HomeMy WebLinkAboutCT 00-06; BRESSI RANCH; GEOTECHNICAL RECOMMENDATIONS CONCERNING ABANDONMENT OF EXISTING PIPELINE SECTIONS; 2004-01-14REcEiYj
Leighton and Associates, Inc. JAN 14 2004
A LEIGHTON GROUP COMPANY
January 14, 2004
Project No. 971009-009
To: Lennar Communities
5780 Fleet Street, Suite 320
Carlsbad, California 92008-4700
Attention: Mr. Jim Urbina
Subject: Geotechnical Recommendations Concerning Abandonment of Existing Pipeline
Sections, Vicinity of Poinsettia Lane, Station No. 74+00 to 79+50, Bressi Ranch,
Carlsbad, California
References: Kiniley-Horn, 2003, Improvement Plans for Poinsettia Lane at Bressi Ranch,
Project No. CT99-03, Drawing No. 397-2K, 36 Sheets, dated February 12, 2003
Rick Engineering, 1997, Plans for the Improvement of Rancho Carrillo Offsite
Sewer, Project No. PD411, Drawing No. 331-1C, 13 Sheets, dated September 4,
1997
In accordance with the request of representatives of Lennar Communities, this letter presents our
geotechnical recommendations concerning the abandonment of the 16-inch techite pipe, the 24-
inch steel reclaimed waterline, and the 12-inch Ductile lion Pipe (DIP) reclaimed waterline in
the vicinity of Poinsettia Lane, Station No. 74+00 to 79+50, within the Bressi Ranch project
located in Carlsbad, California. We understand that the three pipelines, which are located north
of the City of Carlsbad 12-inch sewer main, are not currently active and will need to be
abandoned as part of the grading and improvements for proposed Poinsettia Lane. Furthermore,
we understand that the City of Carlsbad 12-inch sewer main will be partially relocated (i.e., the
section of sewer main west of Station No. 76+54 will be abandoned and relocated within the
right-of-way of Poinsettia Lane). The section of the sewer main east of Station No. 76+54 will
remain in place.
Based on City of Carlsbad guidelines, we recommend that the entire length of the 24-inch steel
line between Station No. 74+00 and 79+50 be abandoned by completely removing the pipe and
backfihling the excavation with compacted fill. However, it appears that portions of the
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971009-009
abandoned 12-inch DIP line and the 16-inch techite pipe are located relatively close to the active
City of Carlsbad sewer main and are located vertically below the active line. Abandonment by
removing the pipes may not be practical due to the close proximity of the pipes (both
horizontally and vertically) to the active sewer main.
Recent excavations performed on January 13, 2004 and review of the Poinsettia Lane
Improvements Plans (Kimley-Horn, 2003) and the improvement plans for the City of Carlsbad
12-inch sewer main (Rick Engineering, 1997), indicates that the DIP line is located within
approximately 6 to 8 feet of the active sewer main while the 16-inch techite pipe is located
within 13 to 15 feet. Geotechnical observations also indicated that the backfill soils of each of
the lines appears to be relatively well compacted with above optimum moisture contents. We
also understand that the City of Carlsbad or their consultant documented the backfill of the active
sewer main.
Table 1 presents the approximate depths of the active sewer main, the 12-inch DIP line, and the
16-inch techite pipe relative to the existing ground surface.
Table 1
Approximate Depths of Sewer Main or Reclaimed Waterlines
Poinsettia Lane
Station No.
Depth of Active 12-inch
Sewer Main
Depth of 12-inch DIP
Line
Depth of 16-inch
Techite Line
1574+00 10 Feet 6 Feet 7 Feet
76+54* 13 Feet 16 Feet 21 Feet
77+25 14 Feet 22 Feet 26 Feet
* This is the location of the connection point of the existing sewer main and new section of
line that will be placed within the right-of-way of Poinsettia Lane
As indicated in Table 1, the depth of the 12-inch DIP line and the 16-inch techite pipe becomes
deeper as the Station Numbers increase in a west to east direction. At the location of the new
connection point (i.e., Station No. 76+54), both the 12-inch DIP line and the 16-inch techite pipe
are located below the active sewer main.
In order to remove the 12-inch DIP line at the connection point, a vertical excavation on the
order of 16 feet will need to be made starting at a point approximately 5 feet from the centerline
of the active Carlsbad sewer main. A similar excavation will need to be performed in order to
remove the 16-inch techite pipe. The excavation to remove the adjacent DIP and techite pipes
will be deeper toward the east. As a result, it is our professional opinion that removal of the 12-
inch DIP line and the 16-inch techite pipe between approximate Station No. 76+64 (i.e. 10 feet
east of the connection point) to 79+50 could potentially impact the active sewer main, and
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971009-009
therefore, we recommend that both of the pipelines be abandoned in place (i.e., filled with a
sand-cement slurry).
Between approximate Station No. 74+00 and 76+64, the adjacent DIP and techite pipe are
shallow enough (relative to the active sewer main) that we recommend the pipes be removed and
the resulting excavation backfilled with compacted fill. In order to remove the adjacent
abandoned pipes where the active sewer main is greater than 8 feet in depth, a 1:1 excavation
should be started at the centerline of the active sewer main and excavated to a depth of 5 to 7
feet. As an alternative, the fill above the active sewer main may be removed to a depth where
there is a minimum of 8 feet of cover over the sewer main. The 1:1 excavation may be made
along the entire length of the removal area. A vertical excavation should then be made along the
base of the 1:1 excavation by an excavator until the pipes are encountered. The maximum depth
of the vertical excavation is estimated to be approximately 12 feet. Where the active sewer main
is less than 8 feet in depth, a vertical excavation should be made starting 5 feet from the active
sewer main.
The removal sections for the pipes should not be any longer than 25 feet and adjacent sections
should be backfilled to a point at. least 2 feet above the elevation of the adjacent active sewer
main pipe prior to the excavation of the next section. Excavations deeper than the active sewer
main elevation should be backfilled as soon as possible and not left open overnight.
As previously discussed in this letter, we recommend that the entire length of the 24-inch steel
line between Station No. 74+00 to 79+50 be abandoned by completely removing the pipe and
backfilling the excavation with compacted fill.
All backfill soils should be moisture conditioned to at least 2 percent over the optimum moisture
content and compacted to a minimum 90 percent relative compaction (based on American
Standard of Testing and Materials {ASTM} Test Method D1557).
If you have any questions regarding this letter, please contact this office. We appreciate this
opportunity to be of service.
Respectfully submitted,
LEIGHTON AND ASSOCIATES, INC.
a ~'
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AV--- . —
Randall K. Wagner CEG 1612
Director of Geology
Distribution: (6) Addressee
(1) Lennar Communities, Attention: Mr. Michael Ugar
(1.) Lennar Communities, Attention: Ms. Kristine Zortman
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