6 Spread foundations
6.6 Design by prescriptive methods – pads and strips
6.6.1 Introduction
EC7 permits the use of presumed bearing resistance where there is adequate case history data to justify such a method. The method provides allowable bearing resistances (or pressures) which are generally assumed to satisfy both ULS and SLS requirements for conventional structures. The allowable bearing pressures should be compared with unfactored loads from the structure.
The following is based on current practice as would be applied to situations where explicit assessment of settlement would not usually be required to validate the design. The design rules presented are based on the now superseded British Standard as follows:
– BS 800431Table 1 for design of spread footings on sand and clays (silts and gravels).
Table 6.2 Minimum design considerations – spread foundations – Depth of adequate bearing stratum (do soft layers exist at depth?) – Variations in water level and its effect on excavation during construction – Effect of installation on nearby structures
– Effect of future groundwork (e.g. service excavations) on foundation stability and movement – Integrity of foundation once installed
– Environmental risk associated with contamination of aquifers and linking aquifers – Soluble materials within the ground
– Changes in shallow ground conditions with time and depth (summer/winter effects of desiccation and swelling, frost depthb, tree actions on clays, transmitted hot or cold temperatures, scour)72 – Ground movements and reductions in ground strength due to seepage, climate or construction Notes
a Combinations of these limit states must be considered if relevant.
b For frost damage to occur, the soil must be frost susceptible and the foundation un-insulated within the frost depth – refer to BS EN ISO 1379371for frost protection measures.
Similar rules are also available in BS 800431as follows:
– BS 8004 Table 2 for design of spread footings on chalk and also CIRIA C574 Engineering in chalk73.
– BS 8004 Figure 1 and Table 4 for design of spread footings on rocks (excluding chalk and Mercia Mudstone).
– BS 8004 Table 3 for design of spread footings on Mercia Mudstone.
The ‘presumed bearing values’ quoted in BS 800431are compared to unfactored dead plus live loads (giving the representative load). If design resistances are to be assessed for comparison with a ULS design action (Fd, which includes partial factors on actions using Design Approach 1
Combination 2 factors as per Section 2.11.3.4 above) then adjustments can be made to the values quoted in BS 8004 to accommodate the difference in design methodologies.
Before presenting prescriptive design rules, it is noted that there are limitations to their usage. Examples of where prescriptive design should not be used are summarised below in Table 6.3. The list is not exhaustive and the designer must check that the design being undertaken is appropriate.
6.6.2 Clay soils – GC1 design
Prescriptive design of pads and strips bearing on clay strata is well documented for strata with uniform or increasing strength with depth and where the strength of the formation level is of medium strength or better (medium strength has a measured undrained shear strength in the range of 40 to 75kN/m2). Where conditions are more complex then prescriptive design may not be applicable and the designer should consider design by calculation (see Section 6.8). Table 6.3 provides a list of situations where prescriptive design would likely be inappropriate.
For GC1 structures and for initial assessment of foundation size based on average undrained shear strength and unfactored loads the presumed bearing resistance qacan be obtained from Figure 6.1 which is based on BS 800431suggested values. This figure also accommodates the EC7 requirement that if the footing has a bulked factor of safety of three or more then no settlement analysis will be required for conventional structures. The choice of undrained shear strength should be in keeping with BS 8004 and can be taken to be a moderately conservative value. In Figure 6.1 the
‘presumed bearing resistance’ can be compared with the unfactored load (vertical action).
Figure 6.1 has been revised in Figure 6.2 to be in keeping with EC7 terminology whilst maintaining a bulked factor of safety of three thereby limiting the need for calculation of settlement. In Figure 6.2 the design resistance, Rdis the design load with case A2 (Table 2.7a) partial factors applied to the actions and the undrained shear strength, cu;kis the characteristic undrained shear strength of the clay.
Table 6.3 Limitations for use of prescriptive spread foundation design Situation Reason for not using prescriptive design
Large loads Where large loads are present requiring pad or strip foundations of more than 3m nominal width then assessment of settlement becomes more important as does the structural design of the foundation.
Inclined and eccentric loads
Where loads are inclined or eccentric to the centre of the footing then the design resistance will reduce due to geometric effects. Inclined loads also require a check for sliding.
Low strength soils (measured cu, 40kN/m2for clays or SPT N , 10 for coarse grained soils)
– Risk of large settlement
– Need early consideration of temporary works and construction plant stability.
Soil strata strengths reduce with depth below formation level
Such situations occur where weathered or desiccated crusts overlie weaker soils and where soft clays underlie sand and gravel strata. The risk of punching failure needs to consider the gradient of strength below the formation level as does the risk of larger than anticipated settlements.
Location near existing, planned or recently removed trees in clay strata
Need to consider ground heave/settlement to arrive at depth of foundation to overcome effects of trees72.
Locations with water bearing layered soils
In layered water bearing sand (or silt) and clay soils the risk of rapid softening of the clay layers is increased due to short drainage path lengths where footings are placed in excavations or trenches.
Excavation below groundwater level in coarse grained soils
Where excavation is carried out beneath the water, softening/
disturbance of the formation level results in uncertain settlement of the footing. Placement of concrete may also be difficult leading to structural problems – solution may be temporary dewatering.
Uneven or sloping ground Where the spread footing is located close to a slope or an excavation, then the assumptions on which prescriptive design is based may no longer apply. In addition, overall stability may also be the controlling factor in design and stipulation of a bearing pressure should not be finalised until the full design is developed.
Heavily fissured clay Where the clay is heavily fissured and disturbed, design needs to consider the mass strength rather than the intact strength that would typically be measured by small scale laboratory or field tests. An assessment of how the fissuring would affect ultimate bearing resistance would be required to allow the presumed bearing pressure to be assessed.
Displacements Where structures are unusually sensitive to movement then prescriptive design is unlikely to be appropriate.
It is intended that the performance of foundations designed using Figures 6.1 and 6.2 will be similar.
0 200 400 600 800
0 50 100 150 200 250 300 350 Presumed bearing resistance qa (kPa)
Undrained shear strength cu (kPa)
Extremely low, very low and low strength Medium strength High strength Very high strength
Square/circular pad
Strip
Extremely high strength
Fig 6.1 Prescriptive spread footing design for GC1 structures.
Undrained shear strength cu;k (kPa)
Extremely low, very low and low strength Medium strength High strength Very high strength
Square/circular pad
Strip
Extremely high strength
0 200 400 600 800
0 50 100 150 200 250 300 350
Design resistance Rd (DA1, C2) (kPa)
Fig 6.2 Spread footing design to EC7 – clays (bulk factor of safety 3.0)
The reader may note that the value for EC7 design resistance is higher than the BS 800431presumed bearing value for a given soil strength, cuor cu;k. The reasons for this are that:
– The design resistance presented in Figure 6.2 includes Design Approach 1, Combination 2 factors as well as a check that there is a bulk factor of safety of 3.0 on the representative load thereby reducing the need for an explicit settlement calculation.
– The design resistance Rdin Figure 6.2 has a partial factor of 1.3 on variable actions which is absent for BS 8004 presumed bearing pressure
assessment shown in Figure 6.1.
Whilst not necessary for GC1 designs, an assessment of settlement should be carried out for GC2 designs. This settlement assessment will use SLS partial factors on actions and unfactored ground stiffness values.
6.6.3 Sand and gravel soils – GC1 design
Where sand and gravel soils are encountered as competent strata without substantial clay layers close to the formation level and where ground conditions are seen to improve with depth, then the following approach to preliminary sizing of spread footings for GC2 structures and for design of GC1 structures may be taken.
The initial approach to the design of footings on sand and gravel is an empirically based relationship between allowable bearing resistance and SPT N value.
qa¼f N60 for footings constructed above the water table qa¼0.5f N60 for footings constructed at the water table where:
qa is the allowable or presumed bearing resistance in kN/m2
N60 is the SPT N value (assuming 60% energy efficiency – see BS EN ISO 22476 Part 325)
f is an empirical factor as presented in Table 6.4.
Table 6.4 Values of f and qafor spread footing design on sand and gravels Soil density SPT blow count Empirical factor f
Sand Sand and gravel Gravel
Very loose ,10 N/A N/A N/A
Medium dense
10–30 10 20 decreasing to 10
( f ¼ 25 N / 2) 20
Dense 30–50 10 10 20
Very dense .50 qa¼500kN/m2 qa¼500kN/m2 qa¼1000kN/m2 Note
Values quoted are for construction above the water table. Use 50% of values for construction below the water table.
Where the spread footing formation level is below the water table (GC2) then temporary dewatering should be considered to allow excavation and placement of concrete in the dry, thereby negating the need to reduce bearing pressures.
These empirical relationships take account of settlement as well as bearing capacity failure of the footing. The 50% reduction for construction at/below the water table allows for the potential loosening of the formation and the risk of increased settlement at design pressure (i.e. the correction is largely a serviceability limit state precaution).
Design of pad and strips bearing on sand and gravel strata can also be based on characteristic values of w0and bearing capacity factors as presented in Annex D of EC7 Part 11along with assessment of settlement.
This approach is presented below.
6.6.4 Rock
The design of spread foundation on rock must consider the following:
– how the rock mass may deform and what its strength is
– the effect of weak layers, bedding joints, other discontinuities, weathering, decomposition and fracturing
– disturbance of the natural state of the rock by construction activity.
However, spread foundations on rock can normally be designed using presumed bearing resistances and the settlement assessed on the basis of comparable experience related to rock mass classification.
Presumed bearing resistances for vertical loading of square pads are presented in Figures 6.3a–6.3d.
EC7 is silent on the design of spread foundations on rock in terms of design by calculation. While designers are free to choose an appropriate method, it is likely that they will need to undertake a review as to whether additional partial factors are needed. Rock mechanics calculations often involve material parameters other than w0, c0, cu and qu(e.g. GSI values or Hoek-Brown failure criterion parameters).