Excavators have uncovered literally tons of large boulders under what was the soccer field on the north side of the existing Hildreth Elementary School. The boulders must be removed, and, depending on how many are found, removal will cost an additional estimated $490,000 to $650,000 and will delay the project between five and six weeks. The School Building Committee voted to approve a change order for $420,000 so boulder-clearing work could continue for the next three weeks until the committee meets again, when more funds may be needed.
Discovery of large boulders will add to cost and delay schedule. (Courtesy photo)
In a presentation to the committee at its June 20 meeting, Phil Conroy of Shawmut, the project construction manager at risk, described large outcroppings of boulders as well as a “boulder nest,” where Volkswagen-sized rocks were stacked on top of each other, with fill below and above, as if they had been placed there. Chair SusanMary Redinger said she has heard some lore that the boulders may have been pushed into a hole at that location when the Brown School was built.
The boulders are a problem because the soil in the entire area where the new school is to be built could not support a structure, so plans call for about 800 rammed aggregate piers (RAPs) to be driven vertically into the ground under the entire slab the building will sit on. The RAPs will tighten up the soil and increase its bearing capacity, eliminating the need to export all the unsuitable soil and bring in structural fill, which would have been more expensive.
Shawmut proposed clearing trenches in all areas where the RAPs will be installed, since the drill rig that inserts the piers costs $1100 an hour, a high price to pay to keep a piece of equipment waiting for boulders to be removed. That will be a longer and more expensive process than planned.
The contractor would not provide a lump-sum price for the work because the number of boulders is unknown, so the costs will depend on how many boulders are removed. When questioned about possibly using ground-penetrating radar to detect where the boulders are, Conroy said existing technology doesn’t provide high enough resolution for that to work.
This change order, along with a previous change order to deal with exporting soil containing arsenic, totals $734,000 after markups are applied. Project manager Tom Murphy of NV5 said this amount represents about 34% of the project’s construction contingency. Murphy added that change orders for unforeseen conditions typically add up to between 1.5 and 2% of total construction costs. These two change orders, both considered unforeseen conditions, add up to 1.7%, and the project is still in the early package stage of construction. Murphy cautioned the committee to consider this when discussing value engineering items and bid alternates on the main bid package, saying, “Are we comfortable with going forward with 34% of our contingency gone?” (See bid story.)
As for the schedule delay, Murphy said boulder removal will shift the dates for the entire project, including the scheduled April 2021 move-in date. But he added that the NV5 and Shawmut teams were discussing ways to get the project back on schedule, including paying some contractors a premium to accelerate their work, a cost that would also come from the construction contingency.
