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7/23/2019 3-CIEEE710 Soil Sampling Lecture
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Concept for an integrated geotechnical approach
to site investigations
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Stratigraphy and Finds
Layer Soil Soil Colour Finds
L1 Sandy soil Gray 7.5YR 5/1 Modern Rubbish (filled soil)
L2 Sandy soil Pinkish white 7.5YR 8/2 Modern rubbish (filled soil)
L3 Sandy soil Reddish yellow 7.5YR 7/6 Modern rubbish (filled soil)
L4 Sandy soil Gray 7.5YR 6/1 Modern rubbish (filled soil)
L5 Loamy soil Reddish yellow 5YR 6/6 Nil (original decomposed soil)
L6 Loamy soil Reddish yellow 5YR 6/8 Nil (original decomposed soil)
L7 Loamy soil, with
somedecomposed bed
rock texture
Light red 2.5YR 6/8 Nil (original decomposed soil)
Test Pit Wall Photograph
Western Wall Section
Test Pit Wall Drawing
Western Wall Section Drawing
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Rotary Drilling
To drill holes in almost all ground conditions for
sampling and installation of instruments 75 mm to 150mm dia boreholes can be drilled (most
common depth 1 to 30 m)
Hole is advanced either by rotating bit or downhole
hammer Cutting is removed by the flushing medium(drilling
fluid)which is pumped down to the drill bit with a pump
Casing is usually required in unstable soils and deep
holes Common drilling fluid; bentonite in water
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B t
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Bor ng too s or
Wash Boring
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Flushing Medium
Purpose is to remove particles and to cool the drill bits Air flush (operates at 1000 m3/min)
Water flush (operates at 24-50 m3/min)
Mud flush include bentonite and polymer based muds Mist and foam flushes (mixture of air, water and mud)
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Piston Sampler
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Boring
tools
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Penetration Test (SPT) Sampler
Area ratio about 100%
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Sample Quality ClassificationSample Quality Application
Class 1 Index tests, moisture content, density,
strength, deformation and
consolidation characteristics
Class 2 Index tests, moisture content anddensity
Class 3 Index tests and moisture content
Class 4 Index tests
Class 5 Strata identification only
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li h d
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Common Sampling Methods
Pl i b i
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Planning boring program
How Many Borings & How Deep?
No hard-and-fast rule exists for determining
the number of borings or the depth to whichborings are to be advanced.
But guidelines exist in Textbooks
Design manuals
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No. Of BoringsProject Spacing (m)
One story house 25 30
Multistory buildings 15 25
Highways/Railways 250 500
Earth Dams 25 50
Residential sub-division 60 - 100
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How Deep (Bridges)?
Boring depth is governed by various factors, including: Foundation type Foundation load Channel relocation, widening, dredging? Scour?
Rules of Thumb Generally speaking, 50- 80 is reasonable Local experience is helpful Look at nearby structures if available
If no experience or other info available, plan for longfirst hole, then adjust.
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How Deep (Retaining Walls)?
Boring depth is governed by various factors,
including: Wall type (Fill vs. Cut)
Rules of Thumb :
Fill Walls: Depth = Wall Height +/- Soil Nailed Walls: Depth = Through Nailed Area,
plus 10 Drilled Shaft Walls: Depth = Exposed Wall Height plus
150% of Wall Height
Soil Nailed Walls Drilled Shaft Walls
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Depth of Boring ( Shallow foundation)
Soil Condition Minimum Depth of Boring (m)
Poor 6 x S0.7 + Df
Average
5 x S0.7 + Df
Good3 x S0.7 + Df
S = No. of stories
Df = Tentative depth of footing (m)
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Depth of Boring(continued)
For roads and airfields
Up to depth of 2 3 m
For embankment roads up to depth = 1.5 2.0 xheight of embankment + 2 3 ft.
For Pipelines 1 2 m below
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Ground Water Measurement In sand or gravels
- Clean the hole
- Use steel tape coated with chalk
In silt or silty sand
- Can be measured in several days
In clays
- Piezometer is used
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Piezometers and Monitor well
Observation of Water Tables
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(a) Standard pipe
(b) Casagrande type piezometer
(c) Electric (resistance
vibrating wire) type(transducer)
Electric pore water
pressure transducer
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Typical applications for piezometers are:
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-Monitoring pore water pressures to determine safe rates of fill or
excavation.-Monitoring pore water pressures to evaluate slope stability.-Monitoring dewatering systems used for excavations.-Monitoring ground improvement systems, such as vertical drains andsand drains.
-Monitoring pore pressures to check the performance of earthfill damsand embankments.-Monitoring pore pressures to check containment systems at landfillsand tailings dams.
Standpipe Piezometers
Advantages: Simple, reliable, not electrical, nocalibrated components.
Limitations: Accuracy depends on skill of operator;reading requires a man on site; remote reading notpossible; slower to show changes in pore-waterpressure.
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Advantages of Vibrating Wire Piezometers:
High Resolution: VW piezometers provide a resolution of 0.025% of full scale.
High Accuracy: Slope Indicator's automated, precision calibration system ensures
that all VW piezometers meet or exceed their accuracy specifications.Groutable: The VW piezometer can be directly grouted-in with a bentonite-cementgrout.Rapid Response: VW piezometers offer rapid response to changes in pore waterpressure, whether they are grouted in, pushed into cohesive soils, or embedded in a
sand filter zone.
Reliable Signal Transmission: With properly shielded cable, signals from the VWpiezometer can be transmitted long distances.
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Well Piezometers
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Sand
Sand
Clay
Water drainsdown well,Saturation undetected
Shallow piezometercontains water, but lowerpiezometer does not. Perchedwater table detected.
Perched
WaterTable
Basic WellLoose Cap: usePop (soda) can
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Installation Method
Sand Pack
Surface ConeSoil or bentonite + soil
(2:1) mix for sandy soils.
Carefully packed. Checkevery visit.
Drill air holeIn tube if capIs tight
3+
Bentonite Seal3 to 6 inches
Well Screen or slots
over depth of interest
Backfill withsoil, packed
well
Basic PiezometerLoose Cap: usePop (soda) can
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Installation Method
Sand Pack~ 3 aboveslots
Backfill withsoil, packedwell
Surface ConeSoil or bentonite + soil
(2:1) mix for sandy soils.
Carefully packed. Checkevery visit.
Drill air holeIn tube if capIs tight
Bentonite Seal3 to 6 inches
3+
6 of WellScreen or slots
Fabric Covers or Socks for Wellsand Piezometers
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and Piezometers
Sand andSoil may fallinto holes inWells andPiezometers.
Cover holesWith porous
fabric.
Can useGeotextile,Drain Sock,Womensnylons
Tape
Knot atbottom
For Flooded Sites Use a Surface MarkerAl ith ll d i t
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Along with wells and piezometers
Gauge showingwater heightabove surface
Finding the Water Table
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Finding the Water Table
Wellsshould be used to identify water tables.
Piezometers measure pressure, and not the freewater surface.
Wells work best when they dont penetrate alayer that is perching water or intersect largecracks
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Finding the Water Table with Piezometers
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2 ft.
6 ft.Piezometers
Where is the water table in this soil?
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If you must use a piezometer to identify a water table,then useth t l l i th h ll t i t f ti t
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2 ft.
6 ft.
the water level in the shallowest piezometer for your estimate
Water table