4.2 Building near trees

SCOPE

This Chapter gives guidance on meeting the Technical Requirements and recommendations when building near trees, hedgerows and shrubs, particularly in shrinkable soils.

INTRODUCTION

DESIGN STANDARDS

DESIGN STANDARDS

Design that follows the guidance below will be acceptable for building near trees, hedgerows and shrubs.

STATUTORY REQUIREMENTS

Design should be in accordance with relevant Building Regulations and other statutory requirements

TREES AND HEDGEROWS ADJACENT TO STRUCTURES

Items to be taken into account include:

Dead trees and dead hedgerows should be removed. Unstable trees should be made stable but where this is not possible they should be felled. If in doubt, advice should be obtained from a Registered Arboriculturalist.

Acts of Parliament, planning conditions, conservation area restrictions or tree preservation orders may mean that trees and hedgerows are protected and must be retained. The local planning authority should be consulted.

Most of a tree's root system is within 600mm of the surface and extends radially for distances often in excess of the tree's height. All parts of the root system are vulnerable to damage and once damaged, roots may not regenerate. Extensive root damage may impair the stability of the tree.

Root damage and tree instability can be caused by:

• stripping topsoil too close to trees
• excavating trenches for foundations and services too close to trees
• raising soil levels adjacent to trees, particularly where non-granular materials are used
• compaction of soil around trees by heavy plant
• storage of heavy materials around trees
• covering rooting area with impervious surfaces.

Damage should be avoided by erecting suitable fencing to create a tree protection zone for each remaining tree and ensuring this zone is left undisturbed during construction work.

Further guidance is given in BS 5837 and Appendix 4.2-E.

Direct damage due to the growth of the main trunk and roots of young trees should be avoided by locating structures and services at a safe distance from the trees. Further guidance is given in BS 5837 and Appendix 4.2-E. Where this cannot be achieved precautions should be taken to allow for future growth. For example:

• foundations should be reinforced to resist lateral forces
• walls or structural slabs should bridge over the roots allowing sufficient clearance for future growth or be reinforced to avoid cracking
• pavings and other surfaces should be laid
  on a flexible base to allow for some movement.

FOUNDATIONS (all soil types)

Foundations for all soil types should be designed and constructed in accordance with Chapter 4.1 'Land quality - managing ground conditions' and other relevant Chapters of the Standards (depending on site specific conditions).

Different foundation types should not be used to support the same structure unless the foundations and superstructure design are undertaken by an Engineer (see Technical Requirement R5).

The remainder of this Chapter gives additional guidance that applies when building near trees, hedgerows and shrubs on shrinkable soils as defined in Clause D5(b).

Foundations (shrinkable soils)

Items to be taken into account include:

(a) shrinkage and heave

Shrinkable soils are subject to changes in volume as their moisture content is altered. Soil moisture contents vary seasonally and are influenced by a number of factors including the action of tree roots. The resulting shrinkage or swelling of the soil can cause subsidence or heave damage to foundations, the structures they support and services. Heave precautions are described in Clause D8.

Shrinkable soils are widely distributed throughout the UK. Local geological survey maps may give relevant information.

(b) soil classification

For the purposes of this Chapter, shrinkable soils are those containing more than 35% fine particles and having a modified Plasticity Index of 10% or greater.

Fine particles are defined as those having a nominal diameter less than 60µm, ie. clay and silt particles.

The Plasticity Index (Ip) of a soil is a measure of its volume change potential and is determined by Atterberg Limits tests. These tests are carried out on the fine particles and any medium and fine sand particles. Soil particles with a nominal diameter greater than 425µm are removed by sieving beforehand. The percentage of particles smaller than 425µm is routinely reported for Atterberg Limits tests. This is a requirement of BS 1377, which specifies the test procedure.

The Modified Plasticity Index (I'p) is defined as the Plasticity Index (Ip) of the soil multiplied by the percentage of particles less than 425µm.

Modified Plasticity Index is related to volume change potential as shown in Table 1.

Table 1 Volume change potential

Alternatively the Plasticity Index may be used without modification. For pure clays and other soils with 100% of particles less than 425µm the result will be the same. However, for mixed soils such as glacial tills, use of the modified Plasticity Index may result in a more economic design.

For further information about the modified Plasticity Index refer to BRE Digest 240.

The volume change potential should be established from site investigation and reliable local knowledge of the geology.

Sufficient samples should be taken to provide confidence that the test results are representative of the soil volume change potential for the site. If in doubt use the higher value of volume change potential.

If the volume change potential is unknown, high volume change potential should be assumed.

(c) water demand of trees

Water demand varies according to tree species and size.

Appendix 4.2-A gives the water demand categories of common tree species.

Where the species of a tree has not been identified, high water demand should be assumed.

Where the species of a tree has been identified but is not listed, the following assumptions may be made for broad leafed trees:

• high water demand - all Elms, Eucalyptus, Hawthorn, Oaks, Poplars and Willows
• moderate water demand - all others.

Where trees are not listed in Appendix 4.2-A, information may be obtained from suitable alternative authoritative sources (see Appendix 4.2-F).

Tree identification can be assisted by reference to a tree recognition book (see Appendix 4.2-F).

For the purposes of this Chapter, the zone (i.e. lateral extent) of influence of trees is shown in Table 2.

Table 2 Zone of tree influence

(d) tree heights

Mature heights of common tree species are listed in Appendix 4.2-A. For the purposes of this Chapter, these are the average mature heights to which healthy trees of the species may be expected to grow in favourable ground and environmental conditions. These may be used even when the actual heights are greater.

The mature heights given in Appendix 4.2-A should be used for trees that are to remain or are scheduled to be planted and where ground levels are unaltered. Where ground levels are increased see also Figure 1 and Sitework clause S3(c).

Where there are different species within hedgerows, the mature height of the species likely to have the greatest effect should be used.

For trees which have been or are to be removed, allowance should be made for the fact that the water demand of a tree varies with its size and rate of growth (see Figure 1). The water demand of a semi-mature tree may be as great as that for a mature tree of the same species whereas the water demand for a sapling or young tree will be significantly less.

Figure 1 Tree height H to be used for particular design cases

Where trees have undergone or are to undergo heavy crown reduction or pollarding, the mature height should be used or a Registered Arboricuturalist should be consulted to undertake a site specific assessment.

(e) climate

High rainfall reduces moisture deficits caused by trees and hedgerows, and cool damp weather reduces the rate of water loss from the tree, thus reducing the risk of soil movement. As the driest and hottest conditions in the UK usually prevail in southeast England, the greater risk occurs in that area and diminishes with distance north and west.

For the purposes of this Chapter, the UK has been divided into zones at 50 mile intervals from London. After the foundation depth has been derived from Appendix 4.2-B or 4.2-C a reduction of 0.05m (50mm) may be made for every 50 miles distance north and west of London (see Appendix 4.2-D).

Items to be taken into account include:

Foundations to all permanent structures (including garages, porches and conservatories) should take account of the effects of soil desiccation caused by previous or existing trees and trees which are scheduled to be planted.

The following foundations will be acceptable in shrinkable soils, provided that they are capable of supporting the applied loads without undue settlement, heave precautions are taken as in Clause D8 and their design takes account of Clause D7:

• strip
• trench fill
• pier and beam
• pile and beam
• raft.

Variations to the foundation depths derived from this Chapter may be permitted where other foundation depths are traditionally acceptable or where necessary to take account of local ground conditions, provided that they can be supported by a design in accordance with Technical Requirement R5.

Root barriers are not a reliable means of reducing the effects of trees on foundations in shrinkable soils and are not an acceptable alternative to the guidance given in this Chapter.

Freestanding masonry walls should be constructed on foundations in accordance with this Chapter or be designed to accommodate likely ground movement, for example, by careful use of movement joints and reinforcement.

One of the following methods may be used:

• design in accordance with this Chapter to a depth derived from Appendix 4.2-B or 4.2-C taking account of:
• the site investigation
• the soil volume change potential
• the water demand of the tree
• the appropriate tree height
• the distance of the tree(s) from the foundations
• the geographical location of the site north and west of London
• appropriate heave precautions.

Note: the most onerous conditions should be assumed in the absence of any of the above information.

• design by an Engineer in accordance with Technical Requirement R5, taking account of:
  
• the recommendations of this Chapter
• results of the site investigation
• advice, when necessary, from a Registered Arboriculturalist or other competent person whose qualifications are acceptable to NHBC.

Note: when this method is used and it results in foundation depths or other details less onerous than those derived from this Chapter, the design should be submitted to NHBC for approval prior to work commencing on site.

The distance D between the centre of the trunk and the nearest face of the foundation should be used to derive the foundation depths from Appendix 4.2-B or 4.2-C.

For trees which have been or are to be removed from within 2m of the face of the proposed foundation and where the height on removal is less than 50% of the mature height given in Appendix 4.2-A, it may be assumed that D = 2m.

Note: This is to avoid the anomalous situation where, for example, a "sapling" removed from the foundation line would otherwise require an unnecessarily deep foundation since the D/H value would always be zero regardless of the height H of the tree.

Foundation depths relating to proposed tree planting should be based on one of the following:

• foundation depths derived in accordance with Appendix 4.2-B or 4.2-C, or
• foundation depths shown in Table 3 with limits agreed in the planting schedules to exclude trees within the distances from foundations shown in Table 4, or
• foundation depths shown in Table 5 with limits agreed in the planting schedules to exclude trees within the zone of influence shown in Table 2.

Table 3 Minimum foundations depths allowing for restricted new planting

Table 4 No tree planting zone for minimum depth foundations

Table 5 Minimum foundations depths outside zone of influence

Planting schedules should be agreed with the local planning authority before work commences on site.

The landscape and foundation designs should be compatible.

Shrubs have considerable potential to cause changes in soil moisture content.

The foundation design should consider shrub planting as follows:

• Shrubs whose mature height does not exceed 1.8m and climbing varieties (i.e. those requiring a wall for support) whose mature height does not exceed 5m:
  • use foundation depth from Table 5

• Pyracantha and Cotoneaster whose mature height exceeds 1.8m:
  • use foundation depth from Table 5 and plant at least 1.0 x mature height from foundation, or
  • use foundation depth from Table 3 and plant at least 0.5 x mature height from foundation

• All others:
  • use foundation depth from Table 5 and plant at least 0.75 x mature height from foundation, or
  • use foundation depth from Table 3 - no restriction on minimum distance from foundation.

Planting schedules should be produced by a qualified landscape architect or other suitably qualified person and agreed with the local planning authority before work commences on site.

The landscape and foundation designs should be compatible.

Table 6 - removed April 2005

Non shrinkable soils such as sands and gravels may overlie shrinkable soil.

Foundations may be constructed on the overlying non shrinkable soil in accordance with Chapter 4.4 'Strip and trench fill foundations' provided all of the following conditions are satisfied, as illustrated in Figure 2:

• consistent soil conditions exist across each plot. This should be confirmed by the site investigation
• the depth of the non shrinkable soil is greater than 3/4 depth X, where X is the foundation depth determined using Appendix 4.2-B or 4.2-C, assuming that all the soil is shrinkable
• the thickness T of non shrinkable soil below the foundation is equal to or greater than the width of the foundation B
• the proposals are submitted to and approved by NHBC prior to work commencing on site.

Where any of the above conditions is not met, foundation depths should be determined as for shrinkable soil.

Figure 2 Foundations in non shrinkable soils overlying shrinkable soil

Where foundations are to be stepped to take account of the influence of trees, hedgerows and shrubs they should be stepped gradually in accordance with Chapter 4.4 'Strip and trench fill foundations' with no step exceeding 0.5m (see Sitework clause S3(b)).

Where the foundations are on or adjacent to sloping ground greater than 1 in 7 (approximately 8°) and man-made slopes such as embankments and cuttings they should be designed by an Engineer (see Technical Requirement R5).

Items to be taken into account include:

• slope stability
• potentially enhanced desiccation due to increased run-off and the de-watering effects of the slope and vegetation.

Items to be taken into account include:

Strip foundations up to 1.5m deep should be constructed in accordance with the recommendations of this Chapter and Chapter 4.4 'Strip and trench fill foundations'. Depths should be determined in accordance with Clause D6.

Trench fill foundations up to 2.5m deep should be constructed in accordance with the recommendations of this Chapter and Chapter 4.4 'Strip and trench fill foundations'. Depths should be determined in accordance with Clause D6.

Reference should be made to Clause D8 to establish the precautions necessary to cater for potential heave.

Trench fill foundations deeper than 2.5m will only be acceptable if they are designed by an Engineer (see Technical Requirement R5) taking account of all potential movement of the soil on the foundations and substructure.

The following will need to be taken into account if foundations are to be deeper than 2.5m:

• foundation depths should be designed taking account of soil desiccation and arboricultural advice
• additional heave precautions may be necessary to cater for lateral and shear forces acting on large vertical areas of foundation
• instability of the trench sides can lead to serious construction difficulties
• the foundation is dependent upon a high level of workmanship and detailing:
  • concrete overspill or overbreak in the excavations can result in additional vertical forces being transmitted to the foundation
  • construction joints will need to be detailed to take account of the increased lateral forces
  • compressible material should be correctly placed to avoid excessive heave forces being applied to the foundation.

Pier and beam foundations should be designed by an Engineer (see Technical Requirement R5) and constructed in accordance with the recommendations of this Chapter and Chapter 4.5 'Raft, pile, pier and beam foundations'.

Note: pier depths up to 2.5m may be derived from Clause D6. Pier depths greater than 2.5m require site specific assessment.

Reference should be made to Clause D8 to establish the precautions necessary to cater for potential heave.

Pile and beam foundations should be designed by an Engineer (see Technical Requirement R5 ) and constructed in accordance with the recommendations of this Chapter and Chapter 4.5 'Raft, pile, pier and beam foundations' .

Reference should be made to Clause D8 to establish the precautions necessary to cater for potential heave.

Raft foundations should be designed by an Engineer (see Technical Requirement R5 ) and constructed in accordance with the recommendations of this Chapter, Chapter 4.5 'Raft, pile, pier and beam foundations'  and the following conditions.

Raft foundations will only be acceptable where all of the following apply, as illustrated in Figure 3:

• the foundation depth derived in accordance with Clause D6 is 2.5m or less
• the raft is founded on granular infill placed and fully compacted in layers in accordance with the Engineer's specification and to NHBC's satisfaction. The infill should not be less than 50% of the foundation depth derived in accordance with Clause D6 and should not exceed 1.25m. Site inspections by the Engineer may be required by NHBC to verify the compaction of the fill
• the infill extends beyond the edge of the foundation by a distance equal to the natural angle of repose of the infill plus 0.5m
• the raft is generally rectangular in plan with a side ratio of not more than 2:1
• NHBC is satisfied that the raft is sufficiently stiff to resist differential movements.

Figure 3 Requirements for raft foundations on shrinkable soils

DESIGNING TO ACCOMMODATE HEAVE 

Heave can take place in a shrinkable soil when it takes up moisture and swells after the felling or removal of trees and hedgerows. It can also occur beneath a building if roots are severed or if water enters the ground from leaking drains, water services or changes in ground water conditions.

Items to be taken into account include:

Before the site is cleared, the location, heights and species of trees, hedgerows and shrubs on and adjacent to the site and which may affect proposed foundations should be surveyed and recorded.

If the location of previously removed vegetation is not known, local enquiries and reference to aerial photographs may be necessary. Otherwise the design should assume the worst conditions or an Engineer (see Technical Requirement R5) should be consulted to undertake a site specific design based on all relevant information.

Where root growth is noted within shrinkable soil and where records are not available, an Engineer (see Technical Requirement R5) should be consulted to assess whether heave is likely.

Trench fill foundations should be designed in accordance with Clause D7. Any foundations deeper than 2.5m should be designed by an Engineer (see Technical Requirement R5).

Heave precautions should be used:

• where the foundation is within the zone of influence of trees (see Table 2), and
• where the foundation depth determined in accordance with Clause D6 is greater than 1.5m based on the appropriate tree height (see Figure 1).

Heave precautions for trench fill foundations up to 2.5m should be in accordance with Sitework clause S4(a).

Pier and beam foundations should be designed in accordance with Clause D7.

Heave precautions for piers should be used:

• where the foundation is within the zone of influence of trees (see Table 2), and
• where the foundation depth derived in accordance with Clause D6 is greater than 1.5m based on the appropriate tree height (see Figure 1).

Heave precautions for pier and beam foundations should be in accordance with Sitework clause S4(b).

Pile and beam foundations should be designed in accordance with Clause D7.

Heave precautions should be used for piles and ground beams in accordance with Sitework clause S4(c). In addition the following should be taken into account in the selection and design of piles:

• piles should be designed with an adequate factor of safety to resist uplift forces on the shaft due to heave by providing sufficient anchorage below the depth of desiccated soil. Slip liners may be used to reduce the uplift but the amount of reduction is small, as friction between materials cannot be eliminated
• piles should be reinforced for the length of the pile governed by the heave design
• bored, cast-in-place piles are well suited to this application. Most types have a straight-sided shaft but some construction techniques produce a contoured shaft, similar to a screw profile, to increase load capacity. The design should allow for the enhanced tensile forces in such piles
• driven piles are less well suited to this application and are difficult to install in stiff desiccated clay without excessive noise and vibration. Most types are jointed and, if these are to be used, the joint design should be capable of transmitting tensile heave forces
• piles and ground beams should be designed taking into account the upward force on the underside of the ground beams transmitted through the compressible material or void former prior to collapse (refer to manufacturer's data).

Suspended ground floors should be used in all situations where heave can occur within the area bounded by the foundations. This includes:

• where the foundation depth derived in accordance with Clause D6 is greater than 1.5m based on the appropriate tree height (see Figure 1), unless NHBC is satisfied the soil is not desicated
• where ground floor construction is undertaken when surface soils are seasonally desiccated (i.e. during summer and autumn) unless NHBC is satisfied the soil is not desiccated.

The following types of suspended floor will be acceptable where there is potential for heave.

PRECAST CONCRETE

A minimum void depth should be provided between underside of beam and ground level as shown in Table 10 (see Sitework clause S4(d)).

TIMBER

A minimum void depth should be provided between underside of joist and ground level as shown in Table 10 (see Sitework clause S4(d)). All sleeper walls should have foundations with depths derived in accordance with Clause D6.

IN-SITU CONCRETE

A minimum void depth should be provided between the ground and the underside of slab as shown in Table 9 (see Sitework clause S4(d)). Where proprietary materials are used, they should be in accordance with Materials clause M2  and the design should take into account the upward force transmitted through the compressible material or void former prior to collapse (refer to manufacturer's data).

Raft foundations constructed in accordance with Clause D7 should provide adequate protection from heave.

All foundations not covered in the above clauses, but specifically designed for heave, should be designed by an Engineer (see Technical Requirement R5) taking account of the recommendations of this Chapter and submitted to NHBC for approval prior to work commencing on site.

Drainage should be constructed in accordance with Chapter 5.3 'Drainage below ground' with the following additional precautions to guard against the effects of heave:

• design gradients may need to be greater than the minimum gradients in Chapter 5.3 as these do not allow for possible ground movement. Where sufficient falls to cater for the likely movement cannot be provided, alternative means of catering for the movement should be used, for example taking the excavation deeper and laying the pipework on granular bedding of suitable thickness to reduce the extent of potential movement
• a drainage system capable of accommodating the likely movement should be used
• pipes and services passing through substructure walls or trench fill foundations should be designed and detailed so as to cope with the potential ground movements shown in Table 7.

Table 7 Potential ground movement

Existing land drains should be maintained or diverted. Where the void beneath suspended floors is liable to flooding, drainage should be provided.

Drives and pathways should be designed and detailed to cater for the likely ground movement.

Further guidance is given in BS 5837.

PROVISION OF INFORMATION

It is important that all relevant information needed for the completion of the sitework is readily available to all appropriate personnel.

All necessary dimensions and levels should be indicated and related to:

• at least one benchmark, and
• reference points on site.

Details should be provided with respect to:

• site investigation
• site survey including location and height of trees and hedgerows affecting the site
• site layout
• dimensions, type and depth of foundations
• soil volume change potential
• tree species (including existing, removed and proposed) using English names
• planting schedules
• original and final ground levels
• technical method statements including critical sequences of construction
• location of services
• design of drainage system
• locations and detailing of:
  • steps in foundations
  • movement and construction joints
  • ducts and services passing through the foundations.