3 Abutments Down, 2 to Go

The foundations for the bridge abutments were poured last Friday (1/8).  Above is a photo of the foundation for the east abutment for Bridge 2.  On Wednesday, 1/13, the abutments were poured for Bridges 1, 3, and 4.  The abutments for Bridges 2 and 5 are scheduled for later this morning.  The photo below shows the pour of the west abutment for Bridge 3.

After the concrete is poured and vibrated, it has to be smoothed and finished.  This work looks really easy, but the material is heavy and it takes a skilled hand and eye to do a good job quickly.

Once the concrete has set enough, the forms are removed.  In about 2 weeks, the concrete will be fully set.  The pre-fabricated bridges will arrive and will be placed atop these abutments.  The area around the abutment will be back-filled and boulders will be placed aesthetically around the abutment and streambank for erosion protection.  The path will be graded to provide a smooth transition between path and bridge.  Above is the east abutment for Bridge 1.  I thought it looked like an upright piano, so I gathered up wood chips for the black keys and asked Terry to play us a little tune.  The sound from this thing is awful, but how often do you get to play a concrete piano while standing in a dry streambed?

Pouring the Foundations

On Friday, 1/8, the foundations for the bridge abutments were poured.  Access to the location of the 1st bridge was the most difficult and concrete had to be brought in with a skip loader.  But even the skip loader could only get to the abutment on the east side of the stream.

Access is so limited on the west side that the only thing small enough is a Bobcat.  The Bobcat scooped concrete from the skip loader and took it to the other side of the streambed.  Doesn't it look like a baby bird being fed by its mommy or daddy?

And here is the Bobcat putting concrete into the form for the west abutment for the 1st pedestrian bridge.

The other bridges are MUCH more accessible, and the cement mixer was able to drive down the stream corridor and pour straight into the forms.  The photo above shows concrete being poured into the form for the east abutment for bridge #5.  The crew member on the right is vibrating the concrete so it will fill the form evenly and bond strongly to the rebar.  The photo below shows the mason getting the concrete smooth and level.

Getting Ready to Pour

The pits have been dug and the rebar placed for the footings for all the bridge abutments.  The inspector from the City's Department of Development Services was out on Thursday to check it all out, and the concrete is scheduled to be poured later today.  The photos above are at the location of the 1st pedestrian bridge -- where the stream flows out of the north lake and over the weir.

The 2 photos above show the locations of the 2nd and 4th bridges. You can see the rebar sticking up out of the holes, ready for concrete to be poured around it.  The bridges are being manufactured in Alabama and trucked to Long Beach ready to be secured to the abutments.  The only bridge construction that has to be done on-site is attaching the hand rails.  The weathering steel of the bridges will add a beautiful rustic flair, while proving secure crossings and clear views of the stream--a big improvement over the crumbling, slippery, and wobbly original bridges.

The Hard Stuff

The bridges are being manufactured and the poured-in-place concrete abutments for the 5 new bridges have been approved by the City.  The longer the concrete allowed to sit, the stronger the abutment will be.  So the contractor is taking advantage of the holiday lull to get ready to pour concrete.  The photo above shows crew members marking out where the abutments will go.

The point bar is starting to take shape as cobbles are brought in and placed.

There are a few places along the stream where we want to let visitors get right up to the water, but we also need to provide some extra-strong protection for the bank.  In those places, the contractor has placed large, angular boulders that lock together to form a solid structure.  There is no grout or concrete holding the boulders together, just gravity and friction--two things that will outlast the strongest grout.

Bioengineering 101

On Friday (12/18/09), Drew Goetting, Principal designer at Restoration Design Group, came down from Berkeley to give us all a lesson in bioengineering.  First, dig a 3-4" deep trench at the toe of the bank slope.  Toss the dirt from the trench upslope to make it easier to fill the wattle.

Next, lay out the coir fabric so the edge extends a few inches upslope past the lip of the trench.  Secure the fabric to the soil with biodegradable staples (in this case, Bio-Stakes made from corn).

Shovel some of the soil from the trench over the coir fabric, then place willow pieces (branches, twigs, and leaves) over the soil.  Add more willows and more soil, making sure that the willows are generally oriented along the streambank and no branches are sticking out perpendicular to the direction of the water flow.  More willows and more soil, until the whole thing is the right height (see below for what "right" means).

Next, imagine that you are making a giant California roll, using the coir fabric as your sushi mat and the willows and dirt as your rice, crab (or krab), and avocado.  Kick and push the mixture into place--it is pretty rugged and will withstand some rough treatment.  The goal is an evenly compacted roll whose top is fairly level.  The finished top of the willow wattle should be above the water surface when the stream is flowing.

Pull the coir fabric fairly tightly over the soil and willows.  Secure the edge of the coir fabric with more biodegradable staples.

Pound some willow stakes straight through the entire thing.  The stakes should go at least a couple of feet into the ground under the wattle. 

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The result is a willow wattle.  The fabric will hold the soil and protect the bank on a temporary basis.  Meanwhile, some of the willows inside the wattle and some of the willow stakes will sprout and their roots will grow to hold the streambank.   After a while, the coir fabric and the staples will biodegrade and all that will be left will be willows, creating a sustainable streambank and a vegetative screen of riparian habitat.

Fine Grading

About half of the stream has dried out enough so that heavy equipment can go back in.  The contractor is doing the fine grading, getting the elevation of the streambed just right and contouring the banks so they are ready for installation of the bioengineering bank strengthening.  The contractor has also installed a couple of areas of boulder reinforcement like what is shown in the photo above.  The stream restoration consultants will come in tomorrow to look at this boulder reinforcement to make sure it is properly done.  Once the restoration consultant is satisfied with the contractor's work, this bank treatment will be replicated at a few more locations along the stream.  The boulder treatment is being used at spots where erosion potential is greatest--immediately upstream and downstream of the pedestrian bridges, and at a couple of places where the walking trail comes very close to the stream.  The restoration consultant will also be giving the contractor detailed instructions in planting willow poles and stakes, installing coir logs, and building willow wattles.  Some of the willows will even be harvested from within the Nature Center itself.

And the rains came...

The weekend rains put quite a bit of water into the streambed.  The contractor's crew was wading through water and mud today pumping the water out.  Once the water is gone, it will take another couple of days for the streambed to dry out enough to take equipment back in.  Hopefully, we won't lose too many more days to rain.