HomeMy WebLinkAboutCT 13-03; ROBERTSON RANCH-RANCHO COSTERA; GEOTECHNICAL RECOMMENDATIONS REGARDING SHALLOW GROUNDWATER AND SATURATED ALLUVIUM; 2015-03-06LGC Valley, Inc.
Geotechnical Consulting
March 6, 2015 . Project No. 133023-03
Mr. Kevin Brickley
Toll Brothers
725 Town and Country Road, Suite 500
Orange, California 92868
Subject: Geotechnical Recommendations Regarding Shallow. Groundwater and Saturated
Alluvium, Planning Ared PA-2 and Robertson Road Station Number 10+50 to 12+25
Robertson Ranch, Carlsbad Tract No. 13-03, Carlsbad, California
Reference: LGC Valley, Inc., 2014, Geotechnical and Environmental Recommendations for Robertson
Ranch West, Carlsbad Tract No. 13-03, Carlsbad, California, Project Number 133023-03,
dated April 29, .2014
Based on geologic mapping and observations made during the remedial grading removals within Planning
Area PA-2 and in the northwestern portion of Robertson Road (between approximate Station Number
10+50 and 12+25), shallow groundwater was encountered at an approximate depth of 5 to 6 feet below the
existing ground surface within the alluvial soils. This letter has been prepared to provide our geotechnical
recommendations ôoncerning the remediation of the shallow groundwater and saturated alluvium condition
within the proposed grading limits of PA-2 and along a portion of Robertson Road next to Tamarack
Avenue:
Robertson Road Shallow Groundwater/Saturated Alluvium Recommendations
In the areas where removals of the alluvium have already been started and where standing water is present
in the excavation, we recommend the following:
1) The standing water should be pumped out as completely as possible. A slot trench/excavation can
be made along the southwest side of the excavation to facilitate the removal of the water in the
excavation. .. .
2). The excessively wet and soft alluvium should be removed and the resulting bottom back-dragged
or smoothed out to create a relatively level bottom.
3) Stabilization geofábric (Mirafi 600X or equivalent) should then be placed in the excavation
bottom with 12inch minimum overlaps of adjacent fabric layers and as smooth as possible.
If the removal excavation bottom is excessively yielding or pumping, a 1-foot layer of crushed
rock (3-inch minus material) should be placed in excavation and spread-out over the geofabric
using lightweight construction equipment.
If the excavation bottom is not excessively yielding or pumping, 1-foot lifts of sandy soil should
be placed over geofabric and spread out and compacted-using lightweight construction equipment
until a firm bottom is attained.
2420 Grand Avenue, Suite f2, • Vista • CA 91081 • (760) 599-7000 • Fax (760) 599-7007
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5) Fill soils can then be placed and compacted using standard construction equipment until finish
grade is reached.
In the area where removals of the alluvium have not been made, we recommend the following:
1) Removals of the near surface alluvium should be made to within I to 2 feet of the groundwater
surface (estimated to be 5 to 6 feet below the existing ground surface).
If significant yielding, pumping, and/or rutting occurs, the removal bottom should be smoothed
out and a stabilization geofabric (Mirafi 600X or equivalent) should be placed in excavation
bottom with 12-inch minimum overlaps of adjacent fabric layers.
If only minor yielding, pumping, or rutting occurs, the stabilization geofabric can be omitted.
3) Fill soils that are not excessively moist should then be placed and compacted in the excavation
until finish grade is reached.
If the stabilization geofabric is placed in the vicinity of the storm drain (at approximate Station Number
10+80), it should be placed at an elevation of 59 feet (mean sea level) or lower so that the geofabric is not
disturbed during the installation of the storm drain pipe.
PA-2 Shallow Groundwater/Saturated Alluvium Recommendations
In the area where removals of the alluvium have already been started, we recommend that:
The standing water in the removal bottom should be pumped out. A slot trench/excavation can be
made along the south side of the excavation to facilitate the removal of the water in the excavation.
The excessively wet and soft alluvium should be removed and the removal bottom back-dragged or
smoothed-out using a lightweight piece of construction equipment to create a relatively level
bottom.
Stabilization geofabric (Mirafi 600 X or .equivalent) should be placed along the bottom of the
excavation with minimum 12-inch overlaps of adjacent pieces of geofabric.
Relatively dry sandy fill soils should then be placed and compacted over the geofabric in the
excavation until a firm surface is obtained.
Fill soils (including alluvial soils that are not overly wet) can then be placed and compacted until
finish grade is reached.
In the area were removals have not been made yet we recommend that:
The removal of the alluvium should be made to within 1 to 2 feet of the groundwater elevation or
when minor rutting starts to occur.
A stabilization geofabric (Mirafi 600X or equivalent) should then be placed in the excavation
bottom and sandy fill soils placed over the geofabric until a firm bottom is attained.
Fill soils (including alluvial soils that are not overly wet) can be placed and compacted until finish
grade is reached.
Project No. 133023-03 Page 2 March 6, 2015
If you should have any questions, please do not hesitate to contact us. The undersigned can be reached at
(760) 215-5540. S
Respectfully submitted, S
•
LGC Valley, Inc
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Randall K Wagner, CEO 1612
Senior Project Geologist •
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RKW
Distribution: (4) Addressee
(1) Toll Brothers, Attention: Mr. Peter Kim
Page 3 March 6, 2015