Factors causing Subsidence of Buildings, especially Tree Roots in Clay Soils

Subsidence is described as “ a movement involved in the site, normally downwards, so that damage occurs to the building standing upon it”.

Some 150,000 homes in the UK Southeast have suffered from subsidence between 1980 and 1995. The main causes are: -

Source Number

Subsidence effect

Subsidence Cause

1

Erosion

River or Sea

2

Mining

Isolated or Wave. Along the North Downs escarpment of Southern England mining has been carried out to extract hearthstones, firestones, building stones and also pure silica sand. Chalk and flints have been mined by the application of pillar and stall methods in Kent. On reaching the mine boundary the pillars were then worked in retreat, allowing the roof to collapse. Often this collapse was not instigated and these uncharted shallow partially extracted workings now present engineering problems.

3

Tree Roots

Clay soils

4

Seasonal fluctuations

Affected by climatic changes and vegetation. The Marine clay sediments of the Southeast have a high shrinkage potential, so swell when it rains and shrink during long hot dry summers. The Southeast also suffers the highest summer soil-moisture deficit in the UK.

5

Escape of water

Defective drainage. If there are any defects in a drain, then a root is liable to enter and proliferate and thereby cause a blockage, but an intact drain cannot be penetrated. Inspect any manholes within 3 metres of a tree/hedge.

Over-stressing

a. Overloading

b. Building alongside

7

Undermining

Deep drainage trench where trench bottom is lower than building foundations.

8

Landslip

Movement of the site. Removal of material from the toe of a slope, side long excavations for the construction of roads or placement of foundations can instigate failure. Loading of slopes at the crest by construction can impose additional forces resulting in slope failure, especially on clay subsoil.

9

Compaction of infill/ settlement

Cut and fill sites. If well-graded sand and gravel is used to fill the cut or fill side of site before the construction of the building upon it, then this has a very low compressibility. If it is domestic rubbish at the other extreme, then this is very highly compressible. Filling below solid floors. This is unlikely with sandy granular subsoil, but settlement relating to a clay sub-soil can take place up to 5 years after the building is completed and in exceptional circumstances, this may even extend to 10 years.

10

Inadequate foundations

“Doomed from the start”. Since 1950 building foundations have been placed at not less than 900mm below ground level. Since that time, very few cases of subsidence have been reported to houses with foundations of such a depth purely due to this foundation depth.

Assumed depth of foundations for properties older than 1930 is 0.6metres,

1930-1950 is 0.7metres,

1950-1976 is 0.9metres,

1976-1986 is 1.0metres,

since 1986 without trees at time of construction is 1.0metres for sand, 1.25metres for sandy/clay and 1.5metres for clay subsoil.

The influence of trees on House Foundations in clay soils:-

Ranking

Tree Species

Maximum Tree Height in metres

Maximum Distance of tree from building for 75% of subsidence cases

Minimum horizontal separation distance in very highly and highly shrinkable CLAYS in metres

1

Oak

23

13

23

2

24

15

24

3

24

8

12

4

Common Ash

23

10

12

5

Plane

30

7.5

15

6

Willow

15

11

15

7

Elm

25

12

12

8

Hawthorn

10

7

5

9

Maple/ Sycamore

24

9

12

10

Cherry/Plum

8

6

8

11

Beech

20

9

10

12

Birch

14

7

7

13

Whitebeam/ Rowan

12

9.5

12

14

Cypress

25

3.5

12

Minimum distance (metres) between structure and centre of trunk to avoid direct damage to a structure from future growth of the base of the trunk and tree roots: -

8m to 15m

Over 15m

Buildings and heavily loaded structures

0.5

1.2

Lightly loaded structures such as single storey timber frame, garages, porch, etc

0.7

1.5

Water demand of different conifer and broadleafed tree species – with 5.6 being moderate water demand and 8 being the highest:-

Tree Category

8

7

6

5

4

3

Broad-leaved

Broad-leaved trees not listed have a factor of 4

Crataegus

Salix

Sorbus (simple leaved)

Ulmus

Aesculus

Fraxinus

Platanus

Tilia

Acer

Castanea

Fagus

Malus

Prunus

Pyrus

Robinia

Sorbus (compound leaved)

Ailanthus

Alnus

Betula

Carpinus

Gleditsia

Ilex

Juglans

Laburnum

Catalpa

Corylus

Ficus

Liquidambar

Liriodendron

Magnolia

Morus

Sambucu

Conifer

Conifer trees not listed have a factor of 3

Cupressus

Chamaecyparis x cupressocyparis

Sequoiadendron

Cedrus

Thuja

Juniperus

Taxus

Tsuga

Abies

Araucaria

Ginkgo

Larix

Picea

Pinus

Before you buy another building, please take account of the following future possible causes of its subsidence, as well as any of the other causes of subsidence detailed above: -

• Pour 2 gallons of water onto a flat area of soil. If the water has not gone within 2 minutes then execute the following actions to obtain a sub-soil type. If there are trees/hedges in the garden, then between one of these trees and the building dig a small pit to remove the topsoil before using a 50mm augur to obtain a sub-soil sample between 1.4 and 1.6metres below this pit. If this sample is sand or chalk, then tree/hedge roots are unlikely to affect the building. If the sub-soil is clay or a sandy-clay, then for each tree/hedge the following should be obtained for a Subsidence Risk Factor by a qualified Arboriculturalist as recommended by an Insurance Broker/Company:-

1. A Site Plan showing an outline of the buildings and the likely position of drainage systems as well as any tree(s) or other woody vegetation in relation to the buildings and its drainage systems.
2. A list complementary to the Site Plan detailing the species, age, height, crown size, condition, vigour, future growth potential and distance from the building of each tree/woody vegetation.
3. The probable type of the sub-soil on the site.
4. The likely depth and type of foundations of the buildings.
5. Any recommendations for immediate and/or future action/treatment to any trees/hedge likely to pose a Subsidence Risk to the building.
6. Any recommendations for immediate and/or future action/treatment to any trees/hedges that are considered likely to cause direct damage to the building or drainage system.
7. Any obvious signs of root growth or damage by root growth to the superstructure of any manhole including benching, brickwork, render or pipework.

• If the sub-soil is chalk or sand, then investigate the drains with a CCTV Drainage Survey,
• Read the Land Registry documents to see what the land was used for before building work commenced (possible water-cress farm!). Read the plans submitted to the local authority for any previous building work for its possible changes to foundation depth and material used under the ground floor, external patios, drives and paths.

If tree/hedge/climber/shrub pruning is contemplated to alleviate a possible problem, the objective is to reduce the leaf area of the plant; as the amount of water utilised by this plant and thus the extent of soil drying which can develop is closely related to this leaf area. Thus control of this leaf area provides a means of controlling soil drying. Crown reduction is generally the most effective method of pruning, but other techniques such as thinning or crown lifting may be used where appropriate. This treatment may have to be repeated on a regular basis, which is no problem for a climber, shrub or hedge, but could be too expensive for a tree.

Recommendations

• If the trees are Chamaecyparis x cupressocyparis, then I would recommend removal of branches on one side and erecting a climbing support frame on that pruned side. Then, when the climber has filled the support space, remove the remainder of the branches and its main trunks down to the top of the climbing support frame. This means that the trees become climbing support posts. Leylandii trees cannot be pruned back beyond the green leaf, since the resulting bare branch will not create new leaves and their eventual height can be 120 feet.
•  
• For hedges and shelterbelts, request the 1-2 year seedlings grown in 100cc cells/root trainers from Sevenoaks Coblands Cash & Carry at Dry Hill Lane, Sundridge, Sevenoaks, Kent, TN14 6AA (Tel 01959 561 274) or ask me and my price will be 25% cheaper than their retail price. The roots of these plants when growing will not throttle themselves after about 10-15 years due to their being almost straight down.
•  
• For more mature trees, request them from the Barcham Nursery selection provided by Sevenoaks Coblands Cash & Carry. The roots of these plants when growing will not throttle themselves after about 10-15 years due to their being almost straight down because of the white containers that they are grown in.
•  
• Pages 30 and 134 of Coblands Catalogue obtainable from Sevenoaks Cash & Carry provide excellent guides to the planting and aftercare of Coblands Plants, especially regarding irrigation (irrigation pipe from Leaky Pipe Systems – 01622 746495), mulching (bark from Gardenscape – Tel 0800 854663 or cow manure from Isle of Grain – Tel 01634 250379) and a 1 metre diameter plant free area around the tree during the first 2 years.
•  
• Most gardens can only afford - due to their size - either small fruit trees (obtainable from Ken Muir – Tel 0255 830181), large shrubs or hedges created from trees (Coblands Cash & Carry) which become no higher than 1.8 metres and are at least 1.5 metres away from the building.
•  
• A minor point to remember: when you are sold a property; you are then responsible if it fails.

  The next page shows the damage created by a builder who had run out of topsoil and then used blue clay instead.