Standard Penetration Test or commonly abbreviated as SPT (Standard Penetration Test) is a field penetration test to measure the geotechnical properties of soil.

This test method is often applied to many construction survey projects because of some advantages: simple equipment, easy operation and recording of results, used for many soils and survey depths, low cost...

Purpose

The test is used to evaluate:

- Bearing capacity of the foundation soil

- Relative density of sandy soil

- State of clay soil

- Uniaxial compressive strength (qu) of clay

- Combined sampling for soil classification

Test principle

The test uses a thin-walled sample tube with an outer diameter of 50 mm, an inner diameter of 35 mm, and a length of 650 mm.

This sample tube is brought to the bottom of the borehole and then a slide hammer weighing 63.5 kg is used to drop freely from a distance of 760 mm. The sample tube is driven into three steps, each step is driven 150 mm deep, totaling 450 mm. The number of hammers in each step is calculated and only the total number of hammers in the last two steps is recorded and this number is often called the "N value".

In the case that after the first 50 hammers, the sample tube has not been driven to 150 mm, only 50 values ​​are recorded. The number of hammers reflects the compactness of the ground and is used for calculations in geotechnical engineering.

Experimental equipment

- Sample tube: outer diameter 50.8 mm, inner diameter 34.9 mm, split tube length: 609 mm, driving tip length is 57.1 mm.

- Weight weighing 63.5 kg, free falling on the hammer base.

- Hammer base.

- Orientation slide.

Experimental sequence

- Step 1: Drill a hole to the intended testing depth, clean the bottom, lower the SPT sample tube and install the hammer base, lever, weights, etc.

- Step 2: Mark the hammer on the hammer in 3 intervals, each interval is 15cm (total driving depth is 45cm).

- Step 3: Let the weight fall freely at a height of 76cm, count and record the number of hammers driven for each 15cm interval.

- Step 4: Take the hammer index of the last 30cm as the SPT index.

The normal SPT test distance is from 1 - 3m, depending on the uniformity of the ground.

Experimental results processing

In fine-grained sandy soil, the number of hammer hits N required to lower the standard sample tube to the last 30cm depth can vary depending on the depth of the groundwater level. If N* is the number of hammer blows performed to lower the sample tube to the last 30 cm below the groundwater level in fine-grained sandy soil, the actual N value needs to be corrected according to the following formula of Terzaghi and Pek:

N = 15 + ½( N - 15)

The SPT test results in the engineering geological borehole are recorded directly in the engineering geological hydrogeological observation book with the data N30 (number of hammer blows at the second testing interval - 30cm), N45 (number of hammer blows at the third testing interval - 45cm) - N30 + N45

When establishing the stratigraphic column (diagram) of the engineering geological borehole with SPT test, draw a graph of the change in N value according to the testing depth.

Using the results of the standard penetration test

- Evaluation of the relative density of sand (according to Terzaghi and Pek)

- Determination of soil state and strength of clayey soil under uniaxial stress (qu) - According to Terzaghi and Pek.

The compressive strength of soil under uniaxial stress can be determined depending on the N value, based on the following correlations:

- Clay: qu = N / 4

- Silty clay: qu = N / 5

- Sandy clay and silty soil: qu = N / 7.5

SPT test results depicted on the right of the borehole cylinder

SPT test results depicted on the right of the borehole cylinder

Testing guidelines

- Vietnam Standard: TCVN 9351:2012

- ASTM Standard: D 1586