Types of Earthworks in Permaculture

Permaculture Designers Manual

 

CHAPTER 9 –

EARTH WORKING AND EARTH RESOURCES  IN PERMACULTURE

Section 9.6 – 

Types of Earthworks in Permaculture

 

 

1. BANKS

It is cheaper to construct a bank than to step or retain the earth, if the bank can be made stable.

Wherever a four-way bucket {Bobcat or drott) or power shovel can be used to dish the bank, they will remain stable at greater slopes than a straight cut, as we have frustrated soil slump by giving it the profile it would achieve had it slumped.

Banks or cuts of more than 4m high need careful stabilization.

Unlike dam walls, cuts for roads are not normally compacted, nor do they necessarily have much pressure on them. The greatest cause of slump is water flow through or over banks and the lie of strata in shaly ground.

All normal soils with some clay and stony banks are fairly stable, given that the angle of rest of these materials is preserved, the banks cut somewhat concave and drainage fitted.

In severe slump areas, re-routing of roads or expensive drainage and concrete retaining walls may be last resorts, but Figure 9.10 shows the essentials of water and slump control; only some may be needed but all can be used. 

 

2. BENCHING

A bench is a flat, near-contoured cut made in a slope. Very severe slopes of 30-40° can be bench-cut if a bulldozer can start on safe ground (typically, from a ridge of 20° or so).

Benches are used to make roads and house sites and are very useful in long-term forestry if a steep hill is benched every 100m or so (Figure 9.11).

 

Benches greatly aid access, planting and eventually harvest. The first fast-growing trees to plant are those on the lower (loose soil) side of the benches; these can be nut or fruit trees, for harvest or for seed crop.

Small-holders on very steep hillsides must work with the slopes or expend much energy on carrying water and fertilizer.

 

Orchard and mulch above poultry above garden is the easiest system. All the better if the ridge, or the hill above that system, is planted with mulch-producing trees such as; Casuarina and pine, oak and beech. Thus, mulch and bedding is thrown down-hill as green feed and seed for chickens and they kick-down to the lower fence where the gardener accepts the manures and shredded mulch for the essential (terraced) garden bed. (Figure 9.13)

 

Benches can be fertilized, ripped and sown with legumes or soft-leaved mulch plants and periodically cut or grazed off.

If cut, mulch can be raked to trees on either border. As benches on very steep slopes will rarely be travelled upon, they can slope slightly outwards to d rain. Benching should not necessarily continue across water runnels unless pipes, gabions or culverts are fitted in the watercourses; small watercourses can be bridged.

 

Benches are quickly made to a survey line pegged every 6m (20 feet) or so followed by a bulldozer with blade set to side-cast (angled).

A run with a raised blade will shave back or slightly step the uphill bank. In stable soils, benches can slope into the hill and so form swales to infiltrate water for trees below the bench.

Cross-walls can be made every 20-30m to prevent gutter flow. Such benches are not so much roads as tree shelves and are a blessing in steep country.

A quiet traverse with a donkey or cart serves to distribute plants and fertilizer (or mulch) and to gather in the crop. Narrow (l.5-3.0m) benches are quickly over-hung by tree crop, shaded, stable and accessible (Figure  9.14).

 

Wherever benches cross drainage channels, pipes or rock-lined swales must be made to carry water across, and these (and their maintenance) are the greatest long-term expense, so they need to be well made and durable.

Although the true finesse of earthworks is best entrusted to the experienced machinery driver, designers need to designate stages, spoil areas, topsoil stores, bank slopes and so on.

Also, they must remember that it is expensive for a bulldozer to travel far pushing a load before the blade to create banks (about 6-9 machine lengths is maximum) and that much faster and better work can be done cross-slope and downhill than upslope by any machine but a bucket digger.

Side-casting is a different matter and graders and bulldozers both can bench soils effectively for miles, as no dirt is carried but rather cast out to one side of the blade, which is ANGLED to the direction of travel to do so.

Thus, a great deal of time, fuel, money and timber can be saved if a site is carefully pegged out to suit the machines called in. Imagine yourself as a driver and make it as easy as possible to cut the shapes you want.

On anything but a simple job, two or more machines may be called for, as in preparing a house site and its sewage lines or in digging and carting a day deposit.

Bulldozers can loosen and pile up material but not load it, bench but not trench, or at least not smaller than the blade width.

 

3. TERRACING

Terraced lands, given a reserve of local green manures or composts and  adequate water, are potentially  very stable production systems.

 

Exceptions to this arise when we:

Attempt to terrace in unstable soils or sediments;

Risk hydraulic pressures on hill slopes from impounded or infiltrated water;

Create terraces that are unstable at the bund or wall face;

Extend terracing as annual crop over too large a proportion of the landscape and so lose leaf or tree nutrient input to crop; and

Make very large series of terraces in high rainfall areas, so that run-off is concentrated.

 

With a useful assessment of the above factors in mind, we can gain long-term production from short series of polycultural terraces (wet and dry crops), with stable bunds (either rock-walled or at a 1:3 slope).

Trees, on bunds and between, above and below terrace series, should form 40-60% of the total landscape plan and both soils and installed water inlets and outlets should permit safe and controllable irrigation.

 

The great benefits of terraces are these:

Very easy crop access on slopes;

Easily controlled and effective irrigation  procedures;

Minimal soil loss due to overland water flow. or to slope cultivation; and

A potential gain in silts or nutrients in irrigation or run-off waters and from leaf fall.

As with dams, terracing is most effective where slopes are least, as earth moved versus area of cropland developed becomes impractical or inefficient ns slopes steepen. At about 30° slope, but preferably at 10-18°, terracing becomes worthwhile.

Terrace construction always begins on the lowest terrace level, with the removal and stockpiling of topsoil over the whole area of the lower terraces, and proceeds uphill as each terrace is made, so that the topsoil of the next highest level is cleared on to the preceding lower terrace.

Stockpiled soil at the lowest terrace is finally carted or lifted to the last of the series uphill (Figure 9.15).

Every terrace system is ideally designed to allow perennial bund and terrace wall plants, specifically for wall stability and green manure crop.

 

As for the extent and series size in a terrace system, it is wise to limit both on the basis of:

Heavy rainfall (hence expected run-off) in tropics; and

Expected rainfall harvest in arid areas, where total terrace areas should not exceed one-twentieth of the catchment harvested and where perennial or adapted crop (never water-demanding crop) should form the selected species.

 

Thus, all terrace systems should aim to occupy no more than 30% of tropical or 5% of dry land areas and in the tropics tree crop should be developed to maintain fertility of the terrace areas.

Water catchment areas should be developed to do the same for arid areas, so that run-off brings leaf mulch to terraced slopes.

In humid cool or tropical areas, wet terraces (10-20% of all terrace areas) can be devoted to a fish-plant polyculture, giving yields of fish, shellfish, and water plant products.

Yields of protein from water cultures can exceed all land-based systems if managed at the same levels of husbandry and care.

Wherever people occupy very steep sites, (slopes of 20° or more), especially in areas of high rainfall, it is preferable to abandon broad terracing for a series or 4-6 narrow production terraces, each series carefully drained to spill excess rain down permanently vegetated slopes. (Figure 9.12)

 

Thus, by ridging the terrace tips, mulching paths and staggering path spills in short series. We can get the advantages of terrace on quite steep slopes without risking erosion and soil loss.

Needless to say, machines are inappropriate for such construction and steep slope terracing of this nature is always hand-cut.

 

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