An Arboriculturists Guide to: VacEx
Just VacEx it…
Excavating within Root Protection Areas (RPAs) is a delicate task, particularly when working alongside utilities and infrastructure projects. Careful planning and understanding are essential to ensure tree roots remain unharmed.
As consulting arboriculturists, it’s our responsibility to provide thorough, clear instructions that are easily understood and executed by those carrying out our recommendations. These instructions should be comprehensive enough to help both the client and the contractor responsible for excavation. This approach ensures best practices are consistently applied, minimising the risk of accidental root damage and supporting the tree’s health.
The Need for Detailed Recommendations in Vacuum Excavation
I often find that recommendations involving vacuum excavation lack specific guidance on how operators should handle the process near trees. In my early days in the infrastructure industry, I made similar mistakes in my reports. However, with the rapid increase in VacEx machines—especially as some firms prefer them over 360 tracked excavators—it’s essential to understand both the operator’s mindset and the machine’s limitations.
Initially, my recommendations were vague, relying on the assumption that the task would be carried out as I imagined. I didn’t, however, specify exact methods, as I didn’t fully understand the machine’s limitations. But no one can interpret unspoken expectations. Fortunately, I had many opportunities to supervise projects where I had recommended VacEx use. This gave me insight into the machine’s functionality—and, perhaps more importantly, into the operator’s perspective.
The Realities of Using Vacuum Excavation (VacEx) within RPAs
When a VacEx is used on-site, it’s common to have a single operator with the machine. This can be problematic within an RPA. The operator usually controls the main vacuum hose with a handheld remote control, and most machines also come with a 90PSI airline outlet with a lance. Ideally, two operators are needed—one managing the hose and the other operating an air pick/air spade. In smaller trial holes, one operator might suffice, but for larger excavations, a single person may struggle to manage everything effectively without risking root damage.
VacEx operators often try to expedite the excavation by forcing the hose directly into the ground with the machine’s weight (often on a 32-tonne chassis). This practice can quickly damage roots, tearing the outer layer or snapping roots entirely—especially in clay soils, where even high suction isn’t always enough to displace soil effectively. I’ve seen roots damaged before I could even intervene to say, “STOP!”
It’s important to note that operators aren’t necessarily being careless; these machines are costly (around £1200 + VAT per day), and clients often want the job done quickly. Operators may also lack knowledge of tree biology.
Method Statement for Complex Excavations
Here’s an example scenario to illustrate some of the complexities involved and how they can be managed effectively.
Scenario: Mains Sewer Burst Repair
Location: 1.5m from an 800mm diameter tree stem (9.6m RPA radius)
Pit Requirements: Width: 1000mm, Length: 1200mm, Depth: 1000mm
Site Description: Large tree beside the burst sewer main, located in the middle of a rear garden, with low branches from a second tree between the dig location and an access gate, which is only 1.2m wide.
The soil structure is clay.
Appraisal:
In real-life situations, this type of scenario would be handled reactively without a comprehensive appraisal. However, let’s assume an appraisal has been conducted.
You’re called to recommend an excavation method. After suggesting hand-digging only (and receiving a slap on the face by a disgruntled site team), you determine that VacEx is the best option.
However, there are many constraints to note; no lorry is getting close to where the team needs to dig, due to access. restrictions. Not only that, the low branches would need to be pruned away, and the ground protection to traverse over the root plates of the two trees would essentially need to be a some kind of temporary cellular confinement system or a temporary road system installed. Using normal 8x4 EuroMats will be doing nothing under 32 tonnes of concentrated pressure.
Here, no lorry can access the garden due to width and height restrictions. It’s just as well, since the above-mentioned temporary ground protection comes at an eye-watering cost.
There are, however, standalone pedestrian-sized machines that can access the dig location. But, they often lack the power to vacuum dense clay soils.
Recommendation
So, we need VacEx regardless. There are machines which can be utilised in these scenarios. A pedestrian sized unit which directly connects a hose extension to a lorry-mounted VacEx can be considered. We really need to state the distance of where the dig location is in relation to where the lorry is sited.
In this situation, the dig location is 50m away from where the lorry is parked. The lorry VacEx hose will likely only have an outreach of 3-4m, so we need multiple connections of 10m hose and air line lengths. These are optional extras at an additional cost, and will not arrive on site unless requested.
The small pedestrian unit (like a Brokk or similar unit) is considerably lighter and smaller which can traverse over more affordable 8x4 EuroMats without excessive damage to the ground.
Key Considerations During Excavation
Ideally, a works briefing or toolbox talk should be conducted before excavation begins.
Begin by gently removing the top organic layer using hand tools. Though spades or shovels can still cause harm, a gentler approach reduces risk. The VacEx hose should ideally hover about 100mm above the soil surface, with an air spade used to loosen soil beneath. In dense clay soils, a clay spade or hand tools should be used to dislodge soil around roots carefully.
Operators must remember that roots dry out quickly once exposed, so any root over 25mm in diameter should have its circumference fully exposed as soon as possible and then managed with care. It’s critical to have an arboriculturist on-site to wrap exposed roots to protect them from accidental damage. Personally, I use damp hessian wrapped in damp sheep wool, encapsulated in 1200ga polythene and secured with heavy duty cable ties. The polythene protects the wool and hessian, keeping it clean and reusable.
When reaching dense clay layers, compressed air and suction alone may not be effective. At this point, a clay spade or trench spade (with a long handle) is ideal for dislodging clay without full pit excavation by hand.
For safety, shoring (or shuttering) will likely be required to prevent pit walls from collapsing during repairs. Being so close to the tree, shoring boards may need to be narrow to fit between primary and secondary lateral roots. If the pit is left open for extended periods, roots should be re-moistened regularly.
Once repairs are complete, remember that many VacEx machines may have worked on contaminated sites, so using BS8332-compliant screened topsoil is advisable. This topsoil is readily available from most aggregate suppliers and ensures that no contaminants are reintroduced into the RPA. Some suppliers have handy online calculators, meaning you can give an approximate volume to order, depending on how detailed your method statement is.
Conclusion
It most likely seems like I’m detailing too much, way beyond what an Arboriculturist should be recommending. My concern is, however, that just recommending VacEx could be just as damaging as sending an excavator bucket through the root plate of a tree. Using a VacEx can be highly effective for working within RPAs, but careful planning, thorough instructions, and appropriate supervision are essential. Each step requires consideration of the tree’s health, the operator’s skill, and the machine’s limitations.
I hope this information helps inform your Arboricultural Method Statements in the future that require VacEx.
