What is the lowest cost testing procedure for finding safe bearing capacity and settlement of shallow isolated footings with width ranging from 2 m to 4 m resting on dry deep deposit of loose to medium dense sand?
- ADirect shear test
- BTriaxial test
- CStandard Penetration Test (SPT)Correct
- DPlate load test
Solution
Concept
For granular soils (sand), obtaining undisturbed samples for laboratory testing is practically unfeasible and expensive, so in-situ tests like SPT are preferred to reliably and economically estimate bearing capacity and settlement.
Formula
qnet,safe = 32 × (N - 3) × ((B + 0.3) / (2B))² × Rw × Sa / 25 (Teng's formula for settlement limit)
Step-by-step solution
1. Identify the soil type and conditions: Dry deep deposit of loose to medium dense sand with footing width 2 m to 4 m. 2. In cohesionless soils, obtaining undisturbed samples for laboratory shear strength tests (Direct shear test or Triaxial test) is difficult, unreliable, and cost-prohibitive. 3. A Plate Load Test has significant scale effects because the pressure bulb of a 0.3 m to 0.75 m plate is much smaller than that of a 2 m to 4 m footing, and the test is cumbersome and relatively expensive. 4. Standard Penetration Test (SPT) is conducted routinely inside boreholes at low cost and provides N-values directly correlated to safe bearing capacity and settlement based on IS 6403 and IS 8009.
Explanation
Standard Penetration Test (SPT) is the most economical, widely used, and standard field testing procedure for dry, cohesionless sand deposits to directly determine both safe bearing capacity and settlement using empirical correlations (such as Peck-Hanson-Thornburn or Teng's formulas and IS 6403 / IS 8009 Part 1). Plate load tests are significantly more expensive, time-consuming, and have a severe scale effect for large footings (2 m to 4 m width) in deep sand deposits. Direct shear and triaxial tests require undisturbed sampling, which is practically impossible and extremely costly for cohesionless sands.
Why the other options are wrong
A: Direct shear tests require undisturbed cohesionless samples, which are virtually impossible and very costly to obtain from deep sand deposits. B: Triaxial tests also require undisturbed sampling and expensive laboratory setups, making them cost-prohibitive for routine testing. D: Plate load tests are expensive, time-consuming, and introduce significant scale-effect errors for large footings (2 m to 4 m) on deep sand.
Correct answer: C) Standard Penetration Test (SPT)