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SITE INVESTIGATIONIN-SITUTESTINGFIELDINSTRUMENTATION

SMILY JOAN I. PAMISA


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INTRODUCTION

The physical survey is that part of site investigation which aims todetermine the physical properties of the ground. These arerequired: to classify the soil into groups of materials which willexhibit broadly similar engineering behaviour; and to determineparameters which are required for engineering designcalculations.

Some soils, for example clays, may readily be sampled. If good-quality samples can be obtained, then laboratory testing offersthe best method of determining soil and rock parameters undercarefully controlled conditions.

Other types of ground are either difficult or impossible to sampleand test successfully. In such cases, in situ tests should be used.information may be obtained in situ in at least three ways:

by using geophysical techniques;

by using in situ soil testing techniques

by making measurements using field instrumentation,


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IN-SITUTESTING


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IN-SITU TESTING

The following types of ground conditions areexamples of those where in situ testing is eitheressential or desirable.

very soft or sensitive clays.

stoney soils

sands and gravels

weak, fissile or fractured rock


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COMMONIN-SITUTEST

In situ tests may be classified in a number of ways,including by cost, ease of use, method ofinterpretation, soil types in which they may be used,parameters which can be determined.

COMMON IN-SITU TEST

penetration resistance;

strength and/or compressibility, or

in situ permeability


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PENETRATION TESTING

Many forms of in situ penetration test are in useworldwide. Penetrometers can be divided into twobroad groups. The simplest are dynamicpenetrometers. They consist of tubes or solid points

driven by repeated blows of a drop weight. Staticpenetrometers are more complex, being pushedhydraulically into the soil. The two most commonpenetration tests, which are used virtually

worldwide, are the dynamic SPT, and the staticCPT.


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CONE PENETRATION TEST

the cone penetration test (CPT)is an gouda cpt testing methodused to determinethe geotechnicalengineering propertiesof soils and delineating

soilstratigraphy. It was initiallydeveloped in the 1950s at theDutch Laboratory for SoilMechanics in Delft toinvestigate soft soils. Based onthis history it has also beencalled the "Dutch cone test".Today, the CPT is one of themost used and accepted in soilmethods for soilinvestigation worldwide.
http://en.wikipedia.org/w/index.php?title=Gouda_cpt&action=edit&redlink=1http://en.wikipedia.org/wiki/Geotechnical_engineeringhttp://en.wikipedia.org/wiki/Geotechnical_engineeringhttp://en.wikipedia.org/wiki/Soil_mechanicshttp://en.wikipedia.org/wiki/Stratigraphyhttp://en.wikipedia.org/wiki/Delfthttp://en.wikipedia.org/wiki/Geotechnical_investigationhttp://en.wikipedia.org/wiki/Geotechnical_investigationhttp://en.wikipedia.org/wiki/Geotechnical_investigationhttp://en.wikipedia.org/wiki/Geotechnical_investigationhttp://en.wikipedia.org/wiki/Delfthttp://en.wikipedia.org/wiki/Stratigraphyhttp://en.wikipedia.org/wiki/Soil_mechanicshttp://en.wikipedia.org/wiki/Geotechnical_engineeringhttp://en.wikipedia.org/wiki/Geotechnical_engineeringhttp://en.wikipedia.org/w/index.php?title=Gouda_cpt&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Gouda_cpt&action=edit&redlink=1


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STANDARD PENETRATION TEST

The standard penetrationtest (SPT) is an in-situ dynamicpenetration test designed to

provide information onthe geotechnicalengineering properties of soil.

Standard Penetration Test, SPT,involves driving a standardthick-walled sample tube into

the ground at the bottom of aborehole by blows from a slidehammer with standard weightand falling distance.
http://en.wikipedia.org/wiki/Geotechnical_engineeringhttp://en.wikipedia.org/wiki/Geotechnical_engineeringhttp://en.wikipedia.org/wiki/Soil_mechanicshttp://en.wikipedia.org/wiki/Soil_mechanicshttp://en.wikipedia.org/wiki/Geotechnical_engineeringhttp://en.wikipedia.org/wiki/Geotechnical_engineering


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STRENGTHAND COMPRESSIBILITY TESTING

Because strength and compressibility parameters are generallyrequired for engineering calculations, many forms of test have beendeveloped with the specific purpose of determining them in particularsoil or rock types. These tests are not as widely used as thepenetration tests described in the previous section, but nonethelessmany are in common usage.

MOST POPULAR TEST iN USE :

Field Vane Test

Pressuremeter Test

Plate Loading Test

Marchetti Dilatometer


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FIELD VANE TEST

The field vane test. Thisis used exclusively tomeasure the undrainedshear strength of soft orfirm clays


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PRESSUREMETER TEST

The pressuremeter test. This isused routinely in France todetermine strength and

compressibility parameters forroutine design, for all types ofsoil and weak rock, but (in its

self-boring form) used in the UKfor special projects in overconsolidated clays, todetermine undrained strength,shear modulus, and coefficientof earth pressure at rest, K0.


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PLATE LOADING TEST

The plate loading test.This is used primarily toobtain the stiffness ofgranular soils and

fractured weak rocks.


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MARCHETTI DILATOMETER

The Marchetti

dilatometer. This is notyet used commercially inthe UK but, at the time of

writing, is becomingmore widely used in

other parts of the world


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PERMEABILITY TESTING

The permeability of a soil can only rarely be obtained withsufficient accuracy from laboratory tests on specimens fromnormal diameter boreholes, and therefore the in situpermeability test is common.

In situ permeability tests can be carried out in soils or rocks, inopen boreholes, in piezometers, or in sections of drillholesealed by inflatable packers.

Three common types of test :

rising and falling head tests; constant head tests; and

packer or Lugeon tests.


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RISINGAND FALLING HEAD TEST

The rising or falling head test is

generally used in relativelypermeable soils. It is usuallycarried out in

a cased borehole or a simplepiezometer such as theCasagrande low-air entry open-

tube type. Wherethe groundwater level existsabove the base of the borehole,the water level in the boreholeor

piezometer tube may either be

reduced or increased. Waterlevel measurements are thentaken at

suitable time intervals until thewater level returns toequilibrium.


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CONSTANT HEAD TEST

Constant head testing isrequired in all soils wherestress changes will result insignificant

consolidation or swelling. When

clay is subjected to an in situpermeability test the effectivestresses

in the soil are modified by theincrease in pore water pressurenormally applied. As the soil

swells istakes in water, and thus testrecords normally indicate ahigher permeability than, infact, exists.


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PACKEROR LUGEON TEST

The rock equivalent of the soilconstant head permeability testis the packer test, alsosometimes

known as the Lugeon test(Lugeon 1933). The test may becarried out in the base of adrillhole using a

single inflatable packer to sealoff the test section, or after thehole is complete, testing may

be carried

out at a variety of depths usinga double packer to seal the testsection top and bottom.


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FIELDINSTRUMENTATION


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The amount of instrumentation used in site investigationdepends on the type of investigation being carried out.In practice the amount of instrumentation used in routine

pre-design site investigation is very limited and normallyconsists only of pore water pressure measuring devices.In the case of investigations for deep excavations inrocks, measurements of in situ stress are also made.

In contrast to this, trial construction, the investigation of the

safety of existing works, and the investigation of failuresto allow the design of remedial works all typically involveconsiderable and quite variable instrumentation. Themain parameters which may require measurement aredisplacement, strain, stress and force; pressure in theform of pore water pressure will be the most frequent

measurement because of the relative importance of thisparameter in geotechnical design.


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USESOF INSTRUMENTATION

Site investigation carried out before design will always requirethe determination of pore water pressures. As a veryminimum, the groundwater level and its seasonal variationsshould be determined, because this information is vital inassessing the geotechnical information provided by boring

and testing, and more importantly because groundwaterconditions play a very significant part in choosing foundationtypes, their levels, and the precautions necessary during theirconstruction.

Despite this, it is rare to see a site investigation report which notonly has an adequate number of measuring points, but also

has records made over a sufficiently long period to ensure thatseasonal fluctuations, artesian pressures, underdrainage, andtidal variations are detected. The importance of goodgroundwater information to designer and contractor is hard tooveremphasize.


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REQUIREMENTSFOR INSTRUMENTATION

The primary requirement of any instrument is that it shouldbe capable of determining a required parameter, suchas water pressure, or displacement, without leading to achange in that parameter as a result of the presence of

the instrument in the soil. It is necessary that anyinstrumentation should be sufficiently duplicated andplentiful to allow for losses, and it is therefore helpful ifthose instruments which are most at risk are cheap.

Obviously, any instrumentation which is installed must be

capable of measuring relevant properties. Relevancerequires sufficient accuracy, correct positioning, and asuitable speed of instrument response to changesoccurring in the soil.


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PORE WATER PRESSURE AND GROUNDWATER LEVELMEASUREMENT

This is the most common form of in situ measurement, andfortunately only one measurement is required at anypoint to define the regime. Quite simple devices areoften used to determine water pressure in the ground,but these devices are unsuitable under many conditions.Hanna (1973) has defined the requirements of anypiezometer as:

to record accurately the pore pressures in the ground;

to cause as little interference to the natural soil aspossible;

to be able to respond quickly to changes in groundwaterconditions;

to be rugged and remain stable for long periods of time;and

to be able to read continuously or intermittently ifrequired.


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TYPESOF PIEZOMETER

Standpipes and standpipe (Casagrande) piezometers

Pneumatic piezometers

The hydraulic piezometer


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STANDPIPESANDSTANDPIPE (CASAGRANDE)PIEZOMETERS

The standpipe piezometer, sometimes

called a Casagrande piezometer, consists of afilter tip joined to a riser pipe.

The filter tip is made from polyethylene orporous stone and has 60

micron pores. The riser pipe is typically madefrom PVC plastic pipe.

This consists of an open-ended tube which isperforated near the base, and is inserted in aborehole. The

space between the tube perforations and thewall of the borehole is normally packed withsand or fine

gravel, and the top of the hole is then sealedwith well tamped puddle clay or concrete toprevent the

ingress of surface water.


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PNEUMATIC PIEZOMETERS

Pneumatic piezometers are used to measurepore water pressure in saturated soils.

Applications include:

Monitoring pore pressures to determinesafe rates of fill or excavation.

Monitoring pore water pressures to

determine slope stability.Monitoring the effects of dewateringsystems used for excavations.

Monitoring the effects of groundimprovement systems such as verticaldrains and sand drains.

Monitoring pore water pressures to checkthe performance of earth fill dams andembankments.

Monitoring pore water pressures to checkcontainment systems at land fills andtailings dams.


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THE HYDRAULIC PIEZOMETER

For the measurement and

control of waterpressures in soil androck, the HydraulicPiezometer is able tomeasure both positive

and negative pressures.

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