Sunday, July 28, 2013

Footbridge near Rt 7 (Sheffield)

Day 1 (July 27th)

One of 3 truckloads headed to the site.

Before today, we had made several trips to the job site to ferry materials from the Tool Shed on Mt Greylock to a location near the job site.  For the largest pieces, we used the power wheelbarrow to carry many at a time.
A full load for the wheelbarrow

Despite a hot, wet summer we had nice weather for the first day of construction.  Our first tasks were to set the sills that the bridge would be resting on and to assemble the various pieces of lumber needed to create the three large timbers necessary to span the banks of the stream.  How big are they you ask?  They need to span a 28ft wide ravine with enough left over to rest securely on the banks--36ft in total.  In addition, they need to be strong enough to carry a reasonable number of hikers.  Using data from the US Forest Service, we found beams 4" wide and 14" deep would be required.

Early in the planning stages (over a year ago) we looked into the possibility of purchasing timbers of this size.  While they could be ordered, the logistics of transporting something this long and heavy (600lbs) to the job site were more than we could support.  So the plan is to laminate plywood and 12ft long 2x14's together at the site set them onto the sills with a bit of rigging.

Today we got the sills started by using a water level (a clear plastic tube filled with water) to make sure the sills on both sides were at the same elevation, we did not want the bridge to be sloped. 
Dave and Jesse use the water level to set the sill height
While the south end of the bridge could be set on sills dug directly into the bank, the north side  required about 10" of excavation to provide a level bearing surface.
North sill is set deeper into the ground

While one team installed the sills, the other began on the beams.  Because we had no workbench available, these would be assembled on the ground next to the trail.  Fortunately,  the AT crosses into a farm field at this location and there was relatively flat open ground to work on.  We laid out the first layer of 2x14's and blocked them up so they sat flat and straight along a taught string we set for that purpose. 
Don and Christine use a string to lay out the beam pieces.
After spreading a liberal amount of construction adhesive, 14" wide pieces of 3/4" plywood were screwed to the 2x14's.  Then another application of adhesive, and the outer later of 2x14's were screwed into place. 
Adhesive is applied
The lengths of all the pieces were arranged so that butt joints were always overlapped by two other pieces.  Finally, at each joint, bolts were installed to reinforce these locations. 
All joints reinforced with bolts.
While theoretically, this should work, we won't be completely certain until everything is in place.  We'll know more after the next project day.

Thursday, July 4, 2013

Rock Steps at Kay Wood Shelter--July 2nd, and 9th 2013

On the blue-blazed side trail to Kay Wood Shelter, we have a short section that due to the available terrain, goes directly down hill for about 30 feet before leveling out at the shelter area.

Because it is straight down the "fall line"--directly down hill, the route water takes when flowing over the ground--and due to use (it's the lowest spot along the slope) this short segment collects a fair amount of water and allows it to run quickly.  The combination of volume and speed of water is a guarantee of erosion, creating a nice little gully, that of course hikers want to avoid, which means they walk along side of it, widening the trail and creating another site for more erosion.

The best way to address this issue, is not to let it happen in the first place.  Trails should cross slopes at an angle climbing/descending more gently.  Ideally, 12% is the maximum desired grade for foot trails to prevent water from getting up enough speed to start washing material off the trail.  For some perspective, an interstate highway specification is for no more than a 7% grade.  Another way to think of grade is the amount of "up" you need to cover in a given distance of horizontal travel.  12% is about 1" of up in 8" of travel.  A 20 foot section of trail at 12% would have a rise of about 28 inches.  This is noticeably steep for a hiker, but hikers and trail managers in New England consider this kind of slope a luxury.  Through tradition, or because of limited available land or geology, or just Yankee stubbornness, a 12% trail is pretty rare around here.

So, you have a steep trail and it is subject to erosion, what can you do?  First off is to find ways to get the water off of the trail at frequent intervals so it doesn't build up speed.  Waterbars are the most often used means--channels cut accross the trail, strengthened by rock or timbers--that sluice the water sideways off the treadway and into the woods.  Unless they get clogged with leaves, dirt, or rocks.  Or the rocks that strengthen them are not large enough or set poorly.  Or hikers walk around the ends pounding down the dirt and providing a new route for water to continue down the trail.  As a Maintainer, you can address the first of these shortcomings.  As a Trail Builder, you can address the second.  The third?  Well, that's up to Hikers.  There are vastly more of them than there are of the other two groups.  Just suck it up and go straight over the water bars--don't walk around them...

So, I've got that off my chest, let's talk about rock steps.  When you have no choice but to go stratight down (or up) and the slope is steep, steps are the only answer--and one of last resort.  Steps take a long time to build.  If you want them to last, build from rock--it never rots.  But, you need to have a source of rock that is suitably sized (big), shaped (has at least one flat side, preferrably two or more) and close to the work site.  You need to be able to set this rock deep into the soil and against other rocks so they will never move again.  This takes time and some skills that are best imparted by working with an experienced crew leader.  Surprisingly, you don't need to be a body builder to work with rock--even big ones.  Learning how to use simple tools and (occasionally, but rarely) a modest amount of basic rigging, regular people can move really heavy things pretty much right where they are needed--very slowly.  There is a level of craftwork that needs to be done, but you learn it by doing it.  The work is hard but honest, in the sense that you are dealing with elemental materials and forces.  The satisfaction is high, there's something to look at after a hard day (or days) in the woods.  This thing you have helped to create has an immediate practical value--and if built right, you can show it to your grandchildren.

OK, on with the project.  We had 8 of us at the trail head on Tuesday morning.  A large crew by our standards--and most welcome.  A short hike on a humid morning brought us to the shelter and project site.

We gathered around the blown out section and discussed our options--either by installing new steps directly into the existing gully, or creating the steps on an undisturbed slope immediately adjacent.  Keeping the trail's footprint small by staying in the gully had some appeal, but the exposed bedrock and likelyhood of continuing water problems favored starting fresh, and we were also able to take advantage of a slightly more favorable slope.
Mining crew at work while Don clears a downed trees

For this project, the rock "mining" was pretty simple.  Just up hill was an even steeper slope/cliff, that had produced what looked like a lot of reasonably sized and shaped pieces that would be easy to transport to their new home.

Some rocks are easier to get to than others.
 A good rule of thumb, "if it isn't too heavy to pick up, it's too small".  We wanted the largest rocks that we could find that were still possible to move.  Of course, to be truely useful, the best rocks have a couple of nice flat surfaces.  Soon, what seemed at first to be an abundance of material, was quickly winnowed down into a few really nice, workable rocks, and a bunch of lesser candidates--which we were going to use anyway.

Once pried out of the hillside where they had been sitting since the glacier left,  the next step is to get the rocks down to where they would be set to create a step.  Given the small work site, we did not want to just send boulders down the hill willy-nilly.  Being a bowling pin is not a good thing.

Anita and Jim coax a rock down the hill
Using a combination of rock bars and pick-mattock, rocks were coaxed/slid/flipped down the hill close to their ultimate location.

To create the most stable stair, each rock should rest on its neighbors below (and in some cases to the sides).   Putting the front edge of the next step onto the back edge of the step below works with gravity to keep everything together.  Simply excavating holes in the soil and dropping rocks in usually results in the rocks creeping downwards or being squeezed out of their position by frost and/or water flow.

Once the bottom rock is set, the slope behind it is excavated to receive the back of the next step.  The art of this process is gauging the size and profile of the excavation to match the incoming rock, and finding the best way to mate the surfaces where the rocks contact each other.
Step number two is landed by Silvia, Dave and Jim.
 More often than not, given our relative inexperience, we had to try several different orientations of the rock to get the best fit, while maintaining a reasonable height of step and level walking surface. 
Don, Jim and Denis coax step number three into place.
If the walking surface of the stair are rounded, slanted or look slippery, hikers will often bypass the steps, continuing the erosion process and widening the trail even further.
Day 1 has 8 steps in place.  4-6 more to go.
To help keep hikers on "the straight and narrow", the sides of the stairs are lined with vertical slabs and the earth on either side of the steps will be covered with "scree" rocks--loosely piled rocks that deter hikers from walking alongside the stairs, increasing the opportunity for more erosion.

Denis and Don install the last few steps
 On Day 2, we landed 6 more steps and installed the vertical slabs and scree rocks.  A few more steps than we had originally planned, but the actual number of steps needed to cover an area depends not only on the slope of the land, but the thickness of the rocks.  Our steps on this project varied from 4" to 8" in height.  At the top of the run, we installed a water bar to direct runoff away from the staircase.
The scree rock is stacked alongside the stairs to reduce walk around.

Work Complete!