When purchasing a new build home, the ground beneath your property is just as important as the walls and roof above it. Many new build developments across the UK are constructed on sites where the natural ground is not strong enough to support a home without first being treated. One of the most common methods used to strengthen weak or filled ground is known as vibratory ground improvement, and the NHBC Standards 2026, Chapter 4.5, sets out detailed requirements for how this work must be carried out.
This article explains each section of Chapter 4.5 in straightforward terms, so that you as a new build buyer can understand what should have happened beneath your home before construction began, and why it matters.
It is worth noting that while a snagging inspection will thoroughly assess the visible finish and quality of your new home, it is not a structural survey and does not extend to assessing ground improvement works or foundation design. However, understanding these standards can help you ask the right questions and ensure your developer has followed proper procedures.
4.5.1 – Compliance
All vibratory ground improvement work must comply with NHBC’s Technical Requirements and be designed by a suitably experienced engineer, working in accordance with established British Standards and codes of practice. The engineer responsible for the foundation design must be independent of the contractor carrying out the vibratory ground improvement work. This separation helps ensure that the design is objective and that proper oversight is maintained. Relevant standards include BS EN 1997 (Geotechnical design), BS EN 14731 (Ground treatment by deep vibration), and BRE Digest 391 (Specifying vibro stone columns), among others.
4.5.2 – Provision of Information
The design and specifications for vibratory ground improvement must be produced in a clear format and shared with all relevant parties. The vibratory improvement proposal, along with supporting drawings, should be submitted to NHBC at least eight weeks before the work begins. All dimensions and levels must be clearly indicated and linked to a benchmark on site. Information issued to site supervisors and subcontractors should cover the dimensions and depth of treatments, the location of services, construction sequences, and any features requiring special attention such as existing sewers or varying water table levels.
4.5.3 – Hazardous Sites and Ground Hazards
Where vibratory ground improvement is proposed on a hazardous site, this must be reported to NHBC in writing at least eight weeks before any sitework begins. The design must take into account the specific characteristics of the site, including any ground hazards. Further guidance on managing ground conditions and building near trees is provided in Chapters 4.1 and 4.2 of the NHBC Standards.
4.5.4 – Desk Study and Site Investigation
Before any ground improvement work begins, the engineer must ensure that a thorough desk study and site investigation have been carried out. The site investigation should be supervised by the engineer and should determine, as a minimum, the depths and properties of natural materials beneath the site, the extent and nature of any filled ground, the presence of existing services and drains, groundwater levels, previous structures, the effect of any sustainable drainage systems, and the occurrence of contaminated substances. The specialist contractor must be satisfied that the investigation provides adequate information to design the ground improvements, and the results should be sent to NHBC before work starts.
4.5.5 – Confirmation of Suitability for Treatment
Before work commences on site, the builder must obtain written confirmation from both the engineer and the specialist contractor that the site is suitable for the proposed ground improvement system. NHBC should also be notified. The engineer and specialist contractor must agree in writing on the design objectives, detailed schedule and programme of work, the tests to be carried out on completion, and the allocation of responsibility for procedures and testing. These written agreements must be made available to NHBC before work begins.
4.5.6 – Suitability of Ground Conditions
Vibratory ground improvement must only be carried out on ground that is suitable for treatment. The NHBC Standards set out specific soil types that fall within acceptable ranges, categorised as Zone A (suitable for deep compaction or vibro-compaction) and Zone B (suitable for stone column or vibro-replacement techniques).
4.5.6.1 – Unsuitable Ground Conditions
Certain ground conditions are generally not acceptable for vibratory treatment. These include clays with a Plasticity Index of 40% or greater, soft clays with an undrained shear strength of less than 30kN/m², ground with peat layers close to foundation level or thicker than 200mm, highly sensitive soils liable to collapse, voided filled ground containing items such as old water tanks or glass bottles, loose or non-engineered fill, ground that is still settling, fill containing more than 15% organic material by volume, and highly contaminated ground where toxic or explosive gas generation may occur. In the case of contaminated ground, stone columns can act as vertical vents for dangerous gases, so proprietary systems that prevent vertical venting, such as vibro concrete plug technology, may need to be considered.
4.5.6.2 – Detrimental Factors
Several additional factors must be considered when specifying vibratory ground improvement. These include partial depth treatment of filled ground, alterations to the oversite level before or after treatment, soils with a Modified Plasticity Index of 10% or greater (where foundations must be designed to accommodate volume changes), obstructions and variations in ground density, the minimum depth of treated soil to allow for the interaction of adjacent foundations, and the potential for stone columns to act as vertical drains allowing water to reach moisture-susceptible strata.
4.5.6.3 – Groundwater Conditions
Groundwater conditions must also be taken into account. Long-term lowering of the water table can cause settlement of existing adjacent buildings, while a short-term rise in the local water table during construction, particularly when a wet process is used, can cause settlement or heave in nearby structures. Surface water sewers should be used for rainwater disposal where possible, and any soakaways must be positioned so that they do not adversely affect the treated ground. The effect of sustainable drainage systems must also be considered.
4.5.7 – Compatibility of the Ground, Design and Treatment
The vibratory ground improvement must be compatible with the treated ground, the site layout, and the design of the home.
4.5.7.1 – Limitations of the Treated Ground
The engineer should avoid siting homes where major changes in ground conditions are expected, consider the vulnerability of the homes’ configuration at junctions and in long blocks, determine the loads the homes will impose, discuss the feasibility of proposals with the specialist contractor, confirm the required load and settlement performance, and advise NHBC at the design stage.
4.5.7.2 – Limitations of Ground Support
The engineer must establish the likely limits of ground movement and account for this in the design. This includes considering the position and spacing of movement joints, the flexibility of masonry mortars, and the use of masonry reinforcement such as bed-joint reinforcement.
4.5.7.3 – Suitable Foundation Types
Only two types of foundation are suitable for homes built on vibratory-improved ground, both of which must comply with BS EN 1992-1-1 for minimum reinforcement: reinforced concrete strip foundations and reinforced concrete raft or semi-raft foundations positioned on a uniformly compacted bed of hardcore. Both types require top and bottom reinforcement, and foundations must be a minimum of 600mm below the surface of the treated ground, founded on firm material of adequate bearing capacity. The reinforced concrete foundation should be designed to span between the centres of adjacent stone columns unless a more detailed structural analysis supports an alternative approach.
4.5.7.4 – Use of Suspended Ground Floors
Suspended ground floors should be provided for all homes where vibratory ground improvement has been carried out, unless the engineer can justify an alternative solution that is acceptable to NHBC.
4.5.7.5 – Notice to NHBC
Where vibratory ground improvement is proposed, NHBC must be informed of the proposed development, the appointment of the specialist contractor, and the proposed start date of treatment.
4.5.8 – Acceptable Methods
Only methods appropriate to the ground conditions and acceptable to NHBC may be used. The two main vibratory methods commonly used in the UK are the dry bottom feed method and the dry top feed method. A third method, the wet bottom feed, is used less frequently but is also acceptable.
The dry bottom feed method is used in weaker soils or where a high water table means the borehole is likely to collapse. The vibrator penetrates the ground by its mass, air flush, and vibration, and stone (generally 40mm) is introduced via a pipe and compacted into the surrounding soil. The dry top feed method is used on cohesive soils where the borehole can remain open. The vibrator forms a borehole, stone fill is introduced in charges of around 500 to 800mm, and each charge is compacted in turn until the column reaches working level, using stone graded at 40 to 75mm.
4.5.9 – Materials for Use as Fill
The stone fill used to form columns must be compatible with the ground conditions and suitable for the vibratory process. Column fill should be a clean, hard, inert material. Limestone fill may not be acceptable in acidic ground conditions. Where fill material is sourced from stockpiles, its uniformity must be checked, as different storage methods can affect particle size and grading. The use of recycled aggregate must comply with BRE Digest 433 or other guidance agreed with NHBC. Certain hazardous materials, including acid wastes, reactive materials, those containing sulfates, organic materials, and toxic materials, require testing before use. Fill containing expansive material is not acceptable. Several types of fill, such as colliery shale, slags, furnace ashes, demolition material, and manufactured aggregates from waste treatment plants, require written agreement from NHBC before they can be used.
4.5.10 – Granular Material
Any granular material used for raising site levels before treatment, or added during deep compaction, must be suitable for compaction and free from hazardous materials unless appropriate precautions are taken. The grading of material for deep compaction should fall within Zone A of the NHBC’s acceptability chart. Well-graded, inert fill that passes a 100mm screen and contains less than 10% fine material of silt or clay size is generally acceptable for raising site levels.
4.5.11 – Sitework
The builder must ensure that the engineer visits the site and provides competent supervision throughout the ground treatment process.
4.5.11.1 – Engineer Checks
The engineer should supervise the work at critical stages, including the inspection of foundation setting out, the installation of columns during the early stages, checking of materials, and any situations where installation data differs from design assumptions or changes to the treatment layout are required.
4.5.11.2 – Location, Depth and Alignment of Columns
Supervision must ensure that each stone column reaches the required minimum depth and is correctly located. Stone columns should be positioned centrally under the foundations they support, or in a predetermined staggered arrangement at a maximum of 2m centres and at the intersection of adjacent reinforced concrete strips. Any missing stone columns must be replaced, and any columns misaligned by more than 150mm in any direction must also be replaced. The location of all stone columns must be checked by the engineer’s representative before the specialist plant leaves the site.
4.5.11.3 – Managing Unforeseen Circumstances
Any unforeseen changes or trends affecting site conditions must be recorded and reported to the engineer immediately. A change in the anticipated depth of a compaction point in excess of 25%, or a variation of over 50% in the quantity of backfill used in compaction points of the same length, must be reported to the engineer and specialist contractor no later than the end of the working day on which they occur. In all such cases, the builder and NHBC must be advised immediately about proposed remedial measures.
4.5.12 – Adjacent Excavations
The builder must ensure that foundations are not disturbed by adjacent excavations for drainage or service trenches. The minimum clearance between excavations and foundations must not be less than the depth of the excavation minus the depth of the structural foundation. Particular attention is needed where excavation takes place below the water table.
4.5.13 – Verification of Completed Treatment
Once the ground treatment is complete, the engineer must require the specialist contractor to verify that the work is satisfactory.
4.5.13.1 – Suitable Testing
Tests should be carried out to establish the degree of ground improvement, load-bearing characteristics, and settlement potential. The specialist contractor should predict results in advance based on their experience, agree acceptable results and tolerances with the engineer before testing, and confirm that the final results are acceptable. Where results differ significantly from predictions, further investigation may be needed. Test methods include 600mm diameter plate tests (at least one per day per rig), dummy footing or mini zone tests (at least one per week per rig on normal sites, more frequently on geotechnically challenging sites), full zone tests using isolated pad or strip footings loaded for 24 hours, in-situ tests for granular materials, and trial pits to verify that columns are fully formed to the required depth and diameter.
4.5.13.2 – Written Confirmation of Completed Treatment
On completion, the engineer should use the test results to verify that the treated ground meets the design requirements, advise the builder and NHBC in writing of the treatment’s effectiveness, and inform the builder of any special precautions for positioning services beneath or adjacent to the home.
4.5.13.3 – Recording of Work
A comprehensive record of all works must be made available to NHBC, including information about the treatment, depth of fill, volume of stone used, any on-site changes, and any other relevant details.
4.5.14 – Further Information
For more detailed technical guidance, the NHBC Standards reference several additional publications including BRE Special Digest 1 (Concrete in aggressive ground), BRE Digest 391 (Specifying vibro stone columns), BRE Digest 433 (Recycled aggregates), and BS EN 771-1 (Specification for masonry units).
Why This Matters to You as a New Build Buyer
As a buyer, you are unlikely to witness the ground improvement works first-hand, as they take place long before you visit the property. However, understanding these standards is valuable for several reasons. If your home is built on treated ground, you should be able to request documentation from the developer confirming that the work was carried out in accordance with NHBC requirements. If you experience any signs of settlement or structural movement after moving in, knowing that these standards exist gives you a basis for raising concerns under your NHBC warranty.
While a snagging inspection from New Build Inspections focuses on the visible quality and finish of your new home rather than the ground beneath it, our inspectors are experienced in recognising surface-level indicators that may suggest deeper issues, such as cracking patterns or signs of movement. If our team spots anything of concern during your inspection, we will always advise you on the appropriate next steps, which may include seeking a structural engineer’s assessment.
If you have any questions about the condition of your new build home, or would like to arrange a professional snagging inspection, please do not hesitate to get in touch with us.
