Forbes-Laird Arboricultural Consultancy Ltd
Principal Consultant:
Julian Forbes-Laird
MICFor, MRICS, MEWI, M.Arbor.A, Dip.Arb.(RFS)
BA(Hons),
Associate Directors:
Patrick Stileman
Ben Abbatt
BSc(Hons), MICFor, M.Arbor.A, Dip.Arb(RFS)
BA(Hons), MICFor, MRICS, CEnv, M.Arbor.A, Dip.Arb(RFS)
www.flac.uk.com
To Richard Nicholson
B.Ed, F.Arbor.A, Dip.Arb(RFS)
ADVICE Executive Consultant:
Neil Keenan Date 16.09.16 Instruction Casterton Road FLAC Instruction ref CC36‐1031 Client Millstream Estate Services Ltd Subject Sycamore trees adjacent to Clanranald Further to my site visit of 13th inst. and to subsequent correspondence, I write in accordance with instructions to set out my advice in connection with the matter referred above. 1. The absence of a soil investigation report poses substantial difficulties in identifying the causal mechanism both for the original damage, attributed to the now‐removed Wellingtonia, and for any subsequent damage, which presently appears to be minor (some might say insignificant). 2. I turn now to background information which I consider to be germane to the present enquiry. There are two means by which tree root action can damage structures: a) direct damage, i.e. pressure on typically lightly‐loaded structures arising from radial or longitudinal root expansion; and b) indirect damage, i.e. loss of loadbearing strength from soils subject to volumetric change (specifically, contraction) due to water abstraction by tree roots. The second of these is known as subsidence and for this to occur the soils in question need to be shrinkable (i.e. soil volume relates to its water content), such as clay or peat (the latter being quite rare in urban areas). 3. Subsidence has an antonym: heave. This typically occurs where structures are built (founded) on shrinkable soils that, prior to construction of the structure, were at a reduced volume due to persistent moisture deficit through long‐term moisture abstraction by trees. Heave occurs where the desiccating tree or trees are removed, thereby enabling the soil to recover its natural volume so as to cause damage to the structures by expansion. Registered Office: Dendron House • 74 Barford Road • Blunham • Bedford • MK44 3ND • T 44(0)1767 641648 • F 44(0)1767 660330 • E [email protected] Company No: 5253618
Logos relate to the Principal Consultant
4. Heave is a very complex phenomenon that commonly occurs both in three dimensions and over relatively long periods of time: decades, rather than, for subsidence damage, months. 5. So, to return to the details of the matter, there is presently no evidence that the relevant soil is a shrinkable clay; whether any damage caused by the Wellingtonia was via the direct or indirect mechanism; what the mechanism(s) is (are) by which the minor subsequent damage has occurred; whether this minor damage can certainly be attributed to the action of the sycamores, be this through the direct or indirect mechanism; or whether, if a shrinkable clay is present, the minor damage results from heave recovery of soil volume following removal of the Wellingtonia. 6. The reason that all this matters, is that if the minor subsequent damage is caused by heave, precisely the worst thing to do is to remove additional trees which, due to their role in abstracting soil moisture, are (in this scenario) moderating the effects of heave. In other words, given the current paucity of the evidence base, the sycamores are potentially reducing the likelihood and/ or intensity of heave reaction following removal of the Wellingtonia. 7. In light of the foregoing, I do not consider that there is sufficient evidence against the sycamores to sponsor a successful application for consent for their removal under the TPO. From the available evidence, the most that it appears can be said is that their roots are probably present within the soil of the adjoining property, and it is possible that some of these may be distorting, or may go on to distort exterior hard surfaces. The remedy for this would typically be root pruning, which I would not consider would be injurious to the trees if undertaken with care. As such, I doubt that either the LPA or, on appeal, the Planning Inspectorate, can be persuaded to consent to their removal. 8. However, it is the case that the majority of the sycamores have high to very high future growth potential and it is further the case that they are located within a sliver of land that is unsuitable for bearing large trees. Indeed, the third tree along from Casterton Road is already growing hard up against the facing stones of the retaining wall by the Rock House car parking: this tree, at least, should be removed. Anyway, it follows that at some point in the future the removal/ replacement of the sycamores will be both necessary and desirable. 9. Whilst the LPA can probably argue for the present that this point is not yet reached, if you were to commission a soil report and if this demonstrated that the site lies on a shrinkable clay soil, this would begin to tip the balance towards precautionary removal to prevent damage to Clanranald. Should you elect to go down this route, I would be happy to look over the soil report and provide further advice on a pro bono basis. This completes my advice on this matter for the present. Julian Forbes-Laird
Director