'How on Earth do giant street trees grow in those puny sidewalk openings?!'
That question is usually answered with: 'through cracks in the pavement!" Without evidence to support that answer, however, people are never really satisfied with it, and I admit, I haven't been either. Imagine how animated I was when I snapped the above background image recently during a Certified Arborist inspection of a curb and sidewalk replacement in Midtown Manhattan. Replacements are common throughout New York City to repair trip hazards.
How the earth face and roots above stood still in the wake of curb demolition is a story in itself. That story is recounted below because it has valuable implications for demolition methods in the future to reduce impacts to mature city street trees.
The work on July 6-7, 2016 involved 3 mature street trees at 330 W. 55th St. in Midtown Manhattan. Below are representative photos of their canopies, trunk sizes, and the sidewalk bed conditions in which they grew.
To everyone's great surprise (the contractor's most), the granite curb here was 18" deep and encased in 12" footing. 'Direct' excavation of the curb would not be possible and thus removal would not be as straightforward as normal.
The contractor thus had to devise a way with his crew to remove the large granite slabs with minimal impact to tree roots. The following describes the 'Drop' Procedure taken and its negligible impacts to involved tree roots.
'DROP' PROCEDURE - 3 Steps
1. Excavation required was streetside (north) of the curb ONLY, as shown. All demolition was by manual jackhammering.
2. Excavation depth of only 12"-14 ” allowed crew to break, then remove, northern segment of concrete footing for 100' length. Width of excavated asphalt road surface was ~18", which approximates normal width of roadway excavation for curb removals.
'DROP' PROCEDURE (cont'd)
3. After the broken half of footing was removed, with a pry bar the 12’ long granite slabs simply pivoted northward, and fell over easily.
Slabs’ tight fit /wedged condition and heavy weight facilitated ‘dropping’ of the curb.
After the first slab was pryed away, the rest fell like dominos – peeling away from tree beds and root zones, cleanly, in less than 30 minutes.
Hats off crew!
Note: there is only a 2 minute lapse between this image and one above it when slabs were still upright.
This long view shows how clean the soil face was for 100'. Below is an askance view of the foreground length.
Exemplary profiles like this of intact tree roots for entire 100' work length was unbelievable, but amazing to see. It was quite an accomplishment by any measure.
With these zero-impact results documented, I question:
Why the ‘Drop’ removal procedure employed by the contractor isn’t specified more, or, is not standard procedure for curb removals?
Was I just fortunate to witness a procedure that never or very rarely happens?
If contractor had known curb was so deep he would have brought in large excavation equipment to remove it and caused the usual levels of soil/root disturbance. As it happened, he was 'stuck' with manual jackhammering and debris removal because that is what his NYC DOT permit, set to expire, would allow. (In other words, he was under the gun to finish work started.)
What’s not in question?
That a surefire way to keep most, if not all, street tree roots intact is to pry away and ‘drop’ curbs in the course of removing them, especially when maximum root preservation is critical for saving a tree, instead of digging them out directly from their installed location. The whole drop process could be easily speeded up with mechanical equipment for a large percentage of manual manuevers observed.
More LINE Insights
1. TREE ROOT PROFILES - Above images are also a demonstration of the fact that majority of roots of any given tree in the landscape are relatively shallow near the surface, and lie within the first 18" of soil depth.
2. CONTAINED ROOTS - The linear rooting pattern of street trees, as shown in above images, is consistent with root growth in other contained soil situations more commonly seen, such as in:
a. CITY PLANT CONTAINERS - above-ground, built or free-standing, where plant roots hug container walls, and
b. NURSERY CONTAINERS - big and small, in which roots routinely grow in circles just inside the container at soil perimeter. All who have ever bought a contained plant is familiar with such contained root growth.
Shaker Contracting, Bronx, New York
Most of the content in this post was adapted from NYC Parks Certified Arborist Report Type 1 submitted by MVR High Line Arborist in review of the 330 W.55th St. work.