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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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