5 Spread foundations

COMMENTARY ON Clause 5

This clause applies to the design and construction of:

pad foundations;
strip foundations;
raft foundations;

Underpinning is covered in Annex B.

5.1 Choice and design of spread foundations

5.1.1 General

5.1.1.1 The design of spread foundations should conform to

BS EN 1997-1:2004+A1:2013, Clause 6, and Clause 4 and Clause 5 (this clause) of this standard.

NOTE Information about shallow (i.e. spread) foundations can be found in the ICE manual of geotechnical engineering (2012), Volume II, Chapter 53 [1].

5.1.1.2 Selection of a suitable type of spread foundation should take into account:

the magnitude and disposition of structural loads;
the bearing resistance of the ground;
the settlement characteristics of the ground;
the differential settlement that the supported structure can tolerate; and
the need to found in stable soil.

NOTE Adjacent pad foundations may be combined or joined together with ground beams to support eccentric loads, to resist overturning, or to oppose horizontal forces. Walls between columns may be carried on ground beams spanning between pad foundations.

5.1.1.3 When the plan area covered by pad and strip foundations exceeds more than about one half of the superstructure footprint, consideration should be given to using a raft foundation instead.

NOTE Some key features of pad, strip, and raft foundations are summarized in Table 6.

Table 6 Some key features of spread foundations

Type	Shape on plan	Aspect ratio	Used to support:	Constructed in:
Pad	Square, circular, or rectangular	L ~ B	One or two columns	Mass or reinforced concrete
Strip	Rectangular	L >> B	Load-bearing wall or several closely-spaced columns	Mass or reinforced concrete
Raft	Square or rectangular	L ~ B	Entire structure or a substantial part of it	Reinforced concrete

5.1.2 Pad foundations

COMMENTARY ON 5.1.2

A pad foundation is a spread foundation whose length (L) and breadth (B) on plan are of similar magnitude (L ~ B). Pad foundations are commonly used to transmit structural loads onto a suitable bearing stratum at shallow depth below ground level. They are the simplest of all foundations and are generally used where groundwater is absent or can be readily controlled.

5.1.2.1 Pad foundations may be of various shapes, including circular, square, or rectangular. The shape of a pad foundation should be chosen to accommodate the effect of eccentricity arising from imposed moments and shear forces on the column and the method of construction.

NOTE Deep pad foundations (>3 m deep) may be used to carry heavy column loads.

5.1.2.2 The thickness of a pad foundation should not be less than 150 mm.

5.1.3 Strip foundations

COMMENTARY ON 5.1.3

A strip foundation is a spread foundation whose length on plan (L) is very much greater than its breadth (B), i.e. L >> B. Strip foundations are commonly used to support the walls of buildings. They are generally used where groundwater is absent or can be readily controlled.

5.1.3.1 On sloping sites, strip foundations should be founded on a horizontal bearing and stepped where necessary to maintain adequate depth.

5.1.3.2 The thickness of a strip foundation should be not less than 150 mm.

5.1.3.3 The breadth of a strip foundation should be chosen, taking into account normal construction tolerances (see 5.2), so as not to overstress the ground beneath it.

5.1.4 Raft foundations

COMMENTARY ON 5.1.4

A raft foundation is a spread foundation whose length (L) and breadth (B) on plan are similar to that of the superstructure. Raft foundations are commonly used to support large or heavily loaded structures. They can prove to be a very cost-effective foundation solution.

5.1.4.1 Raft foundations may be used to support:

lightly loaded structures on low strength natural ground, where it is necessary to spread the load across horizontally variable ground, fill, or weaker zones;
structures that are sensitive to differential settlement;
structures on ground where mining or other forms of subsidence are likely to occur and are thus a flexible structure and foundation is required;
low rise dwellings and lightly framed structures on soils that are susceptible to excessive shrinking or swelling; or
heavy structures that could otherwise be supported on many isolated foundations, occupying a large part of the structure's footprint, when a more economic design is required

5.1.4.2 Settlement of raft foundations may be reduced by providing piles that redistribute loads from the upper layers of the ground to deeper strata.

NOTE Information about rafts can be found in the ICE manual of geotechnical engineering (2012), Volume II, Chapter 56 [1].

5.2 Actions and design situations

The design of concrete spread foundations cast directly onto the ground should take into account the permitted sectional deviations given in BS EN 13670.

5.3 Design considerations

COMMENTARY ON 5.3

Design and construction considerations for spread foundations are given in BS EN 1997-1:2004+A1:2013, 6.4. The design considerations given in this clause are more specific examples of the issues that can affect the performance of a spread foundation.

5.3.1 General

5.3.1.1 The design of spread foundations should consider:

the items listed in BS EN 1997-1:2004+A1:2013, 6.4 and in 4.2.2 of this standard;
when relevant, the additional design considerations for spread foundations on rock given in BS EN 1997-1:2004+A1:2013, 6.7;
changes in groundwater conditions;
long-term stability;
sensitive clays and loose water-bearing sands and soils that change structure when loaded;
the effect of any excavation on soil properties, particularly for foundations greater than 3 m deep;
in low strength soils particularly, the potential for damage to services and drains caused by relative movement between the foundation and the ground when they are supported partly on each;
the layout and design of service pipes and ducts to allow for their future maintenance without the need to break through the foundation;
the potential for underground services, such as drains and water mains, to be damaged by shrinkage and swelling of clay soils;
in connection with extensions of existing buildings:
differential movement of the foundations between new and existing structures;
where cracking and subsequent remedial work is not acceptable, provision of joints between extensions and existing buildings;
stability of existing foundations where they abut the foundations of an extension; and
the possibility of damage caused by the presence of chemicals.

5.3.1.2 Specialist advice should be sought where sensitive clays and loose water-bearing sands and soils that change structure when loaded are expected.

5.3.1.3 Whenever possible, the centre of area of a foundation or group of foundations should be located directly beneath the centre of gravity of the imposed load. When this is not possible, the effects on the structure of tilting and settlement of the foundation should be considered.

5.3.1.4 Where foundation support is provided by a number of separate bases these should, as far as practicable, be proportioned so that differential settlement is minimal.

5.3.2 Depth of foundation

5.3.2.1 Strip foundations of traditional brick and masonry buildings should be founded at a depth where the anticipated ground movements will not impare the stability or serviceability of any part of the building, taking due consideration of soil type and the influence of vegetation and trees on the ground.

5.3.2.2 The depth to the underside of foundations should be not less than 750 mm on low shrinkage clay soils, 900 mm on medium shrinkage clay soils, and 1 000 mm on high shrinkage clay soils. These depths might need to be

increased in order to transfer the load onto satisfactory ground or where trees are located nearby.

NOTE 1 Information about low-rise buildings on shrinkable clay soils can be found in BRE Digests 240 [38] and 241 [39].

NOTE 2 Attention is drawn to the Building Regulations 2010 [40], which state that «except where strip foundations are founded on rock, the strip foundations should have a minimum depth of 450 mm to their underside to avoid the action of frost. This depth, however, will commonly need to be increased in areas subject to long periods of frost or in order to transfer the loading onto satisfactory ground.»

5.3.3 Trees and other vegetation

Consideration of trees and other vegetation in the design of spread foundations should conform to 4.2.3.4.

5.3.4 Frost heave

COMMENTARY ON 5.3.4

Traditionally in the UK, foundations have been taken to a depth of at least 450 mm below ground level to avoid frost heave.

The design of foundations to avoid frost heave should conform to BS EN ISO 13793.

5.3.5 Heating

5.3.5.1 The design of a shallow foundation should take into account possible ground movement due to shrinkage of clay caused by:

boiler installations;
furnaces and kilns;
underground cables and services;
ground storage energy systems;
other artificial sources of heat; and
fill or other soils containing combustible materials.

5.3.5.2 Where excessive heat would otherwise be transmitted, the installation should be isolated from the soil and the foundation by a suitable form of construction. Where the installation is small, insulating materials might be adequate but some form of forced ventilation or cooling by circulated water might be required.

5.4 Calculation models

5.4.1 Bearing resistance – Models based on ground parameters

COMMENTARY ON 5.4.1

BS EN 1997-1:2004+A1:2013, Annex D, presents a sample analytical method for bearing resistance calculation that does not include depth, ground inclination, or rigidity factors. This clause gives an alternative method that includes these factors.

5.4.1.1 General

The design of spread foundations from ground parameters should conform to BS EN 1997-1:2004+A1:2013, 6.5.2.2.

5.4.1.2 Coarse soils (sands and gravels)

5.4.1.2.1 As an alternative to the sample analytical method given in BS EN 1997-1:2004+A1:2013, D.4, the following expression may be used to calculate the ultimate bearing resistance of a spread foundation (R_v) on coarse soils using effective stress parameters:

R'_v / A' = c' N_cb_cs_ci_cd_cg_cr_c + q' N_qb_qs_qi_qd_qg_qr_q + 0.5γ 'B' N_γb_γs_γi_γd_γg_γr_γ

(17)

where:

c', q', and γ' are as defined in BS EN 1997-1:2004+A1:2013, D.4;

B' is defined in BS EN 1997-1:2004+A1:2013, D.4; and

the various coefficients N_c, b_c, s_c, etc. are defined below, after Poulos et al. [41]

NOTE Care is required in assessing submerged weight density term γ', particularly when groundwater is close to the foundation.

5.4.1.2.2 The bearing coefficients in equation (17) should be calculated from:

N_q = e^π×tan^φtan²(45 ° +φ/2)

for φ > 0°

for φ = 0°

N_c = (N_q - 1) cotφ

(18)

5.4.1.2.3 The shape factors in equation (17) should be calculated from:

s_q = 1 + (B / L)tanφ

s_γ = 1 – 0.4(B / L)

s_c = 1 + (B / L)(N_q / N_c)

(19)

5.4.1.2.4 The load inclination factors in equation (17) should be calculated from:

for φ > 0°

for φ = 0°

(20)

5.4.1.2.5 The base inclination (also known as «foundation tilt») factors in equation (17) should be calculated from:

b_q ≈ b_γ

b_γ = (1 – α tanφ)²

for φ > 0°

for φ = 0°

(21)

5.4.1.2.6 The ground inclination (also known as «surface inclination») factors in equation (17) should be calculated from:

for φ > 0°

for φ = 0°

for φ > 0°

for φ = 0°

(22)

5.4.1.2.7 The depth factors in equation (17) should be calculated from:

d_q = 1 + 2tanφ(1 - sinφ)²tan^-1(D / B)

d_γ = 1

for φ > 0°

for φ = 0°

(23)

5.4.1.2.8 The rigidity factors in equation (17) should be calculated from:

for φ > 0°

for φ = 0°

(24)

where:

φ is angle of shearing resistance of the soil;

a is 0.0663 for a smooth foundation or 0.1054 for a rough foundation;

b is 9.3 or 9.6 for a smooth or rough foundation, respectively (when φ is entered in radians); alternatively, b = 0.162 or 0.168, respectively (when φ is entered in degrees);

B is the breadth of the foundation on plan;

L is the length of the foundation on plan;

D is the depth to the underside of the foundation;

A' is the effective area of the foundation;

H is the horizontal force applied to the foundation;

V is the vertical force applied to the foundation;

m is (2 + B/L) / (1 + B/L) for loading in the direction of B or (2 + L/B) / (1 + L/B) for loading in the direction of L;

α is the inclination of the underside of the footing from the horizontal;

ω is the inclination of the ground surface below the horizontal in the direction away from the foundation;

I_r is G / (c' + σ'_v tan φ);

G is the soil's shear modulus of elasticity;

c' is the soil's effective cohesion; and

σ'_v is the vertical effective stress on the foundation.

NOTE 1 The expressions given for N_c and N_q are identical to those in BS EN 1997-1:2004+A1:2013, Annex D. The expressions given for the shape, load inclination, and base inclination factors are also identical to those in Annex D, except for s_q and s_γ. The expressions given for d_q, g_q, and r_q, etc. are missing from Annex D.

NOTE 2 The expression given for N_γ is one of many that have been proposed in the geotechnical literature. The expression given (after Poulos [41]) is generally conservative when compared to other expressions.

5.4.1.2.9 The effects of load combinations involving large inclinations of force or large moments should be assessed using more advanced calculation models than those given in BS EN 1997-1:2004+A1:2013, D.4, or this clause.

NOTE Information about more advanced bearing resistance models can be found in the ICE manual of geotechnical engineering (2012), Volume I, Chapter 21 [2].

5.4.1.2.10 A concrete foundation cast directly against the ground may be considered «rough»; pre-cast concrete foundations should be considered «smooth».

5.4.1.3 Fine soils (silts and clays)

5.4.1.3.1 The ultimate bearing resistance of a spread foundation on fine soils should be calculated as the smaller of its undrained and drained bearing resistances.

5.4.1.3.2 Under undrained (usually short term) conditions, the ultimate bearing resistance of a spread foundation on fine soils should typically be calculated using total stress parameters.

5.4.1.3.3 Under drained (usually long term) conditions, the ultimate bearing resistance of a spread foundation on fine soils should be calculated from effective stress parameters, as described in 5.4.1.2.

5.4.1.3.4 As an alternative to the sample analytical method given in BS EN 1997-1:2004+A1:2013, D.3, the following expressions (after Salgado et al. [42]) may be used, for design situations in which there is no load or ground inclination, to calculate the ultimate bearing resistance of a spread foundation (R_v) on fine soils using total stress parameters:

R_v / A' = N_cc_us_cd_c + q_vb

N_c = π + 2

(25)

where:

A' is the effective plan area of the foundation;

c_u is the undrained shear strength of the fine soil;

s_c is shape factor;

d_c is depth factor;

q_vb is total overburden pressure at the underside of the foundation;

B is the breadth of the foundation;

L is the length of the foundation; and

D is the depth to the underside of the foundation.

5.4.1.3.5 This expression should not be used if the ground surface, the applied load, or the base of the foundation is inclined.

NOTE 1 When the ground surface is inclined, omission of a ground inclination factor from the expression for R_v above is potentially unsafe.

NOTE 2 When the load is inclined, omission of a load inclination factor from the expression for R_v above is potentially unsafe.

NOTE 3 When the base is inclined, omission of a base inclination factor from the expression for R_v above is potentially unsafe.

5.4.2 Sliding resistance

Any earth pressure that is included in the sliding resistance of a spread foundation should be calculated at a strain level that is compatible with that assumed in the calculation of shear resistance along the foundation base.

5.4.3 Settlement

COMMENTARY ON 5.4.3

The magnitude of the settlement that will occur when foundation loads are applied to the ground depends on the rigidity of the structure, the type and duration of the loading, and the deformation characteristics of the ground.

The settlement of a spread foundation may be calculated using any of the following, as appropriate:

methods based on the theory of elasticity;
methods based on one-dimensional consolidation;
for foundations on sand, the empricial method given in BS EN 1997-2:2007, F.3;
methods using non-linear stress-strain models; or
numerical models.

NOTE Guidance on the calculation of settlement for a shallow (i.e. spread) foundation can be found in the ICE manual of geotechnical engineering (2012), Volume II, Chapter 53 [1].

5.4.4 Combined bearing and settlement check using prescriptive measures

COMMENTARY ON 5.4.4

The design of many simple foundations has traditionally been checked against «allowable bearing pressures» which are normally very conservative estimates of the ultimate bearing resistance of the ground, selected on the basis of soil and rock descriptions. The settlement of a spread foundation that has been designed using allowable bearing pressures is commonly assumed to be acceptable.

In BS EN 1997-1, «allowable bearing pressures» are now called «presumed bearing resistance» and this method of design is termed a «prescriptive method».

5.4.4.1 General

Spread foundations may be designed using prescriptive methods based on presumed bearing resistance.

5.4.4.2 Presumed bearing resistance of coarse soils

5.4.4.2.1 Suggested values for the presumed design unit bearing resistance (q_Rv,pres,d) of spread foundations on coarse soils, and located a distance above the water table at least equal to the foundation's breadth, may be estimated from:

(26)

where:

N_γ,k is a bearing coefficient based on equation (18), using the characteristic angle of shearing resistance of the soil;

B is the breadth of the foundation;

γ_s,k is the characteristic weight density of the soil; and

γ_Rv,SLS is a partial factor on bearing resistance.

5.4.4.2.2 For foundations not exceeding 1 m in width that are subject primarily to permanent laoding, a value of γ_Rv,SLS ≥ 2.0 should be adopted.

5.4.4.3 Presumed bearing resistance of fine soils

5.4.4.3.1 Suggested values for the presumed design unit bearing resistance (q_Rv,pres,d) of fine soils may be estimated from the expression:

(27)

where:

c_u,k is the characteristic undrained shear strength of the soil; and

γ_Rv,SLS is a partial factor on bearing resistance.

5.4.4.3.2 For foundations not exceeding 1 m in width that are subject primarily to permanent loading, a value of γ_Rv,SLS ≥ 3.0 should be adopted.

5.4.4.4 Presumed bearing resistance of rocks

Suggested values for the presumed design unit bearing resistance (q_Rv,pres,d) of square pad foundations on rock (for settlements not exceeding 0.5% of the foundation width) may be obtained from BS EN 1997-1:2004+A1:2013, Annex G.

5.5 Materials

5.5.1 Concrete

Concrete and related products incorporated into spread foundations should conform to 4.3.6.

5.5.2 Steel

Steel and related products incorporated into spread foundations should conform to 4.3.7.

5.5.3 Timber

Timber and related products incorporated into spread foundations should conform to 4.3.8.

5.5.4 Fill

Fill incorporated into spread foundations should conform to 4.3.3.

5.6 Durability

5.6.1 Concrete

The durability of concrete used in spread foundations should conform to 4.4.2.

5.6.2 Steel

The durability of steel used in spread foundations should conform to 4.4.3.

5.6.3 Timber

The durability of timber used in spread foundations should conform to 4.4.4.

5.7 Ultimate limit state design

5.7.1 General

5.7.1.1 The ultimate limit state design of spread foundations should conform to 4.6 and this subclause (5.7).

5.7.1.2 The ultimate limit state design of a foundation should conform to BS EN 1997-1:2004+A1:2013, 6.5.

5.7.2 Bearing

The design value of the ultimate bearing resistance of a spread foundation (R_d) should be verified according to BS EN 1997-1:2004+A1:2013, 6.5.2, and conform to expression (6.1) of that standard.

5.7.3 Sliding

5.7.3.1 The design value of the ultimate sliding resistance of a spread foundation (R_d) should be verified according to BS EN 1997-1:2004+A1:2013, 6.5.3, and conform to expression (6.2) of that standard.

5.7.3.2 For drained conditions, R_d should be calculated according to expression (6.3a) of BS EN 1997-1:2004+A1:2013 in preference to (6.3b).

5.7.3.3 For undrained conditions, R_d should be calculated according to expression (6.4a) of BS EN 1997-1:2004+A1:2013 in preference to (6.4b).

5.7.4 Overturning

Overturning of a spread foundation should be prevented by verifying ultimate limit state EQU in accordance with BS EN 1997-1:2004+A1:2013, 2.4.7.2 and the UK National Annex to BS EN 1997-1:2004+A1:2013.

5.7.5 Global stability

The global stability of a spread foundation should conform to BS EN 1997-1:2004+A1:2013, 6.5.1.

5.8 Serviceability limit state design

The serviceability limit state design of a foundation should conform to BS EN 1997-1:2004+A1:2013, 6.6.

5.9 Structural design

5.9.1 General

5.9.1.1 The structural design of a foundation should conform to BS EN 1997-1:2004+A1:2013, 6.8.

5.9.1.2 Spread foundations may be constructed using reinforced or plain (i.e. unreinforced or mass) concrete.

5.9.1.3 The design of reinforced concrete spread foundations should conform to BS EN 1992-1-1.

5.9.1.4 The design of plain (i.e. unreinforced) concrete spread foundations should conform to BS EN 1992-1-1:2004+A1:2014, Section 12.

5.9.1.5 Foundations that act as retaining walls should conform to BS 8002, particularly when:

founded on sloping ground; or
steps occur between adjacent ground floor slabs or finished ground levels.

5.9.2 Pad foundations

The thickness of the foundation should not be less than 150 mm.

5.9.3 Strip foundations

Reinforcement should be provided in strip foundations wherever an abrupt change in load or variation in ground support occurs.

5.9.4 Raft foundations

The structural design of a raft foundation should take into account the reduction in strength caused by holes, ducts, etc. used to accommodate service pipes, drains, and such like.

5.10 Execution

NOTE Attention is drawn to Regulation 22 of The Construction (Design and Management) Regulations, 2015 [3], with regards to health and safety requirements for excavations.

The execution of concrete spread foundations should conform to 4.9.

5.11 Testing

5.11.1 Testing of a spread foundation should conform to prEN ISO 22477.

5.11.2 Plate loading tests should conform to prEN ISO 22476-13.

5.12 Supervision, monitoring, and maintenance

Supervision, monitoring, and maintenance of a spread foundation should conform to 4.11.

5.13 Reporting

Reports for spread foundations should conform to 4.12.