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GEOTECHNICAL INVESTIGATIONS

TOOLS OF THE TRADE

Presented by Heather Davis

THE GROUND BENEATH

No engineering structure is better than the

materials on which it has been built.

We are generally concerned with the strength,

volumetric change and permeability of any

materials we deal with.

ROCK is the solid material forming part of the

surface of the earth.

SOIL is a weakly cemented accumulation of

mineral particles formed by the weathering of

rocks.

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DIFFERENT PROFILES

Bedrock Residual Trans-

Ported

PM

Trans-

ported

PM

Bedrock Bedrock Residual

PM Pebble Marker

Bedrock

ROCK

Igneous e.g. basalt, andesite, granite - formed

when molten magma or lava solidifies

Metamorphic e.g slate – formed by the alteration

of other rocks

Sedimentary e.g sandstone, shale – formed from

the debris of other rocks and living matter

Unconfined compressive strength of the intact or

un jointed material of >1000kPa

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ROCK WEATHERING

State of the weathering

fresh or unweathered

slightly weathered

moderately weathered

highly weathered

completely weathered

ROCK HARDNESS

Hardness Description UCS MPa

Very soft rock Material crumbles under firm blows

of knife

1 – 3

Soft rock Firm blows with pick point indents 3 – 10

Medium hard rock Firm blows with pick point will

break the rock

10 – 25

Hard rock 25 – 70

Very hard rock Breaks with difficulty

Rings like a bell when struck with

hammer

70 – 200

Very very hard rock >200

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ROCK DISCONTINUITIES

Discontinuity spacing

Joint filling

Roughness of

discontinuity planes

Grain size

Rock type

SOIL

BOULDERS >200mm

COBBLES 60mm -200mm

GRAVEL 2mm – 60mm

SAND 0.06mm – 2mm

SILT 0.002mm – 0.06mm

CLAY <0.002mm

Unconfined compressive strength of <1000kPa

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RESIDUAL SOIL

Material formed by the IN SITU decomposition

(chemical weathering) or disintegration

(mechanical weathering) of rock to a degree of

“softness”.

A residual soil, GENERALLY, passes into or

grades into the solid parent rock

Know the parent rock

Relic structure visible

TRANSPORTED SOIL

This is soil that has been transported by a

natural agency such as water, wind or gravity

during relatively recent geological times and

which has not undergone lithification or

pedogenesis.

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ORIGIN : TRANSPORTED

Name Agency of

transport

Source rock Soil type Potential problems

Talus Gravity Any rock Unsorted angular

gravel and boulders

Slope instability

Hillwash Sheetwash Acid/basic

Arenaceous

Argillaceous

Clayey sand, clay, silt, Collapsible grain

structure

Heave

Compressibility

Alluvium Streams or

gulleys

Whatever is within

catchment

Gravel, sand, silts,

clays

All possible

problems

Lacustrine Stream

depositing in

pan, lake etc

Mixed Sand, silt, clay Compressibility

heave

Estuarine Rivers and

tides

Mixed Sand, silt, clay Quick sand

High sensitivity

Aeolian wind mixed Sand Collapsible grain

structure

PEDOGENIC MATERIAL

Ferricrete

Silcrete

Calcrete

Manganocrete

Soils which have become strongly cemented by iron oxide,

silica, calcium carbonate etc.

If the degree of alteration is not too great e.g. scattered

nodules in a matrix – we do not call it pedogenic

If the degree of alteration is such that the material is

totally altered then it is a pedogenic material

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The Bad Boys of the Rock World

Dolomite

“The development of dolomite land continues to

present a challenge in South Africa. While

opportunities exist in the development of such

land, the adverse effects relating to the formation

of sinkholes and subsidences, whether naturally or

as a result of development, cannot be ignored.”

SANS 1936

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Subsidence caused by mining activities

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Failure along discontinuities

The Bad Girls of the Soil World

Expansive clays

those materials where variations in moisture

content result in volumetric change of the soil

skeleton.

Highly compressible clays

those clays which undergo a large volumetric

change with an increase in imposed load causing

high settlement.

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Collapsible soils

those partially saturated materials which exhibit

additional settlement upon wetting up generally

without any increase in applied pressure.

Dispersive soils

those materials in which the clay content has a

high percentage of sodium. The clay fraction

readily breaks down to form a suspension in water.

Investigation Methods

Desk Study

Geological and topographical maps

Google Earth images

Stereo aerial photographs

Previous investigations

Brink books (Engineering Geology of Southern

Africa)

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Desk Study

Site Walk-over

Outcrops, borrow pits, stream courses,

etc.

Condition of existing structures

Talk to locals

Calcretes –

Co

ega ID

Z

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Sun

days R

iver siltston

e quarry, C

oeg

a

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Desk Study

Site Walk-over

Test holes

Test pits

Auger holes

Boreholes

Excavation of test pits with a Tractor Loader

Backhoe (TLB)

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Provides information to approx. 3m

Usually moved to site on a flatbed truck

Can handle most site conditions

4x4 TLBs available for rugged terrain

Can penetrate most soils and even rock if the

rock mass is discontinuous.

SANS 1200D – specifications for excavation

classes with a tracked excavator but classes can

be inferred for the use of a TLB

Excavation of test pits with a tracked excavator

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Provides information to approx. 5m to 6m

Usually moved to site on a flatbed truck

Can handle most site conditions although not

the fastest thing on tracks.

Can damage road surfacing

Can penetrate most soils and even rock if the

rock mass is discontinuous.

A smaller bucket or ripper can be used in harder

layers

SANS 1200D – provides specifications for

excavation classes with a tracked excavator

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Large diameter auger holes

An auger piling rig

augers a 750mm

diameter hole.

Profiler lowered in

bosun’s chair

Can be used to a

depth of approx 20m

or refusal.

Any water/seepage

restricts access

Considerable health

and safety issues

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Drilling of rotary cored boreholes

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Provides information to in excess of 50m.

Usually carried out to a pre-determined depth or

to when a certain amount of competent rock is

proven.

NX size boreholes are common. Double

barrelled core tool (NWD4) used to give a

50mm core and a 76mm diameter hole

Can sample the soil with thin walled tubes

Can install piezometers in the holes to monitor

groundwater and carry out Standard Penetration

Tests (SPTs).

Percussion boreholes

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Drilled using down the hole air percussion

hammers

No undisturbed sample – just chips

Great reliance on interpretation

Conventionally used for investigation of

dolomitic terrain.

Increasingly being used in other terrain.

Conventional systems – info ever 1000mm

Jean Lutz system – info every 20mm

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Project Number:

Project Name: Date:

Investigation: Dolomite Stability Borehole: V11 Sheet 1 of 2

Min

Sec

0 : 36 None Good

0 : 49 None Good

0 : 54 None Good

0 : 57 None Good

0 : 52 None Good 5,0

0 : 26 None Good

0 : 15 None Good

0 : 18 None Good

0 : 29 None Good

0 : 57 None Good 10,0

1 : 18 None Good

1 : 24 None Good

1 : 08 None Good

1 : 16 None Good

0 : 54 None Good 15,0

1 : 23 None Good

1 : 01 None Good

0 : 51 None Good

0 : 53 None Good

0 : 59 None Good 20,0

1 : 09 None Good

1 : 00 None Good

1 : 11 None Good

1 : 16 None Good

1 : 12 None Good 25,0

1 : 35 None Good

1 : 40 None Good

1 : 38 None Good

1 : 41 None Good

1 : 31 None Good 30,0

1 : 16 None Good

1 : 39 None Good

1 : 34 None Good

0 : 56 Medium Medium

0 : 09 Slight None 35,0

0 : 07 Slight None

0 : 08 Slight None

0 : 04 Slight None

0 : 06 Slight None

0 : 09 Slight None 40,0

0 : 07 Slight None

0 : 14 Slight None

0 : 10 Slight None

0 : 36 Slight None

3 : 24 Slight None 45,0

Profiled by: R Govender Compressor 19Bar Coordinates

Drilling Contractor Hennie Erwee Boorkontrakteur Diameter 165 mm x 25°51'34.23"S

Drilling operator Jack Date drilled y 28°15'2.50"E

Date profiled z28/10/2015

DESCRIPTION INTERPRETATION

SOFT ROCK SYENITE

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TRANSPORTED SOILBrown silty sand with

gravel.

PENETRATION AIR SAMPLE

DE

PT

H (

m)

TIME LOSS RECOVERY

Interpretted as

interbedded CHERT AND

WAD.

No sample recovery,

slight air loss.

Penetration rates

indicates the presence of

wad

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P.T.O P.T.O

Yellow brown silty sand

with grey angular

fragments (5mm)

TRANSPORTED SOILRed brown clayey sand

with minor gravel.

SLIGHTLY WEATHERED

CHERT

Brown clayey sand with

angular to subrounded

grey and white chert

fragments (2-10mm)

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Desk Study

Site Walk-over

Test holes

Geophysics

Gravity

Ground penetrating radar

Seismic refraction / reflection / cross-hole / CSW

Resistivity

Inve

stig

atio

n M

eth

ods

Gravity

The aim of a gravity survey is to detect

underground structures by means of the

disturbance they produce at the surface in the

earth’s gravitational field.

Relatively cheap

A large area can be covered relatively quickly

Non-invasive

Non destructive

Has been tested on the Moon

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Ground penetrating radar

Uses radar pulses to image the subsurface.

Non destructive

Uses electromagnetic radiation on the

microwave band

Applications – detection of sub surface objects,

changes in material properties, detection of

voids etc etc

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Seismic Methods

An elastic pulse or a continuous elastic wave is

generated near the surface of the ground.

The resulting motion of the ground at nearby

points on the surface is detected by geophones

Measurements of the time of arrival at the

geophones provides the pulse velocity as the

waves are reflected or refracted back from

surfaces of different properties below the

ground

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Resistivity

investigates variations of electrical resistance by

causing an electric current to flow through the

sub surface using electrodes connected to the

ground.

The variations depend on the size, shape,

location and electrical resistivity of what lies

below.

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In Situ Tests

In situ testing

SPT - standard penetration test

CPT - cone penetration test

VST – vane shear test

DPSH – dynamic probe super heavy test

Plate load test

Dia

gra

ms:

Man

ual

on

sub

surf

ace

inve

stig

atio

ns

FH

WA

NH

I-01-0

31

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SPT

Number of blows taken to advance a split

spoon sampler through 6 increments of 75mm

by dropping a 63.5kg hammer though a height

of 760mm

Last 4 increments used to calculate the number

of blows required to drive 300mm – the SPT N

value

Need a borehole

A compressed sample is, also, gained.

CPT

A 60º cone, usually equipped with a friction

sleeve, is pushed into the ground at a rate of

20mm/sec.

Measurements made continuously of cone

penetration resistance, total penetration

resistance and the side friction resistance of the

friction sleeve.

Depth limitations

No sample

Not suitable for gravel, boulders and rock

horizons

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DPSH

Number of blows to advance

a 60⁰ disposable cone

through 300mm by dropping

a 63.5kg hammer though a

height of 762mm.

Continuous readings.

Economic, quick and can be

done in areas with limited

access.

Will refuse in gravel layers,

cobbles, boulders and

cemented pedogenic layers

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Consider type/ types of foundations

Consider if there is cut to fill.

Consider if there are going to be basements

IT’S NOT ROCKET SCIENCE

- depth of investigation

For spread or pad footings

For spread or pad footings information

regarding the soil and rock within the

zone of material that will be affected by

the stress imposed on it by the structure

is needed.

This “stress bulb” extends to a depth of

approximately 2B (where B is the width

of the footing).

It is considered prudent to extend the

investigation to a depth of at least 2B

below the envisaged founding depth

or until competent rock (soft rock or

better) is encountered.

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For piled foundations

Should a piled foundation

solution be required then the

depth of investigation

should extend to at least 3

pile diameters below the

end of the pile or at least

5m into competent rock.