This is an attempt to create a stand alone self learning module on site investigation. Fasten your seat belts. Sit back, relax and enjoy.
Site InvestigationNarrated by:
N. Sivakugan & Kate Johnson
Relatore
Note di presentazione
Site investigation plays an important role in the early days of most civil engineering projects. The idea is to obtain adequate information about the soil conditions at the site, at minimal cost. We do in situ (or field) tests, and take samples for laboratory tests.
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Some unsung heroes of Civil Engineering…
… buried right under your feet.
foundations soil exploration
tunneling
Relatore
Note di presentazione
All structures need proper foundations. To design good foundations, it is essential to know the geotechnical characteristics of the underlying ground. The only way to know them is to test the soil.
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A good site investigation is a prerequisite.
Relatore
Note di presentazione
A good site investigation is a prerequisite for most infrastructure developments, railways, bridges, buildings and dams.
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?
Need good knowledge of the soil conditions
proposed structure
Problem Soilse.g., reactive clays, soft soils, sink holes, etc.
Relatore
Note di presentazione
Building on a site without knowing the soil conditions can be very dangerous. There could be unexpected problems in the future due to ground movements caused by pockets of weak soils, reactive clays, and sink holes formed in karst terrain. By the way, sink holes are solution cavities formed in limestones.
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Soil data required:
Soil profile
Index properties
Strength & compressibility characteristics
Others (e.g., water table depth)
- layer thickness and soil identification
- water content, Atterberg limits, etc.
- c’, cu, φ’, Cc, Cr, OCR, …
Relatore
Note di presentazione
One of the major objectives of the site investigation exercise is to define the soil profile. Here we determine the different soil layers and their thicknesses. Index properties such as water contents and Atterberg limits are reasonably easy to obtain and are quite useful in classifying the soil. They can also be used in empirical correlations to estimate other soil properties. We often need strength and compressibility characteristics such as cohesion, friction angle, overconsolidation ratio and so on.
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Desk Study
Aerial photographsTopographical mapsExisting site investigation reports (for nearby
sites)Other info. from local councils, literature
First stage of site investigation. Negligible cost. Look for any freebies (i.e., info available currently)
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Site Reconnaissance
Site accessTopographySite geologyConditions of adjacent structuresAny obvious problems foreseen?
A site visit and chat with locals.
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CLAY Bore hole75 mm dia10-30 m depth
Trial Pit1-2 m width2-4 m depth
back hoe drill rig
Relatore
Note di presentazione
We can drill bore holes using drill rigs or dig trial pits using backhoes. Trial pits are cheap and are of good value, but they are limited in depth. They give the opportunity to have a good look at the soil and stratifications within the top couple of meters. When we want to go deeper, bore holes are quite effective. We can do in situ tests within these bore holes and collect undisturbed samples from clay layers.
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Trial PitEnables visual inspection, locating strata boundaries, and access for undisturbed block samples.
A Very Large Trial Pit
Relatore
Note di presentazione
This is a fairly large trial pit for a job in Hong Kong. It needs dewatering.
In clay layers, generally we collect undisturbed samples from various locations within bore holes. These are later used for consolidation and triaxial tests in the laboratory. We use thin walled samplers and piston samplers to obtain good quality undisturbed samples.
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Undisturbed Clay Samples
Required for triaxial, consolidation tests in the lab.
sampling tube
soil (%) 100..
....2
22
×−
=DI
DIDOAR
area ratio
AR<10%
Good quality samples necessary.
Thicker the wall, greater the disturbance.
Take good care in transport and handling.
Relatore
Note di presentazione
Area ratio AR is used to quantify the sample disturbance, which understandably increases with wall thickness. For good quality samples it should be less than 10%. Wrapping in plastic bags, waxing the ends of the sampling tubes are some simple techniques adapted to preserve the moisture content.
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In Granular Soils …
Very difficult to get undisturbed samples.
∴ Go for in situ tests.
e.g., penetration tests
80-90% of foundation designs are based on penetration tests
Relatore
Note di presentazione
It is very difficult to get undisturbed samples in granular soils. Therefore, we go for in situ tests to determine the soil characteristics.
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Penetration Tests
Measuring the soil resistance to penetration by a probe.
φ’, E …
drill rod
GL
Relatore
Note di presentazione
In penetration tests, we drive a tool attached to the drill rod into the soil and measure the soil resistance. This resistance is empirically correlated to geotechnical parameters such as friction angle, soil stiffness etc. Standard penetration test, known as SPT, and cone penetration test, known as CPT, are the two common in situ penetration tests. More than 80% of the foundations designs are carried out based on SPT or CPT results.
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How many bore holes?
120 m
Proposed site for a multi-storey shopping complex
Not enough bore holes; soil profile and properties not well defined..
bore hole
Relatore
Note di presentazione
At this site, the four bore holes at the corners are not enough to adequately define the soil conditions at the site. Probably, the geotechnical engineer was too stingy.
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How many bore holes?
120 m
Too many bore holes and blows the budget.
Relatore
Note di presentazione
Here, the soil is well defined, but too many holes make the site investigation a very expensive exercise. The client will not be happy.
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How many bore holes?
120 m
About right?
trial pit
Relatore
Note di presentazione
Here, we strike a good balance. Trial pits are quite cheap, and they certianly have a place in site investigation.
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How many bore holes?
The number of bore holes depends on:
type and size of the project
soil variability
budget for site investigation
Typically spaced at 20-40 m for non-residential buildings.Locate the bore holes where the loads are expected.
proposed building
Relatore
Note di presentazione
The $60,000 question is how many bore holes do we need for a particular site. There is no easy or definite answer. It depends on many factors such as type of the job, available funds, soil variability and so on. Every additional bore hole can cost considerable money. On the other hand, not having enough bore holes means we don’t have adequate information about the underlying soil. Always try to stretch your dollars, and get good value for the money spent on site investigation.
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How deep to explore?
Explore the soil to a depth where the stress changes become insignificant
Relatore
Note di presentazione
In bore holes, it will cost $40 - $50 for every additional meter. Therefore, we should only explore the soil to the minimum depth necessary.
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Standard Penetration Test (SPT)65 kg hammer
Count the number of blows required for 300 mm penetration
Blow count or
N-Value
760 mm dropanvil
split spoon sampler
drill rod
Relatore
Note di presentazione
Standard penetration test, fondly known as SPT, was developed in North America in 1927 and is still the most common in situ test. Here, we drive a 51 mm diameter split-spoon sampler into the ground using a 65 kg hammer dropped by 760 mm. The number of blows required to drive the sampler by 300 mm is known as the blow count or N-value.
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Standard Penetration Teststill has some
value
AR = 112%; use for classification
mainly for granular soils; unreliable in clays
samples (disturbed) collected in split-spoon sampler
done within bore holes at 1.5 m depth intervals
N-value correlated to φ’, E …
soilI.D. = 35 mm O.D.= 51 mm
Relatore
Note di presentazione
SPT works well in granular soils. In clays, the results are unreliable due to pore pressures developed during driving. The N-values in granular soils are correlated to friction angle, Young’s modulus and relative density. The blow counts are determined at 1.5 m depth intervals. The samples collected in the split spoon can be used for classifying the soil. They are too disturbed for any meaningful test requiring undisturbed samples.
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Standard Penetration Test
Relatore
Note di presentazione
On left is a schematic diagram of the SPT setup.
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SPT Hammer
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SPT Corrections
(N1)60 = CER CN N
Overburden correctionEnergy
correction
Correctedblow count Measured
blow count
Relatore
Note di presentazione
The blow count measured in the field is a crude measure of soil strength and stiffness. There are few variables that can influence this value. Therefore, we apply some corrections to make the N-value more meaningful. The two most important corrections are for overburden and energy rating of the hammer. There are other corrections for bore hole diameter, drill rod length and so on.
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SPT Correlations in Clays
N60 cu (kPa) consistency visual identification
0-2 0 - 12 very soft Thumb can penetrate > 25 mm
2-4 12-25 soft Thumb can penetrate 25 mm
4-8 25-50 medium Thumb penetrates with moderate effort
8-15 50-100 stiff Thumb will indent 8 mm
15-30 100-200 very stiff Can indent with thumb nail; not thumb
>30 >200 hard Cannot indent even with thumb nail
Use with caution; unreliable.
not corrected for overburden
Relatore
Note di presentazione
Although SPT is not recommended for clays, there is still some value. This table relates N60 values in clays to the undrained shear strength and consistency.
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SPT Correlations in Granular Soils(N)60 Dr (%) consistency
0-4 0-15 very loose
4-10 15-35 loose
10-30 35-65 medium
30-50 65-85 dense
>50 85-100 very dense
not corrected for overburden
Relatore
Note di presentazione
These are for granular soils, and can be used as a rough guide.
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Cone Penetration Test (CPT)
Dynamic cone penetration test(DCPT)
Static cone penetration test(SCPT)
closed end; no samples
gives continuous measurements
pushed into the ground @ 2 cm/s
using cone instead of split spoon
similar to SPT; hammer driven
gives blow counts @ 1.5 m depth intervals
Relatore
Note di presentazione
Cone penetration test, known as CPT, originated from Netherlands in 1930s. A cone attached to the drill rods is forced into the ground and the resistance is measured. Dynamic cone penetration test is similar to SPT, and we measure the blow counts. Unilke the split spoon sampler in SPT, we cannot get samples from CPT. We have a DCPT rig at JCU, which we use for consulting jobs. In static cone penetration test, the cone is pushed into the ground at the rate of 2 cm/s, and measurements are taken continuously.
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Dynamic Cone Penetration Test
Better than SPT or SCPT in hard soils such as dense gravels
Siva
As crude as SPT; relies on correlations based on blow counts
Simple and rugged.
SPT
DCPT
Hollow (split spoon)
Solid (∴no samples)
Relatore
Note di presentazione
Dynamic cone penetration is as simple and as crude as SPT. In dense gravels, which can damage the split spoon sampler, DCPT is a good alternative.
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Static Cone Penetration Test
10 cm2 cross section
cone resistance (qc)or tip resistance (qT)
sleeve friction (fs)
friction ratio, fR =fsqc
× 100 %
Typically 0 ———— 10%.
granular cohesive
Relatore
Note di presentazione
The 600 cone used in static cone penetration test has 10 cm2 cross section. When pushed into the ground, we measure the cone resistance and sleeve friction continuously and plot with depth. The friction ratio, which is the ratio of sleeve friction to cone resistance, expressed as a percentage, can vary between 0 and 10 percent.
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Piezocone (CPTU)A modern static cone; measures pore water pressure also.
Piezocone with leadsPorous stone for pore pressure measurementPushed into the ground
Relatore
Note di presentazione
Piezocone is simply static cone, which can also measure pore water pressure.
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Pushed in @ 20 mm/s rate
Continuous measurements of qc, fs and u.
Relatore
Note di presentazione
The cone is pushed into the ground at a rate of 20 mm/s, and the cone resistance, sleeve friction and the pore water pressure are measured continuously.
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Interpreting SCPT Data
See next slide
Relatore
Note di presentazione
From the cone resistance, friction ratio and pore water water pressure, you can easily identify the soil using the charts in the next slide.
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Interpreting SCPT (Piezocone) Data
Soil Behavior Type (Robertson et al., 1986; Robertson & Campanella, 1988)1 – Sensitive fine grained 5 – Clayey silt to silty clay 9 – sand2 – Organic material 6 – Sandy silt to silty sand 10 – Gravelly sand to sand3 – Clay 7 – Silty sand to sandy silt 11 – Very stiff fine grained*4 – Silty clay to clay 8 – Sand to silty sand 12 – Sand to clayey sand*
*Note: Overconsolidated or cemented
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Sounding - Shelby County, TN (U.S.A)
Courtesy: Professor. P.W. Mayne, Georgia Inst. of Technology
Relatore
Note di presentazione
This is just an example of piezocone data plotted with depth. On the right, you can see the soil profile, identified using these plots, and the charts you saw in the previous slide.
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SCPT Correlations
k
vocu N
qc σ−=
cone factor (15-20); varies with cone
In Clays,
In Sands,
E = 2.5-3.5 qc (for young normally consolidated sands)
Relatore
Note di presentazione
Undrained shear strength in clays, and Young’s modulus in sands can be obtained from the cone resistance, using these empirical correlations. Nk, known as cone factor, varies from cone to cone. Check the manufacturers literature or you may have to calibrate it.
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qc/N Relation in Granular Soils
qc in kg/cm2 (1 kg/cm2 = 98.07 kPa)
Relatore
Note di presentazione
Sometimes we either have CPT data or SPT data available and it is useful to be able to go from one to the other. In other words, what is the cone resistance corresponding to N value of 28? It depends on the average grain size. You can use this graph as a guide to estimate qc/N60 ratio, for a specific grain size.
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φ’ from SPT/CPT in Granular Soils
After Peck et al. (1974) After Meyerhof (1976)
Relatore
Note di presentazione
From the penetration resistance you obtain from SPT or CPT, you can estimate the friction angle of the granular soil using these charts.
Pressuremeter TestExpand a cylindrical probe inside a bore hole.
Gives strength, modulus, K0, cv…Most rational of all in situ tests
For all soils
cylindrical probe
Siva
pressuremeter
guard cell
Relatore
Note di presentazione
Pressuremeter was developed in France in late 1950s. It is one of the best in situ testing devices around today. It is a cylindrical probe with guard cells at both ends. The probe is lowered into the bore hole, to the desired depth, and is expanded using air. From the resistance to expansion, most geotechnical parameters can be obtained. It works well in sands and clays. It is quite popular in Europe.
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Dilatometer TestAdvance @ 20 mm/s. Test every 200-300 mm.
Nitrogen tank for inflating the membrane.
Can identify soil (from a chart).
Gives cu, K0, OCR, cv, k, soil stiffness .
60 mm dia. flexible steel membrane
Similar to the cone
Relatore
Note di presentazione
Dilatometer was developed in Italy by Professor Marchetti. It consists of a steel blade having a thin, circular expandable membrane mounted on one face. It is pushed into the ground and the membrane is expanded using air. From the resistance offered by the soil to expansion, several soil parameters can be determined.
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Cone PressuremeterCombines piezocone and pressuremeter.Uncommon; specialised.
pressuremeter
piezocone
Relatore
Note di presentazione
This is from Fugro, an international geotechnical firm. They have developed this cone pressuremeter, exploiting the advantages of both piezocone and pressuremeter.
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Vane Shear Test
bore hole
soft clay
measuring (torque) head
vane
vane
For clays, and mainly for soft clays.
Measure torque required to quickly shear the vane pushed into soft clay.
∴ undrained
torque undrained shear strength cuh≈2d
d
Typical d = 20-100 mm.
Relatore
Note di presentazione
Vane shear test is very useful in determining undrained shear strength of clays, mainly soft clays. The vane consists of four rectangular blades, mounted perpendicularly to a shaft. This is pushed through the bottom of a bore hole for testing. A torque is applied to the vane, shearing the clay along a cylindrical surface, and thus freely rotating the vane. From the torque applied, the undrained shear strength can be determined. A scaled down version is used in the laboratory to determine the undrained shear strength of soil samples.
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Vane Shear Test
Test in Progress Failure surface
Plate Loading TestLoad a square plate (300 mm x 300 mm) to failure.
Plot pressure vs. settlement. Extrapolate to prototype.
Loading arrangement makes it expensive.
Good on random fills; indicateses an average behaviour.
Doing it in Sri Lankan style.
platepressure
settle
ment
Relatore
Note di presentazione
In a plate load test, we incrementally load a square plate of say about 300 mm x 300 mm, and plot the settlement against the applied pressure. This gives an overall picture on what happens when we load a larger footing of say 5 m x 5 m.
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Common In Situ Testing Devices
In bore holes
DMTVST
SPT
CPTPMT
Relatore
Note di presentazione
In summary the common in-situ testing devices are: Standard Penetration Test (SPT) <5sec> Vane shear test (VST) <10sec> Pressuremeter Test (PMT) <15sec> Cone Penetration Test (CPT) <22sec> Dilatometer Test (DMT) <27sec>
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Pocket penetrometer
Push into the clay, and..
..read the strength
A simple hand-held device for measuring unconfined compressive strength (qu = 2 cu) of a clay.
Must for every practicing geotechnical engineer.
Used in trial pits and samples. very rough
Relatore
Note di presentazione
Pocket penetrometer to a geotechnical engineer is like a stethoscope to a doctor. It is an easily affordable and handy device all geotechnical engineers should have. It gives undrained shear strength of clays. Just push it into the clay and read off the scale. Being so cheap, it has a price. It is unreliable.
Relative Test Accuracy
Rela
tive
Cost
per
Tes
t
Cost versus Accuracy
Standard penetration test
Dynamic cone penetration testStatic cone penetration test
Pressuremeter test
Pocket penetrometer test
Relatore
Note di presentazione
Good things no cheap. Cheap things no good. The most reliable and accurate tests often come at a cost. The pocket penetrometer and heel print tests cost nothing, but their use also is limited. May be, SPT strikes a good balance!!