Japanese Knotweed - Not a weed to ignore!

Japanese Knotweed is a serious consideration for Lenders, Developers, Purchasers, Landowners, Planners and Surveyors. The impact of the discovery of Japanese Knotweed on land and buildings can prove to be significant.

Source: Charles Lyndon

Anyone who has a garden will be more than aware of the speed in which weeds will grow, which if left uncontrolled can become unsightly and overgrown very quickly. Having acquired an overgrown allotment a few years ago, which I spent many hours clearing and digging I can tell you with authority that weeds are almost impossible to eradicate and therefore need to be regularly controlled. Most varieties of weeds are harmless if regularly managed, with the exception of the odd thorny or irritant types of weeds. There is however one particular type of weed that has received increased publicity over recent years, due to the size and rate of growth. There are plenty of opinions in relation to the risk and the extent of damage that Japanese Knotweed can cause to buildings/structures and there are plenty of examples of people affected by it which has resulted in denial of mortgage applications, disputes with insurers and extensive costs in trying remove or control its growth. On the other side of the coin, recent research by AECOM challenges popular opinion and suggests ‘Japanese knotweed is no more of a threat to buildings than other plants’. The research of members of the Royal Institution of Chartered Surveyors (RICS) and the Property Care Association (PCA) who have interacted or dealt with Japanese Knotweed in one way or another found that ‘Only between 2% and 6% of respondents reported any co-occurrence of Japanese knotweed and structural damage to buildings. Our paper also concluded that where Japanese knotweed is associated with damage, it is likely that the plants will have exacerbated existing damage, rather than being the initial cause of the damage’. The results of the research are interesting and well worth a read; (Link).

Despite research by AECOM and others that suggest that Japanese is not the problem that the media would have us believe, we do live in a risk averse society. To those who buy/sell/rent and generally live in property I suspect that they will be un-swayed in their opinion and instead choose to panic at the mere mention of the words Japanese Knotweed in a similar way to which many people react to the words ‘Asbestos’ or ‘Subsidence’ etc. For those involved with property surveys and inspections it is essential to be able to identify Japanese Knotweed and to be able provide appropriate advice. This article is therefore written to provide some basic information about Japanese knotweed which can be used to supplement further reading.

Japanese Knotweed (Latin name - Fallopia japonica) was introduced into the UK as an ornamental plant by the Victorians. It originated from Asia in countries such as Northern China and Japan where it grew in harsh habitats on the slopes around volcanoes. When introduced into the UK the conditions were far more fertile than those in Asia allowing the plant to thrive. Japanese Knotweed is a Perennial Plant, meaning that it will grow for many seasons with the plant dying back in the winter and re-growing the following spring. Japanese Knotweed is capable of growing 10cm per day and it is highly invasive and capable of exposing weaknesses in buildings, foundations, concrete and tarmac. It has the capability of regenerating from minute rhizomes (a root or creeping stem), therefore there is a significant risk of spreading the plant from digging and other disturbance. Effective removal of Japanese Knotweed therefore requires a specialist, which as you would expect can be expensive.

As stated previously, Japanese Knotweed is a serious consideration for Lenders, Developers, Purchasers, Landowners, Planners and Surveyors. The impact of the discovery of Japanese Knotweed on land and buildings can prove to be significant. Land values can be reduced to take into account remediation works. It is therefore worth knowing how to identify Japanese Knotweed to firstly establish its presence and if identified how to deal with it. Devon County Council provided an excellent guide to the identification of Japanese Knotweed which is summarised below. The original link to the article is no longer active however the images and information below are still relevant:

How to identify Japanese Knotweed

  A Typical Japanese Knotweed Leaf

In the early spring red/purple shoots appear from the ground and grow rapidly forming canes. As the canes grow the leaves gradually open and turn green:

The plants are fully grown by early summer and mature canes are hollow with a distinctive purple speckle and form dense stands up to 3 metres high:

The plant flowers in late summer and these consist of clusters of spiky stems covered in tiny creamy-white flowers:

During the late autumn/winter the leaves fall and the canes die and turn brown. The canes remain standing throughout the winter and can often still be seen in new stands in the following spring and summer:

The rhizome is the underground part of the plant. It is knotty with a leathery dark brown bark and when fresh snaps like a carrot.  Under the bark it is orange or yellow.  Inside the rhizome is a dark orange/brown central core or sometimes it is hollow with an orange, yellow or creamy outer ring, although this is variable:

Japanese Knotweed and the Law

In 2016, the Environment Agency withdrew its Japanese Knotweed Code of Practice due to new government guidelines. This was replaced in March 2017, by the Invasive Non-Native Specialists Association (INNSA) new Code of Practice. Access to the new code is not as straightforward as the EA Code however you can request a copy from the following: (Link)

Below is a summary of the raft of legislation that relates to Japanese Knotweed which is taken from the Environment Agency’s Japanese Knotweed original Code of Practice.
  
Japanese Knotweed is classified as controlled waste and its disposal is strictly regulated. For example soil containing Japanese Knotweed roots/rhizomes is classified as contaminated waste and can only be taken to a licensed landfill site. Failure to dispose of Japanese Knotweed appropriately may lead to prosecution under section 34 of the Environmental Protection Act (EPA) 1990.  Also, although it is not a criminal offence to have Japanese Knotweed on your land, allowing it to grow onto neighbouring land may constitute a nuisance and as such may provide grounds for a civil action from those affected.

Other relevant legislation includes Section 14(2) of the Wildlife and Countryside Act 1981 states that '…if any person plants or otherwise causes to grow in the wild any plant which is included in Part II of Schedule 9, he shall be guilty of an offence'. Japanese knotweed is one of the plants listed in Schedule 9. Also, waste must be transferred to an authorised person, in other words a person who is either a registered carrier or exempted from registration by the Waste (England and Wales) Regulations 2011. A waste transfer note must be completed and signed giving a written description of the waste as per regulation 35 of the Waste Regulations. The Hazardous Waste Regulations 2005 contain provisions about the handling and movement of hazardous waste.

Japanese Knotweed continues to receive an increased amount of negative publicity which makes it increasingly important for those undertaking property surveys and inspections and giving property advice to be able to identify its presence and give appropriate and proportionate advice.  This article should serve as a good starting point and hopefully generate interest for further reading and research for built environment and related professions. 

Author: Gary O’Neill

Please feel free to share this article and other articles on this site with colleagues, friends and family who you think would be interested

Information/opinions posted on this site are the personal views of the author and should not be relied upon by any person or any third party without first seeking further professional advice. Also, please scroll down and read the copyright notice at the end of the blog.

Retaining Walls - Part 2 - Construction Types

A retaining wall is not randomly constructed, but should be selected and designed to suit individual ground and site conditions

In my last article, I provided some rather dramatic images of retaining wall failures and demonstrated that design and consequently selection of an appropriate construction method is crucial. This is because there are many considerations that will affect the type, size and construction of a retaining wall which in most cases require the advice of a professional such as a Civil/Structural Engineer. Finally I provided a graphic showing the many pressures/forces that occur behind a retaining wall which can significantly influence the stability of a retaining wall. You can read my previous article from the following (Link).

Once thorough ground/site investigations have been completed there are a number of different types of retaining wall that may be selected. Basically, a retaining wall is not randomly constructed, but should be selected and designed to suit individual ground and site conditions. There are various types of retaining solutions such as embedded retaining walls (e.g sheet piling) and reinforced soil walls, however, for the purposes of this article I plan to focus on gravity retaining walls (also known as mass retaining walls) which rely solely on their own weight to stand up and withstand all of the loads and pressures that they will be exposed to. There a wide selection of gravity retaining wall solutions including Masonry (unreinforced), Concrete, (sometimes with masonry facing), Gabions and Crib retaining walls:

Masonry (unreinforced) Retaining Walls

These types of retaining wall are some of the earliest know retaining structures and are formed from masonry which has been mortared together. The lateral forces from backfill are resisted by the weight of wall itself, and due to their robust nature, they develop little or no tension. Masonry retaining walls are not usually reinforced and are generally economical for heights of up to approximately three metres. 

You will note from the image below that a masonry retaining wall will need to incorporate weepholes positioned at approximately two metre centres in order to help relieve hydrostatic pressure. Construction of a masonry  retaining wall is generally labour intensive, often requiring deep excavation, temporary support, construction and backfilling. Clearly this can provide significant safety issues, can be time consuming and often prove to be very expensive.

Source: Chudley R. & Greeno R (2005), Building Construction Handbook. Elsevier Butterworth Heinemann

Masonry Retaining Wall - Source: http://www.archiexpo.com/

Concrete Retaining Walls

Concrete retaining walls provide some flexibility in design as they allow wet concrete poured in-situ with the use of temporary timber formwork, or pre-cast concrete units to be used. Mass concrete retaining walls will be designed using similar principles described above for masonry retaining walls and will have the same safety, labour and cost issues.  Concrete retaining walls are typically designed with a horizontal footing and a vertical stem wall.These types of retaining wall are referred to as Concrete Cantilever retaining walls. These walls allow a much thinner stem and therefore less construction material because the cross section of the wall is much thinner. Concrete Cantilever walls utilize the weight of the backfill to provide most of the resistance to sliding and overturning, and because of this are economical up to heights of ten metres, much higher than the possibilities for unreinforced masonry retaining walls. The image below details a section through a Concrete Cantilever wall where you will note how the backfill load applied to the horizontal footing will help to stabilise the wall.  The further image below also demonstrates how concrete pre-cast units can be used in addition to in-situ for Concrete Cantilever retaining walls.

Source: Chudley R. & Greeno R (2005), Building Construction Handbook. Elsevier Butterworth Heinemann

Pre-cast Concrete Retaining Wall - Source: http://groundtechgeo.com.au/

A slight variation to the Concrete Cantilever wall is the use/installation of Monolithic Counterforts. Counterfort  walls are cantilever walls strengthened with counterforts monolithic (formed as a single unit) with the back of the wall slab and base slab. The counterforts act as tension stiffeners and connect the wall slab and the base to reduce the bending and shearing stresses. Counterforts are used for high walls with heights greater than 8 to 12 m. They are also used for situations where high lateral pressures occur, e.g. where the backfill is heavily surcharged.

Pre-Cast Concrete Counterfort Retaining Wall - Source: http://designerpropertiestexas.com/

Gabions

Gabions are welded wire or rectangular wire mesh boxes, which are then filled with rock, and used for construction of erosion control structures and to stabilize steep slopes. Gabion design is based upon mass gravity design as detailed above and therefore relies on the mass and self weight of the Gabions to resist any imposed loads or forces. Stability is achieved through a combination of the self weight of the rocks and its interlocking and frictional strength. Gabions are often constructed with stepped face and because of the voids between the rocks are able to more easily absorb forces by retained earth and flowing water. Generally Gabions are more economical than other retaining structures as they require little maintenance, no skilled labour, minimum foundation preparation and no costly drainage systems.

Gabion Retaining Wall - Source: http://terraqua-es.co.uk/erosion-control/woven-gabions/

Crib Retaining Walls

Crib walls are made up of interlocking individual ‘boxes’ which are usually consist of timber or pre-cast concrete (see image below). Crushed stone or other coarse granular materials to create a free draining structure are used to fill the individual boxes. Crib walls are popular because they are versatile, strong, light weight, cost effective, fast and easy to install. Another advantage of crib walls, due to their form, is that they can be further enhanced within built geo-textile planters, allowing a faster ‘green up’ time than with other retaining solutions. The advantage of using concrete instead of timber is that concrete components provide for long term durability and will not rot or warp, however the component cost is likely to be higher.

Gabion Retaining Wall - Source: http://terraqua-es.co.uk/erosion-control/woven-gabions/

Concrete Crib Retaining Wall - Source: http://www.retainingsolutions.com.au/

I have provided an overview of typical types of retaining wall solutions above however there are numerous other engineering solutions than may be used depending upon ground conditions, loadings, location and the like.  In order to select an appropriate retaining wall, in most cases it will be necessary to seek professional advice, as the consequences of poor or inappropriate design can prove to be disastrous.

Author: Gary O’Neill

Please feel free to share this article and other articles on this site with colleagues, friends and family who you think would be interested

Information/opinions posted on this site are the personal views of the author and should not be relied upon by any person or any third party without first seeking further professional advice. Also, please scroll down and read the copyright notice at the end of the blog. 

Retaining Walls – Spectacular Collapses!

Sudden retaining wall collapse will be a result of the effect of forces/influences behind the wall and often inappropriate design. Not all damage to a retaining wall is as significant as the examples shown in this article and in fact much more subtle movement can occur over a long period of time before a retaining wall will eventually fail

Retaining walls are an important feature in many construction projects, often a necessity, as they can provide a solution where land is uneven or at a gradient. Very few construction sites are completely flat and where cut and fill site levelling is not possible it may be necessary to consider retaining walls as a way of maximising development space, or solving access issues within or around a construction site.

Wordnetweb define a retaining wall as ‘a wall that is built to resist lateral pressure (especially a wall built to prevent the advance of a mass of earth)’. There are many considerations that will affect the type, size and construction of a retaining wall which in most cases require the advice of a professional such as a Structural Engineer. This is because there are lots of pressures/forces occurring behind a retaining wall that can significantly influence the stability of a retaining wall. The photographs below show the dramatic impact of what can happen when retaining walls fail:

Source: wivinah-hefner.blogspot.co.uk

Source: James Lockyer Associates

Source: http://wqad.com/

Source: http://www.thestar.com.my/

Source: http://blogs.agu.org/

Source: http://www.nibiru-planetx.com/

Sudden retaining wall collapse will be a result of the effect of forces/influences behind the wall and often inappropriate design. Not all damage to a retaining wall is as significant as the examples above and in fact much more subtle movement can occur over a long period of time before a retaining wall will eventually fail. It is therefore worth understanding the function of a retaining wall as well as the forces/influences that require consideration during design.

I published an article a few weeks ago on basement construction where I considered the influence of ground water and particularly hydrostatic pressure on the basement structure (link). This is also a significant factor when designing retaining walls, which if not controlled can effectively push against the internal face of the wall and cause it to fail. So in addition to all of the forces that will potentially affect a retaining wall, other factors for consideration will include the type and nature of the soil, the types of wall construction to be used, materials to be used in the wall construction, ease and speed of construction, the provision of expansion joints, health & safety issues and obviously cost! As you can imagine there is an awful lot to think about and it is no surprise that professional advice may be needed. The diagram below shows a range of different forces that require consideration during the design of a retaining wall:

Source:Own

In my next article, I will discuss retaining wall construction and consider different types of retaining wall.


Author: Gary O’Neill

Please feel free to share this article and other articles on this site with colleagues, friends and family who you think would be interested

Information/opinions posted on this site are the personal views of the author and should not be relied upon by any person or any third party without first seeking further professional advice. Also, please scroll down and read the copyright notice at the end of the blog.