6 Spread foundations
6.9 Design by calculation – SLS
6.9.1 SLS – general
Further to Section 2.11.4, verification of serviceability limit states is to:
– take into account displacements caused by actions – consider displacements in light of comparable experience – involve calculations for settlement estimation in soft clays
– consider total displacement of a foundation and differential displacement of parts of a foundation
– consider the contribution of adjacent loading to stress increase in the ground
– consider the possible range of relative rotations
– take account of load distribution and ground variability when assessing differential settlements and relative rotations
– involve calculations for settlement estimation in firm and stiff clays – consider that differential settlement normally occurs even if calculations
predict uniform settlement.
For conventional structures on clays, the ratio of the bearing capacity Rdto serviceability loading Edshould be checked prior to undertaking settlement calculations:
– If Rd/Ed3 then calculations may not be necessary.
– If Rd/Ed,3 then calculations should be undertaken.
– If Rd/Ed,2 then calculations should consider non-linear stiffness.
However, in general the inequality introduced in Chapter 2 (Section 2.11.4.1) applies:
EdCd
In the following sections, the serviceability limit state is discussed in terms of settlement and heave as these are explicitly addressed in EC7. However, it should be kept in mind that the serviceability limit state can be more than just an assessment of vertical movement (e.g. horizontal movement, vibration).
6.9.2 SLS – settlement
Total settlement (stor more generally Ed) is comprised of immediate and delayed components. The components are typically represented as:
st¼s0þs1þs2 where:
s0 is the immediate settlement s1 is the consolidation settlement s2 is the creep settlement.
Note that the additional settlement due to self-weight compaction of soil is to be assessed where applicable, and the possible effects of self-weight, flooding and vibration on fill and collapsible soils, and the effects of stress changes on crushable soils should be considered.
Total (or immediate) settlement can be calculated by selecting appropriate values for stiffness parameters, for example an undrained elastic modulus and Poisson’s ratio for short term behaviour for an overconsolidated clay.
The assessment of differential settlement must also be carried out by assessing the likelihood of variability of total settlement stdue to the following:
– Variable loading across the structure (e.g. core and column loading).
– Variable foundation levels across the structure (e.g. part of the structure with a basement or on piles).
– Variable ground conditions (e.g. infilled excavation beneath part of building footprint).
– Settlement trough resulting from a flexible foundation (e.g. a flexible raft).
Settlement should be evaluated using commonly recognised methods. For instance, the stress-strain method (compute stress distribution, compute strains from stresses and stiffness moduli, integrate strains to compute a settlement) as implemented in commercial geotechnical software may be appropriate for computing total settlement. Ground stiffness models may be either linear (often for over-consolidated soils) or non-linear (often for normally-consolidated soils), as appropriate.
Separation of the consolidation and creep components and their rates typically requires the interpretation of consolidation and in situ permeability tests. Caution should be exercised when evaluating these components for organic and soft clays.
The depth of ground considered for calculation of settlement should relate to the foundation size/shape, variation in ground stiffness and spacing of foundations. Typically, stress changes to a depth equal to two to three times the building dimension (i.e. not the individual pad or strip dimension) should be considered in uniform ground (more for very soft soils) or to the depth of a
rigid boundary if closer (e.g. competent rock). Nevertheless, the designer should make an assessment as to where stress increase below a foundation is no longer significant.
While not covered in this Manual, designers of buildings where there are vibratory sources or dynamic loading should consider how such loading could result in geotechnical movement beyond those considered for monotonic loading. The reader is referred to additional information in Design and Structures and Foundations for Vibrating Machines82for an introduction to design, CP 2012: Part 1: 1974 CoP Foundations for Machinery83and DIN 4024: 1988 Machine Foundations, flexible structures which support machines with rotating elements84.
6.9.3 SLS – ground heave
Heave is comprised of immediate and delayed components and is to be characterised as being caused by:
– reduction of effective stress (e.g. unloading, excavation or rise in groundwater level)
– undrained heave (e.g. due to excavation of overlying layers) – external sources (such as frost action or effects of trees).
6.9.4 SLS – effect of trees
For structures founded on clay soils subject to the effects of trees (proposed, present or recently removed) careful consideration of ground movements must be carried out to address potential future movements (settlement, heave or sideways). Guidance for construction in the UK is given in the NHBC Standard Part 4 Foundations72. Chapter 4.2 of the NHBC document provides guidance for construction in the vicinity of trees with attention paid to the criteria in Table 6.6.
The NHBC document72provides illustrative details of how to accommodate tree induced ground movement on foundations by means of increased depth of foundation and measures that may be used to isolate the foundation from
Table 6.6 Key considerations – building near trees
Type of tree Different trees have different water demands and thereby have the potential to cause varying levels of suction to varying depth below ground level.
Nature of ground Clay soils are the key soil type at risk. The NHBC document72splits clay soils into 3 categories (high, medium and low volume change potential).
Depth of foundation and distance from tree
The deeper the foundation and the further the foundation is away from the tree the lower the risk.
shallow settlement/heave movements as well as horizontal ground movement and pressures.
6.10 Interaction with structural design
The structural design of spread foundations is covered in the Institution’s Manual for the design of concrete building structures to Eurocode 285. In addition to the general statements made elsewhere in this Manual on design interaction between structural and geotechnical engineering the following issues are relevant to spread foundations:
– The movements of spread foundations are to be considered to ensure they do not lead to an ultimate limit state in the supported structure.
– Foundations and supported structure must be designed to accommodate or resist heave in the ground that has the potential to swell.
– Serviceability must be checked using SLS loading and an appropriate distribution of bearing pressure.
– Allowance should be made for variable ground causing differential settlement unless the structure prevents it.
– The distribution of loads and ground variability is to be considered when assessing differential settlements and relative rotations.
– The stiffness of a spread foundation will affect the distribution of bearing pressure and a suitable method of modelling the distribution should be adopted. For a stiff foundation a linear distribution of bearing pressure may be adopted, while for a flexible foundation, the distribution can be derived using a continuum or spring model. Ignoring structural stiffness tends to lead to over-prediction of differential settlement.
– An estimate of tilt caused by settlement under eccentric loading can be found by adopting a linear bearing pressure distribution and then calculating the settlement at each corner.
– Ground-structure interaction may be generally undertaken using the modulus of sub-grade reaction spring or continuum models (a layered elastic half space representing the ground). When either the ground or the structure is complex more sophisticated models may be necessary (e.g.
finite element analysis).
– Table 6.7 illustrates the general hierarchy of ground-structure interaction models and when they may be used appropriately.
– Also refer to Section 7.14 on interaction with tunnels.