Building with Stone
by Rob Adair, White Mountains NEMBA
Throughout New England we are blessed (or cursed, depending on your perspective) with lots of rock. Boulders, slabs, baby heads, or rock gardens - it's pretty rare when a ride doesn't encounter at least one of these elements. Stone is the most durable material you can build trail features with, and while it is time-consuming and laborious to move, once firmly in place it will be there until the next ice age comes along.
Stone can be used to add technical challenge to trails, such as steep rolls or rock drops; to armor areas of heavy wear like switchbacks and mud holes; and to provide an erosion-free trail surface in places such as stream crossings. Properly built rock features should require little if any maintenance.
Preparation and placement
Once you have your work site figured out, gather available rocks and put them in an out of the way location. It's a bit of a pain to move stones 3 or 4 times before you use them to make room for other rocks that you're rolling through (don't ask me how I know this). On the other hand you need to know what you have to work with, so at least taking inventory of what's on hand is necessary. Make sure everyone is warned before rolling rocks downhill to avoid creating human bowling pins.
Remove duff and organic material down to mineral soil so the stones will have a solid base to rest on. When working on an incline, start from the bottom and "key" a big base stone deeper into the ground to anchor it firmly in place and act as a foundation. There are many ways to orient the stones: as flat flagstone pavers, as boulder causeways, or in a "pitched" orientation (with the rock's long dimension vertical into the ground). The IMBA references given below address these various options really well. When placing rock armor, be sure to get the joints between stones tight enough that they don't become tire traps. Orient them so they are not parallel to the line of travel whenever possible. Filling the joints with rock and gravel is usually a good idea also. If gravel is not readily available on site, small rocks can be crushed using a sledgehammer, but be sure everybody is wearing safety glasses.
When building technical trail features, include bypass lines for those who don't want to ride them. NEMBA's philosophy in our Vietnam property is the main trail bypasses the technical feature, which is the optional line. It's always a good idea to keep fall zones clear of potentially injurious objects like stumps, sharp rocks, etc. This becomes even more important for higher features. A good rule of thumb is the higher the feature, the wider the fall zone.
Moving the stone
Rock is really heavy, typically 160 lbs per cubic foot or more. A 4' x 2' x 1' hunk of granite will weigh in excess of 1,200 pounds, so using mechanical advantage is essential for large stones. Probably the easiest way to move large rocks is by using two or more rock bars. Rock bars are used as simple levers - the force applied on one arm of the lever and the lifting load produced at the other end are proportional to the lengths of the lever arms. Position a small rock as a pivot point, or fulcrum, so that you have a 10:1 ratio of lever arm lengths and 100 lbs. of effort will lift 1,000 lbs. of stone. It really takes at least two people with bars working together to efficiently move large rocks.
In some instances a block & tackle or come-a-long is helpful for moving or lifting. A come-a-long suspended from a stout tree can quickly lift a big rock into place. If there are no trees where you need them, a tripod built from downed hardwood trees works quite well and is easy to put together. Be very careful not to overload the system and use heavy-duty rope and webbing specifically designed for rigging - a snapped cable or sling is like a giant weedwacker that could cause serious injury. To minimize risk of injury due to cable snap-back, drape a few coats over the strained line to absorb any suddenly released tension force (blankets or tarps also work well). We always use a rock bar to prod the stone along to avoid putting excessive loads on the come-a-long.
Large stones will not typically slide on the ground very well, so reduce friction by putting them on a "stone boat", such as an old car hood, or by using rollers and rails cut from deadfall. Snow cover can make transport easier when using a stone boat or toboggan.
Besides the previously mentioned items, it is very easy to pinch fingers or flop a rock onto your leg. Wear leather gloves and keep your hands and feet out of the way. Don't put yourself in a position where a slip of a bar or a sling failure will end up with part of you under the rock. This is easy to do and it can hurt. Steel-toed boots and eye protection are essential.
Finish work - Just like fine carpentry, attention to finish details separates the men from the boys. Minor things like chinking joints with rock and gravel and scattering organic materials and duff so they look natural make for a professional looking job. Restore gravel and rock mining holes by covering with leaves, moss and pine needles. These niceties don't take long to do but go a long way toward making the trail complement the surroundings.
IMBA is a fantastic resource; besides their excellent Trail Solutions book, which is highly recommended, most of the same great photos and descriptions can be found at:
As always, make sure your land manager is OK with your plans before investing your time and effort. Rock on!