W6107fd f15.pdf - form 15 footing & slab design · OPW16/0220

Construction & Maintenance Note 2 - Planting of trees, shrubs and garden beds (H).

It is known that shrubs and trees have root systems that can dry clay-based foundation
soils to a greater degree and to greater depths than the drying effect caused by the
weather and climate (weather and climate are normally referred to as seasonal variation).

This excessive drying of the clay-based foundation soils can result in shrinkage of the
foundation soils and subsequent footing movements can be up to 2 to 3 times the ground
movement caused by seasonal variations.
Excess ground movement is likely to cause cracking in masonry walls.

It is noted that timber framed walls clad with say weatherboards or fibrous sheeting is
more tolerant to the effects of ground movement than masonry walls.

Footings are normally designed to cope with ground movements generated only by
seasonal variations in ground moisture content. A footing designer must know in
advance the extent of vegetation to be allowed for in the footing design – this is not
often considered in the footing design unless the area has established trees which are
going to be left after the new structure is built. The cost of the footing system can be
increased significantly if the effect of trees is allowed for in the footing design.

Existing trees are often removed well before the construction of a new dwelling or
structure and the planting of new trees and shrubs and/or the position of existing
vegetation must comply with the diagram below to reduce the risk of structural damage
(see figure 4.1 below).

                                                                                  Mature
                                                                                  height
                                                                                  of trees
                                           2.5 meters minimum to
                                            small flower gardens.
                      Structure designed
Ground level
2m                Foundation soil

               2.5m                        2.5m

                                            Minimum 1 x mature
                                            height of tree and shrub.

                                            Minimum 1.5 x mature height
                                            or group trees or shrubs

Position of gardens, shrubs & trees to slabs & footings – H Reactivity Soils.




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