Foundation Engineering

 👉 The Primary objective of soil exploration is:

• Determination of the nature of the deposits of soil, depth and thickness of various soil strata.
• Location of Ground water table and obtaining soil and rock samples from the various strata.
•  The determination of the engineering properties of the soil and rock strata that affect the performance of the structure.
• Determination of the in-situ properties by performing field

Methods: Test Pits

                 Boring

                 Standard Penetration Test (SPT) [N Value]

                Cone Penetration Test (CPT): Static cone penetration test (SCPT)[qc value]

                 Dynamic cone penetration test (DCPT)

                 Pressuremeter Test (PMT)

                 Dilatometer Test (DMT)

                 Vane Shear Test (VST)

                Geophysical Exploration: Seismic reflection survey

                                                         Seismic refraction survey

                                                         Seismic Cross-hole survey

                                                         Resistivity Survey

              Plate Load Test

              Pile Load Test

  

👉 Standard Penetration Test (SPT): IS 2131-1981

• The Standard Penetration Test (SPT) is widely used to determine the parameters of the soil in-situ. The test consists of driving a split-spoon sampler into the soil through a bore hole at the desired depth.
•  The split-spoon sampler is driven into the soil a distance of 450 mm at the bottom of the boring
• A hammer of 63.5 kg weight with a free fall of 750 mm is used to drive the sampler.
• The number of blows for a penetration of last 300 mm is designated as the “Standard Penetration Value” or “Number” N.
• The test is usually performed in three stages. The blow count is found for every 150 mm penetration.
• The blows for the first 150 mm are ignored as those required for the seating drive.

• Number of blows for the first 150 mm penetration is disregarded due to the disturbance likely to exist at the bottom of the drill hole
• The test can be conducted at every 1m vertical intervals (Not more than 1.5 m)

👉The refusal of test when

• 50 blows are required for any 150 mm increment.

• 100 blows are obtained for required 300 mm penetration.

• 10 successive blows produce no advance.

👉Two corrections due to:

(a) Overburden pressure (granular soil)

(b) Dilatancy (for saturated fine sands and silts)

👉The corrected N value is given by (overburden correction)

                  

                                   N′ = (CF). N

                 where N′ = corrected value of observed N

                CF = correction factor for overburden pressure

👉Dilatancy Correction

                         

                            N′′= 15+0.5(N′-15)       if N′ > 15

Cone Penetration Test (CPT)

There are two types of CPT

(i) Dynamic cone penetration test (DCPT)                 (ii) Static cone penetration test (SCPT)

Static Cone Penetration Test

• The Static cone penetration test, which is also known as Dutch Cone test, has been standardized by “IS: 4968 (Part-III)-1976 - Method for subsurface sounding for soils - Part III Static cone penetration test”.

• The equipment consists of a steel cone, a friction jacket, sounding rod, mantle tube, a driving mechanism and measuring equipment.

• The cone have an apex angle of 60° ± 15′ and overall base diameter of 35.7 mm giving a cross-sectional area of 10 cm2.

• The friction sleeve should have an area of 150 cm2 as per standard practice.
• The sounding rod is a steel rod of 15 mm diameter which can be extended with additional rods of 1 m each in length.
• The driving mechanism should have a capacity of 20 to 30 kN for manually operated equipment and 100 kN for the mechanically operated equipment.

Dynamic Cone Penetration Test

• The dynamic cone penetration test is standardised by “IS: 4968 (Part I) – 1976 - Method for Subsurface Sounding for Soils-Part I Dynamic method using 50 mm cone without bentonite slurry”.

• The equipment consists of a cone, driving rods, driving head, hoisting equipment and a hammer.

• The hammer used for driving the cone shall be of mild steel or cast-iron with a base of mild steel and the weight of the hammer shall be 640 N (65 kg).

• The cone shall be driven into the soil by allowing the hammer to fall freely through 750 mm each time.

• The number of blows for every 100 mm penetration of the cone shall be recorded.

• The process shall be repeated till the cone is driven to the required depth.