Raft Foundation: Types, Design and When to Use One

A raft foundation, also called a mat foundation, is a single reinforced concrete slab that carries all the columns and walls of a building and spreads their load over the whole footprint. It’s the usual choice when the soil is weak or varies across the site, when separate footings would cover more than about half of the plan, when the settlement between columns has to stay small, and when the building has a basement, above all one below the water table.

The guide ends with a worked example: a raft under twelve columns, its contact pressure worked out by hand with the rigid method and its bearing capacity checked with Rischio’s engine.

What a raft foundation is

A raft is a shallow foundation: like a footing, it bears on the soil near the surface and is checked with the same bearing capacity and settlement methods, but it joins all the columns into one slab. In the United States it’s usually called a mat foundation and in the United Kingdom a raft, and the two names mean the same thing.

With the columns joined, the load spreads over the largest area the building has, so the pressure on the soil drops, and the slab’s stiffness shares the settlement between neighbouring columns, bridging soft pockets in the ground that would let a single footing sink. When the soil is good enough for separate footings, they’re usually the simpler foundation; isolated, spread and combined footings covers them.

When to use a raft foundation

A raft is chosen for one or more of these reasons:

The soil is weak or variable. With a low allowable bearing pressure, footings grow until they nearly touch, and on ground that changes across the site each one settles differently.
Footings would cover more than about half of the building’s plan. This is a rule of thumb: past that point one slab is simpler to excavate, form and reinforce than many large footings with narrow strips of soil between them.
The differential settlement has to stay small, as under buildings with brittle finishes, machinery or tanks.
The building has a basement. The raft becomes its floor, and below the water table it keeps the water out and resists its uplift. The soil dug out for the basement also offsets part of the building’s weight, which is the idea behind the compensated raft below.

When even a raft would settle too much, because the compressible soil is thick or lies deep, the load has to go down to firmer ground on piles, or be shared between a raft and piles. Pile foundation design works out a pile’s capacity, and when the problem is only the top few metres, ground improvement can treat the soil in place.

Types of raft foundation

The five types of raft, each in section under two columns.

Rafts differ in where they put their thickness and how they get their stiffness:

A flat plate is a slab of uniform thickness. It’s the simplest to build and suits evenly spaced columns with similar, moderate loads. The heaviest column sets its thickness, so it carries extra concrete everywhere else.
A flat plate thickened under the columns adds a drop below each column, or a pedestal on top of the slab, where punching shear is highest, and keeps the rest of the slab thinner.
A beam-and-slab or ribbed raft runs beams along the column lines in both directions, with a thinner slab between them. The beams give it the stiffness for heavier loads and longer spans. They can hang below the slab in trenches cut in the ground, or stand above it with the floor built over them.
A cellular or box raft joins a bottom slab and a top slab with walls in both directions, often the basement’s own walls. The hollow box is far stiffer than a solid slab of the same weight, and it carries heavy buildings on poor ground, usually with deep basements.
A piled raft bears on the soil and on piles at once. The raft carries part of the load in bearing and the piles cut the settlement, so it needs far fewer piles than a pile foundation designed to carry everything; they usually go where the settlement would be largest, under the heaviest columns and towards the centre.

Compensated or floating rafts

Before the excavation, the soil at the level of the raft’s base was already carrying the weight of the soil above it. Once that soil is dug out, only the part of the building’s pressure that exceeds the weight removed is new load on the ground. That excess is the net pressure:

q_{net} = q_{gross} - \gamma D_f

where $q_{gross}$ is the pressure at the base, the building and the raft spread over its area, and $\gamma D_f$ is the vertical stress the excavated soil applied at that depth (with layers, the sum of each layer’s unit weight times its thickness).

When the two are equal, the raft is fully compensated, or floating, and the soil below carries about the stress it always did; the settlement is small, mostly the recompression of soil that swelled during the excavation. When the building weighs more, the raft is partly compensated and settles under the net pressure. That’s how many heavy buildings stand on soft clay: most of their pressure replaces soil that was dug out.

In the worked example below, the 3.0 m of soil removed for the basement weighed 54 kPa, which takes the raft’s average gross pressure of 93.7 kPa down to a net 39.7 kPa; a basement about 5.2 m deep would balance it completely. Geotechnical reports give allowable pressures as gross or net, and gross vs net bearing pressure explains how to tell which one a report means.

How a raft foundation is designed

Against the ground, a raft is checked for bearing capacity, for settlement and, below the water table, for uplift. As a concrete slab it’s checked for punching shear and flexure under the contact pressure, which comes from the rigid method or from a model on springs.

Bearing capacity of a raft

A raft’s ultimate bearing capacity comes from the same general equation as a footing’s, with the raft’s width $B$ , length $L$ and depth $D_f$ , and with $q = \gamma D_f$ the overburden at the base. Rischio’s bearing capacity method gives every factor:

q_u = c' N_c F_{cs} F_{cd} F_{ci} + q N_q F_{qs} F_{qd} F_{qi} + \tfrac{1}{2} \gamma' B' N_\gamma F_{\gamma s} F_{\gamma d} F_{\gamma i}

In sand the last term grows with the width, so a raft 16 m wide has a $\gamma$ term roughly ten times that of a footing 1.6 m wide on the same ground. Its ultimate capacity runs into thousands of kilopascals: 2,857 kPa in the worked example, under a maximum pressure of 107.5 kPa. On sand the bearing check passes with a wide margin, and the raft is sized for settlement.

In clay under undrained loading, $\phi = 0$ makes $N_\gamma = 0$ , and the width drops out of the capacity:

q_u = 5.14\, c_u \left(1 + 0.195\,\frac{B}{L}\right)\left(1 + 0.4\,\frac{D_f}{B}\right) + q

A soft clay with $c_u = 25$ kPa gives a raft of the example’s shape about 160 kPa above the overburden, however large the raft is, and a factor of safety of 3 leaves it about 53 kPa of net pressure. On soft clay the bearing capacity can govern, and for a compensated raft the factor of safety is taken on net pressures:

FS = \frac{q_u - \gamma D_f}{q_{gross} - \gamma D_f}

The free bearing capacity calculator works out $q_u$ with Rischio’s engine for a footing of any size, a raft’s included, with no account needed.

Settlement and differential settlement

Most rafts are sized by settlement, and a raft settles differently from a footing in two ways. The stress it adds reaches much deeper, to a depth of the order of its width, so a compressible layer 15 m down that a footing would never feel is loaded by a raft 16 m wide. And its stiffness spreads the settlement: a flexible raft on uniform ground settles into a shallow dish, deepest at the centre, while a stiff one settles more evenly and tilts if its load is off centre.

What cracks a building is the difference in settlement between columns, measured as the angular distortion: the differential settlement divided by the distance between the two columns, usually limited to about 1/500 for ordinary buildings. Since a raft keeps that difference small, it’s common to accept more total settlement under a raft than under separate footings. Foundation settlement works through the immediate and consolidation methods and their limits, and on clay the parameters come from the oedometer test.

Contact pressure: the rigid method

The structural design of a raft starts from the soil pressure under it. If the raft is stiff compared with the soil, it stays plane as it settles, and the pressure under it varies linearly. That’s the conventional rigid method that ACI 336.2R sets out for mats. With the origin at the raft’s centroid, the pressure at a point $(x, y)$ is

q = \frac{Q}{A} \pm \frac{M_y\,x}{I_y} \pm \frac{M_x\,y}{I_x}

where $Q$ is the total column load, $A = BL$ the raft’s area, $M_y = Q\,e_x$ and $M_x = Q\,e_y$ the moments of the load about the centroidal axes, and $I_y = BL^3/12$ and $I_x = LB^3/12$ the second moments of area of the plan, with $L$ along $x$ and $B$ along $y$ . The eccentricities come from the resultant of the column loads:

e_x = \frac{\sum Q_i\,x_i}{Q}, \qquad e_y = \frac{\sum Q_i\,y_i}{Q}

At the corners the pressure reduces to $\frac{Q}{A}\left(1 \pm \frac{6e_x}{L} \pm \frac{6e_y}{B}\right)$ , and the whole base stays in compression while $6e_x/L + 6e_y/B \le 1$ . The pressures then load strips of the raft along the column lines, each designed as a beam with the columns pushing down and the soil pushing up.

The method needs the raft to be rigid. ACI 336.2R checks it by comparing the column spacing in a strip with $1.75/\beta$ , where

\beta = \sqrt[4]{\frac{k_s\,b}{4\,E_c\,I}}

with $k_s$ the modulus of subgrade reaction, $b$ the width of the strip and $E_c I$ its flexural stiffness. Spacings shorter than $1.75/\beta$ allow the rigid method; longer ones call for a flexible analysis.

Flexible rafts and the subgrade reaction

A raft that isn’t rigid bends under its loads, and the pressure follows its deflection, higher under the columns and lower between them. The usual model is Winkler’s: the soil acts as a bed of independent springs, and the pressure at each point is proportional to the settlement there,

q = k_s\, w

and the raft becomes a plate on springs, solved with finite elements. The spring stiffness $k_s$ depends on the size of the loaded area and on the soil down to the depth the raft stresses, so a value from a 0.3 m plate load test can’t be applied to a 20 m raft as it stands. It’s usually backed out of a settlement estimate, $k_s = q/s$ .

Structural checks: punching shear and flexure

Punching shear usually sets a raft’s thickness. Each column pushes down on a small area while the soil pushes up over the whole slab, and the concrete can fail around the column on a truncated pyramid. ACI 318 checks it on a critical perimeter at $d/2$ from the column faces, $d$ being the effective depth. For an interior column of sides $c_1$ and $c_2$ , with the factored column load $P_u$ and the factored soil pressure $q_s$ :

V_u = P_u - q_s\,(c_1 + d)(c_2 + d), \qquad v_u = \frac{V_u}{b_0\,d}, \qquad b_0 = 2\,(c_1 + c_2 + 2d)

The stress $v_u$ has to stay within the concrete’s two-way shear strength, which ACI 318 gives from $\sqrt{f'_c}$ , the column’s shape and the perimeter. Edge and corner columns have three- and two-sided perimeters, one reason a raft is carried past its outer columns. A thicker raft raises both $d$ and $b_0$ ; drops under the heaviest columns do the same with less concrete.

For flexure, the raft bends like a floor turned upside down: its bottom is in tension under the columns, and the soil’s push puts the top in tension between them. The strips of the rigid method, or the moments of a plate model, give the reinforcement both ways, top and bottom.

Uplift below the water table

A raft below the water table is pushed up by the water pressure on its base, $u = \gamma_w h_w$ , with $h_w$ the depth of the base below the water. The finished building usually outweighs it, but during construction, with the raft cast and the floors not yet built, a rising water table can lift it. Water up to the ground surface would push $3.0 \times 9.81 = 29.4$ kPa on the base of the example’s raft, 3.0 m down, more than the 24 kPa the raft weighs. Dewatering continues until enough of the structure is in place, and where the weight will never be enough, tension piles or ground anchors hold the raft down. The water pressure also loads the slab, which has to span between its columns and walls against it.

How a raft foundation is built

Investigate the ground with boreholes or CPTs taken below the depth the raft will stress, and record the groundwater levels; how to read a geotechnical report explains where each result lives.
Excavate to the formation level, supporting the sides where the basement is deep (shoring design) and dewatering where the water table is above the formation.
Inspect the formation, replace loose or softened soil, and cast a thin layer of lean concrete, the blinding, to keep it clean and dry.
Lay the waterproofing where the basement needs it: a membrane under the raft and up the walls, or watertight concrete with sealed joints.
Fix the reinforcement: the bottom mat on spacers that give its cover, the top mat on chairs, any shear reinforcement at the columns, and the starter bars for the columns and walls. ACI 318 asks for 75 mm of cover where concrete is cast against and permanently in contact with the ground.
Form the edges, the construction joints and the openings, with waterstops at the joints of a basement and sleeves for the services.
Pour, compact and finish the concrete in large, planned pours. A thick raft heats up as the cement hydrates, and if its core and its surface cool at different rates it can crack, so big pours need a temperature control plan.
Cure the surface for the time the specification sets, backfill against the walls, and keep the dewatering on until the structure outweighs the uplift.

Advantages and disadvantages of a raft

A raft spreads the load over the whole footprint, evens out the settlement between columns and gives a basement its floor and its barrier against water, and it’s quick to build, with one excavation level and one large pour instead of many separate footings. Its disadvantages:

On good ground it takes more concrete and steel than separate footings.
Heavy column loads need a thick slab or drops, with dense reinforcement that takes time to fix.
Large pours need a steady supply of concrete, planned joints and control of the heat of hydration.
Below the water table it needs waterproofing, an uplift check and, often, dewatering during construction.
On a thick compressible layer it can settle more than footings would, since it stresses the soil to a greater depth.

Worked example: a raft under twelve columns

A building with one basement stands on twelve columns. Check whether it needs a raft, work out the pressure under the raft with the rigid method, and check its bearing capacity.

Columns on a grid of three 6.00 m bays (lines A to D) by two 7.00 m bays (lines 1 to 3), with the service loads in the table.
Raft: 20.00 m by 16.00 m, reaching 1.00 m past the outer column lines, 1.00 m thick in reinforced concrete at 24 kN/m³, with its base at $D_f$ = 3.00 m.
Soil: a loose to medium dense silty sand with $\gamma$ = 18 kN/m³, $\gamma_{sat}$ = 20 kN/m³, $\phi'$ = 30° and $c'$ = 0, and the water table 5.00 m below the ground.
The geotechnical report gives an allowable bearing pressure of 100 kPa for footings, set by a settlement of 25 mm.
$\gamma_w$ = 9.81 kN/m³.

With the origin at the raft’s centre, $x$ along its 20 m and $y$ along its 16 m, lines A to D are at $x$ = −9, −3, 3 and 9 m, and lines 1 to 3 at $y$ = −7, 0 and 7 m. The column loads, in kN:

Line	A	B	C	D	Total
3 ( $y$ = 7 m)	1,100	1,900	2,200	1,300	6,500
2 ( $y$ = 0)	1,800	3,000	3,400	2,000	10,200
1 ( $y$ = −7 m)	1,000	1,700	1,800	1,100	5,600
Total	3,900	6,600	7,400	4,400	22,300

The example's raft in plan: the grid, the column loads and their resultant.

Step 1: footings or a raft?

At 100 kPa, footings would need

\frac{Q}{q_a} = \frac{22{,}300}{100} = 223 \text{ m}^2

which is 70% of the 320 m² the building covers, before counting the footings’ own weight. That’s well past half the plan, so the building goes on a raft.

Step 2: the resultant and its eccentricity

The moments of the column loads about the raft’s centroidal axes, line by line:

\sum Q_i\,x_i = 3{,}900(-9) + 6{,}600(-3) + 7{,}400(3) + 4{,}400(9) = 6{,}900 \text{ kN·m}

\sum Q_i\,y_i = 5{,}600(-7) + 10{,}200(0) + 6{,}500(7) = 6{,}300 \text{ kN·m}

e_x = \frac{6{,}900}{22{,}300} = 0.309 \text{ m}, \qquad e_y = \frac{6{,}300}{22{,}300} = 0.283 \text{ m}

The resultant sits 0.31 m towards line D and 0.28 m towards line 3, where the heavier columns are. Since $6e_x/L + 6e_y/B = 0.093 + 0.106 = 0.199$ , far below 1, the whole base stays in compression.

Step 3: is the raft rigid?

Take $k_s$ = 10,000 kN/m³, on the stiff side for this sand under a raft this size (a softer value makes the raft look even more rigid). For concrete with $f'_c$ = 28 MPa, ACI 318 gives $E_c = 4{,}700\sqrt{28} = 24{,}870$ MPa. Per metre of strip, $I = 1.0^3/12 = 0.0833$ m⁴:

\beta = \sqrt[4]{\frac{10{,}000 \times 1.0}{4 \times 24.87 \times 10^6 \times 0.0833}} = 0.186 \text{ m}^{-1}, \qquad \frac{1.75}{\beta} = 9.4 \text{ m}

The column spacings, 6.0 m and 7.0 m, are shorter than 9.4 m, so the rigid method applies.

Step 4: the pressure from the columns

The raft’s area and the second moments of area of its plan:

A = 20 \times 16 = 320 \text{ m}^2, \qquad I_y = \frac{16 \times 20^3}{12} = 10{,}667 \text{ m}^4, \qquad I_x = \frac{20 \times 16^3}{12} = 6{,}827 \text{ m}^4

At the corners, $x = \pm 10$ m and $y = \pm 8$ m:

q = \frac{22{,}300}{320} \pm \frac{6{,}900 \times 10}{10{,}667} \pm \frac{6{,}300 \times 8}{6{,}827} = 69.69 \pm 6.47 \pm 7.38 \text{ kPa}

Corner	$x$ (m)	$y$ (m)	$q$ (kPa)
D3	10	8	83.5
A3	−10	8	70.6
D1	10	−8	68.8
A1	−10	−8	55.8

These are the pressures for the raft’s structural design, which leaves out the raft’s own weight: it rests directly on the soil beneath it and bends nothing.

Step 5: the pressure on the soil

The soil does carry the raft’s weight: $24 \times 1.0 = 24$ kPa over the whole base, or $W = 24 \times 320 = 7{,}680$ kN. The total load at the base is $V = 22{,}300 + 7{,}680 = 29{,}980$ kN, with the same moments, so the eccentricities shrink to $e_x = 6{,}900/29{,}980 = 0.230$ m and $e_y = 6{,}300/29{,}980 = 0.210$ m, and every corner gains 24 kPa:

Corner	$q$ (kPa)
D3	107.5
A3	94.6
D1	92.8
A1	79.8

The pressure on the soil under the rigid raft, with its own weight: a plane from 79.8 kPa at A1 to 107.5 kPa at D3. The fourth corner, A3, is hidden behind, at 94.6 kPa.

The average is $V/A$ = 93.7 kPa. The 3.0 m of sand removed for the basement weighed $18 \times 3.0 = 54$ kPa, so the net pressure averages $93.7 - 54 = 39.7$ kPa: the excavation offsets more than half of the building’s weight, and the settlement analysis works from those 39.7 kPa.

Step 6: bearing capacity with Rischio’s engine

Rischio’s footing tool has no raft of its own, but a raft’s bearing check is the check of one large rigid footing: its full plan, its depth, the total load at its base and that load’s moments about its centre. That’s how this raft is modelled, as a rectangular footing in the footing tool:

Input	Value
Footing	rectangular, $B$ = 16.00 m, $L$ = 20.00 m
Embedment depth $D_f$	3.00 m
Vertical load $V$	29,980 kN (columns and raft)
Moment along the width $M_B$	6,300 kN·m
Moment along the length $M_L$	6,900 kN·m
Soil	one stratum, $\gamma$ = 18 kN/m³, $\gamma_{sat}$ = 20 kN/m³, $c'$ = 0, $\phi'$ = 30°
Water table	5.00 m deep

Rischio calls the 16 m side $B$ , so its $e_B$ is this example’s $e_y$ and its $e_L$ is $e_x$ . The engine returns $e_B$ = 0.210 m and $e_L$ = 0.230 m, and $q_{max}$ = 107.5 kPa and $q_{min}$ = 79.8 kPa, the same values as step 5. Its ultimate bearing capacity is $q_u$ = 2,857 kPa, so

FS = \frac{q_u}{q_{max}} = \frac{2{,}857}{107.5} = 26.6 \ge 3.0

The engine follows its published method, and $q_u$ can be checked by hand. With $\phi' = 30°$ :

N_q = \tan^2(60°)\, e^{\pi \tan 30°} = 18.40, \qquad N_\gamma = 2\,(N_q + 1)\tan 30° = 22.40

The effective dimensions are $B' = 16 - 2(0.210) = 15.58$ m and $L' = 20 - 2(0.230) = 19.54$ m, so $B'/L' = 0.797$ , and the shape and depth factors are

F_{qs} = 1 + 0.797 \tan 30° = 1.460, \qquad F_{\gamma s} = 1 - 0.4(0.797) = 0.681

F_{qd} = 1 + 2 \tan 30° \,(1 - \sin 30°)^2\, \frac{3}{16} = 1.054, \qquad F_{\gamma d} = 1

The overburden at the base is $q = 18 \times 3.0 = 54$ kPa. Below the base, the water table at 5.0 m splits the depth $B$ = 16 m that sets $\gamma'$ into 2.0 m of moist sand at 18 kN/m³ and 14.0 m submerged at $20 - 9.81 = 10.19$ kN/m³:

\gamma' = \frac{2.0 \times 18 + 14.0 \times 10.19}{16} = 11.17 \text{ kN/m}^3

With $c' = 0$ the cohesion term drops out, and with a vertical load the inclination factors are 1. Keeping full precision in the arithmetic:

q_u = 54 \times 18.40 \times 1.460 \times 1.054 + \tfrac{1}{2} \times 11.17 \times 15.58 \times 22.40 \times 0.681 = 1{,}529.6 + 1{,}327.2 = 2{,}856.8 \text{ kPa}

Rischio’s engine gives the same $q_u$ , 2,857 kPa, with the eccentricities inside $B/6$ and $L/6$ .

The model leaves the soil’s stiffness out, so Rischio skips the elastic settlement and says so in a warning. Its settlement method is a footing’s: it spreads the gross pressure, 93.7 kPa here, over the soil down to $5B$ below the base, 80 m under this raft. A raft’s settlement is estimated from the net pressure, 39.7 kPa, with the soil’s stiffness as it changes with depth and, on clay, its consolidation.

Step 7: what governs this raft

With a factor of safety of 26.6, bearing capacity doesn’t limit this raft on sand. Its design turns on the settlement under the net 39.7 kPa, on the differential settlement between the heavy columns of line 2 and the light corners, and on the concrete: punching at C2, the heaviest column with 3,400 kN, is the first check on the 1.0 m thickness, and the pressures of step 4 load the strips that give the reinforcement.

Frequently asked questions

Why is a raft foundation used?

To spread a building’s load over its whole footprint when the soil is too weak or too variable for footings, to keep the settlement between columns even, and to give a basement a watertight floor. A common rule of thumb calls for one once footings would cover more than about half the plan.

How thick should a raft foundation be?

Thick enough for punching shear at the heaviest column, which usually decides it, and then checked for flexure and for the stiffness the analysis assumes. As typical values only: under a house, a raft can be a thin slab stiffened by beams or thickened edges; under a mid-rise building with heavy columns it’s commonly in the order of a metre; tall towers stand on rafts several metres thick.

What are the disadvantages of raft foundations?

On good ground it uses more concrete and steel than footings, heavy loads need a thick, densely reinforced slab poured in planned stages, and below the water table it needs waterproofing and an uplift check. On a thick compressible layer it can settle more than footings, since it stresses the soil deeper.

How do you make a raft foundation?

Excavate to the formation level, cast the blinding, lay any waterproofing, fix the bottom and top reinforcement with the column starter bars, then pour, compact and cure the concrete. Any dewatering stays on until the structure outweighs the uplift.