4.2 Building near trees

4.2.14 Worked example

The following is an example of how to determine foundation depths using the information in this chapter. 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. Ensure the site investigation includes representative sampling and testing.

Site at Oxford, building near a Lombardy poplar (to be retained) and a sycamore (to be removed).

From laboratory tests:

Plasticity Index, Ip = 36%

Test results also report that 100% of particles are smaller than 425µm.

Therefore:

Volume change potential = medium
(In the absence of tests, assume high volume change potential).

This example is typical of Oxford clay. More than 35% of the particles are smaller than 60µm and therefore the soil is shrinkable. 100% of the particles are smaller than 425µm and therefore I’p is the same as the Ip.

A typical boulder clay also has more than 35% of particles smaller than 60µm and is therefore also shrinkable. However, it may have only 80% of its particles smaller than 425µm, in which case, the I’p is 80% of the Ip.

A typical clayey sand may have less than 30% of its particles smaller than 60µm, in which case, the soil would be non-shrinkable.

Step 2#

Establish the species, mature height and water demand of all trees and hedgerows within the influencing radii.

Lombardy poplar	Sycamore
Mature height = 25m Water demand = high	Mature height = 22m Water demand = moderate

Step 3#

Plot the trees and hedgerows relative to the foundations and draw their zones of influence to determine which trees will affect the foundation design. Use a scaled plan.

Step 4#

Establish the appropriate tree height H to use.

Always use the mature height for remaining and proposed trees and hedgerows. The appropriate height to use for removed trees and hedgerows depends on the actual height when they are removed.

Lombardy poplar	Sycamore
Tree to remain. Therefore: H = mature height = 25m	Tree to be removed Mature height = 22m Actual height = 15m Actual height greater than 50% mature height. Therefore: H = mature height = 22m

Step 5#

Measure the distance D from the centre of the trees or hedgerows to the face of the foundation.

Lombardy poplar	Sycamore
Distance D = 10m from foundation	Distance D = 8m from foundation

Step 6#

Either:

use the NHBC Foundation Depth Calculator App, or
select steps 6C (a) and (b) if using charts in Clause 4.2.12 to derive depths, or
select step 6T if using tables in Clause 4.2.13.

Step 6C (a)#

Calculate D/H value

Distance D from face of foundation (step 5) divided by the appropriate tree height H (Step 4).
Alternatively D/H can be obtained from Clause 4.2.12.

Lombardy poplar	Sycamore
D = 10 = D/H = 0.4 H = 25	D = 8 = D/H = 0.36 H = 22

Step 6C (b)#

Determine foundation depth using the charts in Clause 4.2.12 as follows:

Volume change potential	Chart number
High	1
Medium	2
Low	3

Lombardy poplar	Sycamore
In this example, the volume change potential is medium, then from Chart 2 for broad-leafed high water demand trees at D = 0.4 H Foundation depth = 2.33m	In this example, the volume change potential is medium, then from Chart 2 for broad-leafed moderate water demand trees at D = 0.36 H Foundation depth = 1.50m

The Lombardy poplar is the tree requiring the greater depth (2.33m).

Step 6T#

Determine foundation depth using the tables in 4.2.13 as follows:

Volume change potential	Tree water demand	Table number
High	High Moderate Low	11 12 13
Medium	High Moderate Low	14 15 16
Low	High Moderate Low	17 18 19

Step 7#

Adjust the depth according to the climatic zone.

A reduction may be made for distance north and west of London, but the final depth should not be less than the minimum given in each chart and table.

Oxford is between 50 and 100 miles NW of London. From 4.2.5, a reduction of 0.05m is permitted.

Final foundation depth = 2.33 – 0.05 = 2.28m

Last updated: 2nd January 2024