Correcting SPT N values to Hammer Energy and Overburden Pressure | Formulas and Procedure

Summary

This video explains how to normalize blow counts from a Standard Penetration Test (SPT) by correcting for 60% of the total energy used and adjusting for overburden pressure. The process involves using correction factors (ch, cr, cs, cb) for hammer, rod length, sampler, and borehole diameter. Additionally, it covers calculating effective stress at a given depth using Liao and Whitman's method, which requires total and pore water pressure data. Finally, the corrected blow counts are determined for both energy and overburden pressure to obtain normalized SPT values.

Takeaways

• 🔨 The video explains how to normalize blow counts from the Standard Penetration Test (SPT) for 60% of total energy used.
• 📏 The correction factors applied in the process include ch (hammer), cr (rod length), cs (sampler), and cb (borehole diameter), all provided in a table.
• 🔢 The sum of the second and third numbers from the SPT is used to determine the raw blow count (N). In the example, N equals 7.
• ⚙️ To normalize for energy, the correction factor is calculated as 1.3 (from the hammer) multiplied by the correction factors for rod length, sampler, and borehole diameter (all equal to 1). The final correction factor is 1.3.
• 🧮 The normalized blow count (N60) for energy is calculated by multiplying the raw blow count (7) by the energy correction factor (1.3), resulting in 9.1.
• 🌍 The overburden pressure is normalized using a relationship proposed by Liao and Whitman, requiring the effective stress at a given depth.
• 📉 Effective stress is calculated by subtracting pore water pressure from total stress, which, in this case, is assumed to be zero as no groundwater was encountered.
• ⚖️ The total stress at a depth of 4.3 meters is calculated by multiplying the assumed unit weight of the soil (18 kN/m²) by the depth, resulting in 77.4 kPa.
• 🔢 The correction factor for overburden pressure (Cn) is calculated using the formula provided by Liao and Whitman, resulting in a value of 1.115.
• ✅ The final normalized blow count is calculated by multiplying the energy-normalized blow count (9.1) by the correction factor for overburden pressure (1.115), giving a result of approximately 10.1.

Q & A

What is the purpose of normalizing blow counts from the Standard Penetration Test (SPT)?

The purpose of normalizing blow counts from the SPT is to correct the results for variations in energy input (such as 60% of total energy) and to account for overburden pressure to standardize and compare the blow count results across different conditions.

What formula is used to normalize the blow counts for 60% of total energy?

The formula to normalize the blow counts for 60% of total energy includes several correction factors: ch (hammer), cr (rod length), cs (sampler), and cb (borehole diameter). These factors are used to adjust the blow count to reflect consistent energy conditions.

What are the correction factors mentioned in the script, and what do they represent?

The correction factors are ch (hammer type), cr (rod length), cs (sampler type), and cb (borehole diameter). They are used to adjust the blow count based on specific test conditions like the type of hammer used, the length of the rods, the type of sampler, and the borehole diameter.

How is the corrected blow count (N60) calculated?

N60 is calculated by multiplying the raw blow count (N) by the correction factor for energy (Cr), which is determined based on the type of equipment used in the test. For example, if N is 7 and Cr is 1.3, N60 = 7 * 1.3 = 9.1.

What is the next step after normalizing the blow count for energy?

The next step is to normalize the blow count for overburden pressure, which requires calculating the effective stress at the test depth using the total stress and pore water pressure.

How is the total stress calculated at the test depth?

The total stress is calculated by multiplying the unit weight of the soil by the depth at which the test is conducted. In the example, the unit weight is assumed to be 18 kN/m², and the depth is 4.3 meters, resulting in a total stress of 77.4 kPa.

How is effective stress determined, and what role does pore water pressure play?

Effective stress is determined by subtracting the pore water pressure from the total stress. If no groundwater is present (as in this example), the pore water pressure is zero, making the effective stress equal to the total stress (77.4 kPa in this case).

What formula is used to find the correction factor for overburden pressure (Cn)?

The formula for the correction factor for overburden pressure (Cn) is Cn = 9.78 * √(1 / effective vertical stress). In the example, with an effective vertical stress of 77.4 kPa, Cn is calculated to be approximately 1.115.

How is the final corrected blow count determined after applying both energy and overburden pressure corrections?

The final corrected blow count is determined by multiplying the blow count corrected for energy (N60) by the correction factor for overburden pressure (Cn). In the example, N60 is 9.1, and Cn is 1.115, so the final corrected blow count is approximately 10.1.

What assumptions were made in the example to simplify the calculations?

Several assumptions were made: the unit weight of soil was assumed to be 18 kN/m², there was no groundwater encountered (so pore water pressure was zero), and specific correction factors for equipment like the hammer and sampler were used based on common conditions.