Question: Explain various suitable field tests that are conducted during soil exploration at a site having sandy starter
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Subject : Geo-technical Engineering 1

Topic : Compaction Of Soils & Soil Exploration

Difficulty : Moderate

mumbai university ge1(75) • 55 views
 modified 10 weeks ago  • written 10 weeks ago by
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Field tests are following:-

1) Standard Penetration test(SPT)

2) Dynamic core penetration test(DCPT)

3) Static core penetration Test(CPT)

1) Standard Penetration Test :-

i) The test essentially consists in driving a split spoon sampler of 50mm outside diameter into the soil

ii) The sampler should be driven at the bottom of the bore hole which is cased i.e. a pipe is forced into the bore hole and test conducted at the bottom of the bore hole

iii) In case the strata are stiff such as gravel and sandy gravels, a soil conical $60^{\circ}$ shoe is fitted to the sampler

iv) The sampler is to be attached to the drill rod and driven by blows of a hammer

v) The hammer generally weights 65 kgs with a free fall of 750 mm. generally 30 blows per minute are given

vi) The sample is initially driven a distance of 150mm below the bottom of bore hole and then a further distance of 300mm

vii) The number of blows required to drive this distance is known as standard penetration value N

viii) The number of blows required effect the first 150mm of penetration , called the seating drive is disregarded

Diagram:-

2) Dynamic Core penetration Test(DCPT):-

i) In this test, a core which has an apex of $60^{\angle}$ and attached to drill rods is driven into soil with hammer of 65 kg, falling freely from height of 750mm

ii) The blow count for every 100mm the cane is continuously recorded

iii) The cone is driven till refusal or up to the required depth and drilling rods are withdrawn, leaving the core behind in the ground

iv) The number of blows required for 300mm penetration is rated as the dynamic cone resistance

v) This test gives a continuous record of $N_{cd}$ with depth No. sample, however, can be obtained in this test

vi) Dynamic core penetration test are performed either by using a 50mm diameter cone with bentonite slrry or a 65mm diameter cone with bentonite slrry.

vii) When bentonite slurry is used the setup has an arrangement for the circulation of slurry So that drill rod is eliminated

viii) The dynamic test is a quick test and helps to cover a large area under investigation economically

V Diagram:-

3) Static cone penetration Test:

i) This test is used in place of SPT, particularly for soft clays and slits and finds to medium sand consists of deposits.

ii) The penetrometer used consists of a cone with an apex angle of 60 and a base of 20 cm and is used as follows :-

a) Initially the cone and the friction jacket assembly is in stationary position

b) The cone is then pushed into the soil at the rate of 20m/s by thinner sounding rod of a steady rate till a color engages the cone : The force Q offered by soil for penetration is measured on pressure gauge And tip resistance point resistance, qc can be calculated qc=Qc/Ac, where Ac is the base area.

c) The sounding rod is pushed further pushing the friction jacket and the cone assembly together

The total force $Q_{f}$ required for this is again need on the pressure gauge

$Q_{l}=Q_{c}+Q_{p}$

Where $Q_{f}$=force required to push friction jacket and skin friction fs is calculated:

fx=$\frac{Qf}{Af}$

Af=surface area of friction jacket