HomeMy WebLinkAboutAgenda - 02-10-1998 - Attachment 3e~~{ac i~ vr~~f7''r' v~
MEMORANDUM
TO: John Link, County Manager
FROM: Paul Thames, PE, County Engineer
DATE: February 5, 1998
SUBJECT: Effects of blasting operations at the Orange County Regional Landfill
As per your request, I have produced the following report on potential effects of blasting activities at the
landfill.
In general (that is, not specific to the landfill), the use of explosives to reduce or remove rock formations
on and below the surface of the ground has been known to cause incidental seismic (ground roll) damage
to neazby man-made structures, including water wells. For example, blasting operations have been
known to: 1) crack building footings, foundations and walls; 2) cause wells to collapse entirely or
otherwise crack well liner pipe; 3) disrupt, either permanently or temporarily, the flow of water to the
well from water bearing fractures; and 4) suspend or re-suspend sediments in wells and water bearing
fractures. The extent and shape of the area impacted by seismic effects of blasting operations and
intensity of those effects at any particulaz location within the impact azea can and do vary greatly. There
is evidence that blasting operations using ammonium nitrate based explosives can cause damage to wells
and septic tanks as far distant as 2000 to 3000 feet from the blast center. Some of the factors which,
singly or in combination, control the magnitude of the seismic effects include: a) the underlying geology
of the vicinity -isotropic or homogeneous rock formations aze very efficient at transmitting vibration
whereas heavily foliated or fractured rock formations are very efficient at dampening vibration; and b)
the operational procedures and protocols of each blasting event, including the diameter, depth; spacing
and number ofbore-holes; the positioning of explosive material within bore-holes; the quantity, type
and mix of explosive material (fast ignition explosives such as dynamite cause minimal ground roll and
slow ignition explosives such as ammonium nitrate cause more extensive ground roll); the time delay
intervals between individual detonations in a single blast event; and weather/temperature conditions.
As incidental damages are not uncommon during blasting operations, most contractors and engineers
involved blasting operations typically use dynamite or other fast ignition explosives to minimize the
extent of the area affected by the blast. Some contractors monitor blast effects with seismographs and
by conducting pre-blast inventories or inspections of structures located close to the blasting operations
{usually a minimum radius of 500 feet).
According to Gayle Wilson, during the last nine years in which he has overseen the operation of the
landfill, there have been only two grading/excavation projects requiring the blasting of rock. The first
eras at the onset of the Phase I and II construction of the lined (south of Eubanks Road) portion of the
landfill (late 1994-95). The second blasting project, during preparation for the construction of the Phase
III and IV sections of the lined portion of the landfill, began in mid-December and concluded in early
January 1998. Both blasting projects were conducted under a blasting plan required by and approved by
the state. In both cases, seisomographs were utilized to verify that the seismic effects of the blast were
not excessive. There was one damage claim from the first blasting project. That claim was denied as
groundless by the Town of Chapel Hill's insurance carrier. In the second blasting project, the highest
seismograph readings were approximately twenty-five percent of the limit set by the US Bureau of
Mines for protection of drywall and plaster (the weakest of all building materials) construction.
However, the noise limits were exceeded on at least two occasions by more that two hundred and fifty
percent. As of this date, no damage claims have been filed against the Town of Chapel Hill relative to
this second blasting operation. I have been advised that no additional blasting will occur at the landfill.
If I may answer any questions or provide additional information, please advise.