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SEMBODAI RUKMANI VARATHARAJAN ENGINEERING COLLEGE
DEPARTMENT OF CIVIL ENGINEERING
FOUNDATION ENGINEERING
BYKARTHIVELU
SITE INVESTIGATION &SELECTION OF FOUNDATION
Scope and objectives Methods of exploration Depth of boring – Spacing of bore hole Sampling techniques Penetration tests (SPT and SCPT) Bore log report – Data interpretation strength parameters and Liquefaction potential Selection of foundation based on soil condition.
Site Investigation It decides
suitability construction material & method
It needs for Efficient Safe Economic -design & construction
It involves (i) Site reconnaissance (ii) Site exploration
(i) Site Reconnaissance
Inspection of the site
Topographical features
Excavation, cutting
Quarry, escarpment
Water level in the well & stream
Evidence of erosion on land slides
(ii) Site Exploration
Detailed information aboutOccurrence & extant of soil & rock strataNature & engineering properties of soil &
rockLocation & variation of ground water
Types:GeneralDetailed
General Explorationapproximate picture of sub-soil condition (low
cast) by test pits & boring, sub surface penetration & sounding and geophysical methods.
It gives Depth, extent & composition of soil strata depth of rock Water level Approximate strength & compressibility Preliminary selection of foundation Engineering properties by disturbed sample
Detailed Exploration
For large engineering works
when heavy loads, complex & costly
foundations involved
Soil properties- shear strength, compressibility,
density index, permeability by testing
undisturbed sample (lab) & by field test
Ground Water Level
Important for deep foundation & water-logged areas
Identified byOpen wellBore hole
• Uncased- observation after 12-24 Hr• Cased- casing raised to 30 cm &
observed after 24 Hr
Depth Of Exploration
Significant depth: depth up to which the increase in pressure due to structural loading likely to cause perceptible settlement or shear failure.
Significant depth-Net increase in vertical pressure of initial or of surface intensity press bulb of a isobar1 to 2 times the width of loaded area
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Depth Of Boring
1. Determine the net increase of stress, under a foundation with depth as shown in the Figure.
2. Estimate the variation of the vertical effective stress, ', with depth.
3. Determine the depth, D = D1, at which the stress increase is equal to (1/10) q (q = estimated net stress on the foundation).
4. Determine the depth, D = D2, at which /' = 0.05.5. Unless bedrock is encountered, the smaller of the two depths,
D1 and D2, just determined is the approximate minimum depth of boring required. Table shows the minimum depths of borings for buildings based on the preceding rule.
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For Hospitals And Office Buildings
Isolated spread footing or raft = 1WPile foundation=10-30m, at least 1 width of base
structureRetaining wall=greater of
Number & Spacing Of Holes
For small & less important building- one at centreFor 0.4 hectare building- one at each cornerFor large area- penetration test at every 100m in
grid patternFor road- in z profile along centre line & pitch
line at 100m (30m-500m)For dam- at 50m spacing along one or both
abutment top line in upstream side & few holes in down line
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Approximate Spacing of Boreholes
There is strict rules for spacing of bore holes.
The above values can be changed by considering the subsoil condition
Method Of Site Exploration
Open excavation
Boring
Sub surface sounding
Geophysical method
Open Excavation
Pits & trenches
Suitable for shallow depth (3m) & all type of
soil
Lateral support requires when depth increases
or water exists
Cost increases with depth
Boring
For more depth
Methods Auger boring Auger & shell boring Wash boring Percussion boring Rotary boring
(i) Auger Boring
Suitable in cohesive & soft soil
Highly disturbed sampleUsed in highway projectsType
Hand operated upto 6mMechanically operated
more than 6m
Post hole Auger Helical Auger
(ii) Auger & Shell BoringSuitable for soft to stiff clayUndisturbed sampleFor stiff & hard clay – shellFor sandy soil – sand pumpFor rock – chisel pit with drill rodTypes
Hand operated- upto 25mMechanically operated–upto 50m
Generally hole requires casing
Sand Pump
(iii) Wash Boring
Simple & fast methodSuitable for all type of soil other than boulders
& rockDrill rod is raised & dropped & also rotated -
choppingWater is forced under pressure - jettingSoil stratification guessed by colour of wash
water & rate of progress
(iv) Percussion Boring
Suitable for soil & rockHeavy chisel or pit suspended by cable or drill
rodWater is added if not presentSlurry is bailed out in intervalDisturbed sample
(v) Rotary Boring
Very fast method Suitable in soil & rockA drill pit fixed to lower end of the drill rod &
rotated (lowered continuously)Water solution of bentonite forced in to the
hollow drill rodReturning water contains the cuttingsUsually no casing requires
Water solution of bentonite inside the drill rod
Slurry of solution & broken rock
casing
Drill pit