to Medium

Very Low

Cohesion Between Particles

Very Low Medium to High

Very High

Organic material Content

Low Medium to High

Medium to High

Shrink & Swell Potential

Low Medium to High

High

SOIL CLASSIFICATIONUnified Soil Classification System

The first letter tells the general type of soil• G: Gravel• S: Sand• M: Silt• C: Clay• O: Organic Material

The second letter is supplementary description

• W: Well Graded• P: Poorly Graded• M: Silty• C: Clayey• L: Low Plasticity• H: High Plasticity

SOIL CLASSIFICATION

SOIL CLASSIFICATION

SOIL CLASSIFICATIONFor Coarse Grained Soil

• Well Graded

Coefficient of Uniformity Cu>4 for gravels and Cu>6 for sands.

Cz must be between 1-3.

• Poorly Graded

One range of particle is present excessively in soil sample.

• Gap Graded Large and small range of

particles are present excessively.

SOIL CLASSIFICATION

SOIL CLASSIFICATION

Hydrometer Analysis

SOIL CLASSIFICATIONFor Fine Grained Soil

SOIL CLASSIFICATION• The liquid limit (LL) is defined as the moisture content

at which soil begins to behave as a liquid material and begins to flow.

• The plastic limit (PL) is defined as the moisture content at which soil begins to behave as a plastic material.

• The plasticity index (PI) is a measure of the plasticity of a soil.

• PI = LL – PL

Plastic Index Properties0-3 Non-Plastic3-15 Slightly Plastic15-30 Medium Plastic>30 Highly Plastic

SOIL CLASSIFICATIONFor Fine Grained Soil

SOIL CLASSIFICATION

SOIL PHASE DIAGRAM

SOIL PHASE DIAGRAM Moisture ContentThe water content is defined as the ratio of

mass of water to the mass of soils.

%100*MMw

or

%100*

d

w

s

w

MMw

SOIL PHASE DIAGRAM Void Ratio e = Vv/Vs Porosity n = Vv/V Dry Density p = Ws/V Relative density ID = emax- e/emax-emin

The degree of compaction can be measured by relative density of soil.

MOISTURE CONTENT & DRY DENSITY

SOIL PHASE DIAGRAM Specific GravityDefined as the ratio of the weight of a given

volume of soil solids to the weight of an equal volume of distilled water

w

s

w

s

WWG

Soil Type Specific

GravityGravel 2.65-

2.68Sand 2.65-

2.68Silty Sand 2.66-

2.70Silts 2.66-

2.70Inorganic Clays 2.7-2.80

INTERNAL ANGLE OF FRICTION

Angle of internal friction (friction angle) A measure of the ability of a unit of rock or soil to withstand a shear stress.

It is the angle (φ), measured between the normal force (N) and resultant force (R), that is attained when failure just occurs in response to a shearing stress (S).



Soil Type USCS

Min (Degree)

Max (Degree)

Well Graded Gravel GW 33 40Poorly Graded Gravel

GP 32 44

Well Graded Compacted Sand

SW 30 38

Inorganic Silt (Low Plasticity)

ML 27 41

Inorganic Silt (High Plasticity)

MH 23 33

Inorganic Clay (Low Plasticity)

CL 27 35

Inorganic Clay (High Plasticity)

CH 17 31

INTERNAL ANGLE OF FRICTION Active earth Pressure

Coefficient Ka =1-sin ø/1+sin ø Passive Earth

Pressure Coefficient Kp = 1+sinø/1-sinøLateral Earth

PressurePa= ½*y*Ka*h2

STANDARD PENETRATION TEST The number of blows

required for drill rod to penetrate 300mm below the ground surface.

First 150 mm penetration is neglected.

Most commonly used in-situ test.

Specially for cohesion less soils, which cannot be easily sampled

STANDARD PENETRATION TESTUsefulness Relative density of

cohesion less soils. Angle of shearing

resistance of cohesion less soils.

Unconfined compressive strength of cohesive soils

STANDARD PENETRATION TESTN Value Correction From Terzaghi and Peck recommended the following

correction for dilatancy.

Where NR is recorded value, for NR=15, Nc=NR

From Peck, Hansen and Thornburn correction for overburden pressure

Where is overburden pressure>24 KN/m2

SPT VALUES & SOIL PARAMETERSN values Condition Relative

DensityAngle of Internal Friction

Unit Weight (pcf)

0-4 Very Loose 0-0.2 25°-30° 70-1004-10 Loose 0.2-0.4 30°-35° 90-11510-30 Medium 0.4-0.6 35°-40° 110-13030-50 Dense 0.6-0.85 40°-45° 110-140Over 50 Very Dense 1 45° 130-150

SPT VALUES & SOIL PARAMETERS


N values Condition

Unconfined Compressive Strength (Tsf.)

0-2 Very Soft 0-0.252-4 Soft 0.25-0.504-8 Medium

Stiff0.50-1.00

8-15 Stiff 1.00-2.0015-30 Very Stiff 2.00-4.00Over 30 Hard Over 4.00


BEARING CAPACITY For Strip Footing qult= CNc+yDNq+0.5yBNy (Tsm) For Square Footing qult= 1.3CNc+yDNq+0.4yBNy (Tsm) For Circular Footingqult= 1.3CNc+yDNq+0.4yBNy (Tsm) For Raft Foundationqult= 5.7C(1+0.3B/L) (TSM)

BEARING CAPACITY Pile CapacityPpu =Ap (40N) Lb/B < Ap (400N) Where,N = Statistical average of the SPT N55 in a zone of about 8B above to 3B below the pile pointB =Width or Diameter of pile point in meterLb = Pile Penetration DepthAp= Base are of pile Ppu = Ultimate Pile Tip Capacity

FOUNDATION DESIGN CRITERIA

Two main characteristics The foundation has to be safe against overall shear failure

in the soil The foundation cannot undergo excessive settlementLocation and Depth of Foundation Depth should be at least 50 cm below natural ground level Must be placed below the zone of volume change. In fine sands and silts, foundation must be placed below the

zone in which trouble may caused by frost Depth of foundation in river should be below the scour level.

FOUNDATION TYPES

• Shallow Foundation (Df/B<1)

• Deep Foundation (Df/B>4)

FOUNDATION TYPES

Shallow Foundation (D/B<1)Distributes the loads on the ground laterally. Types1.Strip Footing/Continuous Footing2.Spread or Isolated Footing3.Combined Footing4.Strap or Cantilever Footing5.Mat or Raft Foundation

FOUNDATION TYPES Strip Footing/Continuous Footing1.Provided for load bearing wall2.Provided for closely spaced column.

FOUNDATION TYPES Spread or Isolated Footing1.Provided for supporting an individual column.

FOUNDATION TYPES Combined Footing1.Supports two closely spaced column.2.Even distribution of loads where column is

loaded eccentrically .

FOUNDATION TYPES Strap or Cantilever Footing1.Economical than combined footing.2.The distance between the columns is large.3.Used where allowable soil pressure is high.4.The strap is designed as a rigid beam.

FOUNDATION TYPES Mat or Raft Foundation1.Required where allowable soil pressure is low.2.Reduce differential settlement.3.Distance of columns is low.

TYPES OF FAILURE OF SOIL

TYPES OF FAILURE OF SOIL General Shear Failure1. Brittle type stress-strain behavior. (like dense sand)2. Sudden failure in soil takes place and failure surface

of soil extends to the ground surface.3. Ultimate load can be easily located.4. A bulging of ground surface adjacent to foundation

can be easily located

TYPES OF FAILURE OF SOIL Local Shear Failure1. Observed in case of sand and clayey soil of medium

compaction.2. Significant compression of the soil just beneath the

foundation3. Ultimate load can not be easily located.4. Slight bulging of ground surface adjacent to foundation

can be located.

TYPES OF FAILURE OF SOIL Punching Shear Failure1. Observed in fairly loose soil.2. Poorly defined shear plane.3. Soil zones beyond the loaded area is little

affected.4. Beyond the ultimate load failure, Load-

settlement graph is linear.5. The failure surface in soil will not extend to

the ground surface.

TYPES OF FAILURE OF SOIL

FOUNDATION TYPES Deep Foundation (D/B>4)Distributes the load vertically. Types1.Buoyancy rafts (hollow box foundations)2.Caissons3.Cylinders4.Shaft foundations5.Pile foundations

FOUNDATION TYPES Pile foundationsUsual length -10-20mUsual load- 30-300 ton Types1. End Bearing Pile2. Friction Pile3. Combination of

Friction and End Bearing Pile

FOUNDATION TYPES End Bearing PileUltimate bearing capacity of the

pile depends entirely on the underlying material

Friction Bearing Pile In cohesion less soils, such as

sands of medium to low density, friction piles are often used to increase the density and thus the shear strength.

Combination of Friction and End Bearing Pile

FOUNDATION FOR DIFFERENT TYPES OF SOIL Sand 1.Footings : Easy to Construct and Economical2.Retaining Structures: Must be used because

sand cannot support themselves.3.Deep Foundation (Piles) : Uses Friction

Resistance but low in bearing capacity.

FOUNDATION FOR DIFFERENT TYPES OF SOIL (CONT.) Sand Problems1.Bearing capacity may cause problem but in most

cases it is sufficient2.Excessive settlement in wet and loose deposits.3.Overburden Pressure is usually low.

FOUNDATION FOR DIFFERENT TYPES OF SOIL(CONT.) Silt & Clay1.Footings : Economic but may have problems

with bearing capacity in saturated clays.2.Retaining Structures: Clays are self

supportive up to a certain height. Must be used if it exceeds the critical height.

3.Deep Foundation (Piles) : If bearing capacity is low piles may driven to rock

FOUNDATION FOR DIFFERENT TYPES OF SOIL(CONT.) Silt and Clay Problems1.Low bearing capacity.2.Generally low shear strength when wet.3.High consolidation in soft clays.4.Swelling is possible.5.Over-consolidated soil may have cracks and

fissures.

FOUNDATION FOR DIFFERENT TYPES OF SOIL(CONT.)

Soil Problems Solution

SandSettlement

1. Loose sand must be compacted.

2. Lowering water table may result in sand densification

Bearing Capacity 1. Compaction increases cohesion and friction2. Using Pile foundation

FOUNDATION FOR DIFFERENT TYPES OF SOIL(CONT.)

Soil Problems Solution

Silt and Clay

Consolidation 1. Lowering water table.

2. Pre-Loading3. Drive pile to rock

Bearing Capacity

1. Compaction2. Use of Deep foundation

Expansion or Swelling

1. Treat or stabilize soil2. Maintain Constant water table.3. Alter soil nature4. Include Swell pressure in design

THANK YOU ALL!

Engineering

Soil Properties and Foundation