Annex B (normative)

Underpinning

COMMENTARY ON Annex B

This annex applies to the design and construction of underpinning.

B.1 General

COMMENTARY ON B.1

Underpinning is commonly employed to rectify distress caused to a building by excessive movement of its foundation, to extend foundations into the ground to facilitate future construction work, to accommodate additional loading applied to an existing building, to allow adjacent ground to be lowered, or to change the support system.

The design of underpinning should conform to BS EN 1997-1, and Clause 4 of this standard and this annex.

B.2 Choice of structure

NOTE Underpinning may be provided by:

• extending the depth of existing pad or strip footings;
• constructing ground beams beneath existing walls by stooling (and supporting them on piers or piles);
• constructing needle beams through existing walls (and supporting them on piers or piles);
• constructing piles that are drilled through existing foundations.

The choice of underpinning system should consider the loads to be carried, the sensitivity of the structure and the ground, and working conditions.

NOTE 1 Guidance about underpinning can be found in the ICE manual of geotechnical engineering (2012), Volume II, Chapter 83 [1].

NOTE 2 See Guidelines on safe and efficient basement construction directly below or near to existing structures [93].

NOTE 3 Guidance on health and safety in geotechnical engineering can be found in the ICE manual of geotechnical engineering (2012), Volume I, Chapter 8 [2].

NOTE 4 Further information about underpinning can be found in Underpinning and retention (2nd edition) [94].

B.3 Design considerations

B.3.1 Before underpinning is commenced, a comprehensive inspection, carefully recorded and photographed, should be undertaken to identify the condition of the structure that is to be underpinned and the condition of those structures adjacent to it.

B.3.2 The structure to be underpinned should be carefully examined for indications of differential foundation movement and for inherent weaknesses which might be accentuated during the process of underpinning, and the structure should be temporarily supported or strengthened. Particular care should be exercised when underpinning piers, columns or walls pierced by openings.

B.3.3 Underpinning systems should be designed in accordance with good soil mechanics practice taking into account all vertical and lateral loads imposed upon them, including particularly transient conditions that might arise during construction.

B.3.4 The whole foundation, including that part modified by underpinning, should continue to perform satisfactorily during and after underpinning. If the changed support conditions give rise to excessive differential movement, jacks should be installed temporarily or permanently to correct this. If a structure subsides unevenly, the whole structure should be underpinned and partial underpinning should not be used.

NOTE Guidance on condition surveys can be found in Appraisal of existing structures [95], Inspection manual for highway structures, Volumes 1 and 2 [96], [97].

B.4 Materials

B.4.1 Concrete

Concrete and related products incorporated into underpinning should conform to 4.3.6.

B.4.2 Steel

Steel and related products incorporated into underpinning should conform to 4.3.7.

B.4.3 Timber

Timber and related products incorporated into underpinning should conform to 4.3.8.

B.4.4 Soils, rocks, and rock masses

Particular attention should be paid during the ground investigation for underpinning to determine:

• ground and groundwater conditions below and adjacent to the structure;
• conditions responsible for any excessive movement so that appropriate underpinning measures can be undertaken;
• the ground's load-bearing capacity where the underpinning is to be founded; and
• the disposition and effect of adjoining foundations and services.

B.5 Durability

B.5.1 Concrete

The durability of concrete and related products used for underpinning should conform to 4.4.2.

B.5.2 Steel

The durability of steel and related products used for underpinning should conform to 4.4.3.

B.5.3 Timber

The durability of timber and related products used for underpinning should conform to 4.4.4.

B.6 Ultimate limit state design

B.6.1 The ultimate limit state design of underpinning involving spread foundations should conform to 5.7.

B.6.2 The ultimate limit state design of underpinning involving pile foundations should conform to 6.7.

B.7 Serviceability limit state design

B.7.1 The serviceability limit state design of underpinning involving spread foundations should conform to 5.8.

B.7.2 The serviceability limit state design of underpinning involving pile foundations should conform to 6.8.

B.8 Structural design

B.8.1 The structural design of underpinning involving spread foundations should conform to 5.9.

B.8.2 The structural design of underpinning involving pile foundations should conform to 6.9.

B.9 Execution

B.9.1 The execution of underpinning should conform to 4.9 and this subclause (B.9).

B.9.2 Before excavation is commenced, the live loads on the wall, pier or structure should be reduced as much as practicable where they are large in relation to the dead load, the adjoining owner's consent being obtained where necessary.

B.9.3 All excavations necessary for underpinning should be well supported, using concrete, steel, or timber, and strutted to an approved design, e.g. by an appropriate temporary or permanent geotechnical process, to prevent the surrounding ground from moving before it is supported by the permanent work.

B.9.4 Either steel trench sheeting or concrete poling boards should be used if the shoring cannot be withdrawn. Excavations for access to construct underpinning should be similarly treated and restricted to that essential for the work. The support removed when opening up underpinning excavations will increase the load on adjoining foundations unless it is fully compensated by shoring. These adjoining foundations should not be weakened or undermined by the excavation of access trenches or approach excavations.

B.9.5 Where underpinning will not give rise to excessive ground pressure, the procedure should be to execute underpinning in a series of legs, the length of each leg depending on the general character and condition of the structure to be underpinned, the intensity of the loading and the nature of the ground below. Generally, in brick and/or stone walls of normal type, each leg should be 1.0 m to 1.4 m in length; in walls capable of arching, the length of each leg may be increased accordingly.

B.9.6 Each series of legs should be planned to provide sufficient support between the legs under construction, and to ensure that the loads from the unsupported portions of the wall are distributed throughout the length of the wall. Attention should also be given to positions of openings and piers immediately above the foundations, so that sections of the structure carrying local heavy loads are not left unsupported. No fresh series of legs may be commenced until the preceding underpinning is completed and finally pinned. The sequence in which the underpinning is carried out should be executed in such a way as to avoid differential settlement and ensure load transfer.

B.9.7 The area of open excavations should not exceed 25% of the building's footprint. This limit should be reduced if the building comprises a number of isolated piers.

B.9.8 Where underpinning is required on account of the settlement of the foundation, or where the safe ground pressure is likely to be exceeded during the underpinning operations, the structure to either side of an underpinning leg should be needled and the load transferred to temporary bearings on ground capable of carrying the additional load or other support. The siting of these bearings should be determined to avoid undermining by any operations necessary to complete the underpinning leg. When this is complete, underpinning can commence.

B.9.9 Alternatively, it might be possible to introduce stooled reinforced concrete transfer beam prior to undertaking underpinning.

B.9.10 The construction of the underpinning leg should be commenced immediately after the bottom of the excavation has been exposed. In all cases where the bottom is likely to be affected by exposure to the atmosphere, the last section of the excavation should not be taken out unless the underpinning can proceed forthwith. The bottom of the excavation supporting the underpinning should be sealed with concrete immediately after inspection has shown it to be satisfactory.

B.9.11 Before the construction of a leg, the underside of the old wall or foundation should be cleaned and levelled ready for the new pinning. All joints against legs already constructed should be thoroughly cleaned. The underpinning leg should be constructed as quickly as possible up to within 75 mm to 150 mm of the underside of the old foundation ready for final pinning. The top of the new work should be left smooth and flat to facilitate the final pinning.

B.9.12 As soon as the concrete or brickwork is strong enough to support the load to be placed upon it, the final pinning should be carried out. The final pinning should consist of a fairly dry concrete mix, meaning that only sufficient water has been added to moisten the mixture so that it will remain a ball when squeezed in the hand (the maximum size of aggregate should be 10 mm). The mix should be rammed in hard to make solid contact with the soffit of the underpinned structure.

B.9.13 If the width of the foundation to be underpinned is greater than 1 m, it is advisable to leave the outer half more than 150 mm below the underside of the old foundation to facilitate the pinning up of the furthest part.

B.10 Monitoring

B.10.1 During underpinning, frequent checks for movement or distress in the structure should be made.

B.10.2 In most cases careful inspection is adequate but in special circumstances a detailed schedule of dilapidations should be prepared.

NOTE Crack monitoring, levelling and plumbing might be necessary as the work proceeds.

B.11 Reporting

Reporting for underpinning should conform to 4.12.

Annex C (informative)

Specific formations

C.1 London Clay

London Clay was deposited in marine conditions in the Eocene epoch (30 million years ago). London Clay is made up of various silty clay and sandy clayey silt units, separated by glaucontic rich horizons. Very fine sand and silt dustings, partings, and lenses are frequent in the siltier clays; and sand layers occur in the sandy clayey silts. Phosphatic and claystone nodules are quite common throughout the deposit.

The London Clay in central London is one of the most highly investigated soils in the world.

NOTE A summary of the characteristics of London Clay can be found in Some characteristics of London Clay [98] and The London Clay at T5 [99].

C.2 Gault Clay

The Gault Clay comprises a sequence of clays, mudstones, and thin siltstones with bands of phosphatic nodules of Middle and Upper Albian age. It outcrops in East Anglia, Wessex, Dorset, North East Kent, Surrey, and Hampshire.

Gault Clay causes a number of serious geotechnical problems, including ancient and recent landslides, and can contain sufficient sulfate and sulfuric acid for potential chemical attack on concrete. Seasonal shrinkage and swelling of this highly expansive soil can result in damage to buildings.

NOTE Guidance on the engineering geology of Gault Clay can be found in BGS Technical Report WN/94/31, Engineering Geology of British Rocks and Soils – Gault clay [100].

C.3 Lambeth Group

The Lambeth Group (previously known as the Woolwich and Reading Beds) is a complex sequence of gravels, sands, and clays that vary considerably both horizontally and vertically and whose properties range between those of an engineering soil and a rock.The Lambeth Group underlies much of south-east England, particularly in London and Hampshire, and, as a result, is frequently encountered in major construction projects. CIRIA identified the Lambeth group as one of the «economically important UK soils and rocks» (CIRIA C583 [101]).

NOTE 1 Guidance on the engineering properties of the Lambeth Group can be found in CIRIA C583 [101].

NOTE 2 Guidance on the engineering geology of the Lambeth Group can be found in BGS Open Report OR/13/006, Engineering Geology of British Rocks and Soils – Lambeth Group [102].

C.4 Glacial soils and tills

Glacial deposits are widespread throughout the world and are frequently encountered in the upland parts of the United Kingdom. Glacial tills and soil are amongst the most difficult to engineer, owing to their marked variation in both thickness and engineering properties.

NOTE 1 Guidance on the classification of glacial tills can be found in Chapter 4 of CIRIA C504 [103].

NOTE 2 Guidance on the engineering properties of glacial tills can be found in Chapter 5 of CIRIA C504 [103].

NOTE 3 Information about issues relevant to glacial soils can be found in the ICE manual of geotechnical engineering (2012), Volume I, Chapter 31 [2].

C.5 Problematic soils

C.5.1 Problematic soils include materials that display siginificant volume change, a distinct lack of strength, or are potentially corrosive. Problematic soils are profoundly influenced by the climatic regime in which they were developed. (See ICE manual of geotechnical engineering, Volume II [1] for further information).

C.5.2 Problematic soils include (but are not limited to): arid soils, tropical soils, glacial soils, collapsible soils, expansive soils, non-engineered fills, organics/peat soils, sulfate/acid soils, and soluable ground.

NOTE 1 Information about problematic soils and their issues can be found in the ICE manual of geotechnical engineering (2012), Volume I, Section 3 [2].

NOTE 2 Information about shrinkable (also known as expansive) soils can be found in the ICE manual of geotechnical engineering (2012), Volume I, Chapter 33 [2].

C.6 Chalk

Chalk forms the downland of southern England, the Wolds of eastern England, and the white cliffs of Antrim, East Yorkshire, Dover, and from the Seven Sisters to Dorset. The chalk is the UK's most important aquifer for potable water supply. A great deal of construction and infrastructure development is built on chalk.

NOTE 1 Guidance on the description and classification of chalk can be found in CIRIA C574, Chapter 3 [N2].

NOTE 2 Guidance on the mechanical properties of chalk can be found in CIRIA C574, Chapter 4 [N2].

C.7 Mercia Mudstone Group

The Mercia Mudstone group of rocks underlies much of northern, central, and southern England and parts of Northern Ireland. The engineering properties of the rocks and the derived soils are important as they are frequently encountered in excavations and as founding strata.

NOTE 1 Guidance on the description and classification of Mercia mudstone can be found in CIRIA C570, Section 2, Geological background [104].

NOTE 2 Guidance on the engineering properties of Mercia mudstone can be found in CIRIA C570, Section 5, Correlation of engineering properties to the SPT and Section 6, In-situ properties and behaviour of Mercia mudstone [104].

NOTE 3 Guidance on the engineering geology of Mercia mudstone can be found in BGS Report RR/01/02, Engineering geology of British rocks and soils: Mudstones of the Mercia Mudstone Group [105].

C.8 Lias Group

The Lias Group encompasses an important group of geological materials, comprising clay-rich mudstones interlayered with limestones. The outcrop of the Lias extends in a continuous band from the coast of Dorset in a north-north-easterly direction to Yorkshire, with outlying areas in Somerset and South Wales.

NOTE Guidance on the engineering geology of the Lias Group can be found in BGS Internal Report OR/12/032, Engineering Geology of British Rocks and Soils – Lias Group [106].

Annex D (informative)

Archaeological finds

A key element of the UK Government's policy for managing the historic environment in England is the «presumption in favour of sustainable development» [107]. To achieve this, building foundations are often constructed above important archaeological deposits or, in the case of piled foundations, through them.

The UK Government's National Planning Policy Framework [107] sets out core planning principles regarding management of change to the Historic Environment in England, including «conserve heritage assets in a manner appropriate to their significance, so that they can be enjoyed for their contribution to the quality of life of this and future generations».

Scottish Planning Policy in relation to archaeology finds requires planning authorities to protect archaeological sites and monuments as an important, finite, and non-renewable resource and to preserve them in-situ wherever possible. Where in-situ preservation is not possible, planning authorities require developers to undertake appropriate excavation, recording, analysis, publication, and archiving before and/or during development [108].

In Northern Ireland, the desirability of preserving archaeological sites and their settings is a first principle in assessing and determining planning applications for construction schemes. Consequently, the avoidance of known or suspected archaeological sites is a key element of the design process. Any archaeological remains found on a construction site should be preserved in-situ as the primary option. Where this is not possible then full recording (through appropriate excavation and survey methods) followed by timely and suitable public dissemination of the information and academics alike is to take place.

The UK Government's Planning Practice Guidance gives further advice on enhancing and conserving the historic environment [109].