ST8201-CS 890706 (2) GINN, INC
July 6, 1989
Mr. Scott Balch, P.E.
L. H. Lacy Company
ENGINEERS
P.O. Box 541297
Dallas, Texas 75354-1297
Re: Denton Tap Road Improvements from Belt Line Road to Sandy
Lake Road - Concrete Riprap failure beneath the north ends of the
Grapevine Creek bridge structures.
Dear Mr. Balch:
On Monday, June 26, 1989 Mr. Stan Reed, P.E. and myself visited
the site of the failure and observed the exposed full depth of
failed concrete in several areas along the main failure crack.
In our observation we noted that no steel reinforcing was visible
in the concrete. On Wednesday, June 28, 1989 I revisited the
site and upon further examination of the failed concrete riprap I
found and photographed evidence of horizontal reinforcing steel
on the bottom of the concrete. A noncompliance notice regarding
the nonconformity of the reinforcing steel with the Drawings and
Specifications was served June 28, 1989.
The above and a description by our resident project
representative of the method of concrete placement used in the
construction of this concrete riprap leads us to believe that the
steel reinforcing bars are improperly located beneath the
concrete. Based on the foregoing and the degree of failure
present in this particular case, it is our belief that the
concrete riprap (RRB) on the north bank of Grapevine Creek under
the bridge structures is unsuitable and not in conformance with
the Specifications. In accordance with the Contract General
Conditions of Agreement, Article 2.14 DEFECTS AND THEIR REMEDIES,
you are hereby directed to remove the concrete riprap on the
north side of Grapevine Creek and rebuild it in accordance with
the Drawings and Specifications, or provide an acceptable remedy
which will bring this item into conformity with the Drawings and
Specifications.
After viewing and photographing the failed riprap during several
site visits, and conferring with our structural engineer, we have
concluded that the failure is attributable to fact that
stormwater flowed into voids at the top of the concrete riprap
between the two bridge structures and filled an extensive network
of voids present beneath the riprap (which your subcontractor had
previously tried to fill with concrete) and pressurized the voids
to a higher pressure than that of the flow in the creek during
the high water. As the high water in the creek receded, the
17103 Preston Road · Suite 100 · LB II8 · Dallas, Texas 75248 · Phone 214/2484900
water behind the riprap could not escape and a differential
pressure developed which was sufficient to fail the improperly
reinforced concrete riprap. A rapid loss of the water which had
occupied the voids beneath the riprap occurred at failure'of the
concrete and probably generated flow velocities within the void
network sufficient to further erode the earth beneath the riprap.
Additionally, the degree of apparent rotation of the lower
portions of the failed riprap may indicate weakened or failed
toe-wall sections.
It is our opinion that the failure of the concrete riprap is due
to the means and methods employed in its construction.
Therefore, since similar means and methods were used by your
subcontractor on the balance of the concrete riprap on this and
the adjacent project, we will require that sections of each panel
of concrete riprap (including toe walls) on both projects,
exclusive of the north side of Grapevine Creek, be exposed for
examination by Ginn, Inc. and Reedcon, Inc., our structural
engineer.
In accordance with Division 1, Section 01010 - SUMF~%RY OF WORK,
1.13 INSPECTION OF WORK, Paragraph C, all of the costs of re-
examination, including testing and analysis, will be borne by the
Contractor if the re-examined work fails to meet the requirements
of the Contract Drawings and Specifications. If the Re-examined
work complies with the Drawings and Specifications, the Owner
shall bear the costs of re-examination, testing and analysis and
restoration of the work.
We will be happy to meet with you at the project site to indicate
the areas which are required to be exposed for re-examination.
Please respond to this letter in writing indicating your plan of
action regarding the above including a time and place convenient
for a meeting regarding the above.
Sincerely,
John C. Karlsruher, P.E.
Project Engineer
cc:
Russell R. Doyle, P.E.
H. Wayne Ginn, P.E.
Randy Fleming
Stan Reed, P.E.
File 378 (a:378rip)