sembodai rukmani varatharajan engineering college department of civil engineering foundation...

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

10

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.

11

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

14

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