Walkways – Bridges and Causeways

Walkways – Bridges and Causeways

Being able to cross the water, on a bridge or causeway, will allow you to see the water garden from another viewpoint. You can make the most of this by arranging the planting to provide focal points that are not visible from the edge. In some circumstances, a bridge may be a focal point itself, its role as a means of crossing the water being secondary. Occasionally, in Japanese-style water gardens for example, it may be the primary focal point.

 

CHOOSING A BRIDGE

Walkways - Bridges and Causeways There are many different kinds of bridge, but for the average garden there are three principle types that can be used as the basis for a wide variety of structures.

The simplest comprises a single flat slab or rock that will span a narrow stream or watercourse and may be equipped with a handrail. A traditional wooden bridge can be of almost any design imaginable, usually being constructed in one piece or from several sections bolted together. The third type is the stone clapper bridge, which is best suited to a water feature associated with a rock garden.

A clapper bridge is a very simple structure that normally consists of a pair of spans, two slabs being supported by a flat stone at each end and a strong central pier, which comprises two or three stones laid lengthways. Water passes under each span of the bridge, which should be higher than the surrounding ground. Traditional clapper bridges are often mounted by steps, but in the garden, a simple link from one bank to the other is sufficient. This in no way detracts from the attractiveness of a well built clapper bridge.

 

CONSTRUCTION PRINCIPLES

Irrespective of the method of construction used, a bridge requires firm foundations. These will not only ensure the safe passage of people across the bridge, but also provide stability. Even a modest bridge can be of considerable weight, and settlement may occur if it is built without sufficiently strong foundations.

To be safe, dig the footings for each pier to a depth of 60cm (24in). Piers can be precast, but for mostdomestic gardens, it is simpler to cast them in position. Indeed, for most bridge structures, excavating a hole and filling it with concrete is adequate. On clay soils, where shrinkage may occur, some form of reinforcement should be incorporated into the concrete.

If precast piers are being used, dig the footings so that they are 15cm (6in) larger all round than the piers. Stand each pier in its hole, then pour some concrete around it. Check that the pier is level in both directions and also in relationship to its neighbours. Finally, backfill the hole with more concrete to secure the pier.

If the bridge is a wooden structure, it is usually bolted to the piers. The simplest method of doing this is to use expansion bolts. Drill countersunk holes for the bolts and their nuts through the wooden bridge bearers, then lay these in place on the piers. Using the holes in the bearers as guides, drill holes in the piers with a masonry bit. Finally, insert expansion bolts and tighten their nuts with a box spanner or socket. This will cause the bolts to expand in the holes, providing a firm fixing.

Where a pier needs constructing in the pond itself, it should have a solid concrete footing. If a liner is being used, it is better to plan for such footings before construction begins so that a proper concrete base, of the correct dimensions, can be laid in the bottom of the excavation. The liner is then laid over the top of this, and the pier built on top of the liner. Laying a footing directly on top of a liner is hazardous, unless the liner is taken down into an additional shallow excavation below the footing, as it is likely to be unstable. Within a lined pond, the only footing that will be secure is a substantial raft of concrete, or a large block, into which the piers are embedded.

When the bridge is a simple slab crossing, or being constructed in the clapper style, the stones should be fixed securely to the piers with mortar. When doing this, check that the bridge is approximately level and, if necessary, make minor adjustments with mortar.

 

CAUSEWAYS

A causeway is generally an extension of the bridge idea, carrying the visitor across a larger body of water, not necessarily by the most direct route. One of the attractions of the causeway is the range of experiences it can provide by zigzagging from one part of the pool to another, often creating an opportunity for the visitor to walk dry shod among reeds and rushes planted in the pool.

The piers that are used to support a causeway consist of short lengths of pipe set into concrete slabs, the wooden uprights of the causeway being dropped into them. Horizontal bearers are bolted to these uprights, and a board walkway screwed to the hearers. Garden designers believe that the way in which the boards are arranged affects your behaviour when using a causeway. When they are placed lengthways, it is said that they encourage you to cross; if they are fastened crossways, you are more likely to linger. Irrespective of the manner in which the boards are fastened, once the causeway has been constructed, they should be covered with fine mesh wire netting to prevent the formation of a slippery surface due to the growth of algae.

To preserve the timber, it should be tanalized. If this is done properly, it will not only give the structure a life of many years, but also will not cause any pollution problems in the pool. Once the preservative has dried, it is safe in water, causing no oily scum or damage to ornamental fish or aquatic plants. Untreated timber has a very short lifespan.

 

MAKING A SIMPLE TIMBER BRIDGE

1. Accurately measure and mark the position of each pier. Excavate a hole slightly larger than necessary to allow for adjustment; 15cm (6in) larger all round is ideal.

2. Ensure that the pier is level in both directions and backfill with concrete. It is essential that it is absolutely stable. Allow the concrete to set before fixing the timber bearers.

3. Use expansion bolts to secure wooden bearers accurately to the concrete piers. The timber should be tanalized and you should allow at least two (rustproof) screws for each fixing.

 

16. March 2011 by admin
Categories: Gardening Ideas, Water Gardening/Water Features | Tags: , , | Comments Off on Walkways – Bridges and Causeways

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