arboricultural appraisal report - burnley 0426... · arboricultural appraisal report ... discussion...

MWA Arboriculture Ltd Bloxham Mill Business Centre Barford Rd, Bloxham Banbury OX15 4FF

Tel: 0191 432 9560 Email: [email protected]

Arboricultural Appraisal Report

Subsidence Damage Investigation at:

64 Bank Parade Burnley

Lancashire BB11 1TS

CLIENT: Crawford & Company

CLIENT REF: SU1404685

MWA REF: SUB150817-181

MWA CONSULTANT: Giles Mercer (B.Sc Hons)

REPORT DATE: 25-08-2015

SUMMARY

Statutory Controls Mitigation (current claim)

TPO No Insured Yes

Cons. Area Yes

3rd Party No

Trusts schemes N/a Local Authority No

Planning N/a Other No

Local Authority: - Burnley Borough Council

Property: 64 Bank Parade, Burnley, Lancashire, BB11 1TS

Client Ref: SU1404685 MWA Ref: SUB150817-181

Introduction

Acting on instructions received from Crawford & Company, the insured property was visited on

25/08/2015 for the purpose of assessing the potential role of vegetation in respect of subsidence

damage.

We are instructed to provide opinion on whether moisture abstraction by vegetation is a causal factor

in the damage to the property and give recommendations on what vegetation management, if any,

may be carried out with a view to restoring stability to the property. The scope of our assessment

includes opinion relating to mitigation of future risk. Vegetation not recorded is considered not to be

significant to the current damage or pose a significant risk in the foreseeable future.

This is an initial appraisal report and recommendations are made with reference to the technical reports

and information currently available and may be subject to review upon receipt of additional site

investigation data, monitoring, engineering opinion or other information.

This report does not include a detailed assessment of tree condition or safety. Where indications of

poor condition or health in accessible trees are observed, this will be indicated within the report.

Assessment of the condition and safety of third party trees is excluded and third party owners are

advised to seek their own advice on tree health and stability of trees under their control.

Property Description

The property is a three storey mid-terrace house of traditional construction (built circa 1850) with stone

walls surmounted by a pitched slate roof.

External areas comprise a small garden to the front and a car parking area to the rear.

The site is generally level with no adverse topographical features.

Damage Description & History

The current damage affects the main building and two storey extension to the rear.

We note from the Technical report that Mr. Geoff Hart, who is head of IT and estates for Brook, that

the damage was first noticed approximately 12 months ago following an inspection of the property.

Suspecting that the movement/damage may be due to defective drains to the rear of the property, Mr.

Hart appointed a contractor to undertake a drainage CCTV survey.

This revealed defects to the system which were subsequently repaired/replaced. Following

repair/replacement of the drains Mr. Hart maintained a watching brief on the damage.



In November 2014 Mr. Hart considered it prudent to arrange a site investigation.

Trial holes were prepared by Woodward Ground workers and inspected by ACS Partnership Chartered

Surveyors.

At the time of the engineers’ inspection on the 9th January 2015 the structural significance of the

damage was found to fall within Category 2 (slight) of Table 1 of BRE Digest 251.

Site investigations

Site investigations were carried out by CET property Assurance on the 4th February 2015.

Two trial pits were excavated, TP1 to the left hand corner of the two storey extension and TP2 centrally

to the rear wall of the main building.

The trial pits (TP) were hand excavated in order to reveal the foundation depth and specification and

then a borehole (BH) was sunk through the base of the trial pit in order determine subsoil conditions.

A CCTV survey of the relevant parts of the property's drainage system was undertaken.

Foundations:

Ref Foundation type Depth at Underside (mm)

TP1 Sandstone over Concrete 800

TP2 Sandstone 1350



Soils:

Ref Description Plasticity Index (%)

Volume change potential (NHBC)

TP/BH1 USF-1.0m

MADE GROUND: Medium compact to compact, dark brown, very silty clay with brick & stone fragments & clinker fragments.

24 Medium

BH1 1.0-1.7m

MADE GROUND: Medium compact to compact, dark brown, very silty clay with brick & stone fragments & clinker nodules.

23 Medium

BH1 1.7-2.3m

Stiff, dark brown, silty CLAY 30 Medium

TP/BH2 USF-1.55m

Firm, light brown, silty CLAY with occasional stone nodules.

48 High

BH2 1.55-1.7m

Firm, light brown, silty CLAY with stone nodules.

n/a n/a

BH2 1.7-2.3m

Stiff, dark brown, silty CLAY with stone nodules.

39 Medium

Roots: Roots were observed throughout TP1 and TP2 and to a depth of 2m in BH1 and BH2.

Ref Roots Observed to

depth of (mm) Identification Starch test

TP1 USF Prunus spp. Positive

TP1 USF Fraxinus spp. Positive

BH1 - Prunus spp. Negative

BH1 - Fraxinus spp. Positive

TP2 USF Fraxinus spp. Positive

BH2 - Prunus spp. Positive

BH2 - Fraxinus spp. Positive

Drains: The drains have been surveyed and defects were identified.

Monitoring: At the time of writing this report no monitoring data was available.



Discussion

Opinion and recommendations are made on the understanding that Crawford & Company are satisfied

that the current building movement and the associated damage is the result of clay shrinkage

subsidence and that other possible causal factors have been discounted.

Whilst the foundations of the wall (TP1) are on made ground, in -situ Mackintosh probe testing of the

material indicates that it is sufficiently consolidated to bear the imposed load and as such the damage

cannot be attributed to consolidation settlement. This is borne out by the relative age of the structure

and the recent appearance of damage.

Site investigations and soil test results have confirmed a plastic clay subsoil of medium to high volume

change potential (NHBC Classification) susceptible to undergoing volumetric change in relation to

changes in soil moisture.

The moisture content profiles indicates a reduction in moisture content between the underside of the

foundations down to a depth of 2000mm below ground level which is indicative of desiccation at this

level.

Suction tests indicate desiccation coincident with the depth of root activity.

The desiccation is at depths beyond normal ambient soil drying processes such as evaporation and is

indicative of the soil drying effects of vegetation.

Shear vane testing of the substrate indicates that it is sufficiently consolidated to bear the imposed load

and as such the damage cannot be attributed to consolidation settlement. This is borne out by the

relative age of the building and the recent appearance of damage.

Roots were observed to a depth of 2.0m below ground level in both TP/BH1 and TP/BH2, samples of

these roots were recovered positively identified (using anatomical analysis) as Fraxinus spp. (Ash) and

Prunus spp.



Our survey of the site identified the Ash tree (T1) and the Ash saplings (TG1). The Ash tree (T1) is the

dominant vegetation and it is probable that this is the source of the recovered roots rather than the

young Ash saplings.

The source of the Prunus sp. roots is the Cherry Laurel (H1).

Based on the technical reports currently available, engineering opinion and our own site assessment

we conclude the damage is consistent with shrinkage of the clay subsoil related to moisture abstraction

by vegetation.

Having considered the available information, it is our opinion that the Ash tree (T1) and the Laurel

hedge (H1) are the material cause of the current subsidence damage.

If an arboricultural solution is to be implemented to mitigate the current damage and allow the soils

beneath the property to recover to a position such that an effective repair solution can be implemented

we recommend that the Ash tree (T1) is removed and that a section of the Laurel hedge is removed to

create a minimum clearance other damage building of no less than3m.

We have given consideration to pruning as a means of mitigating the vegetative influence; however in

this case, we do not consider pruning offers a viable long term solution.

Whilst we do not consider the Ash saplings (TG1) to be a material cause of the current subsidence

damage, they do present a real risk of future damage and accordingly we have recommended that they

be removed to mitigate against any future damage.

Replacement planting may be considered subject to species choice and planting location.

Conclusions

Conditions necessary for clay shrinkage subsidence to occur related to moisture abstraction by

vegetation have been confirmed by site investigations and the testing of soil and root samples.

Engineering opinion is that the damage is related to clay shrinkage subsidence.



There is significant vegetation present with the potential to influence soil moisture and volumes below

foundation level.

Roots have been observed underside of foundations and identified samples correspond to vegetation

identified on site.



Table 1 Current Claim - Tree Details & Recommendations

Tree No.

Species Ht

(m) Dia

(mm)

Crown Spread

(m)

Dist. to building

(m)

Age Classification

Ownership

T1 Ash 17 500* 12 9.3 Older than extension (possibly)

Policyholder

Recommendation Remove (fell to ground level) and treat stump to inhibit regrowth

H1 Laurel(Cherry) 3 200 ms

0.7 1.5 Younger than property

Policyholder

Recommendation

Remove (fell to ground level) a section of hedge to create a minimum clearance of no less than 3m. Stump to be grubbed/ground out.

Ms: multi-stemmed * Estimated value

Table 2 Future Risk - Tree Details & Recommendations

Tree No.

Species Ht

(m) Dia

(mm)

Crown Spread

(m)

Dist. to building

(m)

Age Classification

Ownership

TG1 Ash saplings ≤ 3 ≈ 60 ≤ 1.5 ≤ 0.3 Younger than property

Policyholder

Recommendation Remove (fell to ground level) and either treat stumps to prevent re growth

Ms: multi-stemmed * Estimated value



SITE PLAN

Plan not to scale – indicative only Approximate areas of damage

Front

TP/BH2

TP/BH1 TG1 H1

T1



Images

View of T1 and H1

View of TG1



View of H1 and TG1

View of T1, H1 and TG1

arboricultural appraisal report - burnley 0426... · arboricultural appraisal report ... discussion...

Documents