Settlement and Consolidation
How much a footing will settle and how long the clay below it takes to consolidate.
Distributes the footing pressure into the ground with Boussinesq theory, adds the immediate settlement of every layer and the primary consolidation of the clay layers from their compression index, void ratio and preconsolidation pressure, and follows the settlement with time by Terzaghi’s theory. It shows the stresses with depth, the depth of influence that boreholes must reach, the settlement curve against time and the share of each layer.
Overview
Most foundations are limited by settlement rather than by bearing failure, especially on the soft alluvial clays of central and southern Iraq. The tool answers the two questions a designer is asked: how much will it settle, and when.
Layers are entered from the ground surface down with their own properties, so a sand crust over soft clay over dense sand, the common profile of river plains, is handled directly.
What you enter · What you get
What you enter
- Footing shape, size, depth and net pressure; water table
- Soil layers: thickness, unit weight, modulus, Cc, e0, OCR and cv
- Drainage condition and allowable settlement
What you get
- Immediate, consolidation and total settlement
- Time to 50 % and 90 % consolidation
- Stress profile with depth and depth of influence
- Settlement–time curve and table by layer (CSV)
How it works in the field
Method and equations
Stress below the footing
Boussinesq elastic solution below the centre: Newmark’s corner formula applied to four quarter-rectangles, the closed form for a strip, and the axis solution for a circle.
Immediate and consolidation settlement
Immediate settlement adds Δσ·Δz / E over sublayers of 0.25 m. Primary consolidation uses the recompression index Cs up to the preconsolidation pressure σ′p = OCR·σ′v0 and the compression index Cc beyond it.
Time rate
Terzaghi one-dimensional consolidation for each clay layer with its own drainage path; the average degree of consolidation follows the standard approximations of Tv.
Accuracy and error limits
- Consolidation settlement computed from good oedometer parameters is usually within ±25 %; the time rate is far less certain and real clays often consolidate 2–10 times faster in the field than predicted from laboratory cv because of sand lenses. Immediate settlement from a single modulus is within about ±50 %.
- The method is one-dimensional below the centre: it overestimates settlement for small footings on thick clay (Skempton–Bjerrum correction 0.6–0.9 for overconsolidated clay) and ignores secondary compression, which adds 5–20 % in soft and organic clays over decades.
- To reduce the error: obtain Cc, e0, σ′p and cv from oedometer tests on undisturbed tube samples, drill beyond the depth of influence, and monitor settlement plates during construction to back-calculate the real rate.
Who uses it
Standards and references
- Terzaghi, K. (1943). Theoretical Soil Mechanics. Wiley.
- Newmark, N.M. (1935). Simplified computation of vertical pressures in elastic foundations. University of Illinois Engineering Experiment Station, Circular 24.
- Skempton, A.W. & Bjerrum, L. (1957). A contribution to the settlement analysis of foundations on clay. Géotechnique, 7, 168–178.
- Das, B.M. & Sivakugan, N. (2019). Principles of Foundation Engineering, 9th ed. Cengage.
- Holtz, R.D., Kovacs, W.D. & Sheahan, T.C. (2011). An Introduction to Geotechnical Engineering, 2nd ed. Pearson.
At a glance
- Category
- Engineering & geotechnics
- Runs on
- Phone, tablet and desktop browser
- Export
- Stress profile and settlement–time chart as PNG, JPG and PDF; sublayers as CSV; full PDF report
- Access
- Free
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