- These tests involve the measurement of the resistance to penetration of a sampling spoon, a cone or other shaped tools under dynamic or static loadings.
- The resistance is empirically correlated with some of the engineering properties of soil such as density index, consistency, bearing capacity, etc.
- The values of these tests lie in the amount of experience behind them.
- These tests are useful for general exploration of erratic soil profiles, for finding the depth to bedrock or hard stratum and to have an approximate indication of the strength and other properties of soils, particularly the cohesionless soils, from which it is difficult to obtain undisturbed samples.
The two commonly used tests are
- SPT full name is Standard Penetration Test.
- The standard penetration test is coming under the category of penetrometer tests.
- An empirical penetration correlation is derived between the soil properties and the penetration resistance.
- This test (IS: 2131-1981) is performed in a clean hole 55 to 150mm (millimeter) in dia
- A casing or drilling mud can be used to support the sides of the hole. A thick-wall split-tube sampler, 50.8 mm OD and 35 mm ID, is driven into the undisturbed soil into the bottom of the hole under the blows of a 63.5 kg drive weight with 75 cm free fall.
- The minimum open length of the sample should be 60 cm.
Procedure for Standard Penetration Test:
- Test procedure: The split tube sampler, commonly known as split spoon sampler resting on the bottom of the borehole is allowed to sink under its own weight.
- It is then seated 15 cm with the blows of the hammer falling through a height of 75 cm. Then, the split spoon sampler is further driven by 30 cm or 50 blows.
- The number of blows required to affect each 15 cm penetration is recorded.
- The first 15 cm of the drive is considered to be seat drive.
- The total blows necessary for the second and third 15 cm of penetration is termed as the penetration resistance N.
- In case the split spoon sampler is driven less than 45 cm (total), then the penetration resistance will be for the last 30 cm of penetration.
- The whole sampler may sometimes sink under its own weight when very soft subsoil stratum is encountered.
- Under these conditions, it might not be necessary to give any blow for a sampler, and SPT value should be indicated as zero.
- Corrections to Be Observed N Values.
- Overburden Pressure Correction by Other Workers.
- Correlation of N with Soil Properties.
#1. Corrections to Be Observed N Values-
The observed value of N is correct for
- Correction for Overburden.
- Correction for Dilatancy/submergence.
#1a. Correction for Overburden-
A density classification for sands has been proposed originally, in general terms, by Terzaghi and Peck, on the ground of standard penetration resistance, as shown in columns (1) and (2) of as per below table.
Numerical values of density index, as shown in column (3), were then added by Gibbs and Holtz. But, standard penetration resistance is dependent not only on density index but also on the effective stresses at a depth of measurement; effective stresses can be represented into a first approximation by effective overburden pressure.
This dependence has been first demonstrated in the laboratory by resistance at different depths. Several proposals are made for the correction of measured N values after the work of Gibbs and Holtz. The corrected value (N1) is related to the measured value (N) by the factor CN, in which and was later confirmed in the field. Sand in the same density index would thus give different values of standard penetration resistance at different depths.
Several proposals are made for the correction of measured N values after the work of Gibbs and Holtz. The corrected value (N 0) is related to the measured value (N) by the factor Cn, where
N 0 = Cn.N————————————— (1)
Cn = Normalising correction factor
#1b. Correction for Dilatancy/submergence-
The values N o obtained after applying overburden correction is corrected further for dilatancy if the stratum consists of fine silt and sand below the water table, for values of N greater than 15, using the following expression:
N e = 15 + (N 0 – 15 )
#2. Overburden Pressure Correction by Other Workers-
For a constant density index, the N value increase with increasing effective overburden pressure for which correction has been proposed by Gibbs and Holtz, Peck, Thornburn, Whitman, and others.
Gibbs and Holtz (1957) have experimentally studied the effect of overburden pressure on the values of N. Their modification for air dry or moist sand can be represented by the relation :
N 0 = N ( 500 / 1.42σ + 100 ) ————————————— (2)
- N 0 = Corrected Value for Overburden Effect
- N = Actual Value
- σ = Effective Overburden Pressure (Not Exceed 282 kN/sq.mm)
Peck (1974) proposed that N values be reported at a reference overburden pressure of 100 kN/Sq.m., and the normalized value of N (Corrected for Overburden Pressure) be expressed as follows:
N 0 = Cn.N
- Cn = Normalizing Factor
- = 0.77 log10 (2000/σ)————————————— (3)
- σ = Effective overburden pressure (kN/Sq.m) at the test level.
The above correction is valid for σ > 25 kN/sq.m. It may be noted that at reference overburden pressure of 100 kN/sq.m., Cn = 1. If σ decreases below reference pressure, Cn increases; at σ = 50 kN/sq.m Cn = 1.234. However, if σ increases above the reference overburden pressure of 100 kN/sq.n., Cn becomes less than unity; at σ = 400 kN/sq.m., Cn = 0.54. Eq. 3 is represented by curves of as per above figure adopted by BIS.
Lio and Whitman (1986) has proposed the following expression for Cn
Cn = √(2000/σ ) ————————————— (4)
- Here again at σ = 100 kN/Sq.m, Cn = 1
- σ = 50 kN/Sq.m, Cn = 1.141
- σ = 400 kN/Sq.m, Cn = 0.5
Correlation of N with Soil Properties:
Relation between N and Ø table 2
As per the above table 2 gives the relation between corrected N value and angle of shearing Ø resistance, as suggested by Peck (1974).
Relation between N and Ø
As per the above figure gives the graphical relationship between corrected N value and angle Ø as adopted by Indian Standard (IS: 6403-1981). This value of Ø can then be used for finding capacity factors Nc, Nq, and Ny
As per below table 3 and 4 give some empirical correlations of the soil properties with corrected penetration resistance. The approximate values of 4 are after Meyerhof (1956)
|Penetration Resistance N (Blows)||Approx. Ø (Degrees)||Density Index (%)||Description||Approx. Moist Unit Weight (Kn/m.cubic)|
Table 3 Penetration resistance and empirical corrections of cohesionless Soils
- Ø = 25 + 0.15 Id , with fine greater than 5%
- Ø = 30 + 0.15 Id, with fine less than 5%
Larger values could be used for granular soil with 5% or less fine silt and sand
|Penetration Resistance (Blows)||Unconfined Compressive Strength (T/sq.m)||Saturated Density (T/m.cubic)||Consistency|
Table 4 Penetration resistance and empirical corrections of cohesive Soils
Efficiency of Standard Penetration Testing:
The actual energy effective in the driving of the SPT equipment varies due to many factors.
Hence, in addition to the effective overburden stress at the tested location, the SPT parameter depends on the following additional factors:
- Hammer Efficiency.
- Length of Drill Rod.
- Borehole Diameter.
Advantages and Disadvantages of Standard Penetration Test
Advantages of Standard Penetration Test-
- This very simple and economical
- This test provides representative sample 0for visual review,
- Actual soil behavior is accessed through the Standard Penetration Test values
- The method will help to penetrate dense fills and layers
- The test may be applied for a variety of soil conditions
Disadvantages of Standard Penetration Test-
- The results will vary because of any operator or mechanical variability or drilling disturbances.
- Teat very costly also time-consuming
- The test results from the Standard Penetration Test can’t be reproduced