Introduction:
Penetration tests are commonly used in civil engineering to evaluate the properties of soil and determine its suitability for construction purposes. These tests involve driving a probe into the ground and measuring the resistance encountered at various depths. The values derived from these tests provide valuable information about the soil's characteristics, which can be used for various purposes.

Explanation:
The correct answer is option 'A' - Depth of hard stratum and Strength of soil. The values derived from penetration tests can indeed be used to determine the depth of a hard stratum and the strength of the soil.

Depth of hard stratum:
Penetration tests provide information about the resistance encountered during the test. When the probe encounters a hard stratum, the resistance will increase significantly. By analyzing the data obtained from the test, engineers can identify the depth at which the hard stratum is present. This information is crucial for various construction activities, such as determining the foundation depth or planning excavation works.

Strength of soil:
The resistance encountered during penetration tests is also an indication of the strength of the soil. Soils with higher resistance values typically have greater strength, while soils with lower resistance values are generally weaker. By analyzing the resistance values obtained at different depths, engineers can assess the overall strength characteristics of the soil. This information is important for designing structures and determining the appropriate construction techniques.

Other uses of penetration test values:
In addition to determining the depth of a hard stratum and the strength of the soil, the values derived from penetration tests can also be used for the following purposes:

1. Bearing capacity evaluation: The resistance values obtained from penetration tests can be used to estimate the bearing capacity of the soil. This information is crucial for designing foundations and ensuring the stability of structures.

2. Soil classification: Penetration test data, along with other information such as moisture content and grain size distribution, can be used to classify the soil according to various classification systems. This classification helps in understanding the soil behavior and selecting appropriate construction techniques.

3. Settlement analysis: The resistance values obtained from penetration tests can be used to estimate the settlement characteristics of the soil. This information is important for predicting the long-term behavior of structures and ensuring their stability.

Conclusion:
In conclusion, the values derived from penetration tests provide valuable information about the depth of a hard stratum, the strength of the soil, and other important soil characteristics. This information is essential for various civil engineering applications, including foundation design, construction planning, and structural stability assessment.