18 Root Barrier installation considerations for Japanese Knotweed remediation
CuTex (from GEOfabrics®) is the only geocomposite, permeable root barrier in production, and has been independently tested to qualify its abilities as a barrier for Japanese knotweed. Its special formulation of copper sheet mechanically encapsulated between two high strength geotextiles, is highly effective at safely inhibiting Japanese knotweed rhizomes by releasing Cu2+ ions into a zone around the barrier that causes progressive root collapse.
Below are 18 considerations when using CuTex for Japanese knotweed remediation, to help ensure that all of the benefits of the barrier system are gained.
1. Estimate – calculate how much CuTex Root Barrier will be needed for your project, but always allow for 10% wastage (see below).
2. Laying CuTex – ‘Is there a correct way up?’ It’s a common question, but CuTex can be installed on either side.
3. Backfilling - it's not unusual for a vertical barrier to be pulled downward during the backfilling process. Allowance should be made for this, and it is recommended that for a vertical barrier installed over 1m deep, an extra length is added based on +5% of the depth required.
4. Securing – when installing a vertical barrier, it is suggested that an additional 100mm of barrier is sacrificed to be pierced and pinned to the bank at ground level prior to backfilling taking place. Road pins can be used to anchor the barrier temporarily and should be positioned at between 2-5 metre intervals (piercing the barrier within the sacrificed strip). The remaining strip can be trimmed or, where appropriate, used as an additional overlap when completing the installation.
5. Anchor trenches – particularly suited to battered slopes, provide an alternative to pinning a vertical barrier. An anchor trench can be positioned at the top of the slope to hold the barrier in place.
6. Burial cells – according to current best practice, 2 metres of cover should be applied over a Japanese knotweed burial cell, consequently this cover should be considered when the burial pit is being dug.
- Cell depth – A 4-metre-deep pit with battered sides is recommended. Ideally, a step in the wall profile of the pit should be created (to aid access) where the cell cap should be (2 metres from the surface).
- Soil type – this will determine the recommended angle of batter needed to safeguard against soil collapse. (Further advice on this matter should be sought from a qualified professional).
- Barrier selection:
- The vertical face of the cell – here the 2.6m width CuTex barrier is recommended. Subject to the angle of batter, the excess 0.6m of barrier allows for overlap (joining to the horizontal CuTex cap), the sacrificed strip, any loss due to the barrier being pulled downward.
- The base and cap of the cell – these can be laid in strips (overlapping by a minimum of 0.3m) using either 2.6m or 5.2m width CuTex.
For further information on designing and calculating the dimensions of burial cells, read PCA Guidance Note: Safe & Effective Excavation & Burial.
7. Welding - CuTex can be welded using a handheld or automatic heat welder; no additional materials are needed as the root barrier bonds when sufficiently heated. All joints should be checked using a hooked pick. (Contact PBA Solutions for advice and training where required).
8. Taping - CuTex can be taped using a heavy-duty gaffer tape; it is recommended that 300mm wide tape (available from PBA Solutions) is used and that the barrier should be taped on both sides.
9. Prayer-fold joint – use this where possible when joining the CuTex root barrier using tape, as this creates a concentration of copper at the joint. This concentration of copper will contribute to inhibiting roots from exploiting what would normally be considered the weak point in a root barrier installation.
10. Post-installation – the backfill used must be ‘only’ clean, Knotweed-free material. Care should be taken to remove any sharp objects in the backfill material that could damage the root barrier.
11. Surface protrusion - when a vertical barrier is used to block knotweed, the barrier must be brought to, and preferably protrude above, the surface. It is prudent to consider that you may wish to bond the CuTex up stand to a suitable material on the surface. This could include concrete foundations, gravel boards or other appropriate fixed materials.
12. Site machinery (Excavators & Dumpers) - must not directly track over the barrier. A layer of clean soil, free of sharp objects (>150mm depth), should be spread before any compacting or tracking of the soil takes place.
13. Additional protection - where required, a specialist textile can be used as a protective fleece on either side (or a single side) of the CuTex Root Barrier. Instances include:
- Sharp objects – to ensure increased protection when present.
- Capping - when CuTex is required to cap an area of land impacted by knotweed (on either a temporary or permanent basis), where pedestrian or vehicular access is required.
- Burying rough soil – when used in a burial cell containing large stone, rock, or flint, a protective fleece can be specified to protect CuTex from damage.
14. Collaring pipes - when a service or utility pipe needs to penetrate the CuTex, a collar should be formed to encase the pipe, which can both be bonded to the pipe and the CuTex root barrier sheet. The collar must extend more than 0.5 metres past the barrier (both sides).
When bonding to the pipe, a suitable impact adhesive should be used. The objective is to seal the barrier to the pipe so that knotweed can't penetrate. A double seal is recommended on either side of the barrier.
16. Copper-free sections - 100mm strips on one side (for 2.6m wide rolls) or both sides (for the 5.2m wide rolls) are intended to be used in lapping scenarios. However, where a prayer fold is required to get a concentration of copper, these copper-free sections cannot be used.
For further ‘CuTex Root Barrier System’ help, training, and advice contact PBA Solutions (0203 174 2187).