Standard Penetration Test

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Standard Penetration Test
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Home Foundations Standard Penetration Test (SPT) for Foundation Design

Figure 1: Driving sequence in an SPT test

Figure 2: Typical SPT hammer set up

Figure 3: Split spoon sampler for SPT

Figure 5: Correction factor to N’ value to allow for overburden pressure

Figure 6: Relationship between SPT number, plasticity index, and undrained shear strength of clay soil

Figure 6: Relationship between SPT number, plasticity index, and compressibility of clay soil

TAGS foundations Geotechnical Engineering
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The standard penetration test (SPT) is made in boreholes by means of the standard 50.8 mm outside and 33.8 mm inside diameter split spoon sampler. It is a very useful method for estimating the in-situ density of cohesionless soils, and when modified by a cone end, it can also be used to assess the relative strength or deformability of rocks.
An automatic trip device triggers repeated strikes from a 63.5 kg weight falling freely through 760 mm, driving the sampler to a penetration of 450 mm.The only blows counted as part of the conventional penetration number are those for the final 300 mm of driving (N-value). For the entire 450 mm of drive, it is standard practice to count the blows for every 75 mm of penetration.
By doing so, it is possible to determine the depth of any disturbed soil in the borehole’s bottom and the height at which any obstacles to driving, such as cobblestones, huge gravel, or cemented layers, are encountered. In the test, typically no more than 50 blows are made (including the number of blows necessary to position the sampler below the disturbed zone).
Both the depth at the start of the test and the depth at which it is concluded must be given in the borehole record if the full 300mm penetration below the initial seating drive is not achieved, i.e., when 50 blows are made before full penetration is achieved. Appropriate symbols must be used to indicate whether the test was completed within or below the initial seating drive. The tube is disassembled for analysis of the soil samples after removal from the borehole (see Figure 3).
In gravelly soil and rocks the open-ended sampler is replaced by a cone end. Investigations have shown a general similarity in N-values for the two types in soils of the same density.
The standard penetration test was first developed in the USA as a simple tool to determine the density of soils. The test was adopted by various nations throughout the world, and numerous relationships between the test results and soil characteristics and analytical methods were developed.
According to published data, test methodologies vary greatly across different countries. Non-standard types of hammers and samplers were being utilised, and there were several ways to manage the hammer drop, including free-fall or rope and pulley arrangement.
The two most common types of SPT hammers used in the field are the safety hammer and donut hammer. They are commonly dropped by a rope with two wraps around a pulley (see Figure 2).
There are several factors that will contribute to the variation of the standard penetration number, N, at a given depth for similar soil profiles. These factors include SPT hammer efficiency, borehole diameter, sampling method, and rod length factor.
It therefore became evident that if the test data were to be used for correlation with different soil parameters, as will be explained below, corrections to N-values produced by non-standard techniques would be required. The following is a summary of the correction factors that should be applied to the measured blow-count.
where ER m is the energy ratio and CE is the 60% rod energy ratio correction factor. Correction factors for rod lengths, sampler type, borehole diameter, and equipment (60% rod energy ratio correction) are given in Tables 1 – 4.
In the field, the magnitude of ER M can vary from 30 to 90%. The standard practice now in the U.S. is to express the N-value to an average energy ratio of 60% (≈ N 60 ). Thus, correcting for field procedures and on the basis of field observations, it appears reasonable to standardize the field penetration number as a function of the input driving energy and its dissipation around the sampler into the surrounding soil, or;
N 60 = NC H C B C S C R /60 ——– (2)
where N 60 = standard penetration number corrected for field conditions N = measured penetration number C H = hammer efficiency (%) C B = correction for borehole diameter C S = sampler correction C R = correction for rod length
The blow counts for an SPT test at a depth of 6 m in a coarse-grained soil at every 150mm are 9, 16, and 19. A donut automatic trip hammer and a standard sampler were used in a borehole 152 mm in diameter. (a) Determine the N value. (b) Correct the N value for rod length, sampler type, borehole size, and energy ratio to 60%. (c) Make a preliminary description of the compactness of the soil. Strategy: The N value is the sum of the blow counts for the last 0.304 m of penetration. Just add the last two blow counts. Solution Step 1: Add the last two blow counts. N = 16 + 19 = 35
Step 2: Apply correction factors. From the Tables above; C H = 60% C B = 1.05 C S = 1.00 C R = 0.95
N 60 = NC H C B C S C R /60 = (35 × 60 × 1.05 × 1.00 × 0.95)/60 = 34 Step 3: Use Table 5 to describe the compactness. For N = 34, the soil is dense.
Although the applications of SPT results are entirely empirical, their extensive use has allowed for the accumulation of vast knowledge regarding the behaviour of foundations in sands and gravels. Relationships between N-values and properties like density and shearing resistance angle have been identified.
BS 5930 gives the following relationship between the SPT N-values and the relative density of a sand as shown in Table 5;
Some correlations of the SPT with soil characteristics, in particular the susceptibility of a soil to liquefaction under earthquake conditions, require a further correction to N’ 60 to allow for the effective overburden pressure at the level of the test. In granular soils, the standard penetration number is highly dependent on the effective overburden pressure.
A number of empirical relationships have been proposed to convert the field standard penetration number N 60 to a standard effective overburden pressure σ 0 ‘, of 96 kN/m 2 (2000 lb/ft 2 ). The general form for standard sampler is;
N’ 60 = C N N 60 ——– (3) Several correlations have been developed over the years for the correction factor, C N . In standard geotechnical engineering textbooks, two of these given in Equations (4) and (5) are recommended for use (SI Units);
C N = 9.78√(1/σ 0 ‘) ——– (4) or C N = 2/(1 + 0.01σ 0 ‘) ——– (5)
Values of C N derived by Seed et al (1984) are shown in the Figure below;
The consistency and unconfined compressive strength (q u ) of clay soils can be estimated from the standard penetration number N 60 . It is important to point out that the correlation between N 60 and unconfined compressive strength is very approximate. The sensitivity, S t , of clay soil also plays an important role in the actual N 60 value obtained in the field. In any case, for clays of a given geology, a reasonable correlation between N 60 and q u can be obtained as shown in Equation (6).
Where P a is the atmospheric pressure (in the same unit with q u ).
Stroud (1975) has established relationships between the N-value, undrained shear strength, modulus of volume compressibility, and plasticity index of clays as shown in Figure 6.
It is not advised to use the SPT in place of the direct approach of conducting laboratory tests on undisturbed samples to determine the shear strength and compressibility of clay soils. This is due to the fact that the correlations between the SPT and the strength and deformability of clays have only been established empirically, with no consideration of time effects, anisotropy, or the composition of the soil.
The drained angle of friction of granular soils, ϕ’, also has been correlated to the standard penetration number. Peck, Hanson, and Thornburn (1974) gave a correlation between (N 1 ) 60 and ϕ’ in a graphical form, which can be approximated as;
ϕ'(degrees) = 27.1 + 0.3(N 1 ) 60 – 0.00054(N 1 ) 60 2 ——– (8)
Schmertmann (1975) also provided a correlation for N 60 versus σ 0 ‘. After Kulhawy and Mayne (1990), this correlation can be approximated as;
Where P a is the atmospheric pressure in the same unit as σ 0 ‘.
Terzaghi and Peck also give the following correlation between SPT value, D r , and φ as shown in Table 7.
The SPT can be completed quickly and easily. The equipment can penetrate dense materials and is widely available The engineering characteristics of soils such as bearing capacity and foundation settlement have all been linked to SPT results. However, the majority of these correlations are marginal.
Errors can come from a variety of sources, such as test performance and the use of non-standard equipment. The incorrect lifting and dropping of the hammer, inadequate borehole cleaning prior to the test, and failure to maintain the groundwater level, if one exists, are examples of test performance errors. These mistakes result in N values that are not typical of the soil. For coarse gravel, boulders, soft clays, silts, and mixed soils containing boulders, cobbles, clays, and silts, SPT tests are unreliable.
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N’ 60 (blows/300 mm of penetration)
Unconfined Compressive Strength kN/m 2 (lb/ft 2 )

Standard Penetration Test (SPT)-Procedure,Precautions, Advantages


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The Standard Penetration Test (SPT) is most popular and economical test to determine the surface information, both on land and offshore. This test is widely used to obtain the bearing capacity of soil .
The number or blows required for 12 inches penetration resistance of the soil. It is generally referred as the ‘N’ value and measured in blows/unit penetration.
The Standard penetration test ( SPT ) is widely used to get the bearing capacity of soil directly at a certain depth. The consistency of clayey soils can often be estimated from this test.
When a borehole is extended to a predetermined depth, the drill tools are removed, and the sampler (split spoon) is lowered to the bottom of the borehole.
The sampler is driven into the soil, by hammer blows to the top of the drill rod. The standard weight of the hammer is 140 lbs.(62.3 N) the number of blows required for spoon penetration of three “6” inches (15 cm) intervals is recorded.
The number of blows required for the last two intervals are added to give the standard penetration number at that depth.
1. Erect the tripod over the test hole and assemble the unit.
2. Allow the spoon to rest on the bottom of the hole.
3. Drive the spoon with blows from the hammer falling 75 cm (30 inches), until either 45 cm (18 inches) have been penetrated or 100 blows have been applied.
4. Record the number of blows required to effect each 15 cm (6 inches) of penetration. The first 6 inches is considered as seating drive.
5. The number of blows required for the second and third 15 cm (6 inches) of drive added is recorded as the penetration resistance value N of the soil.

The penetration resistance value, N of the soil is _________________blow .
Formula: q= 3.5 (N-3) [B+0.3/2B]Rw 2 Rd.
q= Allowable net pressure in T/m 2 .
Rw 2 = Water reduction factor= 0.5 [ 1+Z/B] .
Rd = Depth factor = l+D/B C or C= 2.0.
Z = Depth of water table below foundation level.
1. If the test is to be carried out in gravely soils , then the driving shoe is replaced by solid 60-degree cone.
2. Below the water table, care should be taken to avoid entry of water through the bottom of the bore due to loosening of soil happens.
3. Water should be added as necessary to maintain the water table level in the bore hole.
4. If the test is carried out in very fine sand or silty sand below the water table the measured “N” value if greater then 15, should be corrected for the increased resistance excess more water pressure set up during driving and unable to loose immediately.
5. Drilling mud can be sued to support the sides of the hole.
6. Stop dropping weight, when the sample penetrates less than 25 mm under 50 blows.
Ans: SPT stands for “Standard Penetration Test.”
whoah this blog is magnificent i love reading your posts.
Stay up the good work! You know, lots of people are looking round for this
information, you could help them greatly.
What is the formula for correction of N blow. I usually use the Terzaghi and Peck empirical formula of 1948.
N`=15+0.5 (N-15).
Please suggest me.
Prasad yes its good you can use that formula.
When you attain refusal we have to stop
Stop the tet if more than 50 blows are required for any of the intervals , or if more than 100 total blows are required. Either of these events is known as refusal and is to noted on the boring log.
thank you.


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The Standard Penetration test (SPT) is a common in situ testing method used to determine the geotechnical engineering properties of subsurface soils. It is a simple and inexpensive test to estimate the relative density of soils and approximate shear strength parameters.

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Standard Penetration Test, SPT, involves driving a standard thick-walled sample tube into the ground at the bottom of a borehole by blows from a slide hammer with standard weight and falling distance. The sample tube is driven 150 mm into the ground and then the number of blows needed for the tube to penetrate each 150 mm (6 in) up to a depth of 450 mm (18 in) is recorded. The sum of the number of blows required for the second and third 6 in. of penetration is reported as SPT blowcount value, commonly termed "standard penetration resistance" or the "N-value". The N-value provides an indication of the relative density of the subsurface soil, and it is used in empirical geotechnical correlation to estimate the approximate shear strength properties of the soils.
 Correlation between SPT-N value, friction angle, and relative density
The Cone penetration test can be carried out from the ground surface with a need for a borehole. The test is carried out by first pushing the cone into the ground at a standard velocity of 1 to 2 cm/s while keeping the sleeve stationary.
For any depth, the resistance of the cone, called cone penetration resistance q_c , is recorded using the force probes provided for this purpose in the cone. Then the cone and the sleeve and moved and penetrated together into the soil and the combined cone and sleeve resistance, indicated by q_t , is recorded at any depth using tension load cells embedded in the sleeve.
This procedure is repeated and the measurements are made at regular depth intervals during penetration. In addition to the stress on the tip and the sleeve friction, the typical CPT probe measures as well the porewater pressure. Some equiped CPT probles are also able to measures shear wave velocity and temperature. The cone penetration resistan
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