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NHBC Standards Chapter 4.2 – Building near trees

Chapter 4.2 of the NHBC Guidelines focuses on the complexities and essential considerations when building near trees, hedgerows, and shrubs, particularly in areas with shrinkable soils. It provides comprehensive guidance to ensure that structures are protected from the ground movement caused by the interaction between tree roots and cohesive soils like clay. The chapter emphasises the need for designing foundations that can either accommodate this movement or extend to depths where the risk of damaging ground movement is minimal. This is critical as trees and shrubs extract moisture from the soil, causing volume changes that can affect the stability of foundations and, consequently, the structures they support.

The guidelines in this chapter cover various aspects such as compliance with technical requirements, the provision of detailed information in designs and site plans, and specific construction methods. It includes detailed instructions on assessing the impact of trees on shrinkable soils, determining suitable foundation types and depths, and implementing heave precautions to mitigate potential damage. Additionally, the chapter highlights the importance of consulting specialist arboriculturists for tree identification and site-specific assessments, especially in scenarios where standard guidance may not suffice, such as with deeper foundations or complex site conditions. The chapter also considers the implications of climate change on these interactions and provides methods for designing foundations that account for future environmental changes.

 

4.2.1 Compliance

Compliance with the NHBC Guidelines is mandatory when constructing near trees, hedgerows, or shrubs. All foundations in these contexts must adhere to the NHBC Technical Requirements to ensure structural integrity and safety. This chapter provides a framework for acceptable foundation types and depths, aiming to mitigate risks associated with ground movement caused by root activity and soil desiccation. By following the guidance detailed in this chapter, builders can achieve the necessary compliance, resulting in foundations that can withstand the dynamic interactions between trees and soil.

4.2.2 Provision of Information

The provision of accurate and comprehensive information is crucial for building near trees. Design plans must clearly indicate all relevant details, including the types, depths, and dimensions of foundations influenced by nearby trees and hedgerows. The site plan should also show the location and species of existing and proposed trees, ensuring that all affected areas are accounted for. Detailed technical method statements, construction sequences, and site investigations are necessary to inform all stakeholders and guide the construction process effectively. Ensuring that this information is meticulously documented and communicated helps to prevent costly adjustments and ensures the project’s smooth progression.

4.2.3 Building Near Trees

When building near trees, it is essential to account for the physical growth of young trees, the protection of remaining trees and hedgerows, and the removal of existing ones. Before site clearance, a thorough survey must record the location, height, and species of trees and hedgerows. If the history of removed vegetation is unclear, local enquiries and aerial photographs should be used to make informed assumptions. For young trees, foundations must be designed to avoid future damage from growth, incorporating measures such as reinforced foundations or bridging walls over roots. Protection of existing trees involves avoiding damage from construction activities, ensuring root systems are not compromised by soil compaction or excavation. For trees and hedgerows that must be removed, consultation with local planning authorities is required to comply with conservation regulations and statutory requirements .

4.2.4 The Effects of Trees on Shrinkable Soils

This section outlines how the presence of trees, hedgerows, and shrubs can significantly impact shrinkable soils. Key factors include soil classification, shrinkage and heave potential, water demand of trees, tree heights, and their zones of influence. Shrinkable soils, which contain over 35% fine particles and have a Modified Plasticity Index of 10% or greater, undergo volume changes due to moisture fluctuations. These changes can lead to subsidence or heave, causing potential damage to foundations and structures. Accurate soil classification is crucial, with specific attention to the Plasticity Index and the Modified Plasticity Index, which better reflects the soil’s volume change potential by considering the proportion of fine particles.

Trees and shrubs extract moisture from the soil, increasing shrinkage in dry periods and contributing to heave when moisture levels rise. The section provides guidance on understanding the water demand of various tree species and their potential mature heights, which are essential for predicting their impact on nearby structures. For instance, trees with high water demand, such as oaks and willows, exert greater influence on soil moisture levels. Understanding these dynamics is essential for designing foundations that can withstand the resulting ground movement, ensuring the long-term stability of buildings constructed in such environments​.

4.2.5 Foundations in All Soil Types

This section emphasises that foundations must be appropriately designed and constructed to safely transmit loads to the ground without excessive movement, regardless of soil type. It highlights the necessity of ensuring that different foundation types are not used to support the same structure unless designed by an engineer. The guidelines stipulate that freestanding masonry walls should be constructed on suitable foundations or be designed to accommodate potential ground movement through the use of movement joints and reinforcement. This ensures structural integrity across various soil conditions and mitigates risks associated with differential settlement and soil instability​​.

4.2.6 Excavation of Foundations

Excavation of foundations should be conducted in accordance with the design and prepared adequately to receive concrete. If trench bottoms become excessively dry or softened due to environmental factors like rain or groundwater, they must be re-bottomed before concreting. Foundation depths are to be measured from the centre line of the excavation to the ground level as specified in Clause 4.2.9. In cases where significant root activity is unexpectedly encountered at the trench base, consultation with an engineer is advised to determine if deeper excavation is required. This section ensures that the excavation process accommodates the designed foundation depths and addresses any unforeseen ground conditions that might impact the foundation’s performance​​.

4.2.7 Foundations in Shrinkable Soils

Foundations in shrinkable soils must be designed to accommodate the effects of trees, shrubs, and hedgerows, taking into account potential ground movement due to soil desiccation and heave. The type of foundation, distance between the tree and foundation, and the method of assessing foundation depths are critical factors. Foundation types suitable for shrinkable soils include strip, trench fill, pier and beam, pile and beam, and raft foundations. These foundations must support applied loads without undue settlement and include suitable precautions against heave. Root barriers are not considered an acceptable alternative. The design must also account for new tree and shrub plantings, ensuring compatibility between landscape and foundation designs​.

4.2.8 Design and Construction of Foundations in Shrinkable Soils

Designing foundations in shrinkable soils requires detailed engineering to address all potential soil movements. Specific attention must be given to trench fill foundations deeper than 2.5 meters, pier and beam foundations, pile and beam foundations, and raft foundations. Each foundation type must consider plot-specific soil conditions and arboricultural advice. Heave precautions, such as compressible materials and careful detailing of construction joints, are necessary to counteract lateral and shear forces. Proper placement of compressible materials helps to manage heave forces, ensuring the foundations remain stable under varying soil conditions. All designs must comply with Technical Requirement R5 and be submitted to NHBC for approval before construction​.

4.2.9 Foundation Depths for Specific Conditions in Shrinkable Soils

Foundation depths in shrinkable soils must be determined to ensure safe load transmission without excessive movement. This involves considering strip and trench fill foundations in non-shrinkable soils overlying shrinkable soils, proper measurement of foundation depths, granular infill beneath raft foundations, and steps in foundations. For foundations on non-shrinkable soils overlying shrinkable soils, conditions must be met as outlined in Chapter 4.3, with consistent soil conditions across each plot confirmed by site investigation. Granular infill beneath raft foundations should be at least 50% of the determined foundation depth and not more than 1.25 meters deep, ensuring stability and load distribution. Foundation steps should be implemented gradually, with no single step exceeding 0.5 meters to maintain uniform depth​​.

4.2.10 Heave Precautions

Heave precautions are essential for protecting foundations, substructures, and services from excessive movement due to ground heave. This section mandates the incorporation of suitable heave precautions in the design of foundations and substructures. These precautions include providing adequate void dimensions, using proprietary heave materials, and designing foundations such as trench fill, pier and beam, and pile and beam foundations to counteract heave forces. Additionally, suspended ground floors, paths, and driveways should be detailed to accommodate potential ground movement. The guidelines also recommend using void formers or compressible materials to mitigate the impact of heave, ensuring the structural integrity of buildings affected by shrinkable soils and tree roots .

4.2.11 New Drainage

New drainage systems must be designed to accommodate potential ground movement caused by heave. This includes ensuring that pipes and services passing through substructure walls or foundations can handle such movements. Gradients may need to be steeper than those typically recommended in Chapter 5.3 ‘Drainage Below Ground’ to account for possible heave. Where sufficient falls cannot be provided, alternative methods such as deeper excavation and laying pipes on a granular bedding may be used. Additionally, existing land drains should be maintained or diverted, and adequate drainage should be provided in areas prone to flooding beneath suspended floors. This approach ensures the drainage systems remain functional and effective despite ground movement​.

4.2.12 Method of Assessment of Foundation Depths Using Charts

The method of assessing foundation depths using charts involves determining depths to prevent excessive movement due to ground heave. Charts provided in Clause 4.2.12, along with Tables 11-19, help in deriving foundation depths up to 2.5 meters. These charts take into account various factors such as soil volume change potential, water demand of trees, and the appropriate tree height. By plotting the distance from trees to foundations and considering the soil’s Modified Plasticity Index, the appropriate foundation depth can be determined. This method ensures that the foundations are designed to withstand the influences of trees and heave, maintaining the stability and safety of the structures ​.

4.2.13 Method of Assessment of Foundation Depths Using Tables

The method of assessing foundation depths using tables provides a systematic approach to determining the necessary depths to prevent excessive movement due to ground heave. Tables 11 to 19 in the guidelines cater to different soil volume change potentials (high, medium, and low) and tree water demands (high, moderate, and low). By consulting these tables, designers can derive foundation depths up to 2.5 meters, ensuring that the foundations are sufficiently deep to counteract the effects of trees on shrinkable soils. The process involves identifying the relevant table based on the soil and tree characteristics, then using the distance from the tree and the tree height to determine the appropriate depth, ensuring structural stability and compliance with NHBC standards .

4.2.14 Worked Example

The worked example in the guidelines demonstrates the application of the method for determining foundation depths. It illustrates a step-by-step process, starting with the identification of the soil’s volume change potential through laboratory tests and the classification of trees in terms of species, mature height, and water demand. By plotting the trees and their zones of influence relative to the foundations, and calculating the distance-to-height ratio (D/H), the foundation depth is determined using the appropriate chart or table. Adjustments for climatic zones and specific site conditions are also factored in, ensuring the example comprehensively covers practical scenarios builders might encounter. This example serves as a practical guide, reinforcing the methodology outlined in the previous sections .

4.2.15 Further Information

The final section, “Further Information,” provides a list of additional resources and references that offer deeper insights and supplementary guidelines on building near trees. These include various BRE Digests, British Standards (BS), and authoritative texts on tree recognition, soil desiccation, and tree root damage to buildings. These resources are invaluable for professionals seeking detailed technical knowledge and best practices related to foundations affected by tree roots and shrinkable soils. Additionally, contact information for institutions like the Institution of Civil Engineers and the Institution of Structural Engineers is provided, enabling further consultation and expert advice. This section underscores the importance of continuous learning and adherence to established standards in construction practices .

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NHBC-Standards-2024-4-2-Building-Near-Trees