Understanding Soil Bearing Capacity Tests

Soil bearing capacity is a fundamental concept in geotechnical engineering. It refers to the maximum average contact pressure between the foundation and the soil which should not produce shear failure in the soil. Determining this capacity accurately is crucial for designing safe and stable structures. Various tests are conducted to estimate the bearing capacity of soil, both in the laboratory and in the field.

Analyzing Field Tests for Bearing Capacity

The question asks for the most reliable field test to determine the bearing capacity of a soil. Let's look at the options provided:

• Plate load test: This is a field test conducted by loading a rigid plate placed on the soil surface or at the footing level and measuring the settlement. It directly simulates the effect of a footing resting on the soil and provides a direct measure of the ultimate bearing capacity and settlement characteristics for the specific area tested.
• Direct shear test: This is primarily a laboratory test used to determine the shear strength parameters (cohesion, c, and angle of internal friction, φ) of a soil sample. These parameters are then used in theoretical formulas to calculate the bearing capacity. While important, it's a laboratory test on a small, often disturbed sample, not a direct field determination of bearing capacity.
• Sounding test: This is a general term that can refer to various penetration tests like Standard Penetration Test (SPT) or Cone Penetration Test (CPT). These are field tests that measure the resistance of the soil to penetration. The results (like SPT N-value or CPT tip resistance) are then correlated with established empirical relationships to estimate bearing capacity. While useful for characterizing soil layers and estimating bearing capacity, they rely on correlations rather than direct load-settlement measurement under footing-like conditions.
• Cone penetration test (CPT): As mentioned under Sounding test, CPT is a specific type of penetration field test. It measures tip resistance and sleeve friction as a cone is pushed into the ground. Similar to SPT, CPT results are correlated to estimate soil properties and bearing capacity. It provides continuous data but still relies on empirical relationships for bearing capacity determination, which may have limitations depending on the soil type and correlation used.

Why Plate Load Test is Considered Most Reliable in the Field

Among the given options that can be conducted in the field for determining bearing capacity, the Plate load test stands out as the most reliable for the following reasons:

• Direct Measurement: It directly measures the load-settlement behavior of the soil under a loaded plate, which closely mimics the behavior of a foundation. This provides a direct estimate of the ultimate bearing capacity and allows for the calculation of allowable bearing capacity based on settlement criteria.
• Simulates Footing Condition: Although the plate is smaller than a typical footing, the test provides insight into how the soil will behave under load at the tested location and depth.
• Provides Settlement Data: Besides ultimate bearing capacity, the test also provides valuable settlement data at different load increments, which is critical for serviceability limit state design.

While penetration tests (like CPT and Sounding tests) are widely used, faster, and provide continuous soil profiles, their bearing capacity estimates are based on correlations which can introduce variability and uncertainty. The Direct Shear test is a lab test, not a field test for direct bearing capacity measurement.

Therefore, the Plate load test is generally considered the most reliable field test when a direct assessment of the bearing capacity and settlement characteristics at a specific location is required.

Revision Table: Soil Bearing Capacity Determination

Additional Information on Soil Tests and Bearing Capacity

While the Plate load test provides reliable local bearing capacity information, it has limitations:

• It only tests a small area, and the results are primarily influenced by the soil properties within a depth of about 1.5 to 2 times the plate width below the plate. This may not represent the behavior of soil under a much larger foundation.
• It can be time-consuming and expensive compared to penetration tests.
• Scaling the results from the plate size to the actual footing size requires careful consideration and appropriate scaling factors, especially for cohesive soils.

For this reason, geotechnical investigations often combine multiple tests, using penetration tests (SPT, CPT) for overall site stratification and property profiling, and supplementing them with occasional Plate load tests or laboratory tests on collected samples for more detailed bearing capacity analysis and validation.

The allowable bearing capacity used for design is typically determined by dividing the ultimate bearing capacity by a factor of safety and/or considering the settlement criteria. Both ultimate shear failure and excessive settlement must be prevented for a safe foundation design.