This Is An Attempt To Create A Stand Alone Self

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This is an attempt to create a stand alone self learning module on site investigation. Fasten your seat belts. Sit back, relax and enjoy.

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Site

Investigation Narrated by: N. Sivakugan & Kate Johnson

Some unsung heroes of Civil Engineering…

foundation s

soil exploratio n tunnelin g

… buried right under your feet. SIVA

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A good site investigation is a prerequisite.

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proposed structure

Need Need good good knowledge knowledge of of the the soil soil conditions conditions

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Problem Soils e.g., reactive clays, soft soils,

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Soil data required:  Soil profile - layer thickness and soil identification

 Index properties - water content, Atterberg limits, etc.

 Strength & compressibility characteristics - c’, cu, φ ’, Cc, Cr, OCR, …

 Others (e.g., water table depth) SIVA

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Desk Study First stage of site investigation. Negligible cost. Look for any freebies (i.e., info available currently)  Aerial photographs  Topographical maps  Existing site investigation reports (for nearby sites)  Other info. from local councils, literature

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Site Reconnaissance A site visit and chat with locals.  Site access  Topography  Site geology  Conditions of adjacent structures  Any obvious problems foreseen?

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drill rig

back hoe

CLAY

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Trial Trial Pit Pit 1-2 1-2 m m width width 2-4 2-4 m m depth depth

Bore Bore hole hole 75 75 mm mm dia dia 10-30 10-30 m m depth depth 11

Trial Pit Enables visual inspection, locating strata boundaries, and access for undisturbed block samples.

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A Very Large Trial Pit

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In clay layers…

Clay

bore

collect undisturbed clay samples in thin walled sampler (e.g. shelby tube)

Consolidation, Consolidation, triaxial tests in

Undisturbed Clay Samples

Required for triaxial, consolidation tests in the lab. Good quality samples necessary. O. D.2 − I .D.2 AR = × 100 (%) so AAR<10 2 <10 R I . D. il % %

sampling tube

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Thicker the wall, greater the disturbance. Take good care in transport and handling.

area area ratio ratio

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In Granular Soils … Very difficult to get undisturbed samples. ∴ Go for in situ tests. e.g., e.g., penetration penetration tests tests

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80-90% 80-90% of of foundation foundation designs designs are are based based on on penetration penetration tests tests

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Penetration Tests Measuring the soil resistance to penetration by a probe. φ ’, E … GL drill rod

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10 m 0

How many bore holes?

Proposed site for a multi-storey shopping complex bore hole 120 m

Not Notenough enoughbore boreholes; holes;soil soilprofile profileand and properties propertiesnot notwell welldefined.. defined.. SIVA

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10 m 0

How many bore holes?

120 m

Too Toomany manybore boreholes holesand andblows blowsthe the budget. budget. SIVA

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How many bore holes?

10 m 0

trial pit

120 m About Aboutright? right? SIVA

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How many bore holes? The number of bore holes depends on: type and size of the project budget for site investigation soil variability

Typically spaced at 20-40 m for non-residential buildings. Locate the bore holes where the loads are expected.

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proposed

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How deep to explore? Explore the soil to a depth where the stress changes become insignificant

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Standard Penetration Test (SPT) 65 kg hammer 760 mm drop anvi

Count the number of blows required for 300 mm penetration Blow Blow count count or or N-Value N-Value

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drill rod

split spoon sampler SIVA

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Standard Penetration Test still stillhas has some some value value

mainly for granular soils; unreliable in clays N-value correlated to φ ’, E … done within bore holes at 1.5 m depth intervals samples (disturbed) collected in split-spoon sampler AAR = 112%; R = 112%; use usefor for classification classification

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I.D. = 35 mm O.D.= 51

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Standard Penetration Test

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SPT Hammer

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SPT Corrections

(N1)60 = CER CN N Corrected Corrected blow blowcount count

Measured Measured blow blowcount count Energy Energy correction correction

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Overburde Overburde nn correction correction 26

SPT Correlations in Clays not corrected for N60 overburde n

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

stiff

Thumb will indent 8 mm

8-15 50-100

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15-30 100-200 very stiff

Can indent with thumb nail; not thumb

>30

Cannot indent even with thumb nail

>200

hard

Use with caution; unreliable.

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SPT Correlations in Granular Soils (N)60 not corrected for overburden

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0-4 4-10 10-30 30-50 >50

Dr (%) consistency 0-15 15-35 35-65 65-85 85-100

very loose loose medium dense very dense

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Cone Penetration Test (CPT)

Dynamic cone penetration test (DCPT) similar to SPT; hammer driven using cone instead of split spoon

Static cone penetration test (SCPT) pushed into the ground @ 2 cm/s gives continuous measurements

closed end; no samples gives blow counts @ 1.5 m depth intervals SIVA

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Dynamic Cone Penetration Test Simple and rugged. Better than SPT or SCPT in hard soils such as dense gravels As crude as SPT; relies on correlations based on blow counts

Siva

Hollow (split spoon)

SPT DCP T SIVA

Solid (∴ no

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Static Cone Penetration Test

10 cm2 cross section

sleeve friction (fs)

fs friction ratio, fR = × 100 qc % Typically Typically00———— ———— 10%. 10%. granula granula rr

cone resistance (qc)

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cohesiv cohesiv ee

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Piezocone (CPTU) A modern static cone; measures pore water pressure also.

Porous Piezocone Pushed stone into with the for pore pressure leads ground measurement SIVA

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Pushed Pushedin in @ @20 20mm/s mm/s rate rate Continuous Continuous measureme measureme nts ntsof ofqqcc,,ffss and andu. u.

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Interpreting SCPT Data

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See next

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Interpreting SCPT (Piezocone) Data

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Soil SoilBehavior BehaviorType Type(Robertson (Robertsonetetal., al.,1986; 1986; Robertson Robertson&&Campanella, Campanella,1988) 1988) 11––Sensitive fine grained 5 – Clayey silt to silty clay 9 – sand Sensitive fine grained 5 – Clayey silt to silty clay 9 – sand 22––Organic material 6 – Sandy silt to silty sand 10 Organic material 6 – Sandy silt to silty sand 10––Gravelly Gravellysand sandto tosand sand 33––Clay 7 – Silty sand to sandy silt 11 – Very stiff fine grained* Clay 7 – Silty sand to sandy silt 11 – Very stiff fine grained* 44––Silty clay to clay 8 – Sand to silty sand 12 Silty clay to clay 8 – Sand to silty sand 12––Sand Sandto toclayey clayeysand* sand* *Note: Overconsolidated or cemented 35 *Note: Overconsolidated or cemented

Sounding - Shelby County, TN (U.S.A)

0

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qt (MPa) 5 10 15 20 25

Fr 0

0

0

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5 Courtesy: Professor. P.W. Mayne, Georgia 36 Inst. of Technology

SCPT Correlations In Clays,

In Sands,

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qc − σ vo cu = Nk

cone factor (1520); varies with cone

E = 2.5-3.5 qc (for young normally consolidated sands)

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qc/N Relation in Granular Soils

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qc in kg/cm2 (1 kg/cm2 = 98.07 kPa)

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φ ’ from SPT/CPT in Granular Soils

After Peck et al. (1974)

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After Meyerhof (1976)

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Pressuremeter Test Expand a cylindrical probe inside a bore hole. Most rational of all in situ tests Gives strength, modulus, K0, cv… For all soils

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pressurem eter

cylindric al probe

guard cell

Dilatometer Test Advance @ 20 mm/s. Test every 200-300 mm. Nitrogen tank for inflating the membrane. Gives cu, K0, OCR, cv, k, soil stiffness . Can identify soil (from a chart). Similar Similar to tothe the cone cone

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60 60mm mmdia. dia. flexible steel flexible steel membrane membrane

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Cone Pressuremeter Combines piezocone and pressuremeter. Uncommon; specialised.

pressurem pressurem eter eter

piezoco piezoco ne ne SIVA

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Vane Shear Test measuring (torque) head

For clays, and mainly for soft clays. Measure torque required to quickly shear the vane pushed into soft clay.

bore hole

∴ undrained h≈ 2d

van e

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d

soft clay

torque  undrained shear strength cu Typical d = 20-100 mm.

van e

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Vane Shear Test

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Test in Progress

Failure surface 44

Plate Loading Test Load 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.

settlem ent

press ure

plat e Doing it in Sri Lankan style.

Common In Situ Testing Devices

SPT VST

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In bore holes

PM T

CPT

DM T

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Pocket penetrometer A simple hand-held device for measuring unconfined compressive strength (qu = 2 cu) of a clay. very very Used in trial pits and rough rough samples. Must for every practicing geotechnical engineer.

Push Pushinto into the theclay, clay, and.. and.. SIVA

..read ..read the the strength strength 47

Relative Cost per Test

Cost versus Accuracy Pressuremeter test

Dynamic cone penetration test

Static cone penetration test Standard penetration test

Pocket penetrometer test

Relative Test Accuracy

If only they had proper site investigation…

…Tower of Pisa will not be leaning today! SIVA

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Hypertext References: www.fugro.nl Fugro Internationalwww.ce.gatech.edu Georgia Institute of Technology www.pagani-geotechnical.com Pagani Geotechnical Equipment