Dramatic landforms in the Rolawn: Why? conceptual show garden by Tony Smith at RHS Hampton Court 2016. Photo: Annie Guilfoyle
Designer Annie Guilfoyle looks into what you need to know to create landforms and earthworks
Landforms were originally created to be places of meeting and worship, burial grounds or to create a lookout in order to spot rampaging rebels. Long barrows, round barrows and henges are all early evidence of man-made landforms. Possibly the finest example of this is the magnificent Silbury Hill in Wiltshire, the largest man-made mound in Europe.
Man continued to mould and mound the earth in later history to express wealth and status rather than to conceal or cremate. Dartington Hall in Devon boasts the most splendid tiltyard: a series of steep, grass steps created in the 14th century to view jousting competitions. Possibly one of the most intriguing Elizabethan landforms was Lyveden New Bield, created by the recusant Thomas Tresham; he concealed his Catholic beliefs by shaping the land into representational forms, containing secret biblical references.
At Claremont in Surrey, Charles Bridgemen’s 18th-century ampitheatre looks as innovative today as it did back then. And I cannot write about earth moving without a nod to the much-slated Capability Brown and his formulaic landforms that swept the 18th-century countryside, sometimes eradicating the formal gardens that went before. Throughout history, landforms have been created not only for practical or symbolic reasons, but also for aesthetics. Nothing much has really changed, with celebrated designers such asKim Wilkie and Charles Jencks creating masterpieces of sculpted earth up and down the country.
Earthwork planning & design
Before designing, it’s always best to check if planning permission is required, according to Martin Goodall LRTPI, consultant solicitor and planning specialist. He says planning permission for landforms or earthworks in a domestic garden falls into a ‘grey area’ that does not seem to require any planning consent, unless the earthworks require extensive building and/or engineering. When planning the work, it is also essential to consider how this style of landform will be maintained. Can it be cut by hand or machine? Will it be firm enough to walk on? Will it support the weight of a mower?
If designing a landform with sloping sides, it is important that you identify your soil type in order to establish the ‘angle of repose’. This is the maximum angle that your soil will remain stable, before it fails or subsides. Each soil type has a maximum angle of repose – however, this can be influenced by other factors such as moisture content, land stability or vegetation.
Structural engineer Jane Wernick CBE FREng Hon FRIBA, director of engineersHRW, has some general advice for garden designers planning on earth moving. “Your initial considerations are what the landform is to be made of and what its foundation will be,” she explains. “The landform will need to be stable, durable and safe to access.”
Wernick recommends checking if the ground on which the landform is to be built is flat or sloping. If the latter, it will be necessary to consider what would stop it from sliding. “Because the landforms are heavy they add what we call ‘surcharge’ to the ground they sit on. This is significant if the landform is on top of some soil that is being supported by a retaining wall, as the soil will then put a larger horizontal force onto the wall. So an engineer will need to check that the wall can take that extra load.”
Drainage is also important. Is the landform going to be permeable to water, and where will the surface water go? Lastly, she says durability needs to be carefully considered. “It is generally a good idea to seek the advice of a structural or civil engineer,” Wernick says. “They will be able to advise on whether you need to employ them on any landform project that you are about to undertake.”
Building a landform
Award-winning landscape contractor Steve Swatton has a checklist of major considerations for designers embarking on a landform scheme. The first thing to check is if there is access for machinery. “Unless the feature is relatively small, diggers and dumpers are the tools for the job. The amount of earth or material to be moved is usually balanced with the size of machine. Access and working space set the practical limitations and therefore size of plant you can use effectively.”
Next, find out about soil stability. “A sandy soil can erode quickly if used on steep banks and heavy clay is often hard to grade depending on ground/ weather conditions. Landscape matting can reduce erosion and give a stable base for planting.” Slope angle is also very important. “If it’s greater than 45 degrees, it ultimately becomes more difficult to stablise the soil. As the angle increases you may have to consider additional methods of soil stabilisation,” Swatton explains. As for soil depth, “when using rubble or spoil as a sub-base make sure that it is compacted in layers, in the same way as the soil. Allow for adequate soil depth for growing grass, plants or trees.”
It is also important to make sure you are communicating your scheme clearly. “Sections are the best form of understanding and calculating existing ground. A model is also useful to communicate forms to your contractor, as spirals and domes are difficult to show in plan without studying lots of existing and proposed levels. Working out material quantity build up or use can be complicated in a 3D design, so if you’re inexperienced, make it clear so the contractor is aware your calculations are only approximate and they should carry out their own to verify.”