4 General rules
4.1 Choice and design of foundation
184.108.40.206 The design of foundations should conform to BS EN 1997-1 and this clause (4).
NOTE 1 Guidance on geotechnical design, construction, and verification can be found in the ICE manual of geotechnical engineering (2012), Volume II .
220.127.116.11 The design of maritime works should conform to BS 6349.
NOTE Guidance on geotechnical engineering principles, problematic soils, and site investigation can be found in the ICE manual of geotechnical engineering (2012), Volume I .
4.1.2 Spread foundations
18.104.22.168 The design of spread foundations should conform to 22.214.171.124 and Clause 5.
NOTE Spread foundations are generally suitable in, but not limited to, the following design situations:
- where an adequate bearing stratum occurs at shallow depth (typically less than 2 m deep);
- on dense (or denser) coarse soils above the water table;
- on medium strength (or stronger) fine soils.
126.96.36.199 Spread foundations should not be placed on non-engineered fill unless such use can be justified on the basis of a thorough ground investigation and detailed design. Spread foundations should generally not be placed on engineered fill unless that fill is designed for the use of spread foundations.
4.1.3 Pile foundations
188.8.131.52 The design of pile foundations should conform to 184.108.40.206 and Clause 6.
NOTE Pile foundations are generally suitable in, but not limited to, the following design situations:
- on compressible strata overlying bedrock;
- on compressible strata overlying coarse soils of adequate density, there being no other beds of greater compressibility below the coarse soils;
- where the strata consist of fine soils of great thickness capable of supporting the piles by friction;
- where there is a need to minimize movements of a structure owing to swelling or shrinkage of surface soils; and
- where the addition of a foundation load would cause instability of the existing ground (e.g. at the crest of an existing slope).
220.127.116.11 Deep foundations, such as piles, should be considered when no adequate bearing stratum exists at shallow depths to permit economic construction of spread foundations.
18.104.22.168 The design of helical steel pile foundations should conform to Annex A.
The design of underpinning should conform to 22.214.171.124 and Annex B.
NOTE 1 Underpinning is typically used for, but not limited to, the following design situations:
- to rectify distress that a structure has already suffered;
- to extend foundations downwards to enable building and civil engineering works to be carried out; and
- to provide adequate resistance to increased loading on an existing structure.
NOTE 2 Guidance on the use of underpinning can be found in the ICE manual of geotechnical engineering (2012), Volume II, Chapter 83 .
4.1.5 Piled rafts
COMMENTARY ON 4.1.5
There are two different types of piled raft in common practice: raft-enhanced pile groups and pile-enhanced rafts.
126.96.36.199 The design of piled rafts should conform to 188.8.131.52 and Clause 5 and Clause 6.
NOTE 1 Piled rafts are typically used for, but not limited to, the following design situations:
- to minimize total, and particularly differential, foundation settlement;
- to reduce bending moments and shear forces in unpiled rafts; and
- to reduce the thickness of a raft foundation.
NOTE 2 Guidance on the use of piled rafts can be found in the ICE manual of geotechnical engineering (2012), Volume II, Chapter 56 .
184.108.40.206 The design of raft-enhanced pile groups should ensure that stresses within both the piles and the raft do not exceed the elastic range of behaviour.
220.127.116.11 The design of pile-enhanced rafts should ensure that all of the piles mobilise their ultimate resistance before the raft reaches its ultimate resistance.
4.1.6 Piled embankments
18.104.22.168 The design of piles used in piled embankments should conform to 22.214.171.124 and Clause 6.
NOTE Piled embankments are typically used for, but not limited to, the following design situations:
- to support infrastructure over low strength ground (e.g. alluvium);
- to support infrastructure over compressible ground (e.g. loose fill);
- to support infrastructure over collapsing ground (e.g. loess and karst geology); and
- to support infrastructure over unstable areas (e.g. land prone to mining subsidence).
126.96.36.199 The design of embankments should conform to BS 6031.
188.8.131.52 The design of basal reinforcement to piled embankments should conform to BS 8006-1:2010, 8.3.
4.2 Basis of geotechnical design
NOTE Attention is drawn to The Construction (Design and Management) Regulations 2015 , with regards to health and safety requirements for construction works, in particular:
- Regulation 9, duties of designers;
- Regulation 11, duties of the principal designer in relation to pre-construction phase; and
- Regulation 22, excavations.
4.2.1 Design requirements
184.108.40.206 Geotechnical data and investigation
220.127.116.11.3 Geotechnical investigations should conform to BS EN 1997-1:2004+A1rmation about the site from the desk study should be used to supplement that obtained in any new ground investigation.:2013, 3.2.
18.104.22.168.4 Ground investigations should conform to BS EN 1997-2 and BS 5930. Info
22.214.171.124.5 Sampling and testing of contaminated ground and groundwater should conform to BS 10175 and BS ISO 5667-11.
126.96.36.199.7 The number of investigation points should be sufficient to establish ground conditions, and any variability in those conditions, in the vicinity of the foundation.
188.8.131.52.8 For low-rise buildings, the number of investigation points undertaken (n) should satisfy the following, unless the ground conditions can be shown to be sufficiently uniform to justify a greater spacing:
|for A ≤ 300 m2|
|for A > 300 m2|
A is the plan area of each building's footprint.
184.108.40.206.9 Where a development involves construction of several buildings, equation (1) may be applied to the aggregate plan area of the buildings' footprints, provided the ground conditions can be shown to be sufficiently uniform.
220.127.116.11.11 For low-rise buildings, the depth of investigation below the planned base of the foundation (za) should satisfy the following minimum requirements:
bF is the smaller side length of the wall's foundation (on plan).
18.104.22.168.13 Where the base of the foundation is located on or near bedrock, the depth of investigation may be reduced.
22.214.171.124.14 The presence of trees and large shrubs should be noted during the course of the ground investigation, so that decisions can be taken concerning their retention or subsequent removal.
NOTE Guidance on site investigation can be found in the ICE manual of geotechnical engineering (2012), Volume I, Section 4: Site investigation  and in the NHBC Design Guide NF21, Efficient design of piled foundations for low rise housing (2010), Section 4: Site investigation .
4.2.2 Design situations
126.96.36.199 Design situations for foundations should include:
- the conditions given in BS EN 1997-1:2004+A1:2013, 6.3;
- collapse settlement;
- building within the zone of influence of buried highwalls (at the edge of opencast mining);
- building over landfill;
- the influence of global ground movements on foundation performance; and
- all other conditions specific to the site that can reasonably be forseen.
188.8.131.52 To conform to BS EN 1990, exceptional conditions involving local failure (such as a burst water pipe that continues to leak after the burst) should be classified as accidental design situations.
184.108.40.206 Design water levels should be compatible with the drainage provisions.
NOTE Information about exclusion zones around buried highwalls can be found in BRE Report 424 .
4.2.3 Design considerations
COMMENTARY ON 4.2.3
Design and construction considerations for foundations are given throughout BS EN 1997-1. The design considerations given in this clause are more specific examples of the issues that can affect the performance of a foundation.
The design of foundations should consider:
- the design considerations given in BS EN 1997-1:2004+A1:2013, 6.4;
- the possibility of weak layers below the proposed foundation level leading to greater settlement of the foundation than would occur in their absence;
- the possibility of shear failure of the ground supporting the foundation owing to the presence of cuttings, excavations, or sloping ground;
- the possibility of inclined or jointed strata in which the bedding or joint planes dip towards an excavation or natural depression causing stability problems due to failure along bedding planes or joints;
- the possibility of settlement of adjacent structures caused during or after construction of the foundations;
- the effect on the groundwater regime during and after construction;
- the execution (i.e. construction or installation) of the foundation; and
- global ground movements (e.g. due to bulk excavation or filling, water table changes, etc.) that can cause foundation deformations, additional to those induced by superstructure loads, and/or additional forces in the foundation or superstructure.
NOTE Global ground movements can be horizontal or vertical depending on the design situation.
COMMENTARY ON 220.127.116.11
Provision of suitable drainage is vital to ensure the acceptable performance of a foundation. Groundwater control is separate from the control of surface water, such as rainfall run-off.
18.104.22.168.1 Drainage should be provided to prevent:
- surface water from entering and eroding the face of any excavations;
- build-up of water pressures during construction in case they have harmful effects upon the foundations; and
- instability of slopes.
22.214.171.124.2 Surface water drains should be constructed using one of the types of pipe listed in 4.3.10.
126.96.36.199.3 Drainage systems should be designed for ease of maintenance and renewal during the design working life of the structure.
188.8.131.52.4 Drainage systems should be designed with positive outfalls to prevent ponding and include provisions of suitable discharge outfalls or soakaways.
184.108.40.206.5 Where the safety and serviceability of the works depend on the successful performance of the drainage system, the consequences of failure should be considered and one of the following conditions (or a combination of them) should be applied:
- a maintenance programme specified;
- a drainage system specified that will perform adequately without maintenance; or
- a secondary («backup») system specified – for example, a pipe or channel that encloses the primary system – that will prevent any potential leakage from entering the ground beneath or next to the structure.
NOTE 1 Guidance on surface water control can be found in Groundwater lowering in construction , Construction dewatering and groundwater control , and Groundwater control: design and practice .
NOTE 2 Guidance on groundwater control can be found in the ICE manual of geotechnical engineering (2012), Volume II, Chapter 80 .
220.127.116.11 Cyclic loading
The design of foundations subject to cyclic loading should consider the following:
- degradation of ground strength (leading to ultimate limit states being exceeded at loads below those expected from verifications based on static strength);
- degradation of ground stiffness, leading to an accumulation of permanent foundation displacement («ratcheting» effects); and
- amplification of loads or movements owing to resonance.
NOTE Guidance on foundations subjected to cyclic and dynamic loads can be found in the ICE manual of geotechnical engineering (2012), Volume II, Chapter 60 .
COMMENTARY ON 18.104.22.168
Foundations built adjacent to existing trees might suffer deleterious effects from the penetration of root-systems. These effects include increased loading on the structure and penetration of roots into joints or drainage systems.
22.214.171.124.1 The principles and procedures to be applied to achieve a harmonious and sustainable relationship between trees (including shrubs and hedges) and structures should conform to BS 5837.
126.96.36.199.2 Foundations should be designed to accommodate any volumetric changes in clay soils that might be caused by the presence of nearby trees.
NOTE Guidance on building near trees can be found in NHBC Standards, Chapter 4.2 .
188.8.131.52 Environmental considerations
The design of foundations should consider:
- their effect on sensitive species;
- generation and control of noise and dust during construction;
- generation, reuse and disposal of waste materials;
- minimizing the amount of material to be disposed of;
- contaminants entering watercourses (for example, as a result of excavation);
- the use of ground improvement; and
- the carbon footprint of the construction and the use of the structure.
184.108.40.206 Ground improvement
COMMENTARY ON 220.127.116.11
Ground improvement is the modification of ground properties obtained by binding or densifying the ground or creating inclusions in the ground.
Ground improvement involves the enhancement of ground properties, principally by strengthening of stiffening processes and compaction or densification mechanisms, to achieve a specific geotechnical performance (see ICE manual of geotechnical engineering, Volume I ).
Ground improvement includes any method by which natural or disturbed ground has its geotechnical performance altered.
Ground improvement covers many methods of ground treatment, including: modification, chemical alteration, reinforcement with steel or geosynthetics, strengthening by drainage, densification by vibration or consolidation, void filling, settlement or compensation grouting (see Contributions to Géotechnique 1948–2008 ).
Ground improvement techniques can be used to alter ground strength, stiffness, or permeability. Common applications include methods to enhance bearing resistance, control post-construction settlements, increase or decrease ground permeability, improve ground stability, generate controlled ground displacement, and mitigate the risk of liquefaction.
NOTE 1 Comprehensive guidance on the design and execution of the most common forms of ground improvement can be found in Ground Improvement .
NOTE 2 Information about the role of ground improvement can be found in the ICE manual of geotechnical engineering (2012), Volume I, Chapter 25 .
NOTE 3 Guidance on vibrocompaction and vibro stone columns, vibro concrete columns, and dynamic compaction can be found in the ICE manual of geotechnical engineering (2012), Volume II, Chapters 59 and 84 .
NOTE 5 Guidance on grouting for ground engineering (including permeation, rock grouting, hydrofracture, ground compaction, jet grouting, and compensation grouting) can be found in CIRIA Report C514 .
NOTE 6 Guidance on soil mixing can be found in the BRE Design Guide to soft soil stabilisation .
NOTE 7 Information about vibro stone columns can be found in BRE Report BR 391 .
NOTE 8 Information about the design of vibrocompaction/vibroflotation and vertical band drains can be found in the Institution of Civil Engineers' Specification for ground treatment .
NOTE 9 Guidance on the design, construction and control of rigid inclusion ground improvements can be found in the recommendations of the ASIRI National Project .