Loading...
HomeMy WebLinkAboutAGENDA REPORT 1987 0429 CC ADJ ITEM 12E//7 i �-7 ITEM f�•?..=_ MOORPARK _ed"&-L �a�c CLINT HARPER, Ph.D. STEVEN KUENY Mayor City Manager ELOISE BROWN DOPPK °"� CHERYL J. KANE Mayor Pro Tem F ° ° i City Attorney THOMAS C. FERGUSON ° PATRICK RICHARDS, A.I.C.P. Councilmember o Director of JOHN GALLOWAY Community Development Councilmember °9a o R. DENNIS DELZEIT BERNARDO PEREZ °q,,Fo � " City Engineer Councilmember JOHN V. GILLESPIE MAUREEN W. WALL Chief of Police City Clerk MEMORANDUM THOMAS P. GENOVESE Ci Treasurer TO: The Honorable City Council FROM: R. Dennis Delzeit, City Engineer DATE: April 8, 1987 SUBJECT: Utility Trench Backfill Requirements BACKGROUND INFORMATION A request was made by the City Council at the r.eeting on March 11, 1987, for the City Engineer to prepare a report on utility trench backfill requirements. This memorandum was requested because of the concern over recent utility trench failures. These failures are occurring in trenches excavated within existing streets. A. Existing Standard Utility trench backfill requirements are divided into two cases: utility trenches in paved roadway areas or trenches in parkways, medians, and open ground. The requirements for these two cases are summarized in Ventura County Standard Design Number 45-5 (attached) and below: 1. Utility trench requirements under pavement: The backfill within the pipe zone must follow the specifications described in City Standard. The pipe zone is described as the section of trench from the bottom of the excavation to one foot above the top of pipe. The backfill required depends on the load factor specified. From a distance of one foot above the pipe to three feet below the finished surface of the pavement, the backfill consists of excavated material, with all organic matter, rubbish, debris, and other objectionable material removed. R'CCE® s APR © 9 1987 799 Moorpark Avenue Moorpark, California 93021 (805) 529-6864 4 Page -2- This backfill must be compacted to 85% relative density. Any backfill placed three feet or less below the finished surface of the pavement must be compacted to a 90% relative density. The base material and pavement are then constructed according to plans and specification. It should be noted that on encroachment projects which remove pavement, the A.C. pavement replaced is one inch thicker than the existing pavement. 2. Utility trench backfill requirements in parkways, medians, and open ground: The backfill within the pipe zone must follow the specifications described in City Standard. The backfill required depends on the load factor specified. It should be noted that a less stringent load factor is generally required under open ground than under pavement. From a distance of one foot above the pipe to the finished ground surface, the backfill consists of excavated material, with all organic matter, rubbish, debris, and other objectionable material removed. This backfill must be compacted to 85% relative density. B. Existing Standards Throughout Ventura County A survey of several cities throughout Ventura County (Ventura, Oxnard, Camarillo) shows that the existing City specification on utility trench backfill requirements is generally followed. For the County of Los Angeles the backfill requirements are similar to those in the City, except that all backfill must be compacted to 90% relative density. In the City of Thousand Oaks, the utility trench backfill specification requires a cement slurry backfill for all trenches within existing streets. This requirement was adopted because of utility trench failures. It was determined that the cause of these failures was an improperly compacted backfill. By requiring a one sack slurry as a backfill in flat areas and a two sack slurry backfill in steep areas, the backfill attains the required compaction. Specifying a slurry backfill for utility Page -3- trenches has greatly reduced utility trench failures in the City of Thousand Oaks. For utility trenches within state road right of way, Caltrans requires all backfill to be compacted to 95% relative density. At the option of the contractor, slurry cement backfill may be used in place of the 95% compacted fill. In summary, most cities and the County still use the same utility trench backfill requirements as the City of Moorpark. The concept of a cement slurry backfill has worked out well in other areas but does require the hauling of all excavated materials and extensive curing time (up to several days) before the trench can be paved and placed back in service. This results in longer periods of interruption to traffic through the work area. RECOMMENDED ACTION For utility trenches excavated within existing streets, require that utility trenches be compacted to a minimum of 90% relative density above pipe zone to subgrade. At the option of a contractor, subject to City approval, a cement slurry backfill may be used in place of the 90% compacted fill. MOORPARK, CALIFORNIA City Council Meeting of —,N198 7 ACTION: By RDD:CL:1s Encl. cc: Steve Kueny, City Manager John F. Knipe, Asst. City Engineer JN 3454 CM0206.MEM O o O " _ _ E- 2E. O _ _ fi p � a r u U O¢ LL o m