Typically it is the sticks and bones of these wooden bridges which I concern myself with, not the stones which carry them. With how all important this founding is to the service life of the bridge it carries, in this entry we will depart from our typical discussion of the history of wooden bridges and the people who designed and built them, and turn to the stonework, and the soil on which that sits, and has carried them through time, into the present.
This is the branch of bridge engineering which has for me, tended to perk the least amount of interest. Talk of soil science and subsurface profiles and standard penetration tests tends to have my eyes glazing over. At the same time, having read a number of historical proposal documents with richly detailed descriptions of abutment construction, and with having more than once been present when such constructions were being rehabilitated, I’ve been able to see abutment stonework from perspectives not often available to us. With their bridges lifted or temporarily rolled out of place, and with partial excavation exposing inside faces of the stone. There is often more to such an assembly than might be imagined by looking at what can be seen from above ground. This is particularly true for Burr’s and other trusses with original Arches, something often unseen is there to take up the thrust imparted by those Arches.
Stonework, and an awareness of the power and effects of moving water, have almost always been part my understanding of how our world works. My childhood home stands quite literally, on the rim of a deep gorge, carved out over eons by a tributary of the Merrimack River, their confluence not a mile downstream. This seemingly gentle but ten foot wide picture book perfect example of a babbling brook, two hundred feet below the gorge’s rim, stood home to a long abandoned mill site. As seasons turned and flowed into passing years, I would mark each newly unfolding year with a post Spring Freshet exploration of what changes each annual melt swollen flood would bring to the gorge. This would begin with the mill site, constricted in flow by massive dry laid granite walls on either side of the brook, always annually exposing former mill parts, cogs and castings, long buried in the stream bed and unseen for generations, and might end with some massive course change in the brook itself and the falling of trees this would bring.
I was reminded of the stunning ferocity and sometimes almost subtle power of moving water on 29 August 2011 when I trekked tools and materials to Rockingham and its village of Saxton’s River to help with the emergency stabilization of Hall Covered Bridge. The ferocity of moving water was more than evident in the almost impossible to process level of damage which lined both the William’s and Saxton’s Rivers. Seemingly unending property damage, including the loss of homes and livelihoods, and an unfathomable amount of riverbank erosion and immeasurable course changes.
The subtle side of damage done came in the form of earth giving way below my feet. Outwardly normal looking ground just inside the upstream wingwall, which I had already traipsed a number of times schleping crib block and shim stock, suddenly gave way and I found myself feet dangling, hanging by my elbows in a sinkhole. Hours of high water, pressure, and torrent, hammering the normally high and dry wingwall, had forced flow through the dry laid stone, and washed the fines ( Sand / soil particles smaller than average in a mixture of particles varying in size ) out of the approach through the stemwall and out into the flood swollen river. This same abutment suffered heavy impact damage, multiple stones had been dislodged, and had peak water levels held for much longer it seems probable that enough soil would have washed out of the approach, that the stonework in the abutment would have become unstable, and that it and the bridge it carried would have been lost.
This for me exposed the Achilles Heel of dry laid abutments.
Don’t get me wrong, I’m in no way advocating anything like removal and replacement – Quite the contrary. Though many people think of concrete as a forever material, it is not. Were we using the same Roman recipe, (Opus caementicium) which they used in the construction of The Pantheon and Pont du Gard, yes, it might be a super service life material. The recipe we use begins to break down after ninety years and has a max service life of one hundred and twenty five years, beyond that it slowly degrades into a clay like crumbling muddle – ( perhaps this might better be a debate for a future entry ) Soft clods of degrading concrete was another form of wondersome unknown debris I would find in Watt’s Brook Gorge each Spring, some late and last incarnation of the long lost dam – To my mind a Wooden Bridge should be founded on abutments which have at least as long a service life as it does. And time has told us and does prove, that a properly maintained covered bridge could and should, (Sadly most suffer periods of poor maintenance, or worse, neglect) easily reach the two century mark.
So what am I advocating? I’m not entirely sure, I do think, that as with all natural disasters, we learn much about how what we build is effected by them. Much about what needs changing, and much about what doesn’t. In this event we learned the loss of wooden bridges to flood waters, is not just about bridges floating out of place, up and off their abutments, but is as often about undermined abutments crumbling beneath them and quite literally pulling them into rivers.
I’m wondering out loud what we are doing with our lesson learned, and how many others noticed this phenomenon of sinkholes and the loss of fines and are studying what might be done about it?
I for one also wonder if perhaps part of the solution might be, not the typical pumping of high pressure grout in behind the stonework, ( with the potential for puddling and the destructive cycle of freeze thaw that possibly triggers ) but might lie in excavating behind upstream wingwalls and backwalls, ( at least in well laid up cut stone examples ) and retrofitting multiple layers of geotextiles and chipped rubber tires. A well draining buffer to keep the wash from reaching the Fines, to perhaps keep future floods from washing away more of our built heritage.