BS EN ISO 22476-3 specifies the Standard Penetration Test (SPT)
Significance
The Standard Penetration Test (SPT) broadly assesses soil and rock homes in geotechnical engineering. It measures their resistance to a standard sample driven by a weight released from a specific height. This test offers valuable insights into material resistance.
Standard
The experiment will observe the protocol specified in BS EN ISO 22476-three, which specifies the important conditions and tips for engaging in and documenting the effects of a Standard Penetration Test (SPT).
Scope
The principal goal of the test is twofold: to check the Standard Penetration Resistance (N-cost) of the soil at a designated location and to simultaneously provide insights into the soil kind, moisture content, and numerous different factors that impact its engineering houses.
Objective
The most important purpose of the test is twofold. First, it’s miles to determine the usual penetration resistance (N-price) of the soil at a particular location. Secondly, it seeks to provide treasured insight into soil type, moisture content material, and plenty of other critical elements that drastically affect soil engineering residences.
Apparatus
For the experiment, the following apparatus will be employed:
- Standard Penetration Test (SPT) Sampler
- Tripod
- Drop Hammer (63.5 kg)
- Driving Head
- Drill Rods
- Water and sand
- Measuring Tape
- Safety equipment (which includes tough hats, protection glasses, gloves, and many others.) can be utilized.
Theory
The Standard Penetration Test (SPT) estimates soil resistance to penetration. It uses a standard sample dropped from a known height with a specific weight. This test involves driving a 50.8 mm diameter, 500 mm long hollow specimen into the soil using a 63.5 kg hammer released from a height of 760 mm. The recorded number of blows needed to drive the sampler the first 150 mm is referred to as the Standard Penetration Resistance or N-value.
Correlation with soil mechanical properties
While it’s important to acknowledge its numerous limitations, the practice of establishing correlations between SPT findings and soil properties of significance for geotechnical engineering design remains customary. This is because SPT results represent valuable in-situ field measurements, often serving as the only accessible test results. Consequently, the widespread adoption of such correlations has become a common practice in many countries.
The US Army Corps of Engineers’ manual for sheet pile design, developed after the works of Terzaghi and Peck (1948) and Teng (1962), cites an example of a rough correlation between SPT N-values, relative density, and bulk density for coarse-grained materials.
| Relative density | SPT N-value | Bulk density (kg/m3) |
|---|---|---|
| Very loose | 0 – 4 | Less then < 1 600 |
| Loose | Range 5 – 10 | 1 530 – 2 000 |
| Medium | Range 11 – 30 | 1 750 – 2 100 |
| Dense | Range 31 – 50 | 1 750 – 2 245 |
| Very dense | More then > 50 | More then > 2 100 |
Procedure
- We select the test location based on the site investigation report.
- We position a tripod at the testing site.
- We attach the driving head to the drill rods and insert the drill rods into the ground until reaching the desired depth.
- We attach the sampler to the driving head and lower it into the drill hole.
- We elevate the hammer to a height of 760 mm and then release it to impact the driving head. We document the count of blows needed to propel the sampler through the initial 150 mm as the N-value.
- We then drive the sampler an additional 150 mm and record the number of blows required to achieve this as the N-value for the second interval.
- We repeat the test at 150 mm intervals until we reach the desired depth or encounter refusal.
- We gather soil samples and subsequently subject them to laboratory testing to evaluate their attributes and characteristics.
Calculation
The N-value, often referred to as the Standard Penetration Resistance, calculates by dividing the energy applied by the hammer for each blow by the total count of blows required to drive the sampler through the initial 150 mm of its length. It is reported in blows per 30 cm of penetration.
Correlation with soil mechanical properties
Despite its several limitations, it is customary to establish correlations between SPT results and soil properties that are important for geotechnical engineering design. SPT results represent in-house field measurements and are often the only test results available. Consequently, the adoption of such correlations has become a common practice in many countries.
The table below presents an illustrative example of the approximate relationship between SPT N-values, relative density, and bulk density for coarse-grained materials. The U.S. Army Corps of Engineers cites this correlation in the Engineering Manual for Sheet Pile Design, which they advanced after the works of Terzaghi and Peck (1948) and Teng (1962).
Determining the N-value in accordance with the BS EN ISO 22476-three trendy involves the following steps:
Step 1: Computation of the Corrected Number of Blows (Nc)
The Corrected Number of Blows (Nc) is determined by applying the following formula:
Nc = N[(60/tn)^(0.5)]/100
In this formula, N represents the total number of blows, and tn denotes the time taken for the final 30 cm of penetration in seconds.
For instance, if the general form of blows is 25, and the time taken for the final 30 cm of penetration is 15 seconds, then:
Nc = 25[(60/15)^(0.5)]/100
Nc = 25[(4)^(0.5)]/100
Nc = 25(2)/100
Nc = 0.5
Step 2: Calculate the energy ratio (Er)
The power ratio (Er), at its middle, is fairly defined because the ratio of the power transferred to the drill rod in evaluation to the electricity transferred to the pattern all through the penetration technique. It’s really worth noting that you can effortlessly and exactly calculate this ratio by means of using the following formulation:
Er = (Wh – Ws)/(Ws – Wd)
The formula is as follows, with its components:
Er = (Wh – Wd) / Ws
Where:
- Wh represents the energy transferred to the drill rod during the hammer blow.
- Ws stands for the energy transferred to the sample.
- Wd accounts for the energy lost due to friction.
Step 3: Calculate the corrected N-value (N60)
The Corrected N-value (N60) is determined using the following formula:
N60 = Nc/[(Er/60)^(0.5)]
where Nc is the corrected number of blows, and Er is the energy ratio.
For instance, let’s consider a scenario in which the corrected number of blows (Nc) is 0.5, and the energy ratio (Er) stands at 0.8. In this situation:
N60 = 0.5/[(0.8/60)^(0.5)] N60 = 0.5/[(0.0133)^(0.5)] N60 = 0.5/0.1155 N60 = 4.33
Therefore, the N-fee for this unique soil layer is 4.33.
It is of good sized significance to emphasize that, in the realm of geotechnical engineering, the BS EN ISO 22476-three wellknown plays a pivotal position in offering complete instructions and particular corrections for N-price calculations. These corrections are crucially contingent upon numerous essential elements, such as the ideal kind of sampler hired and the period of the rod utilized inside the take a look at. Therefore, this standard serves as a well-structured framework, ensuring the precision and reliability of N-value assessments in various geotechnical contexts. These corrections should be applied to obtain accurate results.
Conclusion
The Standard Penetration Test (SPT) is an critical take a look at in geotechnical engineering to evaluate the houses of soils and rocks. The take a look at results offer valuable statistics for foundation design, slope stability analysis, and different geotechnical applications. The experiment follows the techniques mentioned within the BS EN ISO 22476-three and affords accurate and reliable statistics for engineering evaluation and layout.
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