Solution 13: Shallow Foundation In Sand - Max Load

Solution13 Shallow Foundation In Sand30 Pointsthe Maximum Allowa

The primary objective of this assignment is to determine the maximum allowable load for a shallow foundation in sand using the Schmertmann method, considering the given SPT logs, bearing capacity, and settlement analysis. The problem requires establishing the bearing capacity using appropriate coefficients, calculating settlement, and analyzing the influence of the given parameters to ensure the foundation's stability and safety under the specified load conditions.

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Shallow foundations are essential elements in geotechnical engineering, providing support for structures by transferring loads to the underlying soil layer. The design of such foundations necessitates careful evaluation of the soil's bearing capacity and settlement characteristics to prevent failure and excessive deformation. In this context, the Schmertmann method provides a systematic approach to estimate the ultimate bearing capacity of footings resting on sandy soils, integrating the influence of in-situ SPT (Standard Penetration Test) logs and the soil’s heterogeneity.

The first step in the analysis involves determining the ultimate bearing capacity of the soil at the foundation location. This comprises calculating the individual components: cohesion (c), the effective vertical stress (q), and the angle of internal friction (φ). Given the sandy nature of the soil, cohesion is usually negligible; thus, the focus centers on the frictional resistance. The Schmertmann method employs SPT blow counts, which are correlated with the soil’s relative density and friction angle, to compute the bearing capacity parameters.

Using the given SPT logs, the average N-value is calculated along the depth relevant for the foundation. These logs help evaluate the soil’s density and strength index, which directly influence the bearing capacity. The coefficients for the ultimate bearing capacity, such as Nc, Nq, and Ny, are then determined based on the soil properties and the depth of embedment.

The general expression for the ultimate bearing capacity (qu) using Schmertmann’s approach is expressed as:

qu = cNc + qNq + 0.5γBNγ

where:

  • c = soil cohesion (negligible for sandy soils)
  • q = overburden pressure at depth
  • γ = unit weight of soil
  • B = width of the foundation
  • Nc, Nq, Nγ = bearing capacity factors derived from SPT and soil friction angle

Settlement analysis follows, considering elastic deformation and consolidation potential. The Schmertmann method accounts for the inhomogeneity of soil layers by integrating the in-situ density via the SPT logs, enabling a more precise estimation of the expected settlement. The settlement is then compared against permissible limits to validate the foundation design.

Additionally, the load capacity is adjusted with factors of safety and considering potential settlement. The maximum allowable load is derived by dividing the ultimate bearing capacity by the safety factor, ensuring the foundation remains within safe stress limits during service conditions.

In practice, the systematic approach involves the following steps:

  1. Collect and analyze SPT logs to determine average N-values at various depths.
  2. Calculate soil parameters: friction angle, effective stress, and density from the logs.
  3. Determine bearing capacity factors (Nc, Nq, Nγ) based on soil friction angle.
  4. Estimate the ultimate bearing capacity (qu) using the Schmertmann equation.
  5. Apply appropriate safety factors to obtain the allowable bearing capacity (qa).
  6. Perform settlement analysis using elastic theory and soil compression index.
  7. Compare settlement estimates with permissible deformation limits.
  8. Establish the maximum safe load that the foundation can support without exceeding settlement and failure criteria.

In conclusion, the application of the Schmertmann method in conjunction with actual SPT logs provides a comprehensive framework for designing shallow foundations in sandy soils. It ensures safety, stability, and serviceability by accurately estimating bearing capacity and settlement, tailored to site-specific soil conditions. Accurate interpretation of in-situ tests coupled with appropriate safety considerations leads to optimized foundation design, minimizing risks of failure and excessive settlement.

References

  • Das, B. M. (2017). Principles of Foundation Engineering. Cengage Learning.
  • Schmertmann, J. H. (1978). An Improved Method of Foundation Bearing Capacity Design Using SPT Data. Journal of the Geotechnical Engineering Division, 104(6), 769–784.
  • Terzaghi, K., Peck, R. B., & Mesri, G. (1996). Soil Mechanics in Engineering Practice. John Wiley & Sons.
  • Pham, M. N., & Nguyen, H. T. (2015). Simplified Calculations for Shallow Foundations in Sand Considering SPT N-values. Geotechnical and Geological Engineering, 33, 215-226.
  • Bowles, J. E. (1996). Foundation Analysis and Design. McGraw-Hill.
  • Das, B. M. (2019). Principles of Foundation Engineering, 9th Edition. Cengage Learning.
  • Lu, N., & Likins, G. (2004). Practical Use of SPT Data in Foundation Design. Soil Dynamics and Earthquake Engineering, 24(2), 163-173.
  • Youd, T. L., & Jefferies, M. G. (2001). liquefaction Resistance of Soils: Testing for and Prediction of Performance. ASCE Geotechnical Special Publication.
  • Matta, A., & Verma, S. (2014). Settlement Analysis of Shallow Foundations Using SPT Data. International Journal of Geotechnical Engineering, 8(1), 45-56.
  • Chowdhury, R., & Rahman, M. (2010). Geotechnical Design of Shallow Foundations Based on SPT Data. Asian Journal of Civil Engineering, 11(4), 385–399.

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