Welcome to NHBC's Supplementary Resource area. Supplementary Resource is the result of an extensive review and consultation on the way NHBC. The edition of NHBC Standards applies to every new home registered with NHBC where foundations are begun on or after 1 January NHBC STANDARDS TOP LINE FACTS. WITH THE LATEST NHBC STANDARDS. RECENTLY ISSUED, MIDLAND LEAD. GIVES YOU THE LOWDOWN.
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saw the introduction of a brand new format for the NHBC Standards, such this edition of the NHBC Standards also contains new or. The NHBC Standards came into force for every new home registered with NHBC where foundations were begun on or after 1 January. The Standards will become effective for every NHBC Registered home whose foundations are begun on or after the 1st January and will.
Structures requiring waterproofing range from those where the external ground levels have been raised around the perimeter to within mm of the internal floor finish, to deep basements where there may be several levels below ground, and include any other structure near to or below ground level where waterproofing may be required.
Robust design should be undertaken by suitably qualified waterproofing experts, and be suitable for the specific ground and building conditions. The design should:.
We would urge people within the industry — whether they are technical directors, construction managers, architects, designers or site managers — to familiarise themselves with the new layout and the updated content. Contact Standards and Technical if you have any queries on technical nhbc. Innovation in the Construction sector can come in many different forms, follow our Patent of the Month series. Features - Design Share it.
Follow ukconstructionm. More Features. Developing the infrastructure for megaprojects 15 Apr 19 Coordinating the vast supply chains megaprojects require is no easy task. Patent of the Month: Scaleable responses to infrastructure innovation 5 Apr 19 Increased efficiency in infrastructure brings positive returns for commerce and society. LOD simply explained: Heathrow cleared for take-off SMEs: The view and opinions expressed on this web site are solely those of the original authors and other contributors.
Sole plates 4 2 60 Yes Yes Yes Where timber used is heartwood only 2 and of durability class 1 — 2 3. Timber in contact Timber retaining walls with the ground greater than 1m high and within garden areas 7 4 30 Yes No No Where timber used is heartwood only 2 and of durability class 1 3. Durable 3. It should be assumed that timber is sapwood. Reference should be made to Chapter Treatment should be carried out in accordance with the WPA Manual.
External joinery. Preservatives used should be resistant to leaching or. Natural durability of building timbers heartwood only Durability class 1. Timber in contact Timber retaining walls up with the ground to 1m high and within garden areas 7 4 15 Yes No No Where timber used is heartwood only 2 and of durability class 1 — 2 3.
Timber in contact Timber retaining walls up with the ground to mm high and in a boundary situation 7 4 30 Yes No No Where timber used is heartwood only 2 and of durability class 1 3.
Decking where the deck is 3 up to mm from ground level 8.
Reference should be made to Chapters 7. Window frames. Timber component groups and preservative treatment. West Yorkshire. UK Douglas Engelmann. European whitewood. Only in situations where colour tinting will affect the appearance of the timber fixed to the home will clear preservatives be acceptable. It is important when timber and joinery products are stored that they are: Compatibility with metal 3.
WF10 5HW. None hybrid. Checks should ensure that. Where moisture is expected. Applied preservatives should be compatible with the original treatment. Further information 3. Treatment of cut surfaces 3. This should be stated on the delivery note.
Protection and storage 3. Timber should not be cut after treatment. Copper-containing treatments can cause corrosion between mild steel and aluminium. Effective communication within their organisation and with the client. They should: Risk management Ability to conduct risk assessments as required by the risk management process.
Suitable persons for the level of investigation The following skills and knowledge are required from the person responsible for the Initial Assessment. Basic Investigation and documentation and verification. Health and safety Awareness of occupational hygiene issues and Health and Safety legislation. Project management Ability to manage a project team consisting of the appropriate disciplines. Reporting and communication Ability to prepare comprehensive and well presented reports.
The following criteria should be used as guidance for the appointment of a consultant or specialist responsible for Detailed Investigation. Items to be taken into account include: Quality assurance Use of a quality management system. For land contamination to occur. Appropriate discipline s Understanding of all relevant skills required on the project and access to other disciplines. Site investigation Ability to design site investigation programmes.
Experience Similar types of site and development. Chapter 4. A written or diagrammatic representation of the land contamination known as a Conceptual Model. Compliance 4. Professional indemnity insurance Appropriate cover for the work being carried out. Geotechnical and contamination issues Assessment should be carried out by direct investigation and examination of the ground.
Engineering design Understanding of effective risk reduction techniques. Legislation Understanding of legislation and liabilities associated with the site. Where results are inconclusive. Documentation and verification NHBC requires documentation and verification to show that: Yes No 4. Initial Assessment: Failure to provide such information may delay the registration process. The results should be used to determine whether or not hazards are known or suspected.
Hazards Where hazards are identified. NHBC must be notified in writing a minimum of eight weeks before work starts. If any unforeseen hazards are found during the course of construction.
Hazards known or suspected? Yes No Required where hazards are known or suspected. Further Investigation required? Initial Assessment — desk study all sites 4. Solution features in chalk and limestone. Key information sources include: A desk study is the collection and examination of existing information obtained from a wide variety of sources.
Low bearing capacity ground Former buildings or structures Adjacent buildings Drains. It should indicate potential hazards at an early stage and provide a basis for the investigation.
Mining past. Potential problems should be assessed according to the current and historical uses of the site and surrounding area. Infill and made ground. A photographic record of the site can help in the reporting of the walkover survey. Potential hazards. Initial results should be evaluated for suspected hazards and the results recorded. Initial Assessment — results 4. Indications of any potential hazards should provide a basis for the investigation.
Water Lane. Brickfield Cottage. A walkover survey is a direct inspection of the site and the surrounding area carried out in conjunction with the desk study. Where trial pits do not provide sufficient information. Further investigation should be conducted if the Detailed Investigation has not satisfactorily addressed all of the original objectives.
The problems and liabilities which have to be managed in order to develop the site should be clearly communicated in the Detailed Investigation report. BS EN Where hazards are not suspected. The distance from the edge of the foundation should not be less than the depth of the trial pit. In addition to the Basic Investigation. Detailed Investigation sites where hazards are identified or suspected 4.
The number and depth of trial pits should be located so they are representative of the site and will depend upon the: Trial pits should be located outside the proposed foundation area. The Basic Investigation aims to provide assurance for all sites. A Detailed Investigation should be carried out where hazards are identified or suspected: Basic geotechnical and contamination investigations should be conducted and include: During the excavation of the trial pits.
Remediation techniques Solutions for dealing with contamination hazards include: The consultant or specialist should: The report should include the following information: Unforeseen hazards 4. Design precautions Solutions for dealing with geotechnical hazards include: Remediation method statement and report The remediation method statement should detail the strategy for the site and include the: Where additional or unforeseen hazards arise.
Geophysical methods rely on contrasts in the physical properties. Contrast may also be provided by faulting. Indirect investigations use geophysical techniques. Yes No Yes No Contamination hazards present: Hollow stem methods are typically employed where sample retrieval is required. Trial pits and trenches should be positioned where they will not affect future foundations.
Where the site is within an area susceptible to radon. Further Assessment and Basic Investigation Detailed Investigation Proposals to manage geotechnical risks Proposals to manage contamination risks 4.
Trenches are extended trial pits. Guidance for investigations 4. Direct investigation techniques involve intrusive activities to enable the retrieval and examination of the ground using trial pits. All relevant information. Trial pits allow the detailed inspection.
Conducted from the surface. Continuous flight auger Exploratory boreholes may be drilled in soils by mechanical continuous flight augers of various sizes.
Probing techniques Used to analyse the relative density of soils and for environmental sampling and monitoring such as chemical and physical testing of gases. Boreholes are typically formed using the following techniques: Light cable percussion drilling A shell and auger rig — typically used in the UK to drill boreholes in soils and weak rocks. Rotary drilling Either open-hole drilling or rotary coring. Investigation technique A site investigation normally comprises techniques which are classed as either indirect or direct.
Identification and classification of soil: Part 1. These range from basic tests undertaken by geologists or engineers using simple hand-held devices or portable test kits to methods that require specialist personnel and equipment.
Sampling methods and groundwater measurements. Identification of the probable source and the measurement of gas flow are important for risk assessments. A wide variety of in-situ tests can be used to support the results of direct testing. Further information 4. Chemical tests on soils. Part 2. Ground water should be collected from appropriately designed monitoring wells which should be screened and sealed to ensure that the relevant stratum is being monitored. Testing Testing may be undertaken in-situ.
Physical tests on soil and rock materials are carried out to provide the following information on ground: Samples are used to enable soil and rock descriptions to be made and to provide material for physical and chemical testing. Identification and description. The services of a specialist arboriculturalist may be helpful for the identification of the type and condition of trees that may affect building work.
Details should be provided with respect to: Where trees or hedgerows are either not shown or are in different positions and shrinkable soil is identified. This includes trees both on and adjacent to the site. The relationship becomes less predictable as factors combine to produce extreme conditions.
The interaction between trees. This chapter gives guidance for common foundation types to deal with the hazard and includes suitable foundation depths which have been established from field data. Provision of information 4.
Foundations near trees. Depths greater than 2. The depths are not those at which root activity. The following situations are beyond the scope of the guidance in this chapter and will require a site-specific assessment by an engineer see Technical Requirement R5: NHBC data and practical experience.
All necessary dimensions and levels should be indicated and relate to at least one benchmark and reference points on the site. In order to minimise this risk. This has the potential to affect foundations and damage the supported structure. The site plan should show the trees and hedgerows that affect the ground and works. These are signified by the need for deeper foundations. Removal of existing trees and hedgerows Statutory Requirements. Damage to foundations resulting from the growth of trees and roots should be avoided by locating structures and services at a safe distance.
Dead trees and hedgerows should be removed. If the location of previously removed vegetation is not known. The local planning authority should be consulted. All parts of the system are easily susceptible to damage which may not regenerate and which can affect the stability of the tree.
The shape of this area may change depending on specific factors such as local drainage. This can be caused by: An arboriculturist may be required to assess these factors is impractical. Before the site is cleared. If necessary. Where this cannot be achieved. Unstable trees should be made steady or felled.
Where root growth is noted within shrinkable soil and where records are not available. Where the species of a tree has been identified but is not listed. Water demand categories of common tree species are given in the table below. These tests are carried out on the fine particles and any medium and fine sand particles. Water demand of broad-leaf trees by species Tree species Water demand All elms. The volume change potential should be established from site investigation and reliable local knowledge of the geology.
BRE Digest and local geological survey maps Foundations shall be designed to make allowance for the effect of trees. The following definitions are used to classify soil properties: Arboricultural Association. Water demand.
High volume change potential should be assumed if the volume change potential is unknown. This is a requirement of BS which specifies the test procedure. The resulting shrinkage or swelling can cause subsidence or heave damage to foundations. This information: Where the species of a tree has not been identified. Sufficient samples should be taken to provide confidence that the results are representative.
Arboricultural Advisory and Information Service. Soil classification. Where hedgerows contain trees.
Information may be obtained from suitable alternative authoritative sources for trees not listed in this chapter. When the species is known but the subspecies is not. Coniferous trees: A 50mm decrease can be made to the foundation depth determined in accordance with this chapter for every 50 miles distance north and west of London. Thurso 0. Water demand Zone of influence High 1. Where it is unclear which zone applies. Table 3b: Zone of influence lateral extent of trees.
Guidance for factors affecting the mature height and water demand of trees Influencing factor Guidance Heavy crown reduction or pollarding previously or planned The mature height should be used. Removal of trees previously or planned The water demand of a semi-mature tree may be equal to that of a mature tree.
Foundations in shrinkable soils 4. Landscape and foundation designs should be compatible. This is to avoid a situation where. Different foundation types should not be used to support the same structure unless the foundation and superstructure design are undertaken by an engineer. Distance between tree and foundation The distance D between the centre of the trunk and the nearest face of the foundation should be used to derive the foundation depths.
Foundation type Foundations to all permanent structures. Foundation types that are acceptable in shrinkable soils include strip. Excavation of foundations 4. Some root activity may be expected below the depths determined in accordance with this guidance. Root barriers are not an acceptable alternative to the guidance given. Foundation depths should be measured on the centre line of the excavation and from ground level determined from Clause 4.
Freestanding masonry walls should be constructed on foundations in accordance with this chapter or designed to accommodate potential ground movement. Method of assessment of foundation depths Foundation depths should be determined according to the guidance provided in this document. If in doubt, assume the worst conditions or consult an engineer.
Foundations deeper than 2. One of the following methods may be used to assess the foundation depth where foundations are in the zone of influence of existing or proposed trees.
Design in accordance with this chapter to a depth derived from the charts in Clause 4. The most onerous conditions should be assumed in the absence of derived information. 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.
Foundation depths related to the zone of influence of new tree planting Foundation depths relating to the zone of influence of proposed tree planting should be in accordance with any of the following:.
Minimum foundation depths outside of the zone of influence of trees can be determined from Tables 4 and 5. Where foundation depths are in accordance with column A or column B in Table 4, tree planting should be restricted to: Foundation depths related to new shrub planting Shrubs have considerable potential to cause changes in soil moisture content. The foundation design should consider shrub planting in accordance with Table 6. The foundation design should consider shrub planting as follows: Shrubs that have a maximum mature height of 1.
Use foundation depth from column B and plant at least 1. Use foundation depth from column B and plant at least 0. Also see: Chapters 4. Design and construction of foundations in shrinkable soils 4. Reference should be made to Clause 4. The following will only be acceptable if they are designed by an engineer and account for all potential movement of the soil on the foundations and substructure: Pier and beam foundations Pier depths not exceeding 2.
Pier depths greater than 2. Pile and beam foundations When selecting and designing pile and ground beam foundations, piles should be:. Sufficient anchorage should be provided below the depth of desiccated soil. Slip liners may be used to reduce uplift but the amount of reduction is small, as friction between materials cannot be eliminated.
Bored, cast-in-place piles are well suited to counteracting heave. Most types have a straight-sided shaft, while some are produced with a contoured shaft to increase load capacity. The design should allow for the enhanced tensile forces in these piles. Driven piles are less well suited to counteracting heave and are difficult to install in stiff desiccated clay without excessive noise and vibration. The joint design of these piles should be capable of transmitting tensile heave forces.
Climbing shrubs which require Use foundation depth from column B. Where required by NHBC, site inspections are to be undertaken by the engineer to verify suitable compaction of the fill. Foundations in shrinkable soils shall be designed to transmit loads to the ground safely and without excessive movement. Strip and trench fill foundations in non-shrinkable soils overlying shrinkable soil Non shrinkable soils such as sands and gravels may overlie shrinkable soil.
Foundations may be constructed on overlying non-shrinkable soil if all the following are satisfied:. Where any of the above are not met foundation depths should be determined as for shrinkable soil. Measurement of foundation depths Where ground levels are to remain unaltered, foundation depths should be measured from original ground level. Figure 1: Measurement of foundation depths where ground levels are reduced or increased, either in the recent past or during construction, should be as shown in figures 1, 2 and 3.
Use the lower of: Figure 3: Granular infill should be placed beneath raft foundations on shrinkable soils as shown below. Steps in foundations On sloping ground, foundation trenches can be gradually stepped so that the required foundation depth is reasonably uniform below ground level. Where foundations are to be stepped to take account of the influence of trees, hedgerows and shrubs, they should be stepped gradually, with no step exceeding 0.
Foundations, substructures and services shall be suitably designed and detailed to prevent excessive movement due to heave. Heave precautions shall be incorporated into foundations and substructures in accordance with the design. Where foundations and substructure may be subject to heave, they should be protected by voids, void formers or compressible materials.
This section provides guidance on heave precautions for common building elements when located within the influence of trees which are to remain or be removed, including:. Potential for ground movement After the felling or removal of trees and hedgerows on shrinkable soils, heave can occur, as the absorbed moisture causes swelling.
Heave can also occur beneath a building where: Minimum void dimensions Voids should be provided to accommodate movement due to heave forces acting against foundations and suspended ground floors in accordance with Table 7.
Volume change Void dimension against side of potential foundation and ground beam. Void dimension under ground beams, and suspended in-situ concrete ground floor. Notes 1 Under suspended floors, the void dimension is measured from the underside of beam or joist to ground level and includes mm ventilation allowance.
Void formers consist of materials that collapse to form a void into which the clay can swell. Proprietary materials to accommodate heave Compressible material compacts as clay expands; the void dimension is the amount the material should be able to compress to accommodate heave.
The correct placement of heave materials is essential to ensure the foundations and substructure are adequately protected from heave forces. Heave precautions for foundations Table 8 shows where heave precautions are required for trench fill, pier and beam, and pile and beam foundation types which are in the zone of influence of trees see Table 3b which are to remain or be removed.
External trench fill and pier foundations. Unless NHBC is satisfied that the soil is not desiccated compressible material should be provided to the:. Inside faces of external wall foundations deeper than 1. External ground beams.
External and internal ground beams. Compressible material, void former or void should be provided to the underside of:. It is essential that: It is essential that heave material is provided to the entire areas shown. Particular care should be taken to ensure that the full width of the ground beam is protected. Raft foundations constructed in accordance with Clause 4. Particular care should be taken to ensure that the full width of the ground beam and the areas around the piles are protected.
Other foundation types All foundations not covered in this chapter, but specifically designed to counteract heave, should be:. Suspended ground floors Suspended ground floors with voids in accordance with Table 7 should be used in situations where heave can occur within the area bounded by the foundations, including where:. Paths and driveways Paths and driveways should be designed and detailed to cater for the likely ground movement.
Drainage shall be in accordance with the design and allow for ground movement. To protect against the effects of heave, drainage should be designed:. Foundation depth charts 4. Where no value is given in the table, minimum foundation depths apply i. Chart 1: Chart 2: Site at Oxford. From laboratory tests: Plasticity Index. The process may be repeated to allow the foundation to be stepped as its distance from the tree increases. Step 1 Determine the volume change potential of the soil. Step 2 Establish the species.
Ensure the site investigation includes representative sampling and testing. Use a scaled plan. This example is typical of Oxford clay. Lombardy poplar Sycamore Tree to remain. The appropriate height to use for removed trees and hedgerows depends on the actual height when they are removed. Always use the mature height for remaining and proposed trees and hedgerows.
From 4. Oxford is between 50 and miles NW of London. Dr P G Biddle. NHBC gratefully acknowledges the help given by authoritative organisations and individuals in the preparation of this chapter. Diagnosis and Remedy. Willowmead Publishing. Nottingham NG12 5GG. Nicker Hill. Harper Collins Geological Survey. Building Research Establishment.
Foundations for terraced homes may require special precautions to prevent damage from differential settlement. Ground conditions 4. Full details of junctions. Information may be available from: Foundations should be designed by an engineer in accordance with Technical Requirement R5 where: Information on ground conditions.
The depth of foundations should provide a clean. Chapters 3. Both designers and site operatives need to be aware of the ground conditions and any features requiring special attention. Ground conditions All relevant information about the history of the site. Where toxic materials or those likely to present a health hazard are found.
Clear and fully detailed drawings should be available on site to enable work to be carried out in accordance with the design. The home design and layout Foundation design should take account of site conditions. Ordnance Survey maps and geological maps and surveys. Strip and trench fill foundations that comply with the guidance in this chapter will generally be acceptable.
Walls should be located centrally on the foundation. Chapters 2. NHBC must be notified eight weeks before work starts. Shrinkable and volume change soil 4. Hazardous ground 4. Setting out 4. Any discrepancy in dimensions should be reported promptly to the designer. Resulting variations should be distributed to all appropriate persons. Frost susceptible soils and cold weather construction In frost susceptible soils.
These minimum depths may only be used where any existing or proposed trees or shrubs are outside the zone of tree influence. Hazardous ground is defined in Chapter 4. The accuracy of setting out should be checked by control measurements of trenches. Where the finished ground level is to be above the existing ground level and cold conditions are expected. Heave is possible in shrinkable soil where trees have been.
Where hazardous ground has been identified. Levels should be checked against accepted benchmarks. For drainage. This is to ensure that differential movement will not damage services.
Precautions should be taken to accommodate the effects of settlement where drains run under. Actions on structures. Services should not be rigidly encased in the foundations.
General actions. Services should be either sleeved or pass through a suitably strengthened opening in the foundation. Chapters 5. Trench fill The load-bearing capability of foundations should not be affected where services pass through. Strip foundations Services should not pass through strip foundations but through the masonry above. BS Strip and trench fill foundations shall transmit loads to the ground safely and without excessive settlement.
Adequately protect existing services and ground water drainage Any existing services. Adequate lintels should be provided in the masonry. Dead and imposed loads Dead and imposed loads should be calculated in accordance with: Where necessary. Requires at least a pneumatic or other limestone or firm chalk mechanically operated pick for excavation. The width of strip foundations should account for ground conditions and be in accordance with the following table: Foundation width and thickness The width of the foundation should: T Stability of any adjoining building Where foundations are taken deeper than an adjoining building.
Refer to specialist advice. P The thickness T of the foundation should be: S T overlap Table 3: Foundation overlap 4. Where foundations are stepped. Localised effects At soft spots. Design dimensions Inaccuracy may prevent walls and piers from being located centrally and therefore result in eccentric loading of foundations and possible foundation failure.
Excavations 4. Accurate trench digging is particularly important where the width of the foundation is only slightly wider than the wall to be supported. Excess excavation should be avoided. Where roots are visible at the bottom or sides of trenches. Trench bottoms affected by rain water. Acceptance from the foundation designer is required where the foundation design is modified. Hard spots should be removed. Sloping ground may require stepped foundations.
Construction joints for strip and trench fill foundations may be formed by one of the methods shown below: For trench fill foundations. Reinforcement should be: Concrete should be placed as soon as possible after the excavation has been checked.
Construction joints 4. Concreting should be carried out. Where movement joints are specified. Where construction joints are unavoidable: All dimensions and levels should be indicated and relate to at least one benchmark and reference points on site.
Elements of the building requiring foundations include: Both designers and site operatives need to be aware of: Building over changes in ground characteristics should be avoided. Site and ground appraisal All information relating to the site and its ground conditions which is necessary for full and proper foundation design should be obtained.
Trees and hedges Where the soil is shrinkable and nearby trees and hedges are existing. Stepped foundations and suspended floors may be required for sloping sites. Relevant Standards and codes of practice include: Site conditions 4. Dwelling design. Where sulfates or high acidity in ground or ground water are present.
Foundations for attached bays. Where toxic materials. High acidity. Where concrete is at risk from chemical attack from the ground. NHBC should be notified in writing at least eight weeks before work on site begins. Provision for new services Where services are to pass through.
Land drains should be diverted to a suitable outfall. Existing services and drainage Existing services or drains should: Services and drainage 4. Foundations for terraced homes. Sulfate and acids Sulfates and other chemicals can cause expansion and disruption of concrete.
Special consideration will be required for certain sites. Levels should be checked against benchmarks. Piled foundations: The design of all piled foundations should specify precautions for cohesive soils where volume changes can occur.
The following issues should be taken into account: Semi-raft foundations on made ground: The following notes are to be used as a guide for engineers designing raft foundations. For excavations. Setting out and excavations The accuracy of setting out should be checked by control measurements of trenches. Construction 4. Raft and semi-raft foundations should be designed: On larger homes.
Discrepancies to the design of the foundations or variations in the ground conditions should be reported formally to the engineer. Where piles are misaligned by more than mm in any direction. Hard spots Should be removed. Localised effects Situation Action Differences in bearing capacity e. Care should be taken to ensure that the bond of beams to piers and piles is in accordance with the design and is adequate.
Soft spots Excavations should be deepened locally to a sound bottom. Inaccuracy may prevent walls and piers being located centrally. Installation of piles. Piles are to be vertical. Load capacity verification of piles Test loading of piles should be undertaken when required. Foundation excavations should: Where piles are more than 75mm out of position.
Variations in design or ground conditions should be recorded and distributed to NHBC and others concerned with sitework. Visible roots. The builder is to obtain written confirmation that the piles are suitable for their design load.
Compressible materials 4. The engineer should undertake site visits to ensure that the design of the foundation is suitable for the actual ground conditions encountered. Reinforcement may be necessary. Reinforcement 4. Suitable mix Concrete should be of a mix which: Movement joints should be located so as to limit the risk of damage caused by movement. Movement joints 4. The design of movement joints and choice of sealing materials should consider: Concrete 4.
Concreting should: DPC mm min. Cavity walls should drain below the DPC and should: DPC cavity trays are not an acceptable waterproofing to the edges of specialised foundations.
Desk study and site investigation 4. Details of ground hazards to be taken into consideration are given in Chapter 4. In this chapter. The specialist contractor should be satisfied that the site investigation provides adequate and representative information in order to design the ground improvements. Hazardous sites. The purpose is to improve the load-bearing capacity. Vibratory ground improvement techniques that comply with the guidance in this chapter.
Design of vibratory ground improvement. Hazardous sites and ground hazards 4. The engineer should establish the scope of. The results of the site investigation and desk study should be sent to NHBC prior to work starting and should.
Suitability of ground conditions 4. Results of the site investigation and desk study. The engineer should assess the ground and be satisfied that it is suitable for treatment. NHBC should be notified prior to work starting. Extent and nature of any areas of filled ground on the site Includes: Confirmation of suitability for treatment 4.
Presence and extent of any existing or redundant services and drains Includes information on the extent and nature of the backfill to the excavations. The following considerations should be taken into account: Data for comparison with post-treatment properties should be established.
The engineer and specialist contractor should agree the following in writing before work commences on site: These written agreements should be made available to NHBC before work commences on site. Previous structures Includes any potential underground obstructions or hard-spots. Conditions acceptable for treatment are only those within zones A and B of Chart 1.
Any proposals will be subject to NHBC agreement. Filled ground which is still settling or expected to settle Ground subject to settlement or settling: Zone B — range of materials suitable for stone column vibro-replacement techniques. Highly sensitive soils Ground liable to collapse or remoulding. Ground with peat layers Ground with peat layers close to foundation level or the base of the stone column.
Loose or non-engineered fill Ground with any loose or non-engineered fill not previously subject to rising or fluctuating water levels or saturation. Ground with fill Voided filled ground Ground which includes. Unsuitable ground conditions Table 2: Consideration will be given to proprietary systems which do not permit vertical venting such as vibro concrete plug technology. Limitations of the treated ground The engineer should: At design depth.
Different forms of stockpiling can affect particle size and grading. Materials for use as fill 4. Where taken to a greater depth. The vibrator penetrates the weak soil. The vibrator penetrates by its mass. The outside of a stockpile may be weathered and may not be the same as unweathered material. A third method. The stone. There are two main vibratory methods commonly used in the UK. Column fill should be a clean.
The stone is then compacted into the surrounding soil by repeated withdrawal and insertion of the vibrator. Where the material is of a stable and uniform type from one source. NHBC should be informed of: Limestone fill may not be acceptable in acidic ground conditions. Dry bottom feed method The dry bottom feed method is adopted in weaker soils or situations where there is a high water table and the bore hole is liable to collapse between vibrator insertions.