Standard Penetration Test: Overview

Standard Penetration Test (SPT) is one of the main methods of determining in situ soil structure, bearing capacity, and other engineering properties. In addition, the angle of shearing resistance of cohesionless soils and the unconfined compressive strength of cohesive soils can also be derived from it. Invented in the USA back in 1925, the penetration test was standardized in terms of required equipment and procedure to compare test results from different investigations.

The penetration test is carried out in a borehole, thus providing representative soil samples.

Read on to find out all the essential information about the Standard Penetration Test, its purpose, procedure features, benefits, and drawbacks.

What is the Purpose of the Standard Penetration Test?

The issue of weak foundations of different structures is usually caused by poor investigation of the properties of the soil used for it. Equal settlement of any building or construction is only possible on the high-bearing capacity soil.

Like any other penetrometer test, SPT is an essential tool for engineers on construction sites. Thus, construction stability and a security engineer salary largely depend on their ability to conduct a standard penetration test.

Primary purposes of SPT:

• Extraction of soil sample for identifying its type and nature and multiple depths;
• It allows for determining soil penetration resistance at different levels to identify its properties.

Standard Penetration Tests – The Procedure

Before diving into the description of the SPT procedure, it is essential to note that this test requires precision and specific tools. In addition, the level of engineer’s experience and care on site is crucial for accurate and reliable results.

The procedure of the standard penetration test

• The required sampling depth borehole is pre-drilled (the sample tube is 50.8 mm in diameter and around 650 mm in length).
• The tube is driven into the soil with an anvil of 63.5 kg, dropping precisely from 0.76 m height.
• First, the sampler is driven 150 mm into the pre-drilled hole (counting from its bottom). Then, the n value of anvil blows required to move it another 300 mm is recorded as an N30 count.

Both drilling and stabilization of the borehole require a high level of care.

Factors like the force/energy of the drill rod, its impedance, and the drilling method have an impact on the results.

Relative Density and Bearing Capacity of The Soil

The bearing capacity of the soil represents the maximum load per unit area, which it can support without moving. This factor is closely connected with the soil type, which depends on its relative density. There are five main types: very loose, loose, medium, dense, and very dense soil. The difference is in the bulk density that can be found within 1600 kg/m3 (very loose) and 2100 kg/m3 (dense). Everything that exceeds these limits is called “very dense soil.”

Test also accounts for the nature of the soil, which can be cohesive, cohesionless (non-cohesive), or rocky.

Correlations in Penetration Test

Before using the values obtained from the penetration test in empirical correlations, the value of N has to undergo IS 2131 – 1981 correlations:

Dilatancy Correction

Clays and fine sands below the water table tend to have pore water pressure that drives the resistance of the soil to penetration up. Therefore, if the n value is higher than 15 on sands, the following correlation is required:

Nc = 15 + 0.5 (Nr -15)

Nr is an observed (recorded) value, while Nc is the corrected one.

Overburden Pressure Correlation

Overburden pressure has been proven to affect the standard penetration resistance. The measured n value differs depending on the depth as the confining pressure increases. Hence, the following correction is necessary:

Nc = Cn*N

Cn here is the correction factor for the overburden pressure.

Standard Penetration Test – Pros and Cons

SPT is a reliable and widely used penetrometer test. First, however, it is essential to outline its main benefits and drawbacks:

Advantages

Standard penetration test has the following advantages:

1. It is simple and cost-effective
2. Allows collecting samples for further investigations of the soil
3. If carried out carefully provides an insight into the actual soil behavior on the construction site
4. It applies to multiple soil conditions.

Disadvantages

As for the disadvantages:

1. The reliability of the results strongly depends on the competence of the operator
2. Limited applications for rocky soils
3. SPT is relatively time-consuming.

Closing Thoughts

A standard penetration test is an effective tool for determining the crucial properties of the soil used for construction purposes. Data derived from carefully conducted SPT allows understanding of the type and nature of the soil. The test is pretty simple but requires precise equipment and experienced operators to get the most accurate results.