ES_173527_BR_1059006_Conv_Lot 497 10 HILL STREET SUNSHINE BEACH QLD(C) · MCU18/0006.02

                                                                                                                                                                                                                                                                                                                                                                                                                    Project Job No. 173527
                                                                                                                                                    Root Barriers

                                    TREES AND FOUNDATIONS                                                                                                                                                         Existing Tree




                                                                                                                                                                 Possible Cracking/ Rotation
                                                                                                                                                                 in masonry walls


1. Introduction                                                                                                                                                                                                                              Cracking in
                                                                                                                                                                                                                                             brick work


                                                                                                                                                                                                                                                                                                                                                                                                                                  Dwelling
                                                                                                                                                                                                                                                                             Dwelling
The potential effect of trees on house foundations is now widely documented, however the following                                                                           G.L                                                                                                                                             G.L
is a brief overview on the impacts trees can cause along with some comment in regards to possible
measures in reducing or mitigating these effects.
                                                                                                                                                                                                                                                                                                 Depth determined
                                                                                                                                                                                                                                                                                                 by H's Zone
                                                                                                                                                                                                                                                                                                                                                                                                                   Root Barrier
Trees and large shrubs typically require substantial volumes of water/ moisture to survive, some more so
than others. Therefore when designing or constructing a new dwelling where trees are located within the
vicinity, an allowance for the potential drying effect from trees must be considered.
Whilst still a problem, the concern when designing foundations is not so much the roots of trees passing under
the slab and lifting as most people have seen on pathways etc.. Rather trees in the vicinity drawing on the
extra moisture under the slab area, that would typically remain at a balance where trees do not exist.                                                                                                                                                                                        Figure 2. Case A - Existing tree managed on property.
                                                                                                                                                     Figure 1. Case A - Unmanaged tree on property


With the trees absorbing this additional moisture, the potential for clays to shrink is exacerbated, thus
imposing further stresses on the foundations and the potential for damage where allowances have not
been made.

                                                                                                                                                                                                                                                                                                                                                                                                                     Existing Tree
                                                                                                                                                                                                                                                                 Existing Tree



2. Site Classification & Trees
                                                                                                                                                                                                                                                                                                                                                                                      Boundary Fence
                                                                                                                                                                                                                                       Boundary Fence

In accordance with AS2870-2011, where trees are within the vicinity of a proposed new construction, extension etc..
Trees must be considered when determining the site classification. This can be done in two ways :-                                                                                                                       Cracking in
                                                                                                                                                                                                                         brick work
                                                                                                                                                                                                                                                                                                                                                                                              Adjoining Property
                                                                                                                                                                                                                                            Adjoining Property

a) The site is to be classified as class P (Problem Site) per Section 2.1.3 (e) & (f                                                                                                                                                                                                                                                       Dwelling

   - the site may be subject to moisture changes due to site conditions more severe than normal conditions.                                                                                          Dwelling
                                                                                                                                                                                                                                                                                                     G.L
                                                                                                                                                                    G.L
   - the site may be subject to other factors resulting in foundation movement beyond the reactive soil movements
     resulting from moisture changes due to the normal site conditions.
                                                                                                                                                                                                                                                                                                                                      Depth determined
                                                                                                                                                                                                                                                                                                                                      by H's Zone
or

b) The site is to be classified based on the potential characteristic surface movement of the soils identified (Ys),                                                                                                                                                                                                                                                           Root Barrier
   however the (Yt), potential surface movement due to tree induced suction change is also to be calculated and                                      Figure 3. Case B - Unmanaged tree on neighbouring property                                                                               Figure 4. Case B - Management of tree on neighbouring property using root barriers.
   effectively added to the determined (Ys) value. This clearly increases the (Ys) value, potentially increasing the
   nominated site classification by one or two classes.




3. Designing Foundations in Vicinity of Trees                                                                                                                                                                                                                                             Existing
                                                                                                                                                                                                                                                                                          Dwelling                                            Vacant
                                                                                                                                                                                                                                                           Vacant


The approach for designing foundations where trees are influencing the site conditions can be done in a number
of ways, such as, but not limited to the following :-                                                                                                                                                                                                                                       Root
                                                                                                                                                                                                                                                                                            Barrier
    Remove problem trees - this is the most cost effective option, however does come with limitations. The main being
     time. If trees are to be removed for the sake of reducing the site classification and potential costs associated, it is
     required the site be left for up to a minimum of two full seasonal changes to allow the moisture content within the soil
     to stabilise as the trees are no longer drawing on the moisture, thus potentially creating a rebound effect.

    Design extensively heavier foundations. By doing this you are simply designing foundations to accommodate the exacerbated
     Shrinkage movement caused by trees within the vicinity of the construction. This method can be fairly costly.                                                                                                                                                                                                                          Vacant
                                                                                                                                                                                                                                                            Existing
                                                                                                                                                                                                                                                            Dwelling
    Install an appropriately designed and recognised root barrier system. This is to be installed to a set minimum depth, typically
     2.0 metres or more depending on the sites regional locality. The barrier system is to be positioned between the proposed/ existing
     dwelling and the influence trees. It is not uncommon for the barrier system to be installed on at least three sides per fig.4.
     The root barrier system is to be installed by an appropriately qualified person/s providing Compliance Certification in the way of
     a Form 15 (Compliance Certificate for Building Design or Specification) and Form 16 (Inspection Certificate). Where the contractor
     Cannot provide the nominated certificates, an alternative contractor should be commissioned.
     Examples of typical case scenarios using root barrier systems are shown in the attached figures.




                                                                                                                                                                                                                                                                                        Street




                              STA CONSULTING ENGINEERS
                           Brisbane North (Milton) (07) 3071 7444 Brisbane South (Rocklea) (07) 3071 7444 Gold Coast (07) 5522 1995 Mackay (07) 4960 4800
                                                                                                                                                                                               Client
                                                                                                                                                                                               DJK Electrical Services
                                                                                                                                                                                                                                                                                                                              Site Address
                                                                                                                                                                                                                                                                                                                              Lot 497 10 HILL STREET SUNSHINE BEACH 4567
                                                    Townsville (07) 4766 8741 Rockhampton (07) 4994 9810 Newcastle (02) 4032 6450                                                              TBA SUNSHINE BEACH 4567
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