Sunday, February 16, 2014

Trail Guide: Construction in rocky soils

Rocky or coarse soils are common on hills and mountains in British Columbia. Many trail construction guidebooks say that these soils should be avoided. Avoiding these soils may be impossible in some areas.

Rocky soils make for difficult construction and poor results if the tread or walking surface of the trail has projecting rocks or any rock larger than 4 inches within the top 4 inches of the walking surface. Foot traffic tends to move the finer soil from around a big rock so that it projects above the surface and becomes a tripping hazard.

Rocks in coarse soil can be used to advantage. In northern Europe, many fields have dry-stone walls or fences built from the stones removed from the fields to enable cultivation. Rock found in coarse soils can be used to advantage during trail construction by using dry-stone construction techniques.

The basic tools needed are a rock bar, a sledgehammer (with safety goggles), a shovel, mattock and wheelbarrow. A mason's hammer and chisel make a handy addition.
The rock bar is essential for prying and moving large rocks. It is a good idea to paint the rock bar in some bright color because rusty steel tends to blend into the forest. The wheelbarrow is used for end- hauling or moving soil and rock up and down the trail. The garden wheelbarrow show above can be used for trail work, but there are folding wheelbarrows with a canvas receptacle that are more suitable.

The following sketch illustrates the general technique for making use of rocks to build a trail in coarse soil:

This type of construction is known as half bench construction, because the outside edge of the trail is built up above the ground surface using dry-stone wall methods.

Face rock 1 is embedded and fill rocks 1 are filled in behind. Fill any spaces between the face rock and the fill rock with small stones.

Dry-stone works through the friction of overlapping rocks. Face rocks 2 should overlap face rocks 1. Overlapping face rock 2 on fill rocks 1 will also help to tie the structure.

The technique is really a hand sorting exercise and removal of the larger rocks for constructing the wall and base should leave gravel and finer soil material for the tread or walking surface.

If the trail is being constructed on an uphill gradient rocks can be moved from in front to behind and the finer soil is left to pull over the sub-grade or base. Thin nitrile coated work gloves are helpful to reduce wear and tear on the hands.

Dry-stone relies on the contact and friction between rock. While it is important to get relatively good contact and fitting between the face rocks seeking perfection will slow the work considerably. The outside rock face is usually sloped slightly inward for greater stability. The face rock wall need not have smooth surface. A rough surface looks more natural and little projections, nooks and crannies in the surface encourage mosses and re-vegetation. Filling any spaces between the face rock and the fill rock with small stones increases the stability of the face rock.

Half bench construction using dry-stone offers several advantages. It disturbs less soil and area than full bench construction. The trail is elevated above the surface of the ground and this along with the rock sub-grade helps to drain the trail. Trail surfaces should be out-sloped to help water drain off the trail.  On climbing sections or gradients additional provision needs to be made for cross draining or directing water that tends to flow down the trail, off the trail to prevent erosion. The best method is grade reversals or little dips built into the trail. In half bench construction the height of the retaining wall can be reduced at the dip to enable water to flow off. The second method are water-bars. Water-bars are diagonal structures running across the trail surface at 45 degrees. There are several ways that these can be constructed. Temporary water-bars can be made with a diagonal hump of surface material. Foot traffic wears down this type rather rapidly and they need to be rebuilt before each wet season. Water-bars can also be constructed out of rock. Single line rock water-bars are a line of rocks embedded into the trail at 45 degrees to form a barrier to water flow. These also tend to fill up in front of the rock and can become ineffective if not maintained. Double rock water-bars are a double line of rocks placed in the surface at 45 degrees with a drainage trench of approximately one foot in between. These tend to be self cleaning and if they do get plugged they can be cleaned with the heel of a hiking boot. Gradient reversals or double rock water-bars are the most sustainable solutions.

How frequently should cross drainage be installed in trails? Some guides show tables requiring more frequent cross drains as gradient increases. Generally you need more frequent cross drains as gradient increases. Erosion becomes a serious issue or trails with a gradient of 15% or more. Steep trails with gradients of 20% or 30 % should have cross drains every 10 meters. Assess the terrain, soil and moisture conditions on the slope above the trail. If there is a long slope above the trail with thin rocky soils, you can expect considerable overland flow during snow-melt or rain storms. It the trail is built or sands or gravels with little clay binding material, it will be more susceptible to erosion. A spring may need its own cross drain. Think of a spectrum of one cross drain per 100 meters on very gentle gradients and favorable conditions increasing to one cross drain per 10 meters on steep gradients and likelihood of considerable overland water flow.

Half bench construction should not be attempted in fine soil materials. You need to have rock to build up the outside edge of the trail. End hauling materials in a wheel barrow is a feature of half bench construction. Some spots will lack sufficient large rocks, while others will have insufficient fine materials to make a 4 inch deep tread surface with no 4 inch+ rocks and stones. If stretches of fine soils are found along the route, revert to full bench construction. Rather than waste fine materials, end-haul them down hill for surfacing. While the sketch shows two lifts of dry stone in the retaining wall, it can go from one lift to many lifts if the trail is being built around a rock out crop. Trail construction in rocky soils requires ingenuity to come up with a good solution for the circumstances. There may be no fine materials on some sections, and the flat sides of rocks can be laid to provide a walking surface.


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