Bre411 Si for Low Rise Buildings Direct Investigations

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The aims of direct site investigation

Site investigation is the gathering of all theinformation on ground conditions which mightbe relevant to design and construction on aparticular site. On a site intended for low-risedevelopment, a desk study is the first stage ofinvestigation (see Digest 318). This involveschecking existing records, such as geologicalmaps of the site. The desk study, and a walk-oversurvey (see Digest 348), will indicate theprobable ground and groundwater conditions andthe problems they might pose.

Direct investigations are carried out:● To check the information from the desk study● To obtain any additional information required

to ensure safe and economic construction.

Direct investigations are never routine. On eachsite, the process is tailored to the specific projectand the specific ground conditions.

The type of information provided

The soil profileGround investigations identify the levels of thevarious soil or rock types on the site byidentifying the boundaries between them.Boundaries between soil types are not always

distinct, eg when one type changes gradually intoanother.

Soil classificationGround investigations classify the soils beneaththe site into broad groups: each group containssoils with similar engineering behaviour.

The simplest classification used bygeotechnical engineers divides the ground intofive categories: ● Rock● Granular soil (eg sands or gravels)● Cohesive soil (eg clays)● Organic soil (eg peats)● Fill or made ground.

Broad soil classifications, coupled with simpletests to determine soil parameters or to detect thepresence of chemicals harmful to constructionmaterials, are normally sufficient for low-risebuilding projects.

The natural variability of the soil in eachclassification can be assessed using index tests –see Table 1.

Soil parametersEngineering design and calculation may requirethe determination of soil parameters. In manysituations, soil parameters will not be required for

CI/SfB (A3s)December 1995

Site investigationfor low-risebuilding: directinvestigations

Digest411

The main objective of a siteinvestigation is to examinethe ground conditions sothat the most appropriatetype of foundation can beselected. A typical siteinvestigation begins with adesk study and a walk-over survey to establish the

general geology of the site,and continues with anexamination of thegeotechnical properties ofthe ground.This Digest discusses thevarious techniques fordirect investigation of a siteand provides guidance on

how to select theappropriate method for aspecific location.This is one of a set ofDigests dealing with siteinvestigations. The otherDigests in the set are listedon page 12.


the design of simple low-rise buildings, forexample when ground conditions are clearly verygood (eg in rock) or very bad (eg peat).Parameters need be determined only in difficultground (eg clays). Such situations are, however,common in the UK.

Groundwater conditionsGroundwater conditions are significant becausethey can affect construction in a number of ways.● A high groundwater table can lead to extra

costs and make construction more difficult, egbecause of the need for increased support anddewatering of foundation excavations.

● The presence of chemicals in groundwater(such as acids or sulfates) can lead to damageif foundation concrete is not of an appropriatequality (see BRE Digest 363).

● A high groundwater table implies that pore-water pressure in the soil is high, and thismeans that the soil will be correspondinglyweaker. As well as influencing foundations,high pore-water pressures will adverselyaffect the stability of slopes and the pressureson retaining structures.

Planning direct investigations

Direct methods of site investigation are generallymore expensive than desk studies, walk-oversurveys and other, indirect methods. It istherefore important that investigations areproperly planned, not only to ensure value formoney, but also to ensure that the objectives ofthe site investigation are met.Stage 1: Carry out a detailed desk study andwalk-over survey● Identify the probable ground and groundwater

conditions● Locate areas on the site likely to cause

construction problems, eg areas of fill, oldhedgerows and grubbed-up trees, mineshafts,low-lying ground, etc.

Stage 2: Make an initial design for thestructures and the site● Site the proposed structures so as to avoid as

many problems as possible● Design structural forms with special regard to

anticipated ground hazards. For example,make the structure as flexible as possible ifsignificant settlements are anticipated

● Make preliminary estimates of the types offoundation required, and determine theposition of critical slopes and retaining walls

● Consider the parameters required forgeotechnical design of the foundations.

Stage 3: Plan the direct methods required for

the site investigation● Identify the depths of investigation required at

different locations around the site (seeopposite page). Boreholes should alwayspenetrate completely through made ground orinfilling

● Identify suitable in-situ and laboratory testingmethods for the expected soil conditions andthe parameters required (see Table 1)

● Decide on the number of exploratory holes andthe sampling and testing frequency, makingallowances for the presence of unforeseenground or groundwater conditions (see Table2).

Stage 4: Keep a record of the investigationRecord the basis of the planned site investigationand the expected ground conditions. Thespecialist contractor who carries out the workwill then know if the ground conditions heencounters are unforeseen (in which case it maybe necessary to alter the scope of the field ortesting work).

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BRE Digest 322 considershow site investigations forlow-rise building may best beprocured, and recommendsthat a geotechnical adviserbe appointed to plan,supervise and interpret theresults.

Table 1 Types of laboratory and in-situ testsSoil Suitable

Purpose type test Frequency

Classification Granular, Particle size Every 1–1.5mtests cohesive distribution

Cohesive Atterberg limits Every 1–1.5mIndex tests Granular SPT1 Every 1–1.5m

Probing ContinuousCPT2 Continuous

Cohesive Moisture content Every 1–1.5mUndrained triaxial Every 1–1.5m

Parameter Granular CPT Continuoustests SPT Every 1–1.5m

Cohesive Undrained triaxial Every 1–1.5mOedometer Every 1–1.5mSulfate content 2–3 tests onand pH each soil typeVane every 1mCPT ContinuousSPT Every 1–1.5m

Tests on (Water) Sulfate content On everygroundwater and pH water sample1 Standard penetration test2 Cone penetration test

Desk studies are discussedin detail in BRE Digest 318;walk-over surveys in Digest 348.

Table 2 Number of exploratory holesBoreholes (use wheredepth required >4m) Trial pits

Suggested 1 for every 3/4 dwellings 1 for every minimum 2 dwellingsSuggested 1 for every 2/3 dwellings 1 for everymaximum dwelling


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Depths, numbers and locations of exploratory holes

Borehole depth depends on thestress distribution under thefoundations (Fig 1).

Boreholes should penetrate alldeposits unsuitable for foundationpurposes, such as unconsolidatedfill, peat, organic silt and very softcompressible clay.

Depth requirements should bereconsidered when the results of firstborings are available, and it is oftenpossible to reduce the depth of

subsequent borings or to confinedetailed and special explorations toparticular strata.

The maximum number ofboreholes will depend on thecomplexity of the local geology andthe planned construction project; itmay change as information emergesfrom early investigations.

Although there will be financialconstraints, between 0.5% and 1% ofthe capital cost of the construction is

likely to be available for siteinvestigation. Problem areas needinvestigation, and where the site isparticularly problematic the numberof boreholes must be increased toensure adequate coverage.

Boreholes should be located inpotential problem areas, eg slopesnear proposed site of structures.Where practicable, they should belocated along straight lines tofacilitate section drawing (Fig 2).

Figure 1 Borehole depth depends on the stress distribution under the foundations

Figure 2 Boreholes should be drilled in straight lines to facilitate section drawing

Structure on isolated pad or raft Closely spaced strip on pad footings Structure on friction pile

Site plan

Alignment of boreholes


Ground investigation techniques

Many different methods are now available fordirect site investigations. The problem is to selectmethods which:● Will work in the particular ground conditions

expected at the site● Give the information required to resolve the

particular construction problems that areanticipated and to make the necessary designcalculations

● Are sufficiently economical, given therestricted budget within which most low-rise building projects operate.

This Digest describes only those techniques thatare currently in fairly common use in the UK.Other methods may also be appropriate in specialcircumstances: descriptions of these methods arein BS 1377 and 5930, and in ref (1).

Soil and rock description

The description of soil must be carried out to anagreed and accepted standard. Very detailedengineering soil descriptions require someexpertise, but simple soil descriptions can bedone by following the rules and guidelines laiddown in BS 5930 and Digest 383.

The description of rock is more specialised,and requires the detailed identification of therock-forming minerals(1). However, detailed rockdescriptions will not normally be necessary forthe ground encountered during the investigationof low-rise building sites.

Soil description requires the identification ofthe material making up the soil — termedmaterial characteristics in Digest 383 — andalso the in-situ soil characteristics, including thestrength or density of the soil and the presence orabsence of fabric features, such as laminations,joints and fissures. A full sample descriptiongives information on colour, consistency,moisture condition, fabric and structure,principal soil type and subsidiary soil types. Notall methods of investigation provide theopportunity to determine both sets ofcharacteristics. Table 3 shows the ability ofvarious methods of investigation to provideinformation for soil descriptions.

Trial pitting is by far the most satisfactorymethod for describing the soil because it isgenerally quick and economical. However,although trial pitting is relatively cheap whencarried out to shallow depths, it becomes rapidlymore expensive as the depth increases. Where thedepth to be investigated exceeds 3–4m, the mostcommon methods are boreholes with tube

samples (in clay), and disturbed samples and SPTtests (in granular soils).

An accurate soil description is the mostvaluable outcome of a direct investigation. Itmust be carried out systematically, usingappropriate investigation techniques, but oncethis is done a wealth of information on the likelybehaviour of the soil can be obtained rapidly andat relatively little cost.

Exploratory holes

Trial pitsTrial pits are extremely valuable if the depth ofinvestigation required is less than about 5 m (seeDigest 381). This is the depth that canconveniently and easily be excavated in most soiltypes using a back-actor or 360° slew hydraulicexcavator.

Trial pits are particularly useful in theinvestigation of sites intended for low-riseconstruction. This is because foundations for thistype of structure are generally 0.4–0.45 m wideand 1–3 m deep, so the required depth ofinvestigation is 3–5 m.

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Table 3 Methods for soil and rock descriptionConsistency/ Soil

Method Colour strength Fabric type

Trial pits ✔ ✔ ✔ ✔

Hand-auger holes ✔ ✔5

Window samplers ✔ ✔5

Boreholes ✔ ✔2 ✔4 ✔5

SPT ✔1 ✔3 ✔1

Atterberg limit tests ✔

Triaxial tests ✔

Field vane tests ✔

CPT ✔3 ✔6

Notes:1 Using the disturbed sample obtained from the split spoon2 Tube samples are required in clays, and SPT tests in

non-cohesive soils3 Using the penetration resistance4 Large-diameter (minimum 100mm) tube samples are

required5 Unsuitable for determining the particle size distribution of

very coarse soils6 Using the friction ratio

BRE Digest 381 considersthe way that trial pitexcavations should berecorded, and gives detailedsafety recommendations onsupport and other importantmatters. Digest 383discusses soil description.

Safety warning!

There are very significant safety risks associated withworking in narrow, deep excavations of this sort: do notenter unsupported trial pits deeper than 1.2 m.


Routine trial pit records — ‘logs’ — arenormally taken by ‘profile logging’: the materialis inspected as it is dug by the excavator, and thedepth is determined by dipping the hole with aweighted tape. Lateral variability of the ground isnot recorded, and in-situ fabric and specialfeatures (such as the presence of shear surfaces)may be difficult to detect. The advantage of thissimple method is that, because people are notrequired to go down the hole, there is no need forside support.

‘Detailed logging’ is also common, and ispreferable to profile logging because it allowsclose inspection of the ground. However, forsafety reasons, hydraulic (or equivalent) trenchshores must be used wherever the trench is morethan 1.2 m deep.

Trial pitting is normally relatively fast. Ingood conditions, four to six 4 m-deep trial pitscan typically be excavated and logged per 8-hour shift.

Auger holesAuger holes are normally made by hand-turning avery light auger into the ground (Fig 3) or by usinglight power-auger equipment.

For two reasons, augering is not as widelyused as trial pitting and boreholes:● Auger holes — unlike trial pits — do not allow

the ground to be examined in situ● Auger holes do not allow the depth of

investigation, or the range of sampling and in-situ testing, that conventional siteinvestigation boreholes will give.

Typical auger holes are 75–150 mm in diameter.Short helical augers are generally used, anddisturbed soil is collected from the auger flight asit is brought to the surface. Small continuous-flight auger drilling rigs are available whichallow greater depths of investigation. Smalldiameter (38 mm) undisturbed tube samples maybe taken, using the same equipment as is used intrial pits, but this is unusual (see section on Soilsampling).

The small disturbed samples typicallyobtained from an auger hole are described, andthe descriptions collated to produce a boreholerecord, in the same way as for larger-diameterboreholes.

Window samplersThese are becoming a more readily availablealternative to auger holes. A window sampler is asteel tube, usually about 1 m long, with a series ofwindows cut in the wall of the tube throughwhich to view or take specimens of the soilsampled. It is driven into the ground by alightweight percussion hammer, then extracted

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Advantages of trial pits compared with auger holes

● Trial pits allow a detailed examination of the ground in situ

● They provide some indications of ‘digability’, trench stability and groundwater conditions.

Figure 3 Hand augering equipment

Pros and cons of small-diameter augering

Advantages● There is less disruption to the site than with either trial

pits or boreholes● The equipment is light, and can relatively easily be

brought to the position of boreholes● Under favourable circumstances, the depth of

investigation is greater than with trial pits.

Disadvantages● Auger holes of this type cannot be cased, and

therefore will not stand open in many soils below the water table

● The equipment used is not robust enough to penetratestiff, hard or stony ground

● Compared with other methods, progress is relatively slow and is often made slower by unfavourable ground conditions

● Tube sampling is difficult, and without borehole support it may not always be possible to assess the depth from which disturbed samples originate

● In-situ testing is not possible.


using jacks. Samplers come in a range ofdiameters. In practice, the largest is driven intothe ground; then it is withdrawn and a smaller-diameter sampler is used to sample the soil at thebottom of the hole left by the first sampler. Thissequence is repeated using progressivelysmaller-diameter samplers to obtain a profile ofdisturbed samples down to a maximum depth of8 m.

BoreholesIn the UK, 150 mm or 200 mm diameterboreholes are normally made using lightpercussion (often incorrectly termed ‘shell andauger’) equipment (Fig 4). This is relativelylight, and is normally towed to the boreholepositions behind a light 4-wheel drive vehicle,and operated by a two-man crew.

In clays, the borehole is progressed bydropping a weighted hollow tube (the‘claycutter’) into the hole, so that soil becomeslodged in its base. The claycutter and its contentsare then lifted carefully to the surface (Fig 4). Ingranular soil, a hollow tube (the ‘shell’ or‘bailer’) with a flap valve or ‘clack’ at its base, issurged in the bottom of the water-filled hole,creating a soil/water mixture. Soil drops out ofthis mixture and is collected. Typically, eachdrilling rig carries out 7–15 m of drilling pershift.

Material taken from the drilling tools isusually retained as small ‘disturbed’ samples.These are far less suitable for soil descriptionthan tube samples, since the drilling process willhave:● Changed the strength of the soil, by

remoulding it● Removed fine particles from granular soils● Increased the moisture content of the soil, due

to the presence of water in the hole.

In cohesive soils, tube samples — known as‘undisturbed samples’ — are normally taken atintervals of 1.0–1.5 m by driving a 100 mmdiameter tube into the soil at the bottom of theborehole (Fig 5). These samples are normallytaken to a laboratory for engineering soildescriptions and laboratory testing to be carriedout. In-situ tests, such as the StandardPenetration Test (SPT) or the vane test, can becarried out in the borehole as drilling proceeds.

In granular soils, large disturbed (‘bulk’)samples are taken from the soil, typically atintervals of 1.0–1.5 m.

The foreman driller prepares a daily record ofthe ground and groundwater conditionsencountered during drilling. This record is givento a geotechnical engineer or engineeringgeologist who then carries out an engineeringsoil description on all the samples, and compilesan engineering borehole record by bringingtogether the driller’s daily records, thedescriptions of soil samples, and the results of in-situ tests. This borehole record will containdetailed engineering information on the types ofsoil beneath the site and the depths at which thesoils and the groundwater are found. Furtherinformation may then be obtained by carryingout laboratory tests on disturbed or tube samples.

BackfillingOnce boreholes and trial pits have beencompleted, it is important that they are properlybackfilled. If the soil excavated from trial pits issimply tipped loosely back into the hole fromwhich it has been excavated, it will be in a muchweaker and more compressible state than beforeexcavation took place. Foundations placed overold pits may suffer excessive differentialsettlement, and the sides of temporaryexcavations may collapse. Poorly backfilledboreholes are less of a hazard to spread or stripfoundations, but may affect small-diameter piles,and may allow unwanted ingress of surface wateror mixing of ground and surface water.

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Pros and cons of light percussion drilling

Advantages● Light percussion drilling can be used to make deeper

holes in a wide range of ground conditions. This is the normal method of investigation for civil engineering projects and for medium-rise construction

● This technique is also frequently used for the investigation of low-rise building sites when greater than normal depths of investigation are required (perhaps due to the presence of pre-existing slope instability, or fill, or in cases where deep clay desiccation is suspected).

Disadvantages● Light percussion drilling is considerably more

expensive than shallow trial pitting and shallow augering.


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Figure 4 Light percussion drilling rig and tools

Figure 5 Tube sampling equipment

Courtesy of Wykeham Farrance Engineering

Sample tubes and accessories


Soil sampling

Soil samples are classified as undisturbed ordisturbed. When sampling is carried out badly, itis also possible to obtain unrepresentativesamples, eg samples which do not containrepresentative fractions of all the particle sizesthat are found in the ground. Obviously, suchsamples should not knowingly be taken.

When water is encountered, water samples areoften taken for laboratory chemical analyses.

Undisturbed sampling is normally confined tocohesive soils (like clays), and is carried out bypushing or hammering tubes into the ground.These samples are required for strength andcompressibility testing in the laboratory.Disturbed samples are easier and cheaper toobtain, and are used in all soil types. However,the testing that can be carried out on them will bemore restricted.

All sample types must be sealed as soon aspractical after they are taken from the ground, inorder to minimise the loss of moisture duringstorage and before testing.

Methods for soil sampling in trial pits● Obtain disturbed material from the face with

the excavator bucket during excavation● Hammer 38 mm diameter tubes into the sides

or base of the excavation, using a hand-heldhammer with a ‘jarring link’ (Fig 5)

● Push standard 100 mm diameter tubes(U100’s) into the ground with the excavatorbucket

● Hand-cut ‘block samples’(when time andmoney permit).

Methods for soil sampling when carrying outlight percussion boring● Obtain disturbed material from the drilling

tools as drilling proceeds. Small disturbedsamples (sometimes termed ‘jars’, weighingabout 2 kg) are normally taken from fine-grained soils, such as clays, silts and peats.Large disturbed samples (termed ‘bulk bags’,weighing 25–50kg) are required from coarsegranular soils, such as gravels. Representativesamples cannot normally be obtained fromsoils containing cobbles or boulders.

● Hammer — or, in special circumstances, jack— standard 100mm diameter tubes (U100’s)into the base of a borehole.

Basic laboratory testing

Laboratory tests are carried out in order to:● Examine the natural variability of the soil

(index tests)● Classify the soils that have been sampled into

groups with broadly similar engineeringbehaviour (classification tests)

● Determine parameters and values forengineering design and calculation(parameter tests).

The tests most commonly used during low-risebuilding site investigations are shown in Table 4. Moisture content, Atterberg limit,particle size distribution, pH and sulfate tests canbe carried out on either tube (undisturbed) ordisturbed samples, but good-quality tube orblock samples are required for filter papersuction tests and triaxial and oedometer tests.

In-situ testing

Many different types of in-situ test areavailable(1), but only a few are used in the routineinvestigation of low-rise building sites.

Common types of test● Probing, using either lightweight dynamic

penetrometers or the Cone Penetration Test(CPT) is generally carried out from thesurface, without the need for boreholes

● The Standard Penetration Test (SPT), in lightpercussion boreholes

● The field vane test, carried out from thesurface, in trial pits or light percussionboreholes.

Because it is carried out from ground surface,without the need for a borehole, probing is veryfast and economical. It can provide goodinformation on the variability of soil conditionsacross a site, but does not allow visualexamination and description of the soil.

The SPT is routinely used in conjunction withlight percussion boring, to obtain information onthe density of granular soils. It can also be used toobtain data on the undrained shear strength ofheavily-overconsolidated cohesive soils.

The vane test is used solely to obtain data onthe undrained shear strength of relatively weakand compressible clays. It cannot be used in stiff,very stiff or hard clays, because the equipment islikely to be damaged.

The advantages and disadvantages of thesedifferent test methods are summarised in Table 5 opposite.

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Table 4 Laboratory tests commonly used during site investigations for low-rise building

Class of test Parameter (and example of use) Type of test Type of sample

i Moisture content Moisture content u, d(soil variability)

i, c Atterberg limits Liquid limit, plastic limit u, d(identification of shrinkable clays)

i, c Particle size distribution Particle size distribution u,d(identification of soil type)

i, p Soil suction Filter paper suction test u(identification of desiccated clays)

p Undrained shear strength Undrained triaxial test u(bearing capacity of foundations)

c, p Coefficient of compressibility Oedometer consolidation test u(settlement of foundations)

c, p pH of soil or groundwater pH test u, d, w(specification of foundation concrete)

c, p Sulfate content of soil or groundwater Sulfate tests u, d, w(specification of foundation concrete)

i = Index u = Undisturbedc = Classification d = Disturbedp = Parameter w = Water

Table 5 In-situ tests commonly used during site investigations for low-rise buildingsTest method Applications Advantages Disadvantages

Dynamic probing ●Soil profiling ●Very fast, ●Unable to determine soil type● Identifying soft spots ● Inexpensive ●Cannot penetrate coarse soils●Assessing ground variability ●Able to cover large areas (cobbles) or hard layers

in detailCPT ●Soil profiling ●Very fast, ●Cannot penetrate dense or

● Identifying soft spots or cavities ● Relatively inexpensive coarse granular soils,●Assessing ground variability ●Able to cover large areas hard layers or rocks●Assessing soil type, undrained in detail

strength and compressibilitySPT ●Density and effective angle of ●Simple, rugged equipment ●Test affected by boring

friction in sands and gravels which works in all ground disturbance●Assessment of undrained

shear strength in clayField vane ●Determination of undrained ●Allows in-situ strength ●Useful only in soft–firm clays

shear strength of clays determination


Probing is carried out by dynamic or quasi-static methods. Both may be useful duringthe direct investigation of low-rise building sites.For most types of probing it is not necessary tomake boreholes, and the rate at which ground canbe investigated is therefore extremely rapid.However, soil samples are not generallyavailable, so visual soil description is notpossible.

Quasi-static cone penetration testingThe quasi-static cone penetration test (CPT) (2) issometimes considered to be probing, andsometimes to be in-situ testing.

This technique involves hydraulically pushinga 10 or 15 cm2 cone into the ground, at a standardrate of penetration (2 cm/sec), and measuring itspenetration resistance (Fig 6). Modern electriccones are also equipped with a ‘friction sleeve’,which measures the shear stress applied by thesoil as the sleeve passes through it. The ratiobetween cone resistance (expressed in units ofstress) and side shear is termed ‘friction ratio’,and is used to estimate the type of soil throughwhich the cone is being driven – see BS 1377 andBS 5930.

The special electronic and hydraulicequipment necessary for the test is housed in alarge truck so access may be restricted ondifficult sites. Light-weight tracked or tractor-mounted equipment is also available for use insoft or difficult ground conditions. There are alsolimitations on the hydraulic thrust available topush the cone into the ground, so that penetrationof dense coarse soils may be impossible.However, progress is normally extremely rapidand a great deal of information can be obtained,not only concerning the soil profile, but also itstype and its strength or density.

Figure 7 shows the results from an electricCPT. These are best interpreted by specialists,experienced in this type of work. Theinformation which can be obtained includes:● The soil profile● Estimates of the soil types encountered,

including consistency (in cohesive soils) anddensity (in granular soils)

● Preliminary estimates of the properties of thesoil, including the undrained shear strengths ofclays, the effective angle of friction ofgranular soils and their compressibilities

● Evidence of the presence of voids beneath thesite.

Additional sensors can also be incorporated intothe CPT to assess other ground properties,including contamination, but generally atconsiderable extra cost.

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Figure 7 Results from a typical cone penetration test

Figure 6 Electric cone penetration test equipment

The equipment is housed in a large truck


Dynamic probingDynamic probing is much less sophisticated thanCPT. It involves driving a steel rod (often with aspecially shaped and hardened tip) into theground, by using repeated blows of a hammer ofa specified mass falling through a fixed distance.The number of blows required for each incrementof penetration (eg 100 mm) is recorded, andplotted as a depth v blow-count log (Fig 8).

Different types of dynamic penetrometer areused around the world. Several types are incommon use in the UK. Table 6 gives details ofcommon dynamic penetrometers, showing thevariations in the energy that they deliver. Ingeneral, higher energies per blow are required toachieve greater depths, to penetrate stronger anddenser soils, and to penetrate coarser granularsoils. The apparatus is very light, and can be taken tosite in a conventional van and quicklymanhandled into position by one or two people.In terms of metres probed per day, rates of outputare high, so that the cost per metre for probing ismuch lower than that for boring or trial pitting.The information given by probing is, however,very restricted, and is difficult to interpret in theabsence of results obtained with more definitivetechniques, such as boring or trial pitting.

The most useful application of probing is indetermining the variability of a site, by makingprobe holes on grid lines at regular intervalsacross a site. Areas where unusually low or highpenetration resistances are encountered can thenbe the subject of further investigation by trialpitting or boreholes. Another application of thistechnique is to determine the thickness of soft orloose material overlying more competentground, perhaps where this is the proposed levelfor foundations. Probing will rapidly determinethe depth of a thin layer of poor ground across awide area, with little disruption and at low cost.

Lightweight dynamic probes cannot penetratehard layers or very coarse soils, such as cobblesand boulders. In such circumstances their usewill be restricted, since the aim of anyinvestigation should be to investigate the depthof all soft, loose and unconsolidated materials,even when they are overlaid by stronger soils.

The Standard Penetration TestThe Standard Penetration Test (3) involvesdriving a 52 mm outside diameter ‘split spoon’open-drive sampler (Fig 9) into the bottom of aborehole, with repeated blows of a hammer of62.5 kg mass falling 760 mm. In the UK anautomatic trip hammer ensures a correct heightof fall for the hammer weight.

The number of blows necessary to drive thesplit spoon six increments of 75 mm are countedand recorded. The blows for the first twoincrements of 75 mm are discounted, becausethey are considered to be affected by boringdisturbance. The final four blow counts, for thelast 300 mm of the test, are added together to givethe penetration resistance, N (reported asblows/300 mm). In granular soil the N value iscorrected for overburden pressure.

Field vane testThe field vane test uses a cruciform rectangular4-bladed vane, which is pushed on thin rodsahead of the borehole in order to reach claywhich is relatively unaffected by drillingdisturbance. Equipment at the ground surface(Fig 10) allows the vane to be rotated via the rodswhilst the torque being applied to it is measured.The maximum torque is recorded, and can beinterpreted in terms of the undrained shearstrength of the clay (ref (1) and BS 1377).

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Figure 8 Dynamic probing apparatus and results

A detailed account of theSPT test and its use isavailable from theConstruction IndustryResearch and InformationAssociation (CIRIA) (3).


References and further reading

[1] Clayton, CRI, Matthews, MC, and Simons, NE.Site investigation. Oxford, Blackwell Science Ltd, 1995.[2] Meigh, AC. Cone penetration testing. ConstructionIndustry Research and Information Association GroundEngineering Report. London,CIRIA/Butterworths.[3] Clayton, CR . The Standard Penetration Test (SPT):methods and use. Construction Industry Research andInformation Association Report CP/7. London, CIRIA, 1993.[4] ISSMFE. Report of technical committee onpenetration testing of soils. In Penetration testing. London,Institution of Civil Engineers, 1989.Health and Safety ExecutiveEntry into confined spaces. Guidance note GS5. London,HMSO. Protection of workers and the general public during thedevelopment of contaminated land. London, HMSO, 1991.British Standards InstitutionBS 1377:1990 Methods of test for soils for civilengineering purposesBS 5930:1981 Code of practice for site investigationsOther BRE Digests318 Site investigation for low-rise building: desk studies322 Site investigation for low-rise building:procurement348 Site investigation for low-rise building:the walk-over

survey363 Sulfate and acid resistance of concrete in the ground381 Site investigation for low-rise building: trial pits383 Site investigation for low-rise building: soil

descriptionAcknowledgementThe Building Research Establishment gratefullyacknowledges the assistance of Professor CRI Clayton ofthe University of Surrey in the preparation of this Digest.

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Figure 9 Standard penetration test equipment

Figure 10 Field vane test equipment

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Table 6 Details of common dynamic penetrometersTest specification

Factor DPL DPM15 DPM DPH DPSH

Hammer mass (kg) 10±0.1 30±0.3 30±0.3 50±0.5 63.5±0.5Height of fall (m) 0.5±0.01 0.5±0.01 0.5±0.01 0.5±0.01 0.75±0.02Mass of anvil + guide rod (max), (kg) 6 18 18 18 30Rod length (m) 1±0.001 1±0.001 1±0.001 1±0.001 1±0.001Mass of rod (max), (kg) 3 6 6 6 8Rod eccentricity (max), (mm) 0.2 0.2 0.2 0.2 0.2Rod OD (mm) 22±0.2 32±0.2 32±0.2 32±0.2 32±0.3Cone apex angle (degrees) 90 90 90 90 90Cone area (nominal), (cm2) 10 15 10 15 20Cone diameter (mm) 35.7±0.3 43.7±0.3 35.7±0.3 43.7±0.3 50.5±0.5Mantle length (mm) 35.7±1 43.7±1 35.7±1 43.7±1 50.5±2Number of blows:per x cm penetration N10:10 N10:10 N10:10 N10:10 N20:20Standard range of blows 3–50 3–50 3–50 3–50 5–100Specific work/blow (kJ/m2) 50 98 150 167 238ReferencesBS 1377:1990 — — — ✔ ✔

International Reference Test Procedure (4) ✔ — ✔ ✔ ✔


Bre411 Si for Low Rise Buildings Direct Investigations

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