Remediation

Tree roots Opera Australia building

When the roots of street trees were suspected of causing damage to a building at Alexandria occupied by Opera Australia, the City of Sydney commissioned Costin Roe Consulting to investigate and report.

Like most suburbs of inner Sydney, Alexandria showcases streets lined with trees of considerable size and age. The narrowness of inner-Sydney streets will often mean that trees, only small when first planted, have reached maturity within close proximity to residential buildings and commercial properties.

Tree roots Opera Australia building

Large street trees grow in close proximity to the Opera Australia building in Alexandria, Sydney. The encroachment of tree roots is a common problem in densely built inner-city areas.

The City of Sydney values the trees on Sydney’s public and private land as public assets. The city’s Urban Forest Plan 2013 commits to a sustainable Sydney by 2030 and ongoing improvement in the diversity of the urban canopy. So, when trees become a problem for the built environment, remedial works must be undertaken with great sensitivity, and in close consultation with the City Of Sydney. Costin Roe Consulting are experts at devising remedial engineering solutions where commercial requirements and stakeholder interests need to be managed within the broader context of environmental preservation.

At 58-66 Euston Road, Alexandria, a commercial property occupied by Opera Australia, Costin Roe Consulting found that the roots of two large trees, growing in the street outside the building, had penetrated the ground beneath the basement floor slab.

Just below the front entrance to the building, the basement floor slab had lifted by approximately 100mm over an area of 8 metres x 3 metres. The investigation also revealed that tree roots were blocking an in-ground cast iron stormwater pipe connecting the building downpipe and the street gutter.

Remedial works recommended by Costin Roe Consulting included the removal and replacement of the affected concrete slab, tree root pruning, and the installation of a root barrier to prevent further root growth beneath the floor slabs in the basement car park. To clear the blocked stormwater pipe, tree root coring was recommended, or alternatively the replacement of the blocked section of pipe.

Tree roots Euston Rd Alexandria

Roots were blocking an in-ground stormwater pipe.

Tree roots lifting concrete slab

Two trees roots in excavation outside the storeroom wedged between two concrete slabs, lifting the top slab.

All cracks in structures, foundations, and retaining walls should be investigated by a structural engineer. Cracks allow the penetration of moisture and could indicate or lead to serious structural issues.

Some cracks appear to be worrisome, yet are nothing to worry about, and can be fixed with a simple cosmetic repair. Other cracks might not look so threatening but are symptomatic of grave structural compromise.

“A structural engineering report will properly identify the cause of any cracking and recommend the most effective and cost-efficient remedial actions required,” said Grant Roe, BE(Hons) MEngSc MBA MIEAust CPEng, NER director of Costin Roe Consulting.

“This is not to say that major remediation works are going to be the recommendation whenever an engineer is asked for advice,” Grant said. “True, we are often investigating cracks with the kind of ominous underlying cause which could threaten the structural integrity of a building and might be very costly to repair. But I’m always happy to joke with building owners: That’s not a crack… I’ll show you a crack! when it turns out that cracks of great superficial concern can be fixed with a bit of filler and a slap of paint. The important thing is to ascertain exactly what kind of crack you’re dealing with so that you don’t have any sleepless nights or potentially very nasty and costly surprises.”

Structural cracks

The visual characteristics of a crack can help to indicate its likely severity.

Without discounting the role of forensic engineering in protecting the structural integrity of buildings and reducing risk, there are some general guidelines which even unqualified people can use to decide if a structural engineering report should be called for urgently, or just in good time.

Non-structural cracks (“That’s not a crack…”)

Non-structural cracks could be unsightly and prolific but do not threaten structural integrity. Non-structural cracks can be caused by changes in heat or moisture, minor external forces, and the natural settling of buildings. Here are some of the common causes of non-structural cracks:

  • Concrete shrinkage and creep
  • Pressure from surrounding vegetation or trees
  • Minor shifting or movement of ground and foundations
  • Natural settlement and shrinkage
  • Normal changes in lateral soil pressure (eg: between dryness and rain)

Cracks displaying any of the following characteristics are often non-structural in nature:

  • Hairline cracks that are no more than 1mm to 2mm wide
  • Minor cracks radiating from the corners of doors and windows
  • Cracks running vertically or diagonally
  • Cracks confined to plasterboard or the plaster layer only
  • Cracks up to about 3mm wide and not progressively widening with time

While non-structural cracks in external walls and foundation slabs must be repaired to prevent the continuing penetration of moisture, remediation is generally not expensive or disruptive. For example, cracks can be injected with urethane to provide a long-term seal and prevent further leaks.

Structural cracks (“I’ll show you a crack!”)

Structural cracks

All cracks in structures, foundations, and retaining walls should be investigated by a professional.

Structural cracks are always concerning. Most structural cracks are caused by poor design and construction methods or materials, overly-swollen soil, poor soil bearing quality, overloading, or significant ground movement or subsidence. Structural problems will very often manifest as not just cracks but other potentially telling signs such as sticking doors and windows, doors which no longer hang evenly, and floors which have developed unevenness or a noticeable slope.

The types of cracks which would warrant urgent structural investigation might include:

  • Horizontal cracks running along walls
  • Vertical cracks which are widening at the top or bottom
  • Cracks appearing in a stair-step pattern
  • Significant cracks in foundation walls
  • Any significant cracks in beams and foundation slabs
  • Cracks which form in corners and spread diagonally or horizontally
  • Any cracks wider than approximately 3mm
  • Cracks extending from one level of a structure to another

Not all structural cracks costly to repair

Due to advances in engineering, building products, and building methodologies, the remediation of structural cracks doesn’t necessarily mean removing and rebuilding part of a structure or its foundations. Some structural wall cracks can be repaired by the installation of carbon fibre strips, for example. Problems associated with significantly sinking and settling foundations may be remediated by the use of resistance and helical piers.

“The forensic engineer should also be trying to save money, wherever possible,” said Grant Roe. “All engineers need to stay up-to-date with the latest remediation products and technology so that our recommendations are not just technically sound and thoroughly practical, but also good value.”

Cracks in warehouse floors a special case

The special example of warehouse floors requires additional consideration. Non-structural cracks less than about 0.3mm to 0.5mm can often be tolerated. Cracks of this width are generally not affecting materials-handling equipment. However, cracks greater than 0.5mm in a warehouse floor, while being ‘non-structural’, will often become a durability nightmare by quickly widening – affecting warehouse productivity and causing damage to the wheels of materials-handling equipment.

“All cracks greater than 0.5mm in warehouse floors require careful examination and selection of suitable remediation techniques to mitigate issues with floor durability and warehouse productivity,” Grant said.

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