Foundations

The excavation for the next building down Portage Avenue is a deep, square hole with a drilling rig in the bottom. A steel auger the width of a dinner plate screws into the grey clay, pulls out, and is followed by a cage of reinforcing bars and a pour of concrete. 'That is a pile,' Professor Lindqvist says. 'There will be a few dozen of them under this building. Winnipeg sits on the bed of an old glacial lake, and the clay down there swells when it is wet and shrinks when it dries. You do not want to rest a building on it if you can help it.' Wren asks how deep the piles go. 'Down to where the soil is stiff enough, or to the till and limestone further below. The geotechnical engineer will tell us.'

What foundations do

The foundation is the part of a building that transfers its loads into the ground. It must do three things. It must carry the loads without the soil failing, which would let the building sink or tilt suddenly. It must keep settlement, the gradual sinking of the building as the soil compresses, small and even, so that walls do not crack and floors stay level. And it must resist the other forces that act at the base of a building: frost heave, the swelling and shrinking of clay, groundwater pressure, uplift from wind and, in some regions, earthquakes.

Foundations are usually the first thing built and the hardest to fix. A crack in a wall can be patched; a foundation that settles unevenly can make a building unusable, and repairing it means working under a building that is already there. That is why foundation design starts with an investigation of the ground.

Soils and rock

Soils are classified by the size of their particles. Gravel and sand are coarse-grained: their particles are visible, water drains through them easily, they compress little and quickly under load, and they are generally good for foundations if they are dense. Silt and clay are fine-grained: their particles are microscopic, water drains through them very slowly, and they compress slowly over years. Clays can be stiff or soft, and some clays expand greatly when wet and shrink when dry. Organic soils such as peat compress enormously and are unsuitable for foundations. Bedrock is generally the best bearing material, although some rocks are fractured or soluble.

Much of southern Manitoba lies on the bed of glacial Lake Agassiz, which covered the region at the end of the last ice age. Its sediments are thick, highly plastic clays. They hold water, shrink and crack in dry summers, swell in wet years and can creep. Shifting clay is the reason so many older Winnipeg houses have cracked basement walls and sloping floors, and the reason larger buildings in the city usually stand on piles that pass through the clay to stiffer till or to the limestone bedrock below.

Before designing a foundation, a geotechnical engineer drills boreholes on the site, logs the layers, takes samples and tests them. The report gives an allowable bearing pressure, or a factored bearing resistance, for each soil layer, the expected groundwater level, recommendations for the type and depth of foundation, and warnings about problems such as expansive clays, high water or contaminated fill. For a small house the code allows simple assumptions; for anything larger, a site investigation is essential.

Frost

In a cold climate, frost is a constant concern. When water in the soil freezes, it expands by about 9%, but the real damage comes from frost heave: as the ground freezes, water is drawn up from below to the freezing front, where it forms growing lenses of ice that lift the ground, and anything resting on it, by many centimetres. Fine soils such as silts are the worst, because they hold water and let it rise by capillary action. When the ice thaws in spring, the soil is saturated and weak.

The usual protection is to place footings below the depth of frost penetration. In southern Manitoba, frost can penetrate well over a metre into the ground, and two metres or more under snow-cleared driveways and parking lots, so footings for heated houses are set deep, typically in a full basement, and unheated structures such as garages and decks need deep footings, piles or special insulated designs. The heat lost from a heated basement helps keep the soil around it from freezing.

An alternative is the frost-protected shallow foundation, developed in Scandinavia and now accepted in North American codes. Rigid insulation is laid horizontally around the building at the footing level, trapping the earth's heat and the heat escaping from the building, so the soil under the footing never freezes. This allows a shallow footing, saving excavation and concrete, and is common in Norway and Sweden.

Shallow foundations

Shallow foundations spread the load of the building over enough soil that the pressure stays below what the soil can safely carry. They are used where suitable soil is found near the surface.

• A strip footing, or continuous footing, is a long concrete pad under a wall. It is wider than the wall, so the wall's load spreads over a larger area.
• A pad footing, or spread footing, is a square or rectangular pad under a single column.
• A combined footing carries two or more columns that are too close to have separate pads.
• A mat, or raft, is a thick reinforced slab under the whole building. It is used where soil is weak and the pads would nearly touch, or where differential settlement must be minimized.
• A slab on grade is a concrete floor resting directly on compacted gravel, often with thickened edges acting as footings. It is common for garages, warehouses and houses in warmer regions.

The width of a footing follows from a simple idea: pressure equals force divided by area. If a wall carries a certain load per unit length and the soil can carry a certain pressure, the footing must be at least as wide as the load divided by the pressure.

# Footing sizes from the allowable bearing pressure (service loads)
q_allow = 100.0        # kPa, from the geotechnical report (stiff clay)

# strip footing under a wall
w = 45.0               # kN per metre of wall
B = w / q_allow
print(f"Strip footing: width at least {B:.2f} m -> use {max(B, 0.4):.2f} m")

# pad footing under a column
P = 600.0              # kN on the column
side = (P / q_allow) ** 0.5
print(f"Pad footing: area {P / q_allow:.1f} m2 -> square {side:.2f} m by {side:.2f} m")

# what if the soil is only half as strong?
for q in (100, 75, 50):
    print(f"q = {q:3d} kPa -> pad side {(P / q) ** 0.5:.2f} m")

Strip footing: width at least 0.45 m -> use 0.45 m
Pad footing: area 6.0 m2 -> square 2.45 m by 2.45 m
q = 100 kPa -> pad side 2.45 m
q =  75 kPa -> pad side 2.83 m
q =  50 kPa -> pad side 3.46 m

Halving the bearing pressure increases the footing area by two, but the side of a square pad only by about 1.4. On very weak soils, pads become so large that a mat or piles become more economical.

Deep foundations

Where the soil near the surface is too weak, too compressible or too changeable, the load is carried down to stronger layers by deep foundations.

• End-bearing piles transmit load through their tips to rock or a dense layer, like columns standing in the ground.
• Friction piles carry load by friction or adhesion along their sides, in soils with no firm layer within reach. Many piles in clay carry load both ways.
• Piles can be driven, hammered into the ground as precast concrete, steel H-sections, steel pipes or timber, or cast in place in a drilled hole, as at the site down the street.
• Caissons or drilled piers are large-diameter shafts, sometimes with a belled base, carrying heavy column loads.
• Helical or screw piles are steel shafts with a spiral plate, screwed into the ground with small machines. They are quick and used for decks, additions, cabins and light buildings.

Piles are usually connected at the top by a pile cap, a thick block of concrete under a column, or by a grade beam, a reinforced concrete beam that spans between piles and carries a wall. In expansive clay, grade beams are often built over a void form, a cardboard or foam layer that collapses as the clay swells, so the clay pushes on nothing instead of lifting the beam.

Basements, drainage and water

A basement is a storey partly or wholly below ground. Its walls are both foundations and retaining walls: they carry the building down to the footings and resist the sideways pressure of the soil and any groundwater. Basement walls are usually cast concrete or, less often today, concrete block. Insulated concrete forms, hollow foam blocks filled with concrete, are also used.

Water is the main enemy of a basement. Good practice is to keep water away rather than fight it at the wall: slope the ground away from the building, extend downspouts, place a perforated drain pipe, the weeping tile or footing drain, in clean gravel beside the footing to collect water and lead it to a sump or storm sewer, and protect the wall. Damp-proofing, a coating that slows moisture, is used where groundwater is not a problem; waterproofing, a continuous membrane that resists water under pressure, is needed where the water table rises above the floor. A drainage board against the wall gives water an easy path down to the drain. Inside, a vapour retarder under the slab and insulation on the walls help keep the basement dry and warm.

Exercise 3: reading a geotechnical report

This exercise takes about three hours.

• Find a published geotechnical report for a building in your region; many cities post them with planning applications, or your instructor will provide one.
• Draw the borehole log as a vertical strip, with each soil layer, its thickness and the groundwater level.
• Record the recommended foundation type, the bearing pressure and the frost depth.
• Sketch the recommended foundation in section at 1:50 (1/4 in = 1 ft), showing the footing or pile, the wall, drainage and insulation.
• Deliverable: the borehole strip, the section and a paragraph explaining why the engineer chose this foundation.

Self-critique checklist

• Have I identified the layer the foundation actually bears on?
• Is the bottom of the footing below frost depth, or protected?
• Have I shown how water is kept away from the foundation?
• Have I noted any special soil problems, such as expansive clay?