Graham Prichard & Jason Mckenzie
Asset Management Department Lake Macquarie City Council, Speers Point NSW

Abstract

A suburban precinct has been identified with substantial damage to assets resulting from tree roots. This project aims to remove the worst offending trees and repair the public assets. The community consultation process and technical aspects of the planning and implementation of the street tree replacement are discussed.

Introduction

Lake Macquarie City Council has a number of suburban areas where street trees are damaging concrete footpaths. Often when residents approach Council requesting repairs, the maintenance crews respond that they are unable to effect repairs due to the tree roots.

This project outlines the steps taken in implementing remedial works to address this issue at The Gardens. The success of this approach shows that removal of inappropriately planted trees is a viable option in these situations.

Background

Many of the concrete footpaths in The Gardens precinct are severely damaged from uplifting, resulting primarily from tree roots, compounded by the construction methods and soil type.

The main entrance road (known as The Park Chase) was planted with Platanus acerifolia along both sides, and also has concrete footpaths along both sides. The surrounding streets included in this project were planted with Triadica sebifera, with a concrete path on only one side of the streets. Interestingly, this species has shown itself to be capable of causing major disruption to concrete footpaths from a young age.

Council’s limited resources are allocated on a city wide priority basis for street tree management and footpath repairs/replacement.

Unfortunately the problems present at The Gardens are also present at a number of other areas that were constructed at around the same era. Lake Macquarie City Council has since improved processes for street tree planting and management, prepared detailed guidelines and prevented further occurrences of poor street tree installations.

Location

The Gardens is a relatively new estate (circa 2000) located in eastern Lake Macquarie within the suburb of Valentine.

Figure 1. Locality map with red arrow indicating project area

The project area includes several streets within The Gardens precinct where the damage from street trees has been assessed as most hazardous. Other areas with damage have not been included due to budget constraints.

Governance

All land within The Gardens is zoned R2 Low Density Residential by the Lake Macquarie Local Environment Plan (2014). The precinct is a residential area incorporating several small parks and playgrounds. There are no major thoroughfares, nil industry, retail or other land uses. Bordering the precinct on three sides is the Green Point reserve, comprising approximately 160Ha of natural bushland.

All land within The Gardens is zoned R2 Low Density Residential by the Lake Macquarie Local Environment Plan (2014). The precinct is a residential area incorporating several small parks and playgrounds. There are no major thoroughfares, nil industry, retail or other land uses. Bordering the precinct on three sides is the Green Point reserve, comprising approximately 160Ha of natural bushland.

History

Previous trials

Due to ongoing problems with footpath damage, installations of two products intended to prevent or minimise further damage were made during September 2009. Assessments in August 2011 found no sign of further footpath damage at the installation sites.

The first product, Tripstop®, is a plastic hinge installed during construction in the expansion gap of concrete footpaths. The product is designed to allow the concrete slabs to hinge rather than sheer when concrete slabs subside or are uplifted, preventing the creation of trip hazards.

The second method required the installation of a below ground plastic cell matrix, filled with high quality soil, intended to allow healthy root development in a confined space while preventing uplifting of concrete footpaths or damage to below ground infrastructure.

Figure 2. Installation of structural cell matrix (2009).

As shown in Figure 2, installing structural cells to existing plantings can result in damage to tree roots.

While the trials appear to be effective in preventing damage to footpaths, due to high cost and the need for it to be installed prior to tree planting, the structural cell matrix has only been used in town centre environments. The Lake Macquarie City Council standard specification for concrete footpaths now includes the use of Connelly joints, which exceed the performance of Tripstop® in preventing uplifting of concrete footpaths.

Analysis

Following a risk assessment of the footpaths, it was determined that replacement of some sections was required to address safety. A number of options were canvassed, including replace all the trees, replace some trees, remove all trees without replacing them, do nothing. Residents were consulted about the options, and a project was developed to present a detailed design of the preferred option, replacing trees and footpaths in the worst affected areas.

In deciding to remove and replace the trees, a number of options and factors were considered. Removing the trees and not replacing them was rejected by the residents, who stated that the trees where a major influence on their decision to invest in the area. Indeed, a recent Canadian study (Kardan et al 2015) has found that people living in areas with higher street tree density report better health perception and fewer cardio- metabolic conditions, and that this effect is equivalent to raising the income of each household by $10 000. As it was not possible to repair the footpaths to a satisfactory standard without seriously damaging or removing the trees, replacement was the preferred option.

Council’s landscape architects completed the tree planting design and quantity estimates. Council’s operational division Civilake, undertook the construction of this project.

Council uses a project management framework, in the Sycle software package, for project management.

The Site

Most of the trees have been planted in narrow spaces between the kerb and concrete footpaths, without adequate preparation and without installation of root barriers. Many have been planted directly on top of drainage pits or against street lights. The species selection and planting methods are not compatible with contemporary requirements.

Soils in this estate are poor and topsoil is absent as a result of previous land use as a quarry. Soils are predominantly clay/conglomerate and the area is subject to subsidence from historical coal mining, possibly contributing to the damage to concrete footpaths. The soil in the area does not favour tree root growth. This is evidenced by the surface rooting occurring in gardens and tree roots disrupting concrete paths.

Existing plantings

The precinct contains deciduous trees planted in themed groupings along the streets. An assessment of street trees during 2011 located 474 trees. Information was collected on the species, location, health, type and extent of damage occurring to pathways and infrastructure. The deciduous species show varying performance and a range of issues relating primarily to infrastructure damage, encroachment and poor tree health in some areas.

Constraints

The design and location of replacement trees was problematic, given the constraints present on site. The locations of many of the original trees was inconsistent with the requirements for clearances from stormwater pits, street lights and other assets, vehicular sight distance requirements resulting in a slight reduction in the number of trees being planted. The residents also expressed concern that the replacement trees need to achieve a similar outcome to the themed deciduous plantings, requesting that trees on sides of roads without footpaths also be replaced. Following an evaluation, this was agreed to, as it was noted that those trees were also posing a risk of infrastructure and asset damage.

Engineering guidelines

Lake Macquarie City Council has developed engineering and landscape guidelines that are used to specify requirements for concrete footpaths, street tree planting and other works. Consistent with the Local Government Association Urban Forest Policy (LGA Urban forest policy 2003), Lake Macquarie City Council is integrating tree management expertise with civil design and construction .During the preparation of the landscape plans for this project, Council’s landscape architect prepared the plans to be consistent with the guidelines. A new specification was created for the installation of the root directors, as they were not included in the engineering and landscape guidelines.

Financial analysis

With no capital budget allocated to street tree planting, the status quo option was to accept the liability risk and to continue using maintenance budgets to repair the footpaths and other damage caused by the trees.

Preliminary analysis for this project included estimates of the ongoing maintenance costs of the footpaths and trees, assuming no major renewal or intervention occurred. This analysis found that over a fifty year period, savings resulting from this renewal project related to the trees was approximately $1 000 000  and  for footpaths was in excess of $5 000 000.

A successful application was lodged requesting asset renewal funding to undertake this project.

Risk management

Councils transport planning inspectors assessed the footpaths in accordance with the Statewide Mutual best practice guide footpaths (2014) to ascertain the level of risk posed by the displacements. It was determined that in several streets an unacceptable level of risk was present, with vertical displacements of 80mm and 60mm being amongst the higher figures recorded. Displacements in the range of 30mm-50mm were recorded on approximately 35 occasions. Formal risk assessments were conducted as part of the project management framework.

Figure 3. Damage to footpaths was extensive, creating risk to users.

Project risk assessments were completed using corporate templates in accordance with the Lake Macquarie City Council project management framework.

Asset damage

Damage has been recorded to footpaths, kerbs, road surfaces, driveways, lawns, gardens and a number of other residential assets. Damage has occurred primarily to concrete footpaths including  cracking, displacement and uplifting. Roads, kerbs and other assets have been little affected to date, due to the young age of the trees.

Figure 4. An example of damage occurring throughout the project area.

Future damage would be likely to result from the trees planted on top of storm water inlet pits and against light posts. Damage to private assets has been reported, including driveways, paths, gardens, letterboxes and other fixtures. In accordance with the Statewide Mutual best practice guide trees and tree roots (2013) Council is taking reasonable action to address the issue of damage as far as it is reasonable to do so.

Communication strategy

A communication plan was prepared in conjunction with Lake Macquarie City Council communications team to develop a strategy to communicate with the various stakeholders.

Components of the communication strategy included a letter mailed out to all residences seeking comment on the design and proposed species. The letter invited residents to attend an onsite meeting where the plans were displayed with staff present to answer questions.

A memo was sent to councillors to inform them of the project. The customer service centre (call centre) was provided a summary of the project and details of how to respond to any enquiries. A notice was placed on council’s website immediately prior to tree removal commencing. An additional letter was posted to all residents regarding bringing forward tree removal, due to an issue with damage to larger vehicles resulting from lateral branches encroaching the roads.

In the days prior to on ground works commencing, a letter box drop was carried out with specific information about the footpath and tree replacement works.

Consultation

The Gardens residents group (Lake Group Strata) met with Council staff to discuss options for addressing the problem of footpath and asset damage caused by the trees. The representatives canvassed the options with residents they represent. Concern was expressed that the existing landscape of the suburb should not be lost by total street tree removal and that replacing trees only on the side of streets with footpaths would create a mismatch of trees, reducing the character of the area.

Resourcing process

Following the design and quantification of the preferred option, an internal budget submission was made and duly approved to fund the project. A budget for this project was sourced from the footpath asset renewal fund.

Costs were estimated based on the number of trees, root directors, amount of soil and concrete for disposal and installation, labour, plant and contingencies. For each tree being replaced, it was estimated that on average 6m of concrete footpath would require replacement.

Tree selection

The original species planted were shown to be unsuitable for the location, primarily for excessive size, aggressive root growth and in the case of the Triadica sebifera the tendency to invade nearby bushland. A number of species, compatible with the desired character of the area and practical requirements, were shortlisted.

The narrow space between rear of kerb and footpath and the number of assets and  services  present, restricted the available locations for tree planting and the mature size of tree species. Previously, garbage truck contractors had threatened to take action because of alleged damage to their trucks caused by branches overhanging the vehicular travel path. As Council does not have a formative tree pruning program, the selected tree species were required to have a narrow form to avoid branches impinging on the vehicular travel path. Trees were required to have NATSPEC (Clark 2003) certification, copies of which are required prior to tree planting.

Characteristics required:

Narrow  form, deciduous/autumn  colour, non-invasive  roots, non-weedy, small  fruited, non-malodourous, overall small stature, hardy, attractive, available.

Species considered, but not selected included:

Species Reason for non-selection
Pistacio chinensis Crown spread, mature size
Lagerstroemia cultivars Requirement for formative pruning
Pyrus cultivars Crown spread, potential weediness

Planting details

Due to the close proximity of various assets and the large number of issues with tree roots from the original trees, it was decided to install modular root directors.

Lake Macquarie City Council planting guidelines were adapted to provide a drawing for the installation of the root directors and tree planting.

Figure 5. Typical planting layout (adapted from Lake Macquarie City Council landscape design guidelines 2013).

Removal of existing trees and footpaths

The project was initially set up to run across two financial years during June/July. However, in April 2015 an intense low pressure storm system caused extensive damage to the Lake Macquarie area, uprooting thousands of trees and resulting in major disruptions to scheduled works. Fortunately the project trees had been removed prior to the storm and no damage occurred in the project area.

A contract arborist was engaged for tree removal with 185 trees removed and stumps ground. This exposed the residents of the gardens estate to a baron streetscape devoid of colour, character & proportional height as against the existing rendered two-storey dwellings within this estate.

Figure 6. When sections of damaged concrete were removed for temporary repairs in 2013, large tree roots

The works were rescheduled and commenced in July 2015.

Lake Macquarie Councils Civilake construction crew initiated works by removing sections of damaged concrete path and previous maintenance asphalt patches. Construction work zones at times stretched up to 300m in length. With a work zone of such length causes some safety, disturbance and access issues to adjacent residents, construction staff were required to manoeuvre at times to manage access and traffic flows throughout the construction process. Signage and barrier mesh was put in place in accordance with the traffic management plan. As the work zone progressively moved with the project, rolling along through each street identified for planting and concrete footpath replacement, these issues of safety, access and disturbance to residents required daily adjustments and setups.

With concrete paths removed and the ground beneath exposed, it was clear as to the amount of tree roots within the top 200mm of the surface area surrounding the location of the previously removed trees (figure 6). This prevented excavation of new tree pits and installation of formwork and new concrete paths. An arborist was again engaged to stump grind tree roots within the project site prior to tree pit installation and concrete path replacement.

Tree pits were excavated to allow for installation of root directors, with additional area to support future tree root growth. A circular root director was chosen after consideration of a number of factors such as aesthetics, the width of the root directors, the available space for plant installation, the depth of the root directors, and provision of a 100mm raised flange creating a mowing edge to the outside and a mulched edge to the inside.

Tree pits were back filled with certified organic soil material, with excavated spoil used to support and stabilise the root director. The organic soil was left to settle for a period while concrete footpath replacement was completed, with tree pits topped up and trees installed prior to mulching. Trees were monitored for stability within the tree pit with consideration to install additional tree stakes & ties, however, this was not required.

Concrete footpaths were replaced & constructed prior to tree installation. All joints between new and existing concrete paths were doweled, and all joints adjacent tree pit locations fitted with a Connolly joint expansion system. (Lake Macquarie City Council engineering guidelines). This system reduces the potential for vertical lift at the joints of concrete paths, if paths are displaced by tree roots it allows a path section to rise as one creating a roll over as opposed to a disruption.

Construction methods

During removal of the failed paths, it became obvious that substantial root growth was present that required removal to allow the concreting of the new paths, and planting of the new trees. To prevent damage to private assets and service conduits that could occur by removing the roots with an excavator, the roots were ground out with a motorised root grinder.

The distance between kerb and failed footpath alignment was noted to be variable, therefore the RDC900 root director, a Citygreen product, was used as the larger RD1000 would not fit in all of the planting locations.

Existing failed footpaths were removed in sections corresponding to street blocks. Each section was then replanted and the footpath replaced before commencing on the next section. This reduced the length of time each section was barricaded and therefore the reduced disruption to the local community.

Tree species and nursery supplier were identified and an order placed to hold the required quantity of trees (Pyrus calleryana “Chanticleer” and Prunus cerasifera “Oakville Crimson Spire”). These selections are  narrow in form and consistent with the existing planting theme throughout the gardens estate.

Tree installation with 45ltr size specimens was undertaken throughout the winter period of July to early August. Installation of deciduous species during this time reduced potential planting shock allowing the newly planted street trees to adjust to their new environment prior to the growing season.

Trees are monitored for potential bacterial & fungal diseases, and maintained with an establishment period of 13 weeks with tasks such as fertilising, watering, mulching, directional pruning and replacement of damaged or dying specimens.

Sustainability

Lake Macquarie Council supports a sustainable approach of recycle where possible. All trees removed were shredded to mulch and stockpiled for re-use where appropriate. All concrete was transported to a masonry recycle plant. All excavated spoil for tree installation was used as backfill to concrete paths and as a supportive substrate to the tree root directors. All remaining spoil was transported to the local waste management facility as the Environmental Protection Authority guidelines prevent excavated spoil to be reused at other allocations outside of the construction work zone, unless treated and tested.

During the design process, it was decided not to incorporate a stormwater water system, due to concerns over maintenance of private stormwater pipes linked to the tree pits.

Conclusion

The time between the removal of the trees and replacement with new trees was longer than originally planned due to delays resulting from the April storm. While all of the residents had approved the tree planting at all times during the consultation, a number subsequently contacted Council to request a tree not be planted on their verge. This change is attributed to the length of time between tree removal and replacement and people becoming used to the treeless streets during winter.

This project has been successful in demonstrating that it is better to remove trees and replace them properly, than to try and ameliorate damage that is going to continue and worsen over time. Of course, if the trees have a great value, then it is easier to make a case for preserving the trees at an increased cost.

Acknowledgements

The authors would like to acknowledge the assistance and dedication of Council staff including the Civilake work crews, Landscape Architects, Communications and Asset Management personnel.

References

  • Clark, R (2003) Specifying Trees – a guide to assessment of tree quality. NATSPEC
  • Kardan O , Gozdyra P , Misic B, Moola F, Palmer L, Paus To, Berman M, (2015) Neighborhood greenspace and health in a large urban center. http://www.nature.com/srep/2015/150709/srep11610/abs/srep11610
  • Lake Macquarie City Council (2013) Landscape Design Guidelines. http://www.lakemac.com.au/page.aspx?pid=109&vid=10&fid=257&ftype=True (accessed 28-7-2015)
  • Lake Macquarie City Council (2013) Engineering Guidelines to the Development Control Plan Part 1 – Design Guidelines. http://www.lakemac.com.au/page.aspx?pid=109&vid=10&fid=3983&ftype=True (accessed 28-7- 2015)
  • NSW Local Government Association (2003) LGA Urban Forest Policy http://www.lgnsw.org.au/policy/natural- resource-management/street-trees-and-urban-forests (accessed 3-8-2015)
  • Statewide Mutual Best Practice Guide Footpaths ver. 5 (2014)
  • Statewide Mutual Best Practice Guide Trees and Tree Roots ver. 7 (2013)

 

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