Geotechnical principles underpinning Site Investigation.

Geotechnical principles underpinning

Site Investigation

Hard Ground:Bulk (mass in geological

language) properties dominate the design &

stability.

Soft Ground:Material Properties dominate

the design & stability

• It is a process site investigation is equivalent to Research, the product of which is information which has to be

interpreted and evaluated for the site

• DevelopmentOutput usually presented in the form of a

report (printed and pdf) and, increasingly, with the data being in electronic format (AGS Format).

Site investigation…

site investigation components

1. desk study and site reconnaissance

report

2. site works and laboratory testingfactual report interpretative report evaluative report

3. construction and post-construction monitoringreport

Desk Study and Site Reconnaissance

• 1888

Desk Study from published records - Coventry

1937

1962

1999

East India Docks - 1 October 1808 looking south – data by research on Google

Site ofMillennium Dome

or O2 Arena

Sequentially the site ofa)East India Export Dockb)Mulberry Harbour constructionc)Power Station d)Golf Rangee)Housing development

River Thames

Web Study!

Essential Tools in undertaking a site Reconnaissance

.

• Whitby• Sandsend

• circa 1925

• (photograph from • a local resident provided during• site reconnaissance)

Site Reconnaissance - attracting information

Site to be developed

back scarpat top of unstable slope

site reconnaissance

Oct 2001 aerial photograph with breaks in slope marked from field mapping

aerial photography from desk study with superimposed site mappingshowing form lines on unstable slope

area of previous

photographs

merry hilldudleywest midlands1984 aerial photograph(before failure)

key to observationson siteafter failure

canal

Toe feature

Turkey Merry Hill

canal

Down-slope direction

Down-slope

Yalova Turkey – Google Earth photo of suspected landslip siteComparison with known suspected site at Dudley

Typical Techniques used for

Intrusive Investigation/fieldwork

Trial pitsAdvantages

Cheap Quick Trends in strata can be

detected Identifies Construction

problems Ease of excavation can

be assessed Excavation stability can

be observed Groundwater problems

evaluated Some in situ tests can

be undertaken Large samples can be

takenDisadvantages

Causes substantial disturbance

Access constraints for machinery

Limited depth can be penetrated

Slope before trial pit excavationAmpleforth Abbey

Note tension crack in ground

• hand penetrometer

hand vane

Simple tests on samples from trial pits

trial pit record

Cable Tool Percussive(Shell and Auger) Boring

Rotary Drilling

Rock core

FieldworkDrilling

Window sampling

boring techniques

Cable Tool Percussive(Shell and Auger)boring

CABLE TOOL PERCUSSIVE RIG

AdvantagesSimple technologySuitable for a wide range of ground

conditionsCan penetrate to considerable depthsSamples obtained for laboratory testingIn situ testing can be undertakenInstruments can be installed in holes

DisadvantagesConstraints due to width / heightSubject to obstruction by concrete or

rockCan only penetrate a short distance

into rock

7m

Typical Techniques used for Intrusive Investigation

CABLE TOOL PERCUSSIVE RIG, example of daily log record


Rotary Drilling rigs

Lorry mounted

minuteman


Rotary drilling

Advantages continuous coring in rock capable of considerable

depth can core clays can probe for voids in rock

Disadvantages size of rig constrains access

(Typically 8m-9m high but can be 2m- 5m)

difficulties in gravel small diameter


Stratford upon Avon TheatreAchieved success despite severe access constraints


• borehole sampling devices

• “undisturbed” sample• U 100


From Gosling and Baldwin, Ground Engineering March 2010

UT 100 prototype

Open tube samplers for cohesive soils

U open tube samplers:• They are driven into the ground by a sliding hammer• The number of blows is recorded• Samples are 450mm long• Sample disturbance (due to

friction on the inside of the sample) is roughly proportional to the thickness of the sampler: the thinner the sampler the less the disturbance; however the higher the chances of getting the sampler damaged.

• They are available in D=38mm; 75mm; 100mm.

Open-tube (U100) sampler

• Area ratio

• For U100 samplers the area ratio is generally approximately 30%.

• Thin-walled samplers have area ratios of about 10% and so sample disturbance is less.

%1002

22

cD

cDwD

Core box

Rotary drilling Example of borehole record


cable percussion/rotary drilling combined borehole record


Open tube samplers: disturbance

For a comprehensive summary on the problem of sample disturbance using tube samplers, look at the PhD thesis of Eyre Hover (University of Warwick) titled: The investigation of tube sampling disturbance using transparent soil and particle image velocimetry.Using transparent soil and PIV it is possible to reconstruct the field of strains induced by the pushed in sampler.

Samples• Sample interval specified in the contract• A typical sampling pattern for a cable

percussive borehole may include the following:• Small disturbed samples at changes of

geological horizon• Small disturbed samples at the base of each

U100 sample (shoe sample)• U100 sample every metre in cohesive material• SPT and bulk sample every metre in granular

material, bulk sample should cover at least the sample interval of the SPT.

Samples• Remember it is not possible to achieve 100%

recovery of undisturbed samples in a light cable percussive borehole.

• Even if you take continuous U100 samples, the bottom of each U100 sample (shoe) will be

• The U100 sample is typically 450mm in length• Water samples at every water strike and at the

start of each shift if there is standing water

Obstructions and Boulders• Light cable percussive boring rigs can drill through some

solid obstructions and boulders of rock by using a chisel.• This a heavily weighted chisel which is dropped

repeatedly on the boulder, breaking it into smaller pieces which are removed from the borehole using the shell

• It is usually necessary to reduce the size of the casing to pass through the boulder

• If boulders are anticipated then it is necessary to start the borehole at a large diameter (250mm to 300mm) at the surface

Window Sampling first began to appear during the 1980s and is now one of the most cost effective means of rapidly assessing ground conditions across site areas for both geotechnical and environmental consultants.

The process uses either drop weight or hydraulic hammer to drive 1m or 2m long sample tubes into the ground. The sample tubes have open slots or “windows" along their length to allow for logging and sampling.

Window Sampling (most commonly now – windowless with a plastic liner)


Windowless sampling, which is now probably more common than window sampling, involves driving sample tubes containing plastic liners into the ground. The sample is retained within the liners which can be split on site or taken to a laboratory for analysis. This method of sampling reduces the potential for cross-contamination of soils during the drilling process.




Advantages Size allows use in

constricted access Continuous sample

obtained Sampling for logging

and lab testing Little surface

disturbanceDisadvantages Limited depth of

penetration Small sized samples Typically does not allow

use of casing


“window” sampling record


standard penetration test (SPT)equipment


Standard Penetration Test (SPT)

• Undisturbed samples cannot be taken in granular material with light cable percussive rig.

• The degree of compaction of sands and gravels is assessed using SPT

• Small diameter sampler pushed into the ground using a drop weight (63.5 Kg) falling repeatedly from a known height (760mm).

• Data relative to the first 150mm of penetration are discarded because the soil is considered disturbed.

• The number of blows to penetrate the ground for 75mm is recorded. This operation is repeated four times for a total of 300mm.


• SPT or ‘N’ values are calculated as the number of blows to penetrate the ground of 300mm.

• The degree of compaction of granular soils described by ‘N’ value:

N = 0 to 4 – Very looseN = 4 to 10 – LooseN = 10 to 30 - Medium denseN = 30 to 50 – DenseN = >50 – Very Dense



• SPT’s are sometimes taken in cohesive soils alternating with U100 samples

• For cohesive soils, an empirical correlation between ‘N’ values and unconfined compressive strength values has been established:

N Consistency Unconfined Compressive Strength (kPa) <2 Very soft Under 20 2-4 Soft 20 – 40 5-8 Firm 40 – 75 9-15 Stiff 75 – 150 16-30 Very stiff 150-300 >30 Hard >300



• Consistency of test results can be an issue with SPT testing as it is highly dependent on the skills of the driller

• Look at the comparison of SPT values with depth taken in the same material (chalk with flints) by different drilling companies

Standard Penetration Test (SPT)Typical Techniques used for Intrusive Investigation

However, despite the limitations and simplistic nature of the test, SPT are widely used as the basis for bearing capacity calculations.

Standard Penetration Test (SPT)Typical Techniques used for Intrusive Investigation

Cone Penetration Test (CPT)


Instrumented cone


The stresses at the cone base and along the lateral cylindrical surface are measured. The formulae of the bearing capacity for deep foundations (piles) apply.

Static Cone Penetrometer• Used in soft clays and loose sands where

boring operations tend to disturb in situ values.

• Tube and inner rod with a conical point at the base are hydraulically advanced into the ground.

• Cone cross-section 1000mm2 with an angle of 60o

• Every 300mm depth the cone is advanced ahead of the tube by 50mm and the maximum resistance recorded.

• The tube is then advanced to join the cone after each measurement and the process is repeated.

Advantages• continuous soil profile of strength or density• direct measurement of skin friction• can identify soil type by using friction ratio• quick method• can be used over weak ground• can be taken to considerable depth• can be used for contamination testing

Disadvantages• large vehicle constrains access • normally cannot sample but• an adaptor can allow sampling• can be obstructed in fills

Cone Penetration Test (CPT)Typical Techniques used for Intrusive Investigation

Static Cone Penetrometer

The ratio of sleeve resistance to that of cone resistance is higher in cohesive than cohesionless soils, hence the nature of the soil can be estimated from the friction ratio.

Borehole Vane Test

)(62

32 DHDM

Shearing resistance = M = torqueD = Diameter of vaneH = Height of vane

Borehole Vane Test

• It measures undrained shear strength and remoulded strength in situ.

• Suitable for clays & particularly sensitive clays with cohesions <100kPa.

• Causes little disturbance to the soil mass • Results affected by sandy, silty or organic

content of the clay• Must be used in conjunction with laboratory

testing, not instead of.

Laboratory testing

Shear Box For shear strength measurement in granular soils or cohesive soils in slopes

AN

= AF

=

t

s

t

Dl

sa

sa sb

sb

sc

sc

f

c

The plotted results can be defined by an equation for a straight line:

f = c + tan

Where is the normal stress applied to the soil slip surface and c and are the shear strength parameters (cohesion and angle of internal friction respectively). This is known as the Coulomb equation.

c

f

However, the situation is often complicated by the presence of pore water and thus pore water pressures. From the principle of effective stress we know that ' = - uWhere u is the pore water pressure and ’ is the effective normal stress. We can therefore modify Coulomb’s equation to model effective stresses: f = c' + ' tan 'Note that the constants are also modified to the effective strength parameters. These do not have the same magnitude as the total stress parameters.

Ring shear test

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trols.it/immagini/produ

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F-258

50.jpg

The Bromhead Ring Shear apparatus tests an annular soil sample 5 mm thick with inner and outer diameters of 70 and 100 mm respectively and confines it radially between concentric rings.

It is compressed vertically between porous bronze loading platens and a rotation is imparted to the base plate and lower platen by means of a variable speed motor and gearbox.

This causes the sample to shear, forming a shear surface close to the upper platen, which is artificially roughened to prevent slip at the platen/soil interface.

Torque transmitted through the sample is reacted by a pair of matched load measuring proving rings or load cells bearing on a cross arm.

Ring shear test ' tan ' + 'c = rrr

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trols.it/immagini/produ

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F-Ring-shea

r.jpg

Triaxial Test

Triaxial test schematic

valve

soil

porous plates

q

s1

s3s3

s3s3

pore water pressure apparatus

burette

pistonpoint load

perspex cell

confining pressure, s3= s2

f’ = 29 deg.

c’ = 16 kN m-2

100 200 300 400 500 600 700

100

200

300

0

s’ kN m-2

t kN m-2

The triaxial test measures the strength of soils ……

Triaxial Test

3131 2 p1 3 1 3q q 'p p u

1 2 3 1 30

2vol

V

V

1 3

2 2

3 3dev a r

…… but also their stress –strains behaviour which means how much they deform under a given loading

Triaxial Test

In addressing Site Investigation and its interpretation it is worth bearing in mind Donald Rumsfeld’s suite of knowns and unknowns

• “There are known knowns. These are things we know that we know.

• There are known unknowns. That is to say, there are things that we know we don't know.

• But there are also unknown unknowns. There are things we don't know we don't know.”

Sources & ReadingWaltham, A.C., 2009. Foundations of Engineering Geology. 3rd ed. Chapter 21. Abingdon: Taylor & Francis.

Barnes G. (2010). Soil mechanics principles and practice. Palgrave. chapter 15.

Bell, F.G., 2007. Engineering Geology. 2nd ed. Oxford: Elsevier. pp. 322-328.

Price, D.G., 2009. Engineering Geology-Principles and Practice. 1st ed. Berlin, Heidelberg: Springer-Verlag. pp. 145-150.

British Standards Institution, 2004. BS EN 1997-1 Eurocode 7: Geotechnical Design – Part 1: General Rules. BSI.

Weltman, A.J. and Head, J.M., 1983. Site Investigation Manual. London: CIRIA

Geotechnical principles underpinning Site Investigation.

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