Foundations and retaining structures

Every load in a building ends up in the ground. Foundations spread those loads into soil or rock that can carry them without excessive settlement, and retaining structures hold back soil where the ground level changes. Foundations are also where design assumptions meet the real site: the geotechnical report predicts what the soil will be, but the excavation reveals what it actually is. Constructors are often the first to see the difference, and their decisions about excavation, dewatering, bearing surfaces and backfill directly affect how the foundations perform. This lesson explains how foundations and retaining walls work and what constructors must check.

CON-130 covered soils, site investigation and earthworks. This lesson builds on it with structural behavior.

The geotechnical report and the foundation design

Foundation design starts with the geotechnical investigation: boreholes or test pits, laboratory testing and a report that describes the soil and groundwater and recommends foundation types and design values. Key values for constructors include the allowable or factored bearing pressure for footings, the pile or caisson capacity, the groundwater level, the earth pressure coefficients for walls and shoring, and recommendations for excavation, dewatering, subgrade preparation and backfill.

In the United States, foundation design follows IBC Chapter 18 and the geotechnical report, typically using an allowable bearing pressure for footings with a factor of safety against bearing failure and a settlement limit, while the concrete is designed to ACI 318.

The engineer's design assumes the soil at the bearing level matches the report. If the excavation reveals softer soil, fill, organic material, groundwater or rock at a different level, the geotechnical engineer must be called to review before concrete is placed. Most specifications require the geotechnical engineer or a qualified technician to inspect and approve every footing subgrade before forms and reinforcing are placed.

Shallow foundations

Shallow foundations transfer load to soil near the surface. They are the most economical choice where competent soil exists at a reasonable depth.

A footing works by spreading the column or wall load over enough area that the pressure on the soil stays within the allowable value. The basic relationship is:

Subgrade protection matters as much as design. Footing subgrades disturbed by equipment, softened by rain or groundwater, or frozen will settle. Common controls include excavating the final layer by hand or with a smooth-edged bucket, placing a mud slab (lean concrete) on sensitive soils, keeping excavations dry, and protecting subgrades and new footings from frost.

In the United States, footings must extend below the frost line as set by the local building department, ranging from about 12 in in the South to 48 in or more in northern states and over 100 in in parts of Alaska. Never place concrete on frozen ground, and protect new footings from freezing.

Deep foundations

Deep foundations transfer load through weak upper soils to stronger soil or rock below, or carry load by friction along their length. They are used where shallow soil is weak, loads are heavy, scour or expansive soil is present, or uplift and lateral loads must be resisted.

Pile capacity comes from end bearing at the tip and skin friction along the shaft. Driven piles are often accepted based on driving resistance (blows per unit penetration) and dynamic testing with a pile driving analyzer; drilled shafts are accepted based on the soil or rock observed at the base and the shaft's integrity. Load tests may be required. Pile caps tie groups of piles together under columns and must be built to tolerance, since piles out of position create eccentric loads.

Retaining walls and earth pressure

A retaining wall holds back soil where the ground level changes. Soil pushes sideways on the wall with a pressure that increases with depth, much like water but smaller. The lateral earth pressure depends on soil type, whether the wall can move, groundwater and surcharge loads on the ground behind the wall.

• Active pressure develops when the wall can move slightly away from the soil, as with a free-standing cantilever wall. It is the lowest pressure.
• At-rest pressure applies when the wall cannot move, as with a basement wall braced by floors.
• Passive pressure is the resistance of soil when a wall is pushed into it, as at the toe of a wall or in front of a soldier pile embedment.

Construction issues for retaining and basement walls are among the most common causes of foundation problems:

• Backfilling basement walls before the floor slab and ground floor are in place. A basement wall designed to be propped at top and bottom by floors acts as a cantilever if backfilled first, and can crack or fail. Backfill only when the design allows, or brace the wall temporarily.
• Compacting backfill with heavy equipment close to the wall, which raises the pressure far above design values. Use light compaction equipment near walls.
• Drainage. Hydrostatic pressure from water trapped behind a wall can double the load. Weep holes, drainage board and perimeter drains must be installed and kept clear.
• Surcharges. Stockpiles, cranes, trucks and material storage behind a wall add pressure. Keep them back or have the engineer check.

Underpinning and work next to existing buildings

When excavating next to an existing building, its footings may be undermined. Underpinning extends the existing foundation down to below the new excavation level, typically in short alternating sections (pins) so that the building is never unsupported over a long length. Underpinning, shoring and monitoring of adjacent structures are engineered and need careful sequencing, condition surveys of neighboring buildings before work, and settlement and vibration monitoring during work.

Groundwater and dewatering

Groundwater affects foundations in several ways. Water in an excavation softens subgrades and can cause the base of an excavation in sand or silt to boil or heave if the water pressure below exceeds the weight of soil above. Dewatering lowers the water table so that work can proceed in the dry, using sumps, wellpoints or deep wells. Dewatering itself can cause problems: lowering the water table under neighboring buildings can cause settlement, especially in soft clays or where old timber piles may decay when exposed to air.

Permanent groundwater also pushes upward on basement slabs and outward on walls. A basement below the water table must either resist the uplift with its weight, tie-down anchors or tension piles, or relieve it with permanent drainage. During construction, an incomplete basement may not yet be heavy enough to resist uplift, so dewatering must continue until enough weight is in place, as set by the engineer.