Standard penetration test (SPT): What should you know about it?

A strong foundational base is a crucial aspect of a construction project. The first step towards ensuring a strong structure foundation is to check whether the soil at the construction site is suitable for bearing the project load. This is done by a process known as the standard penetration test or SPT. SPT helps us identify the soil’s properties to make the correct decisions regarding the construction project. Read on to learn more about this test and its utility.

 

Applications of SPT

Knowing the properties of the soil at a particular location can be of great help in many cases. Here are some important applications of SPT:

Soil classification

The process of SPT helps to classify soil into various categories depending upon the number of blows it requires to attain a particular depth. This process is therefore used in construction to determine the suitability of soil.

Foundation design

Knowing the load-bearing capacity of the soil is important for the structural planning of buildings, bridges and roads. SPT helps in the accurate determination of the load-bearing capacity of the soil.

Liquefaction potential

Soil liquefaction is a process by which the strength and stiffness of soil particles get reduced due to rapid movement. The liquefaction potential of soil can also be figured out by SPT, making it an important tool in the field of earthquake engineering as it helps in determining the response of the soil to seismic activity.

Settlement analysis

Settlement refers to the downward movement of soil when faced with excessive load. This can cause major harm to the structure of a building in the long run. To prevent this, it is important to have information about the strength of the soil.

Tunnel and pipeline design

Not only is SPT useful for the suitability of soil for structures to be built off-ground, but it is equally important for underground structural considerations as well. This test determines the stability of the soil to withhold structures such as tunnels and high-pressure pipes.

Slope stability analysis

Not all construction is undertaken on level ground, is it? However, it is risky business to build structures on slopes. This is where SPT comes in. SPT assesses the stability of soil on sloped areas to ensure resistance to landslides.

Retaining wall designs

The data collected by SPT comes in handy when designing foundational structures such as walls. The information collected tells exactly how much pressure the soil will be able to hold so that the structures are designed accordingly.

See also: Soil mechanics: Meaning, soil types, soil behaviour, and applications

 

SPT procedure

The SPT procedure requires specialised equipment and technical knowledge. The three main equipment required includes an SPT drill rig, a split spoon sampler and a drive hammer. Following are the major steps involved in conducting a standard penetration test:

Drilling borehole

The first step of the test is to dig a borehole on the site using an SPT drill rig. The depth of the hole varies depending on the type of construction project the information is being collected for.

Setting up the sampler

Next, a split spoon sampler is lowered into the borehole with the help of drill rods. This is done by dropping a heavy hammer on the top of the rods from a specific height to drive the spoon deep into the soil.

Blow count

The number of hammer blows required to send the split spoon to the required depth is recorded as the ‘N-value.’ This value is the indicator of the strength of the soil and its ability to withstand penetration.

Soil sampling

Once the N-value is found, the split spoon is raised again. The soil samples collected by the sample are then preserved and sent for further testing and analysis in the laboratory.

Depth intervals

To ensure maximum accuracy of results, this process is repeated several times at regular intervals as the borehole gets deeper. The blow counts or N-values of the soil are recorded at various depths to determine its properties.

Documentation

All the data collected during the test and the results of the lab tests are elaborately documented for future reference. The data includes the depth intervals, blow counts, physical condition of the soil, problems encountered while drilling and other useful things.

See also: Soil Compactor: Types, Advantages, and Disadvantages

 

Advantages of SPT in construction

Conducting an SPT at the construction site before beginning the project has several advantages. Here are a few of them:

Cost and time effectiveness

SPT is a viable way to determine the suitability of the soil to withstand the construction process and make decisions accordingly. It can easily be conducted on-site without the need of an extensive laboratory setup, thereby saving both time and money.

In-situ assessment

As SPT is conducted on the site of construction itself, it provides information about the soil as it is in its natural state and environment without any extraneous factors affecting its condition. This yields more accurate and useful results in terms of utility for construction.

Standardised and validated procedure

SPT is a scientific test that has been designed after proper standardisation and validation procedures. It is widely used and accepted as an important tool in the field of geotechnology, making it a reliable process.

Construction planning and risk mitigation

SPT gives us an idea of the properties of the soil that might potentially cause problems during the construction process such as settlement, liquefaction and slope instability. This knowledge helps to devise measures to avoid potential problems before they become unmanageable.

Earthquake engineering

SPT is highly crucial for the safety and resistance of structures to earthquakes as it helps determine the ability of the soil to resist seismic activity. This helps in appropriately designing the structure to best withstand earthquakes.

 

FAQs

What does SPT stand for?

SPT stands for standard penetration testing. It is a procedure used to determine the properties of the soil at a particular site.

Why is SPT conducted?

SPT has various applications in the field of construction and beyond, including structural design considerations, soil classification and determination of risks posed by the soil properties.

Is it necessary to conduct an SPT before beginning construction?

Yes, conducting an SPT before starting the construction process is important for better decision-making regarding structural design and risk mitigation.

What equipment is required for SPT?

SPT largely requires three materials, namely a drill rig, a split spoon sampler and a drive hammer.

Can I perform SPT on my own?

Performing SPT requires proper equipment and technical expertise, which is possessed by civil engineers.

Is a laboratory required for SPT?

No, SPT is conducted on the site itself and does not require an immediate laboratory setup for the calculation of results.

What is the cost of conducting an SPT?

The cost of conducting SPT depends on the complexity of the case and might range from anything between Rs 35,000 to over one lakh.

Got any questions or point of view on our article? We would love to hear from you. Write to our Editor-in-Chief Jhumur Ghosh at jhumur.ghosh1@housing.com