Standard penetration resistance (N-value) is a measure of the strength of soil. It is determined by the number of blows required for a standard split-spoon sampler to penetrate the soil to a certain depth. In very stiff clays, the standard penetration resistance typically falls within a range of 15 and 30 blows per foot.

Explanation:

What is Standard Penetration Resistance?
Standard penetration resistance, often referred to as N-value, is a widely used geotechnical parameter to measure the strength and consistency of soil. It is determined by conducting a standard penetration test (SPT), which involves driving a split-spoon sampler into the soil using a hammer. The number of blows required for the sampler to penetrate the soil by a certain depth is recorded as the N-value.

Very Stiff Clays
Very stiff clays are cohesive soils with high strength and low permeability. They are typically characterized by their high plasticity, low compressibility, and high shear strength. Due to their stiffness, penetrating very stiff clays can be challenging, and the N-value provides an indication of their resistance to penetration.

Standard Penetration Resistance in Very Stiff Clays
In very stiff clays, the standard penetration resistance usually falls within a range of 15 and 30 blows per foot. This means that the split-spoon sampler needs to be driven into the soil with a hammer multiple times, typically between 15 and 30 blows, to achieve a penetration of one foot. The higher the number of blows required, the stiffer and more resistant the clay is.

Implications of Standard Penetration Resistance
The N-value obtained from the SPT can be used to assess various geotechnical properties of the soil. In the case of very stiff clays, the high N-value indicates the following characteristics:

1. Strength: The high resistance to penetration suggests that very stiff clays have a high shear strength, making them suitable for supporting heavy structures.

2. Compressibility: Very stiff clays have low compressibility, meaning they undergo minimal settlement when subjected to loads. This property makes them favorable for construction projects.

3. Permeability: Very stiff clays have low permeability, which restricts the flow of water through the soil. This can be advantageous in preventing seepage and maintaining stability.

Conclusion
In summary, the standard penetration resistance in very stiff clays typically falls within a range of 15 and 30 blows per foot. This range indicates the high strength, low compressibility, and low permeability characteristics of very stiff clays. Understanding the N-value helps engineers and geotechnical professionals make informed decisions during the design and construction of structures on these types of soils.