Geotechnical Engineering Geotechnical Engineering deals with the

application of Civil Engineering Technology to some aspects of earth

Geotechnical Commision of Swedish State Railways (1914-1922) was the first to use the word Geotechnical in the sense that we know it today: the combination of Civil Engineering technology and Geology

Geotechnical Engineering deals with; Design of Foundation Stability of Slopes and Cuts Design of Earth Structures Design of Roads and Airfield

Soil Descriptions Soil is an unconsolidated agglomerate of

minerals with or without organic matter found at or near the surface of the earth crust, with which or upon which civil engineers build their structures

Soil Formation Mechanical Weathering (Physical

Weathering Chemical Weathering Oxidation Carbonation Hydration Desilication Leaching

WeatheringPhysical processes of weathering

Unloading○ e.g. uplift, erosion, or change in fluid pressure.

Thermal expansion and contractionAlternate wetting and dryingCrystal growth, including frost actionOrganic activity

○e.g. the growth of plant roots.

Chemical Process of weatheringHydrolysis

○ is the reaction with water○will not continue in the static water.○involves solubility of silica and alumina

Chelation○Involves the complexing and removal of metal ions .

Cation exchange ○ is important to the formation of clay minerals

Oxidation and reduction.Carbonation

○is the combination of carbonate ions such as the reaction with CO2

Factors affect weatheringMany factors can affect the weathering process such as climate, topography, features of parent rocks, biological reactions, and others.

Climate determines the amount of water and the temperature.

Transportation of Weathering Products

Residual soils- to remain at the original place

Engineering properties of residual soils are different with those of transported soils

The knowledge of "classical" geotechnical engineering is mostly based on behavior of transported soils. The understanding of residual soils is insufficient in general.

Transported soils-

to be moved and deposited to other places.

The particle sizes of transported soils are selected by the transportation agents such as streams, wind, etc.

The transported soils can be categorize based on the mode of transportation and deposition (six types).

Transported Soils (Cont.)(1) Glacial soils: formed by transportation and deposition

of glaciers.(2) Alluvial soils: transported by running water and

deposited along streams.(3) Lacustrine soils: formed by deposition in quiet lakes

(e.g. soils in Taipei basin).(4) Marine soils: formed by deposition in the seas (Hong

Kong).(5) Aeolian soils: transported and deposited by the wind

(e.g. soils in the loess plateau, China).(6) Colluvial soils: formed by movement of soil from its

original place by gravity, such as during landslide

Uses of Soil As a supporting material to bear the loads

of structures resting on earth As a raw constructional material for

construction of earth structures (Dams, levees, roads)

As a processed material (Burnt bricks, concrete mix etc.)

In Pottery (Kaolinite) Kaolinite is also used in paper paint and

pharmaceuticals Bentonite is used in drilling

Principle Soil Types Loam Silt Mud Caliche Marl Hardpan Peat Clays Drift Shale Black Cotton Soil

Three Volumetric Ratios (1) Void ratio e (given in decimal, 0.65)

(2) Porosity n (given in percent 100%, 65%)

(3) Degree of Saturation S (given in percent 100%, 65%)

)V(solidsofVolume

)V(voidsofVolumee

s

v

)V(samplesoilofvolumeTotal

)V(voidsofVolumen

t

v

%100)V(voidsofvolumeTotal

)V(watercontainsvoidsofvolumeTotalS

v

w

e1

e

)e1(V

eVn

s

s

Typical Values of Specific Gravity

Material Gs

Quartz sand

Cacareous sand

Clay minerals

2.65

2.64 – 2.71

2.67 – 2.73

Types of Soil in Pakistan

The Objectives of Soil Investigations Identify & describe pertinent surface conditions

Determine location and thickness of soil and rock strata (subsurface soil profile)

Determine location of groundwater table

Recover samples for laboratory testing

Conduct lab and/or field testing

Identify special problems and concerns

Desk Study To determine the available information and become familiar with

the project Geological maps, topographical maps and other records from

Library Remote sensing data, Satellite imageries, Aerial Photographs Local authorities, statutory bodies, the geological survey, mining

companies Experience of local contractors Maps are used to identify natural features such as streams,

springs, scraps, landslides and vegetation etc. Maps are also used to identify man made surface features such as

fences, power lines, roads and buildings

Site Reconnaissance

To confirm the findings of the desk study Current maps should be marked with site boundaries

and major structures Visually check the ground conditions Look for settlement cracks on existing structures Record the location of transmission lines, trees,

manholes or any other possible obstruction High water marks on old buildings, bridge abutments

etc. Interviews of the local people

Preliminary Design Data

Soil exploration and preliminary design should be started about the same time

For buildingsType, Size and height, depth of basementApproximate arrangement of columns and bearing

wallsApproximate range of column and wall loads

For bridgesType and length of bridge spanApproximate loads on piers and abutments

Steps in Soil Exploration

Boring Sampling Testing

Common Types of Boring

Test (Trial) Pits Auger Percussion Drill Rotary Drill

Test Pits

For shallow visual investigations and to take sample from top few meters of soil

Economical and rapid Excavation with hand tools, backhoe, bull dozer or

caisson drilling equipment 1 to 3 m deep, can be extended to 6m Retaining structure my be used if collapse is

expected Unsuitable for granular soil and below water level Some times trenches are dug

Auger

To drill holes to a shallow depth

(up to 6m) Operated by hand or by power For disturbed or undisturbed

samples Up to 200 mm dia hole Difficult in water bearing sand and

gravels

Percussion Drill

Common method for advancing test holes in all soils and weak rocks for shallow and deep exploration

Holes advanced by jack hammer and chopping bits 150 to 300 mm dia holes can be drilled from 1 to 80 m,

most common depth is up to 30 m Shells, bailers, clay cutters and chisels are boring tools Hole is advanced by repeatedly raising and dropping

the boring tool Casing may be required in unstable soils and deep

holes

Rotary Drilling

To drill holes in almost all ground conditions for sampling and installation of instruments

75 mm to 150mm dia boreholes can be drilled from 1 to 250 mm (most common depth 1 to 30 m)

Hole is advanced either by rotating bit or downhole hammer

Cutting is removed by the flushing medium which is pumped down to the drill bit with a pump

Casing is usually required in unstable soils and deep holes

Common drilling fluid; bentonite in water with slurry density 68-72pcf

Types of Bits

Core bits (for sample recovery)

Diamond or tungsten Drag (to create open holes) Rock rollers Percussion bits

Flushing Medium

Purpose is to remove particles and to cool the drill bits Air flush (operates at 1000 m3/min) Water flush (operates at 24-50 m3/min) Mud flush include bentonite and polymer based muds Mist and foam flushes (mixture of air, water and mud)

Sampling

To obtain specimens for determining the physical, chemical and mechanical characteristics of the soil in laboratory

Samples can be either jar, bag, tube, continuous, core or block

Ideal sampler must be thin walled (less area ratio), have smaller taper angle and smooth walls

Classification of SamplesClass Properties Symbols Technique

1 Classification

Strength

Deformation

Permeability

w, γ, SG, PI, PSD

C, Φ, Cu

E, G, mv

K, Cv

Pushed thin wall sampler

Some thick wall samplers

Some core barrels

2 Classification w, γ, SG, PI, PSD

Thin and thick wall samplers and core barrels

3 Classification W, PI, PSD Clay cutter or auger (dry)

4 Classification PSD, PI Clay cutter or auger (wet)

5 None Cutting and flushing return

Sample Application

Type Size Installation Class Ground

Jar Small Cuttings 5 All

Bag Large Shell, Core 3,4,5 Soil

U100 100 mm dia

0.5 m long

Pushed

jacked

1,2 Clay

Piston 75 – 200 mm dia

0.5 – 1 m long

Jacked 1,2 Clay

Tube 75 – 100 mm dia

0.5 – 1 m long

Jacked 1,2 Clay

Core NX – SX Drilled 2 All

Block Up to 1 m Cut 1 All

Continuous 50 – 100 mm dia Jacked 2 Soil

Sample Disturbance

Should be minimum so that the parameters obtained should be representative

Caused during drilling due to stress relief, swelling, compaction, piping and collapse of the borehole

Caused during sampling by stress relief, remoulding, compaction, segregation, fracture, loss, friction

Caused during transport and storage

Minimizing the Sample Disturbance

SwellingBy minimizing the time of sampling

CompactionBy keeping the casing above the base of borehole

PipingBy maintaining a water balance

CollapseBy using casing or mud or by keeping the hole dry

DistortionBy using smooth clean tubes and thin walled sampler

Common Sampling Methods

Types of Samples

Disturbed Samples

Disturbed samples are those obtained using equipment that destroy the macro structure of the soil but do not alter its mineralogical composition. For determining grain size, Atterberg limits, and compaction characteristics of soils disturbed samples are taken

“…Estimating the nature of the formation from the cuttings is like identifying the cow from the hamburgers.” G.F. Sowers

Types of Samples (Continued)

Undisturbed samples

Undisturbed samples are obtained in clay soil strata for use in laboratory testing to determine the engineering properties of those soils. Undisturbed samples of granular soils can be obtained, but often specialized procedures are required such as freezing or resin impregnation and block or core type sampling Specimens obtained by undisturbed sampling methods are used to determine the strength, stratification, permeability, density, consolidation, dynamic properties, and other engineering characteristics of soils

Testing

Laboratory Testing Field Testing

Laboratory Testing

Property of Soil Type of Test

Grain size distribution Mechanical analysis

Consistency Liquid limit (LL)

Plastic limit (PL)

Plasticity Index (PI)

Unit weight Specific gravity

Moisture Natural moisture content

Field moisture equivalent

Compressibility Consolidation

Laboratory Testing (Continued)

Properties of Soil Type of Test

Shear Strength Cohesive soils Non cohesive soils General

Unconfined compressionDirect ShearTriaxial

Volume Change Shrinkage factorsVolume changeExpansion pressure

Permeability Constant headFalling head

Compaction characteristics Standard proctorModified proctor

California bearing ratio CBR

Field Testing

Purpose of Test Type of Test

Compaction Control Moisture density relation

In-place density

Shear strength (soft clay) Vane test

Relative density (granular soil) Penetration test

Permeability Pumping test

Bearing capacity Pavement

Footings Piles

CBRPlate bearingPlate bearingLoad test

Standard Penetration Test (SPT)

SPT are used for most of the soil types A cheap, simple and rapid test Most suitable for granular soils A split spoon sampler is used which is an open

ended steel cylinder splitting longitudinally Most common size of sampler are

I.D = 1.4 – 1.5 inO.D = 2.0 inLength = 2 ft

Hammer wt. = 140 lbs Free drop = 30 in

Standard Penetration Test (SPT) (continued)

Penetration resistance is reported in no. of blows per feet

The spoon is withdrawn and a representative sample is secured

Samples are kept in airtight jars

Correlations of SPTRelative Density of Sand

N (blows/ft) Relative Density

0-4 Very Loose

4-10 Loose

10-30 Medium

30-50 Dense

>50 Very Dense

Correlations of SPTStrength of Clay

N (blows/ft)

Unconfined Compressive Strength (tons/ft2)

Consistency

<2 <0.25 Very soft

2 - 4 0.25 - 0.50 Soft

4 - 8 0.50 - 1.00 Medium

8 - 15 1.00 - 2.00 Stiff

15 - 30 2.00 - 4.00 Very Stiff

>30 >4.00 Hard

Cone Penetration Test (CPT)

A cone is connected with standard rods and an electric cable

The cone is pushed continuously into the ground and automatic measurements are taken of the force on the tip and sleeves

Depth is also recorded Cone resistance qc and

friction resistance fs are

used together with charts

to obtain soil parameters

Cone Penetration Test (CPT)(continued)

Cone Penetration Test (CPT)(continued) Rod dimensions are 36 mm dia, 600 mm long Cone has 600 angle, surface area = 10 cm2

Mostly used for fine to medium grained soils

Types of Pressuremeter

Pre-bored Pressuremeter (PBP)Menard Pressuremeter (MPM)Elastometer (OYO)High Pressure Dilatometer (HPD)

Self-bored Pressuremeter (SBP)Cambridge self-boring pressuremeter (CSBP)Pressuremeter AutoforuerPushed-in PressuremeterCone PressuremeterStress Probe

Pressuremeters

Pressuremeter are probes that are installed into the ground below the borehole

An expanding section is inflated and the displacement of the expanding section and the pressure required to cause that displacement are measured

Dilatometers•The flat plate dilatometer (DMT) is an in-situ test device with specific advantages that can useful on certain projects. However, it is not yet used as frequently as the cone penetration test (CPT) in engineering practice.

•A 60-mm diameter circular membrane on the flat face (95 mm wide by 220 mm long). This membrane is expanded after the DMT has been pushed to the desired depth.

Other Penetration Tests

DPT (Dynamic Probe) Mackintosh Probe Static Probes

Friction Cone (Dutch Cone)

Piezocone (CPTU)

Geophysical Tests

Ground Penetrating Radar Frequency Domain Electromagnetics Time Domain ElectromagneticsVery Low Frequency EMResistivity Spontaneous PotentialSeismic RefractionSeismic ReflectionMagneticGravityThermalRadioactiveMetal Detectors

Vane Shear Test

Consists of a Vane connected to the surface with rods. The vane is turned by a torque

Used in soft to firm clays Rotation is used to measure the sensitivity of clay A rapid and simple method

Vane Shear Test (continued)

T = cЛ[d2h/2 + d3/6]

where

T = Torque required to shear

the soil

c = cohesion of clay

d = dia

h = height of vane

Ground Water Measurement

In sand or gravels

- Clean the hole by horizontal jets

- Use steel tape coated with chalk

In silt or silty sand

- Can be measured in several days

In clays

- Piezometer is used

How Many Borings & How Deep?

“No hard-and-fast rule exists for determining the number of borings or the depth to which borings are to be advanced.”

But guidelines exist in –

• Textbooks

• Design manuals

How Many Borings? Conventional Wisdom

The number (density) of borings will increase:○ As soil variability increases○ As the loads increase○ For more critical/significant structures

Rules of Thumb:Soft soils, critical structures – 50'Soft Soils - Space 100' to 200'As soils become harder, spacing may be

increased up to 500’

No. of Borings

Source: Sowers 1979

Structure or Project

Subsurface Variability

Spacing of Borings (ft)

Highway Subgrade

Irregular 100-1000 (200, typical)

Average 200-2000 (500, typical)

Uniform 400-4000 (1000, typical)

Multistory Building

Irregular 25-75

Average 50-150

Uniform 100-300

No. Of Borings (continued)

Project

Distance Between Borings (ft.)

Horizontal stratification of soil

Uniform Average Erratic

Min. No. of Borings

1 or 2 story building200 100 50 3

Multi-story buildings 150 100 50 4

Bridge,Pier,Abutment 100 25 1 – 2

Highways 1000 500 100

Borrow pits 1000-500 500-200 100-50

No. Of Borings (continued)

Area (acre) Total No. of Borings Deep Boring

< 10 4 1

10 – 49 8 2

50 – 99 14 4

100 – 200 20 5

> 20024 plus 1 boring each additional 10 acre

6 plus 1 boring each additional 10 acre

No. Of Borings (continued)

Soil ConditionFoundation Footprint Area / borehole

(m2) (ft2)

Poor 100 – 300 1000 – 3000

Average 200 – 400 2000 – 4000

Good 300 – 1000 3000 – 10000

No. Of Borings

Project Spacing (m)

One story house 25 – 30

Multistory buildings 15 – 25

Highways/Railways 250 – 500

Earth Dams 25 – 50

Residential sub-division 60 - 100

Depth of Boring

Highway and airfieldMin. depth 5 ftShould extend below organic soil, muck, fill, or

compressible layers Retaining walls

Deeper than possible surface of slidingDeeper than width of base of wall

Embankment and cutsDeeper than possible surface of slidingEqual to the width at bottom of cuts

How Deep (Bridges)?

Boring depth is governed by various factors, including:Foundation typeFoundation loadLowering of grade line at underpass?Channel relocation, widening, dredging?Scour?

Rules of Thumb Generally speaking, 50’- 80’ is reasonableLocal experience is helpfulLook at nearby structures if availableIf no experience or other info available, plan for long

first hole, then adjust.

How Deep (Retaining Walls)?

Boring depth is governed by various factors, including:Wall type (Fill vs. Cut)Lowering of grade line at wall?Scour?

Rules of Thumb (TxDOT):Fill Walls: Depth = Wall Height +/-Soil Nailed Walls: Depth = Through Nailed Area,

plus 10’Drilled Shaft Walls: Depth = Exposed Wall Height plus

150% of Wall Height

Depth of Boring (continued)

Soil Condition Minimum Depth of Boring (m)

Poor 6 x S0.7 + Df

Average5 x S0.7 + Df

Good3 x S0.7 + Df

S = No. of stories

Df = Tentative depth of footing (m)

Depth of Boring (continued)

For embankments1.5 to 2.0 times the height of embankmentShould be sufficient to check possible shear failure

and probable settlement For dams

Should be such to explore all starta through which piping and seepage may occur

Some borings up to rock bed with minimum 3 m into the rock

Depth of Boring (continued)

For roads and airfieldsUp to depth of 2 – 3 mFor embankment roads up to depth = 1.5 – 2.0 x

height of embankment + 2 – 3 ft. For Pipelines

1 – 2 m below invert level For structural foundation

1.5 x width or up to good soil10 % of the contact pressure

Rocks Rocks are described from observation of natural

outcrops, quarries, cuttings, excavations and rock cores.

Description of rock material for engineering purposes generally follow the sequence below:

Colour Grain Size Texture and Structure State of Weathering Rock Name Strength Other Characteristics or Parameters

Types of Rocks

Spacing of Discontinuities in Rocks

Spacing Structural Features Discontinuities in One Dimension

Discontinuities in Three Dimensions

>2m

600mm-2m

200-600mm

60-200mm

20-60mm

6-20mm

<6mm

Very thick

Thick

Medium

Thin

Very thin

Thickly laminated

Thinly laminated

Very widely spaced

Widely spaced

Medium spaced

Closely spaced

Very closely spaced

Extremely closely spaced

Very large

Large

Medium

Small

Very small

Compressive Strength of Rock Material

Compressive Strength (MN/m2) Description

<1.25

1.25 – 5.0

5.0 – 12.5

12.5 – 50.0

50 – 100

100 – 200

>200

Very weak

Weak

Moderately weak

Moderately strong

Strong

Very strong

Extremely strong

Rock Mass Classification

Terzaghi’s descriptionIntact rockStratified rockModerately jointed rockBlock and seamy rockCrushed but chemically intact rockSqueezing rockSwelling rock

Rock CoringDouble-tube core barrel is typicalDiamond or tungsten-carbide tooth bitSize of core samples varies (NX, NQ, HQ,

etc.)

Rock Quality Designation (RQD)

RQD = ΣLength of core pieces>10 cm long X 100

The core should be at least NW size (2.15 in dia) Should be drilled with double-tube core barrel

Total length of core run

Rock Quality Designation (RQD)(continued)

RQD (%) Rock Quality

< 25 Very poor

25 – 50 Poor

50 – 75 Fair

75 – 90 Good

90 - 100 Excellent

RQD = 115 – 3.3 Jv

Where Jv is sum of the No. of joints per unit length of all joints (Volumetric joint count)

Plotting of RQD Values

QUESTIONS