Many of these structures are among the all too few surviving monuments to Jersey's medieval period. 'Fosses' and 'banques' are earth banks, sometimes dating back to the 14th century. They often have hedges or trees growing on top of them. 'Fosse' is the generic name for all banks, whereas a 'banque' was built to retain earth, so the land will be higher on one side than on the other.
Many of these walls, fosses and banques are in private ownership and the goal of this section is to help landowners recognise the problems of deterioration, and find an appropriate solution.
The problem areas are discussed in detail below in the following order:
- bulging and leaning
- dry stone fallout
- loose rock
- mortar joint erosion
- plant damage
- tree root bulges
- tree root overhangs
- vehicle scouring
Bulging and leaning
- the earth behind a wall pushes forward
- the wall is not strong enough to hold it back
- the bulge or lean appears and continues to develop
- the wall eventually collapses
Walls at risk
A bulge or lean can appear in any retaining wall. Once a wall starts to bulge or lean it can only get worse and could partially collapse without warning. Leaning walls present a more serious problem as the whole wall is likely to collapse. Both conditions are dangerous and require immediate attention. High and long walls pose a greater risk, especially if they hold back a steep bank or if there are additional walls further up the same slope. Large bulges in narrow lanes are particularly dangerous as they are sometimes struck by vehicles, leading to collapse and possible injury.
Bulges and leans are not always easy to predict. They are usually associated with bad drainage behind the wall or compaction of the land above it, coupled with inherently weak construction. Extra pressure is put on the wall by piling up more soil behind it or any other weight increase, such as using farm machinery too close to the top of a wall. Damaging vibration and compaction waterlogging can also be avoided by keeping heavy machinery away from walls. Drainage channels can be dug to redirect any water which is running down from land above.
Badly leaning walls will usually need complete rebuilding. Bulging walls often collapse at the point where the bulge occurs and leave the rest of the wall standing. They can sometimes be rebuilt if the rest of the wall is stable. Often though, more bulges occur along the length of the wall and the whole wall will need rebuilding. It is important to seek expert advice at an early stage. If the wall in question is higher than two metres or has a large bank on top, a reinforced wall may be needed and additional engineering advice must be sought.
Dry stone fallout
- the joints are dry or filled with clay
- single stones become loose and fall out of the wall
- more stones follow and the wall slowly collapses
Walls at Risk
All dry or clay-jointed walls are at risk. Retaining and free-standing walls were both built in this way. Most of the free-standing walls are found between field boundaries and nearly all the retaining walls are along roadsides. The purpose of the retaining wall is to support and protect the steep banks of road cuttings from erosion. This is especially important for roads cut into the sandy soils found in the west of the Island. Without these, the unstable banks would crumble away and much of the field area above would be lost. These walls are concentrated in St Ouen but can be found throughout the Island where they are very important for their:
Some of these dry walls provide the only habitat for rare species of lizard. If a wall is home to lizards contact the States Ecologist as this is of special interest.
Old dry-jointed walls were not built to withstand the vibration caused by modern traffic or the use of heavy farm machinery in the fields above. The condition of these original walls will tend to worsen as more stones become loose and fall out. Many walls can be saved from extensive rebuilding work by regular maintenance but they will all need some kind of reinforcement if they are to survive in the future.
There is a technique available to reinforce a wall without totally rebuilding it. This is only possible where the wall has not lost too many stones and is not bulging or leaning. The work involves the injection of lime mortar into the heart of the wall, which will hold the inner faces of the stones together and preserve the appearance of the original dry wall. The process is carried out by a stonemason and is much cheaper than rebuilding.
When too many stones have been lost from a wall it will need to be rebuilt. Traditional building methods are no longer suitable in most situations due to traffic damage. A stronger structure is now required with more large "through stones" and a lime mortar fill to secure the back of most stones. This construction would appear as a dry wall but perform as a more solid unit. Plenty of small areas in the upper part of the wall should be left as completely dry pockets for plants and wildlife which are only found in deep dry joints. Lime mortar should be used rather than cement for all repairs as it can take some movement in the wall without cracking and supports more plant life.
- rock faces often have many joints and cracks
- rock and soil slopes have many unstable rocks
- loose rocks break away from the face or the slope and fall into the road
Rock slopes at risk
All rock, or rock and soil, slopes that are currently exposed will suffer from weathering erosion. The composition of rock and soil types will govern the rate of this weathering but rock with weak joints and cracks will suffer more from erosion and may shed large stones. Slopes of loose rock held together by soil often erode too quickly for plants to establish. Loose rock is a hazard to the public and requires immediate attention especially where high, steep slopes are involved. Not all rock slopes have always been exposed. Many existing fosses and banques have an underlying structure of rock, others consist of a combination of loose rock and soil. If the surface of these fosses and banques is lost during an earth slide, further erosion can expose the rock face or rock and soil slope.
Prevention and repairs
Erosion of rock can only be controlled by engineering. The most economical approach is to allow the erosion process to continue but to stop loose stones from free-falling to a point where they will cause damage. This can be achieved by securing a mesh to the slope which lets stones fall but contains them against the face. An alternative for shallower slopes is to dig a ditch at the base of the slope and build up a mound alongside. This will prevent falling stones from rolling into the road but is only possible where there is room. Most cases will require the advice of a qualified engineer especially when dealing with high and steep faces.
Mortar joint erosion
- mortar joints are attacked by wind and rain
- the joints start to crumble
- loose mortar is washed away and the erosion becomes deeper
- stones loosen and fall out
Walls at risk
All old walls will show some degree of joint erosion. This becomes visible after a wall has been exposed to the elements for many years. The first joints to suffer are often those in an exposed position at the top of the wall. Erosion of this type is initially a maintenance problem, but if stones become loose the wall will be weakened and a more serious rebuilding job will be needed. High walls and retaining walls should be checked regularly, as they can become dangerous.
Joint erosion happens naturally and should he tackled as a continuous maintenance task. Ideally, the joints of a wall should be cleaned out and refilled before any stones become loose. The ideal time to re-point a wall is when the joints are crumbly to a depth of approximately 3.75cm (1½ inches) into the wall. At this stage the loose mortar is easily removed and the joints can be refilled with a good depth of new mortar. This depth is needed for the repair to be effective. If this work is carried out properly, further problems are unlikely to occur.
Where a wall has been neglected more extensive work will be necessary. Without maintenance, erosion becomes so deep that stones begin to fall out of the wall and eventually the wall will need complete rebuilding. The smaller stones in a wall are first to become loose and fall out. These stones can be cleaned up and repositioned with new mortar. If the holes are left open the stones above are likely to become loose and water will penetrate into the heart of the wall, causing more damage. Coping stones, bricks or cement capping along the top of a wall prevent water from seeping down into the core of the wall. If these are missing or loose they should be replaced or repointed quickly. Groups of loose stones can be carefully repositioned with new mortar but if most of the stones are loose, the whole wall may need rebuilding. The correct type of mortar should be used for repair work. This is a lime mortar if the wall is old, rather than cement mortar and expert advice should be sought before work is undertaken.
- the soil in a banque becomes waterlogged
- the surface layer of mud slides down the road
Fosses and banques at risk
Mudslides are a common problem in the Island. These generally occur on large fosses or banques which have been cut too steeply for a clay or silt soil type. These fosses and banques are usually devoid of trees or large shrubs and they tend to have farmland sloping down towards them. They are concentrated in certain areas of the Island which are identified by previous mudslides. Problems occur following heavy rain when the structure has soaked up so much water a large volume of soil slides down its face, often blocking the road.
Waterlogging can be a result of direct rainfall but more often it arises from water draining down from farmland above the fosse or banque. This type of waterlogging can be prevented by digging a cut-off drain above to prevent any sudden build up of water. Planting the top and the face with deep rooted woody shrubs will also help to prevent a slide by binding the soil together and providing shelter from direct rainfall. Road visibility lines should be preserved when introducing new plants. Farming the land directly above a steep fosse or banque compresses the soil and can cause drainage problems. Ploughing furrows down fields towards them should be avoided wherever possible as water will run down these furrows and saturate them. This also causes erosion of the topsoil, which is often washed into the road as mud.
When a slide occurs any bare earth remaining needs protection or it will be washed away by further rain. The first step is to immediately cover the slope with a sheet which will protect it until it can be seeded and planted up in the spring. Turf can also be an effective treatment.
There are sometimes problems in establishing plants on the exposed slope. Difficult slopes to plant can be covered with a geotextile mat before seeding as this cuts down on further erosion and gives the seeds more of a chance. Plant types and seed mixes as well as turf if needed should be chosen by an ecologist or engineer as only certain species are suitable and these will vary with site conditions.
- plants grow in the wall and open up the joints
- loose stones begin to fall out of the wall
- more plants seed in the damaged areas
- the wall is slowly broken up
Walls at risk
The majority of old walls show some kind of plant growth. This can be an attractive feature and in most cases does not cause serious damage. Only 2 types of plant cause significant damage: buddleia and valerian. Buddleia is very persistent and can grow into a large woody shrub on old or new walls. Valerian grows as a smaller, herbaceous plant in the weaker joints of old walls, usually near the top. These plants readily seed in any joint which is not perfect and the powerful roots will quickly force it open. The damaged joints will then allow water into the wall which will cause more damage. Soon stones will be pushed out and the wall will be slowly broken up by the plants.
The only successful way to prevent buddleia and valerian from invading is by routine maintenance. This involves removing young plants before they cause too much damage. A large number of valerian plants growing on a wall suggests that the mortar joints may need attention. Such action is only essential when many of the joints are seriously eroded. If other walls nearby also have these plants, the seeds will keep blowing across and re-seeding. Only repointing will prevent this. Plants such as ferns can often be left until a full re-pointing job is essential. Weed killers should never be used as they do not last and will affect other harmless plant and animal life.
Where buddleia and valerian have been left to grow, the roots will have pushed deep into the joints and may have loosened stones. The best course of action is to dig the plants out and refill any cavities with mortar. Loose stones can be secured with the correct type of mortar. If the repair is not carried out, water will enter the wall, stones may fall out and more plants will easily colonise the damaged areas.
- the ground beneath a wall is not stable
- the wall foundations move
- the wall sinks in places and cracks open up
Walls at risk
Subsidence can affect any wall but the older the wall is, the more likely it is to fail. Old walls were often built without proper foundations and by now many are weakened by erosion. New walls built on unstable ground can also suffer. The problem is not common but in some cases it can be serious and require prompt attention. The fault can affect the wall in various ways depending on its construction and the type of movement. Usually the wall sinks at one point, causing the top to become uneven. In old walls with weak joints all the joints tend to distort at the point where the wall has sunk. In new walls vertical cracks appear and whole sections become uneven. The higher and longer the wall, the more serious the problem.
Subsidence is not easily predicted as the fault starts underground with movement of the subsoil and foundations. Altering the drainage around a wall can affect the strength of the soil around its foundations and cause failure. If the mortar joints are badly eroded this can also weaken a wall. Foundations of new walls must always account for weak spots in the soil and the foundation strip must be strong enough to bridge these spots if the soil sinks.
If the wall is still stable despite small cracks then serious work is not essential. In such cases filling the cracks with mortar may often be enough. This must be checked as the wall may continue to sink and require more serious attention. If the wall's leaning into the road and cannot be supported it will need rebuilding before it becomes a danger. If the wall is higher than 2 metres, engineering advice should be sought. If it is leaning away or unstable it can sometimes be saved by constructing supports to the inside face but it is important to seek expert advice in such cases.
Tree root bulges
- a tree is growing too close to a wall
- the roots push up against the wall as they grow
- a bulge and crack appear next to the tree
- the bulge grows and eventually collapses
Walls at risk
The roots of a large tree are capable of overpowering any normal wall. If a tree is of a large species and is growing less than 3 metres from a wall without a root barrier, the wall could be at risk. Mortar jointed walls tend to lean and vertical cracks appear. Dry and weak jointed walls bulge and stones are pushed out. In both cases immediate attention can prevent a serious problem developing.
A young tree growing close to a wall might not present a problem until nearing maturity. Many species of tree can be prevented from reaching full size by cutting back the crown, coppicing or pollarding. This will also prevent the root system from reaching full size and reduce the chance of damage. If the problem is more advanced, and roots of a larger tree are already pushing the wall, the soil behind the wall can be dug out and in certain cases the roots can be cut back, allowing a plastic sheet root barrier to be introduced. This will prevent further root growth towards the wall. The cavity between the barrier and the wall should be filled with a free-draining aggregate such as gravel. Reducing the size of the tree crown or coppicing can still be a worthwhile measure as it will reduce the potential damaging effects of wind-rock. Crown reduction, coppicing and root cutting should be carried out by a tree surgeon.
If a small crack or minor bulge has appeared, this can sometimes be repaired and reinforced before introducing the root barrier. Badly leaning and bulging sections of wall will require complete rebuilding. In cases where the roots cannot be cut without endangering the tree, the wall can sometimes be rebuilt leaving a gap for the roots to continue growing. If this is not possible the tree may have to be removed. Expert advice should be sought in these circumstances.
Tree root overhangs
- a tree is growing on top of a steep banque
- the soil around the roots is loosened and crumbles away
- an overhang forms under the tree exposing the root
- the overhang slowly grows, endangering the tree
Fosses and banques at risk
This problem only occurs in steep fosses and banques with trees on top especially when the trees are close together. There is no typical case, as each tree has a different root system and each fosse and banque has a different structure. But, once started most root overhangs grow and will eventually undermine a tree causing it to becoming unstable. The process is caused by tree roots expanding and breaking up the hard soil around them. This is exacerbated by the rocking of a tree in high winds and, if the roots are close to the surface of a steep bank, the face will begin to crumble away. Damage by traffic or cutting back at the base of the fosse or banque will encourage an overhang to develop. An overhang often exposes clay and stones. This is a very difficult surface for new plants to root in, especially in the dry shade under the overhang. With no plants to bind the surface together it usually continues to crumble away. In some cases large stones are pushed out causing a hazard to traffic. These cases should receive immediate attention.
The best way to prevent this problem is to avoid planting large trees directly on top of a very steep bank and to protect those which are already planted from scouring damage caused by vehicles. There are many trees already planted on top of steep fosses and banques but not all will develop an overhang so it is best to wait for the first signs of this developing before taking any action. Routine maintenance is required to identify a problem and take the necessary action.
An overhang should be dealt with as soon as it appears as the repair is much simpler at this stage. The surest way to stop it from growing and save the tree is to bridge the cavity with a structure such as a retaining wall. A simpler approach may involve coppicing or pollarding the trees to allow light and water to penetrate. This will encourage plant growth to stabilise the overhang and reduce the potential danger. Further advice will be required in many situations as they can be unique and complex. If the tree is dead, a different problem can arise as the roots which form part of the structure will rot and can cause the whole thing to become unstable.
- tyres and bumpers scour the base of a banque
- large vehicles damage the middles of the banque
- the banque is slowly undermined and becomes unstable
Fosses and banques at risk
All fosses and banques are damaged by vehicles. This only becomes a problem when the damage recurs in the same place. Narrow lanes with tight bends and inadequate passing points suffer the most, especially if there is a lot of traffic. If a narrow road has adequate passing points, any damage is usually confined to those points. Constant scouring does not give plants time to regrow and the fosse or banque is slowly eaten away. Scouring can cause a more serious problem where large trees are growing directly on top. These trees are more easily undermined after scouring has damaged the face of the fosse or banque and undermining exposes overhanging roots which might cause a tree to become unstable.
Problem areas are indicated by damage that has already occurred. It is important to take action in cases where the scouring is being repeated and the condition of is worsening. Scouring of long stretches of a fosse or banque indicates a lack of passing points. The first action should be to increase the number of passing places, so only these passing points will need serious protection from scouring instead of the whole length of the road. Scouring on tight bends is an isolated problem and will only need treating on the bend.
Reducing vehicle scour on long stretches of road involves laying a kerb at the base of the fosse or banque. This kerb should be laid in the least conspicuous way allowing plants to spill down its face. This will reduce the unsightly appearance of hard edging in country lanes. One method involves placing strips of turf on top of the exposed kerb. These will root into the bank behind and encourage plant growth. The colour and texture of the kerb units should be in keeping with a countryside context and plain grey concrete should not be used. At tight passing points and on corners, extra protection may be needed in the form of large smooth rounded stones, set at intervals, to prevent kerb-mounting. These stones are set into the bank leaving only a rounded face visible.