HomeMy WebLinkAboutCT 04-16; LA COSTA GREENS NEIGHBORHOOD 1.06 & 1.07; GEOTECHNICAL RECOMMENDATIONS REGARDING OVERSIZED MATERIAL IN TRENCH BACKFILL ZONES; 2014-03-12j
LOG
Geotechnical, Inc.
March 12, 2014 Project No. 12043-01
Ms. April Tornillo
Taylor Morrison
8105 Irvine Center Drive, Suite 1450
Irvine, California 92618
Subject: Geotechnical Recommendations Regarding Oversized Material in Trench Backfill Zones,
La Costa Town Center Residential Development, Carlsbad, California
Reference: City of Carlsbad, 2013, Engineering Standards, Volume 3 - Standard Drawings and
Specifications, Chapter 6 - Construction Specifications, Division 2.5 - Earth Backfill
Material for Trench Zone
Introduction
In accordance with your request, LGC Geotechnical, Inc. has prepared this letter to provide geotechnical
recommendations regarding oversized material in trench backfill zones at the La Costa Town Center
- Residential Development located in Carlsbad, California. It is our understanding that, due to the presence of
relatively shallow metavolcanic bedrock, traditional utility trench excavation techniques were not viable in
some areas. Due to the hard rock conditions, "blasting" techniques were utilized in order to excavate to
trench bottom design depths. Although "blasting" of the metavolcanic bedrock enabled trench excavation,
abundant oversized rock materials were generated.
Recommendations
Division 2.5 - Section B of the above referenced City document states that "Earth backfill shall be excavated
materials that is free from organic matter, roots, debris, and rocks larger than 4 inches in the greatest
diameter". Although contrary to the City specifications, it is our recommendation that oversized rocks 6.
inches or less in the greatest dimension may be utilized in trench backfill. The onsite materials generated
from trench excavation should be screened of any oversized rocks larger than 6 inches in the greatest
dimension and the oversized portion not used in the trench backfill. The rock that is less than 6 inches in
greatest dimension should be mixed with finer grained onsite soils to reduce the size of void spaces within
the backfill material It is our opinion that trench backfill compaction requirements and geotechnical'
performance may still be achieved with trench backfill soils containing rocks up to 6 inches in the greatest --..._---
dimension. .:
Our recommendation allowing rocks up to 6 inches in greatest dimension in trench backfill is based
experience with similar projects and follows the Standard Specifications for Public Works Consti
"Greenbook" (2012), Section 306-1.3.1.