3-CIEEE710 Soil Sampling Lecture

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

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3-CIEEE710 Soil Sampling Lecture

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