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1. Aim:
- To conduct the SPT test as per specification
- Collect and identify soil samples in the split spoon sampler
- Record and interpret the test data
2. Introduction:
In the standard penetration test (SPT), a standard split spoon sampler is driven into the soil by a hammer weighing 65 kg, with a free fall of 75 cm. The number of blows of the hammer required to penetrate 300 mm of the sampler (Last 150 mm + 150 mm) is recorded as N-value. This N-value gives the idea of the relative density of granular soils and the consistency of cohesive soil . It may also be used for determining the approximate bearing capacity needed in foundation design.
The split spoon sampler is used for obtaining disturbed representative samples of soil which are needed for visual classification and certain laboratory tests.
3. Apparatus Used:
(i) Split spoon sampler with a drive shoe, and sampler head.
(ii) Drill rod
(iii) Drive weight assembly consisting of
(a) Guide rod with driving head
(b) Hammer of 63.5 kg
(iv) A tripod assembly with a pulley, hoisting and lowering arrangement
(v) Boring equipment-Post hole auger
(vi) Centering guide for keeping drill rod vertical
(vii) Spanners, Wrenches, Measuring tape, Chalk for marking.
4. Procedure:
(i) Locate the exact position on the ground where the test has to be conducted.
(ii) Make a bore hole using the auger
(iii) Stop boring at the depth where the penetration test has to be conducted. Record this depth. Penetration tests are usually conducted at intervals of 1.5 m or change of stratum which ever occurs earlier.
(iv) Centre the tripod over the bore hole such that plumb line attached to the pulley passes through the center of the bore hole.
(v) Inspect the split spoon sampler, clean it and assemble.
(vi) Connect the sampler head of the sampler to a rod using coupling in between.
(vii) Lower the sampler with the drill rod into the bore hole so that the drive shoe just touches the bottom of the bore hole.
(viii) Keep the drill rod vertical and insert the centering guide through the drill rod till the guide is placed on the ground.
(ix) Connect the top portion of drill rod to the driving head of the guide rod using a coupling.
(x) Link the hammer chain with the hook provided on the pulley.
(xi) Raise the hammer by operating the hoist line and insert its hole to the guide rod so that the hammer rests on the driving head.
(xii) Replace the top cap of guide rod. The distance between bottom of top cap and base is 75 cm. This ensures a free fall of 75 cm.
(xiii) Mark the drill rod at heights 15 cm, 30 cm and 45 cm above the ground level.
(xiv) Raise the hammer and allow it to drop freely through a height of 75 cm.
(xv) Count the number of hammer blows. Observe whether the sampler is going down or not with every blow.
(xvi) Do not record the number of blows required to cause first 15 cm of penetration (Seating drive)
(xvii) Record the number of blows required to penetrate the sampler for the remaining 30 cm . This gives the N-value (Field value). Raise the sampler to the surface and open the sampler, measure the length of the sample.
5. Observation:
Diameter of bore-hole -----------------------
Type of boring -----------------
R.L of Ground surface ------------------
R.L of Groundwater table-----------------------
Observation Table:
Depth below GL (m) No. of blows per 30 cm penetration N-value
From To
------ ------ -------------------- -------------
------ ------ -------------------- -------------
------ ------ -------------------- -------------
------ ------ -------------------- -------------
Unit weight of soil----------------
Effective stress--------------------
Observed N-value ------------------------
State of compaction/consistency: Very loose/Loose/Medium/Dense/Very dense/Very soft/Soft/Firm/Stiff
Corrected for overburden pressure--------------
Dilatancy correction ----------------
Note: For more details about SPT Test and N-value correction please visit:
Thank you
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