Engineering & Technology · Geotechnical Engineering & Ground Systems

Shallow and deep foundations and soil-structure interaction

A foundation connects a structure to a ground volume. Its job is not merely to have a pressure smaller than a tabulated number: the structure and ground must remain compatible as they deform.

Review force balance, normal and shear stress, strain, density versus unit weight, fluid pressure and hydraulic head. Read graph axes and convert kN/m² to kPa, metres to millimetres and seconds to days. Ground below the surface is spatially variable; the lessons introduce every soil-specific symbol and keep saturated, hydrostatic and loading assumptions explicit.

Back to the chapter

After reading, you can

Trace loads through shallow and deep foundations while recognizing deformation compatibility and soil–structure interaction.

Follow the load into the ground

Shallow foundations include individual spread footings, strips and mats that distribute load near the surface. Deep foundations such as piles and drilled shafts transfer load to greater depths through shaft interaction, base interaction or both. “Deep” does not mean that every load reaches intact bedrock or that upper layers become irrelevant.

The chosen foundation must address vertical and lateral loads, moments, uplift and movements relevant to its service. Ground shear failure, structural failure of the foundation element and unacceptable settlement are different limits. Groundwater, nearby excavations, scour or erosion and construction disturbance can change the load path and its resistance.

Equilibrium and compatibility must agree

For centred vertical loading with an assumed uniform contact pressure, average pressure is P/A. Actual contact pressure can vary with eccentricity, footing flexibility and nonuniform ground. A larger footing reduces this average at fixed P, but also changes the ground volume affected. It may engage a deeper compressible layer.

Pile shaft and base resistance develop with relative movement and can reach important levels at different displacements. Do not add unrelated maximum values and assume they occur simultaneously. The figure instead supplies one compatible transfer state and checks its axial equilibrium. Pile weight and other effects are deliberately omitted.

In a group, surrounding ground deforms under several elements and their stress fields interact. Group settlement and resistance need not equal a simple multiple of a single isolated pile. Settling soil can also drag downward on a pile: relative ground movement can reverse the direction of shaft action. Construction method and loading history belong with the model.

Depth 0–10 m increases downward at 22 units/m. Axial compressive force N is horizontal 0–600 kN at .65 units/kN. A straight profile decreases from 600 kN at head to 200 at base, corresponding to constant shaft transfer 40 kN/m over 10 m, total 400 kN. Base transfer is 200 kN, so 600=400+200. Pile weight is omitted. Supplied transfers are at one assumed compatible displacement state, not independently added ultimate resistances or allowable loads.

A moving support changes the structure above

Soil–structure interaction is the two-way relationship between ground deformation and structural force distribution. A stiff mat can redistribute load as a softer patch compresses; a flexible support system may deform differently. In a frame with redundant load paths, differential support movement can introduce forces even without an increase in external gravity load.

Simple springs or an elastic ground continuum can help model that interaction, provided their stiffness, scale, loading range and boundaries are justified. A spring constant is a model parameter, not a universal label attached to a soil type. Compare predictions with relevant site and load-test evidence and update the model when the observed mechanism differs.

Check pressure, transfer and distortion separately

For an original centred load of 600 kN, a 2×2 m footprint has average pressure 150 kPa; a 3×3 m footprint gives about 66.7 kPa. These calculations alone do not identify allowable pressure or predict settlement. The increased footprint has changed more than one stress value.

Separately, the constructed 10 m pile in the figure has constant upward shaft transfer 40 kN per metre at an assumed compatible displacement. Integrated transfer is 400 kN. A further 200 kN base reaction balances the 600 kN head load. Axial force decreases linearly from 600 to 200 kN; this is not an ultimate capacity or proof of group behaviour.

For two other supports 8 m apart, suppose supplied settlements are 12 and 28 mm. Their difference is 16 mm and the simple differential-settlement/span ratio is 16/8000=.002. Whether that movement is acceptable requires the actual structure, connection and service criteria. None are supplied here.

Try the reasoning

Why is reduced average pressure not a complete reason to choose the larger footing?
Show the answer and reasoning
Its affected ground volume, settlement, eccentric response and construction conditions can also change.
What does the declining axial-force diagram demonstrate?
Show the answer and reasoning
At the assumed state, load leaves the pile through shaft interaction before the remaining force reaches the base.
Why does multiplying a single-pile resistance by pile count require scrutiny?
Show the answer and reasoning
Group interaction, movement compatibility, construction effects and shared ground can change the mechanism.

What to carry forward

Check equilibrium, failure modes and compatible movements for the complete foundation–ground–structure system.

References

Teaching resources and research supporting this lesson’s explanations and examples.