Sharring experiences in urban infrastructure delivery.

Tree Wells in Sidewalks

By: Tom Davis Category: Sidewalk Features

Downtown Houston has added over a 1000 trees in the sidewalks since the mid 1990’s. Various designs were used that were driven by cost concerns and by salesman’s claims. This post will address the locations where the sidewalk was built of concrete unit pavers. Other lessons learned about tree wells in sidewalks will be posted here in hopes they help you minimize maintenance cost.SettlementAtTree2.jpg

The issue was that the pavers settled at the tree wells and the trees grew and the roots pushed the pavers up. We had uneven and unsightly areas. The settlement issue was caused by the failure of the specified thin plastic layer filled with sand on top of the tree’s root ball to support the pavers that settled into the root ball and the surrounding high-organic planting mix. SettlementAtTree1.jpgGranted, the original construction specifications said to compact the planting mix to 90% Standard Proctor Density. But, the tree would not grow with a compacted root ball and that density test cannot be applied to a high organic soil media. Yes, this issue and the expense it created has been a source of frustration for those of us left behind to maintain it or find another solution.

We were told this was just a maintenance issue and should include it in the budget–forever. But maintenance meant removing and reinstalling the pavers back in the same way hoping for a different result. For not much more one-time expense the re-installation could be with a different support design thereby having a chance for a different result.

The solution we developed addresses two certainties: the trees will grow and the pavers will settle unless supported. New design tree well skirts. Note concrete support for pavers up to perimeter curb.The first image here shows the construction of the new reinforced concrete skirt. Note the “L” cross-section with the lower leg poured out 6″-12″ to support the pavers and maintain a level transition from the top of pavers to the exposed top of the new skirt. That upper part is 6″ wide, stained charcoal grey and broom finished to match the dark paver bands in the original sidewalk. Black star granite fills the area above the root ball, planting mix and irrigation piping. As the dirt settles and occasionally the tree is blown askew the gravel can be re-leveled and tree straighten by the landscaper without rebuilding the sidewalk.

Tree Well with Concrete Skirt & GravelAlong with the concrete supporting the pavers the large, gravel filled well allows for 10-15 years of tree growth with the only maintenance being to occasionally replace minor amounts of gravel.

You can see the new and the original tree wells on Preston Street in downtown Houston Texas. The District installed the new tree wells when many of the trees were replaced after Hurricane Ike in 2008. Others that were badly out of level were also replaced. Some of the original designs are still adequately level and they were not changed. We are pleased that the new design with the stained concrete and gravel does not standout such that there is a glaring difference with the original design.


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