How to avoid dam construction failures

This Landcare Note gives information on how to minimise the risk of either design failure or operational failure of farm dams not located on waterways.

Dams which are to be constructed on waterways must be referred to your local water authority

How to prevent failures

Usually, the causes of the failure can be easily found. The owner may have been over-confident in undertaking planning, and in doing so, failed to include soil testing in the investigatory program.

The other common cause of failure is in the use of inexperienced contractors. Nothing can take the place of a reliable and reputable contractor, and by using experienced machine operators you can reduce the risks of failure dramatically. Their previous jobs can be checked and a good outcome is considered the best recommendation

Soil assessment and testing

It cannot be stressed too heavily that the soil on the actual site should be examined before detailed planning starts.

Many types of soil and subsoil do not “hold” water and it is necessary to confirm the existence of impervious clay to seal the excavation and to form the core of the bank. It is also highly desirable to determine the susceptibility of the soil to tunnel out and cause bank failure. Landcare Note LC0069: Soil materials for dam construction provides further information. Many potential failures can be prevented if the contractor is fully aware of any soil limitations on the site.

A further requirement is to investigate the materials along the centreline of the bank to ensure that the core trench reaches impervious material

Equipment

A bulldozer or a scraper is mainly used when constructing a farm dam, preferably in conjunction with a sheepsfoot roller on larger jobs. Scrapers generally give better bank compaction, but bulldozers are more manoeuvreable.

Strip topsoil

Completely clear and strip at least 150mm of top soil from the excavation and the bank areas. Stockpile it in a convenient place, for later use.

Designing bank and excavation

Design the bank and excavation so that the upstream edge of the pit will be covered when the dam is full. This will help to prevent erosion of the edge of the pit.

Figure 1: Plan view of dam

Core trench

As a preliminary to the construction of the bank, a core trench at least 2.4 metres wide and at least 0.6 metres deep should be cut out along the full length of its centre line. It is essential to site the core trench in a foundation of impervious clay. In many cases the core will need to be deeper than 0.6 metres. It is essential that all soft, weak, coarse and organic materials are removed. The whole remaining foundation area of the bank site should be surfaced ripped. The core trench should then be backfilled and compacted with the most impervious material available, to provide a seepage seal.

Building the bank

Probably the most important requirement of bank construction is to have effective compaction of soil material. The requirement for compaction cannot be overemphasized. Construction should be undertaken when the soil is moist. Autumn or early winter are usually the best times. Construction is often difficult in mid-winter because sites are too wet. It is not advisable to attempt construction in mid-summer, when the soil is too dry and difficult to compact. Even though the soil moisture content may be ideal in late spring, problems can occur when a newly built bank dries out over summer, and failure can result.

Figure 2: Cross section through dam

Start to build up the embankment by placing earth in regular and even layers no more than l00mm thick, with a scraper or bulldozer - 150mm layers can be used if compacted with a sheepsfoot roller.

If only a limited quantity of good quality clay is available, the best of it should be used to progressively build up the clay core. The least suitable materials should be kept for the downstream section of the bank. Do not incorporate any large rocks, logs or other debris into the bank.

Remember - to achieve adequate compaction, the soil must be moist, but not so excessively as to be muddy or slushy.

In many cases a water cart should be used to moisten soil as it is spread on the bank.

The ideal way to compact the embankment is to use a sheepsfoot roller. This will minimise the risk of future failure. However, a bank up to 3 metres (10 feet) high may be satisfactorily compacted with the tracks of a loader scraper provided the soil is: moist; not dispersive; and it is built up in thin layers.

Freeboard

The correct amount of freeboard will vary with the size of the dam, area of catchment and likely wave action.

Minimum freeboard should not be less than 1 metre. Even with good compaction some vertical settling of the bank should be expected. Make a 10% allowance for settlement.

Batters

For banks up to 3 metres (10 feet) high, the standard recommended slope of batters is 3:1 on the upstream side of the bank and 2:1 on the downstream side. Before building batters steeper than this, it is important to ensure that it is safe to do so.

Spillway

A correctly designed spillway is essential. Many dams fail due to faulty design or construction of the spillway. It must be large enough to handle flood flows without water overtopping the bank. Nor should the flows cause erosion of the spillway or disposal area below the dam. If the spillway has a newly formed earthen surface to take overflows from the dam, a heavy grass surface cover should be sown and established as quickly as possible. Do not allow construction equipment or vehicles to travel on the spillway discharge area, the vegetative cover is too important.

A rule of thumb for estimating the width of a spillway: it should equal (in metres) the square root of the catchment area (in hectares). For example, a catchment area of 9 hectares would require a spillway 3 metres wide.

If trickle flows of water are likely to be produced from the catchment during winter and spring, installation of a trickle flow pipe should be considered. See Landcare Note LC0090: Trickle flow pipes for farm dams.

Topsoil

When construction is completed, the stockpiled topsoil should be spread over the bank. Suitable grass species should then be sown to stabilise the bank and prevent it eroding. Trees should not be used on banks because their larger root system can disturb the compacted mass.

Another important feature of placing topsoil back over the bank is that, when grassed, it helps prevent the clay bank from drying out and cracking.

Further Information

• Widely varying circumstances apply on different properties and thus the information contained in this note should be used for general guidance only. It is advisable to seek expert assistance with detailed planning when a decision to construct is made.
• Your local water authority (ie either Southern Rural Water, Goulburn-Murray Water, Murray Sunrasia Water or Wimmera Mallee Water) is able to supply additional information.
• Southern Rural Water produce a set of Farm Dam Notes.
• The following 1993 publication is available from the NRE Information Centre Your dam: an asset or a liability.