# Standard Penetration Test

The soil conditions at the site approximately determine the type of foundation that they can design. For example, engineers typically use shallow foundations, such as spread footings and mat foundations, for low-rise structures with shallow bedrock or soil that have a high bearing capacity. On the other hand, engineers typically use deep foundations, such as drilled shafts and piles, for high-rise structures with deep bedrock or soil that has a low bearing capacity.

In universal, a higher SPT value shows stronger and more compact soil, which may be suitable for shallow foundations. However, it is important to note that other factors, such as the water table, soil type, and the presence of subsurface barriers, can also affect foundation design. Therefore, one should consult with a geotechnical engineer who can use the SPT data in combination with other information to determine the most appropriate foundation design for a specific project.

The Standard Penetration Test (SPT) is a widely used in-situ test to determine soil properties. Cohesionless soils particularly suit it, and the correlation between the SPT value and soil friction angle (φ) is well established. In Bangladesh, the SPT is commonly used for all types of soil. Introduced by the Raymond Pile Company in 1902, the test remains the most common in situ test worldwide. In this article, we will discuss the SPT, its correction, and its correlations with different soil properties.

## 1. The Standard Penetration Test (SPT)

The SPT consists of driving a split spoon sampler into the soil through a borehole 55 to 100 mm in diameter at the desired depth. A hammer weighing 63.5 kg drops onto a drill rod from a height of 750 mm. The number of blows required to produce a penetration of 300 mm is considered the penetration resistance. The blows for the first 150 mm of penetration are not taken into account to avoid seating errors. The blows required to increase the penetration from 150 mm to 450 mm constitute the N-value.

The SPT procedures are detailed in ASTM D 1586 and AASHTO T-206. It is essential to note that several factors contribute to the variation of the standard penetration number (N) at a given depth for similar soil profiles. These factors include the SPT hammer efficiency, borehole diameter, sampling method, rod length, water table, and overburden pressure.

The most common SPT hammers used in the field are the safety hammer and donut hammer. They are usually dropped using a rope with two wraps around a pulley. The configurations of the hammers are shown in Figure.

## 2. SPT Corrections

It is crucial to apply corrections to the SPT values to account for various factors that affect the test results. The corrected SPT value can then be used to estimate soil properties such as shear strength, bearing capacity, and settlement. Several corrections are available, including the overburden correction, the rod length correction, and the borehole diameter correction.

### 2.1 Overburden Correction

The overburden correction is applied to account for the effect of the stress state on the SPT value. The correction factor is determined using the formula:

OCR = (σv/100)^(0.5)

Where σv is the vertical effective stress at the test depth in kPa. The correction factor is then calculated using the formula:

N60 = N * (60/[(60+d)/2])^(OCR)

Where N is the measured SPT value, d is the depth of the test in meters, and N60 is the corrected SPT value.

### 2.2 Rod Length Correction

The rod length correction is applied to account for the effect of the rod length on the SPT value. The correction factor is determined using the formula:

RLC = (Lr/Lo)^0.5

Where Lr is the actual rod length and Lo is the standard rod length (1.5 m). The correction factor is then calculated using the formula:

N60 = N * RLC

Where N is the measured SPT value and N60 is the corrected SPT value.

2.3 Borehole Diameter Correction

The borehole diameter correction is applied to account for the effect of the borehole diameter on the SPT value. The correction factor is determined using the formula:

BDC = (dbo/150)^0.5

Where is the actual borehole diameter in mm? The correction factor is then calculated using the formula:

N60 = N * BDC

## Where N is the measured SPT

### 1. Can the Standard Penetration Test be used to determine the strength of soil?

The Standard Penetration Test (SPT) can provide valuable information about the strength of soil, but it is important to note that the test is primarily used to evaluate the resistance of soil to penetration. The test does not directly measure soil strength properties such as cohesion, friction angle, or compressibility.

That being said, there are established correlations between the SPT N-value and soil strength properties, particularly for cohesionless soils. These correlations allow engineers to estimate strength parameters from the SPT data, making the test a useful tool for geotechnical investigations. However, it is important to recognize the limitations of these correlations and the potential for significant variability in the results depending on factors such as soil type, sampling method, and hammer type.

### 2. Are there any limitations to the Standard Penetration Test?

Any testing method, the Standard Penetration Test (SPT) has its limitations. One major limitation is that the test only provides information about the resistance of soil to penetration, and does not directly measure soil strength properties such as cohesion, friction angle, or compressibility.

Additionally, there are several factors that can affect the results of the SPT and lead to variability in the data, including hammer type, borehole diameter, sampling method, rod length, water table, and overburden pressure. It is important for engineers to carefully consider these factors when interpreting SPT results and to exercise caution when using established correlations to estimate soil properties from SPT data.

### 3. How does the Standard Penetration Test compare to other in-situ testing methods?

The Standard Penetration Test (SPT) is one of several common in-situ testing methods used in geotechnical engineering. Other methods include the cone penetration test (CPT), the vane shear test, and the pressure meter test, among others.

Compared to some other methods, the SPT is relatively simple and inexpensive to perform, making it a popular choice for many geotechnical investigations. However, it is important to recognize that the SPT has some limitations in terms of the type of soils it can effectively evaluate, and that other testing methods may be better suited for certain applications.

Ultimately, the choice of in-situ testing method will depend on several factors, including the specific soil properties being investigated, the project budget, and the availability of equipment and expertise.

### 4. What are the advantages of using the Standard Penetration Test?

There are several advantages to using the Standard Penetration Test (SPT) in geotechnical investigations. One of the main advantages is that the test is relatively simple and inexpensive to perform, particularly compared to other in-situ testing methods.

Another advantage of the SPT is that it provides a quick and easy way to obtain information about the resistance of soil to penetration, which can be useful in a variety of engineering applications. The SPT can also provide valuable data for estimating soil strength properties, particularly in cohesionless soils where correlations between the SPT N-value and soil strength are well established.

Finally, the SPT is a widely recognized and well-established testing method, with a long history of use in geotechnical engineering. This can provide a level of confidence in the data obtained from the test and can make it easier to compare results across different projects and locations.

### 5. How is the data from the Standard Penetration Test used in geotechnical engineering?

The data obtained from the Standard Penetration Test (SPT) can be used in a variety of ways in geotechnical engineering. One common use is to estimate soil strength properties, particularly for cohesionless soils where correlations between the SPT N-value and soil strength are well established.

In addition to estimating soil strength, SPT data can be used to