Standard Penetration Test

Aim: To obtain subsurface information by undisturbed or disturbed sampling.

Apparatus:

1. Tripod
2. Drill rods
3. Hammer of 63.5 kg
4. Samplers

Theory:

One of the most common in-situ tests is the standard penetration test or SPT and currently the most popular and economical means to obtain subsurface information (both inland and offshore). It offers the advantage of low cost, applicability to many soil types, samples are obtained (although disturbed) and a large database from which many useful correlations have been developed Usually SPT is carried out at every 0.75-m vertical interval or at the change of stratum in a borehole. This can be increased to 1.5 m if the depth of borehole is large. Due to the presence of boulders or rocks, it may not be possible to drive the sampler to a distance of 450 mm. In such a case, the N value can be recorded for the first 300-mm penetration. The boring log shows refusal and the test is halted if:

1. 50 blows are required for any 150 mm penetration
2. 100 blows are required for 300 mm penetration
3. 10 successive blows produce no advance

Procedure:

The standard penetration test is conducted in a borehole using a standard split-spoon sampler.

(i) When the borehole (55 to 150 mm in dia.) has been drilled to the desired depth, the drilling tools are   removed and the split-spoon sampler, attached to standard drill rods of required length is lowered to the bottom of the borehole and rested at the bottom.

(ii) The split-spoon sampler is then driven into the soil for a distance of 450 mm in three stages of 150 mm each by blows of a drop hammer of 63.5 kg mass falling vertically and freely through a height of 750 mm at the rate of 30 blows per minute (IS 2131 - 1981). The number of blows required to penetrate every 150-mm is recorded while driving the sampler. If full penetration is obtained, the blows for the first 150 mm is retained for reference purposes, but not used to compute the SPT value because the bottom of the boring is likely to be disturbed by the drilling process and may be covered with loose soil that may fall from the sides of the boring. The number of blows required for the next 300 mm of penetration is recorded as the SPT value. The number of blows is designated as the "Standard Penetration Value” or “Number" N.

(iii) The slit-spoon sampler is then withdrawn and is detached from the drill rods. The split barrel is disconnected from the cutting shoe and the coupling. The soil sample collected inside the split barrel is carefully collected so as to preserve the natural moisture content and transported to the laboratory for tests. Sometimes, a thin liner is inserted within the split barrel so that at the end of the SPT, the liner containing the soil sample is sealed with molten wax at both its ends before it is taken away to the laboratory.

Precautions:

Some of the precautions to be observed to avoid some of the pitfalls of the test are as follows:

• The drill rods should be of standard specification and should not be in bent condition. 
• The split spoon sampler must be in good condition and the cutting shoe must be free from wear and tear. 
• The drop hammer must be of right weight and the fall should be free, frictionless and vertical. 
• The height of fall must be exactly 750 mm. Any change in this will seriously affect the N value. 
• The bottom of the borehole must be properly cleaned before the test is carried out. If this is not done, the test gets carried out in the loose, disturbed soil and not in the undisturbed soil. 
• When a casing is used in borehole, it should be ensured that the casing is driven just short of the level at which the SPT is to be carried out. Otherwise, the test gets carried out in a soil plug enclosed at the bottom of the casing. 
• When the test is carried out in a sandy soil below the water table, it must be ensured that the water level in the borehole is lower than the ground water level in the borehole is always maintained slightly above the ground water level. If the water level in the borehole is lower than the ground water level, "quick condition may develop in the soil and very low N values may be recorded.

In spite of all these imperfections, SPT is still extensively used because the test is simple and relatively economical. It is the only test that provides representative soil samples both for visual inspection in the field and for natural moisture content and classification tests in the laboratory. Because of its wide usage, a number of time-tested correlations between N value and soil parameters are available, mainly for cohesion less soils. Even design charts for shallow foundations resting on cohesion less soils have been developed on the basis of N values. The use of N values for cohesive soils is limited, since the compressibility of such soils is not reflected by N values.

SPT values obtained in the field for sand have to be corrected before they are used in empirical correlations and design charts. IS: 2131 - 1981 recommends that the field value of N corrected for two effects, namely, (a) effect of overburden pressure, and (b) effect of dilatancy.

Calculations: 

(a) Correction for overburden pressure:

Several investigators have found that the overburden pressure influences the penetration resistance or the N value in a granular soil. If two granular soils possessing the same relative density but having different confining pressures are tested, the one with a higher confining pressure gives a higher N value. Since the confining pressure (which is directly proportional to the overburden pressure) increases with depth, the N values at shallow depths are underestimated and the N values at larger depths are overestimated. Hence, if no correction is applied to recorded N values, the relative densities at shallow depths will be underestimated and at higher depths, they will be overestimated. To account for this, N values recorded (NR) from field tests at different effective overburden pressures are corrected to a standard effective overburden pressure.

The corrected N value is given by

`N_{c}^{\prime}=C_{N} N_{R}`

Where,

`N_{c}^{\prime}` = corrected value of observed N value

`C_{N}` = correction factor for overburden pressure

`N_{R}` = Recorded or observed N value in the field

The correction proposed by Peck, Hanson and Thornburn (1974) is given by the equation:

`C_{N}=0.77 \log _{10} \frac{2000}{\sigma^{\prime}}`

Where,

`\sigma^{\prime}` = Effective overburden pressure at the depth at which N value is recorded, in kPa

(b) Correction for dilatancy:

Dilatancy correction is to be applied when N'c obtained after overburden correction, exceeds 15 in saturated fine sands and silts. IS: 2131 - 1981 incorporates the Terzaghi and Peck recommended dilatancy correction (when N'c > 15) using the equation

`N_{C}=15+0.5\left(N_{C}^{\prime}-15\right)`

Where,

`N_{C}` = final corrected value to be used in design charts.

`N_{c}^{\prime}` > 15 is an indication of a dense sand, based on the assumption that critical void ratio occurs at approximately `N_{c}^{\prime}` = 15. The fast rate of application of shear through the blows of a drop hammer is likely to induce negative pore water pressure in saturated fine sand under undrained condition of loading. Consequently, a transient increase in shear resistance will occur, leading to a SPT value higher than the actual one.

Note: The overburden correction is applied first. This value is used as observed N value and then the dilatancy correction is applied.

Correlation of 'N' with engineering properties:

The value of standard Penetration number depends upon the relative density of the cohesionless soil and the UCC strength of the cohesive soil. The angle of shearing resistance (Ø) of the cohesionless soil depends upon the number N. In general, greater the N-value, greater is the angle of shearing resistance. Table below gives the average values of Ø for different ranges of N

N	Denseness	`\emptyset`
0-14	Very Loose	`25^{\circ}-32^{\circ}`
4-10	Loose	`27^{\circ}-35^{\circ}`
10-30	Medium	`30^{\circ}-40^{\circ}`
30-50	Dense	`35^{\circ}-45^{\circ}`
>50	Very Dense	> `45^{\circ}`

The consistency and the UCS strength of the cohesive soils can be approximately determined from the SPT number N. Table gives the approximate values of UCS strength for different ranges of N. 

N	Consistency	`q_{u}\left(k N / m^{2}\right)`
0-2	Very Soft	<25
2-4	Soft	25-50
4-8	Medium	50-100
8-15	Stiff	100-200
15-30	Very Stiff	200-400
>30	Hard	>400

It can also be determined from the following relation

`q_{u}` = 12.5 x N

Where,

 `q_{u}`= UCS strength kN/m2

Observation Results and conclusions: 

Thanks to this YouTube Video : https://youtu.be/aAdCh6Hv6EM