SEMBODAI RUKMANI VARATHARAJAN ENGINEERING COLLEGE DEPARTMENT OF CIVIL ENGINEERING FOUNDATION ENGINEERING BY KARTHIVELU

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 about Occurrence & extant of soil & rock strata Nature & engineering properties of soil & rock Location & variation of ground water Types: General Detailed

General Exploration approximate 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 by Open well Bore 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 <10% of initial 1 5 or 1 10 of surface intensity press bulb of a isobar 1 1 2 to 2 times the width of loaded area

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.

For Hospitals And Office Buildings

Isolated spread footing or raft = 1 1 2 W Pile foundation=10-30m, at least 1 1 2 width of base structure Retaining wall=greater of 1 1 2 width of base 1 1 2 height of 𝑠𝑡𝑒𝑚

Number & Spacing Of Holes For small & less important building- one at centre For 0.4 hectare building- one at each corner For large area- penetration test at every 100m in grid pattern For 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

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

Post hole Auger Helical Auger (i) Auger Boring Suitable in cohesive & soft soil Highly disturbed sample Used in highway projects Type Hand operated upto 6m Mechanically operated more than 6m Post hole Auger Helical Auger

(ii) Auger & Shell Boring Suitable for soft to stiff clay Undisturbed sample For stiff & hard clay – shell For sandy soil – sand pump For rock – chisel pit with drill rod Types Hand operated- upto 25m Mechanically operated–upto 50m Generally hole requires casing Sand Pump

(iii) Wash Boring Simple & fast method Suitable for all type of soil other than boulders & rock Drill rod is raised & dropped & also rotated - chopping Water is forced under pressure - jetting Soil stratification guessed by colour of wash water & rate of progress

(iv) Percussion Boring Suitable for soil & rock Heavy chisel or pit suspended by cable or drill rod Water is added if not present Slurry is bailed out in interval Disturbed sample

(v) Rotary Boring Very fast method Suitable in soil & rock A drill pit fixed to lower end of the drill rod & rotated (lowered continuously) Water solution of bentonite forced in to the hollow drill rod Returning water contains the cuttings Usually no casing requires

Water solution of bentonite inside the drill rod Slurry of solution & broken rock casing Drill pit