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Home My WebLink About3215; LA COSTA AVE WIDENING; TEMPORARY SLOPES IN FORMATIONAL ROCKS; 1998-06-30Engineers/Consultants June 30, 1998 / Mr. Drew Bonolo Project Manager C.A. Rasmussen, Inc. 5205 Avenida Enemas, Suite A Carlsbad, CA 92008 Via fax (760) 436-3420 Subject: Temporary Slopes in Formational Rocks (Santiago Formation) Reference: La Costa Avenue Widening Project Dear Sir, You haverequested that we evaluate the site conditions and provide you with maximum allowable temporary cut slopes (excavation plan) for the construction of crib walls along La Costa Avenue in Carlsbad, California. You have provided us with the following documents for our review: "Report of Geotechnical Services Pertaining to Widening of La Costa Avenue Between Highway 5 and El Camino Real, City of Carlsbad, California", prepared by Testing Engineers-San Diego and dated July 18, 1986. Geologic Maps titled "Alignment Study for La Costa Avenue Widening I- 5 to E.C.R.", Sheets 2 through 9, prepared by Testing Engineers-San Diego. Contract Drawing Sheet 16 titled "Crib Wall Profiles for. La Costa Ave. Widening Interstate 5 to El Camino Real" and Sheet 17 titled "Crib Wall Details for La Costa Avenue Ave. Widening, Interstate 5 to El Camino Real", prepared by Wilidan Associates and dated February 21, 1990. Three separate segments of the crib wall requiring temporary construction slopes are evaluated herein. Wall Segment 1 is between Stations 58+15 and 63+47.5, Wall Segment 2 is between Stations 72+15 and 76+62.5, and Wall Segment 3 is between Stations 85+77.5 and 92+00. At your request I visited the job site on June 26, 1998. During my visit six trenches (two in each Wall Segment) were excavated into the slopes along La Costa Avenue to expose the Formational Rocks (Santiago Formation) .for rock quality assessment. A Hitachi EX-200 Excavator was used to dig the trenches. 500 Sansome Street San Francisco, CA 9411 1-3275 Phone: (415) 434-1822 Fax (415) 956-8502 PO'T'd 960 VGE Sfl' S3IU 1 3OSSU saor O:Pt 966t-o-Nflr Two types of crib walls will be constructed as a part of this project. These walls are described below: Type I Wall (fled-back Soldier Pile Crib Wall) Type I Walls are crib walls that have supplementary soldier piles and tieback support. Our understanding of construction sequence for Type I crib walls is as follows: Using a sloped excavation, a bench will be created at the elevation of the top of the piles. By chilling from the bench, piles will be installed in 24-inch diameter drilled holes filled with structural concrete. After all piles are installed, excavation will be carried out in front of the piles to a depth of feet below the top of piles. Tiebacks will be installed from this lower bench in accordance with the Contract Plans. Excavation will be completed to subgrade for the installation of the crib wall. Since soldier piles and tiebacks will support the lower section of these walls, we believe that the critical section of theteinporary construction slope in this case will be the unsupported slope above the top of the piles. type II Wall ("Plain" Crib Walls) Type II Walls are regular crib walls that will be constructed by excavating to subgrade and then installing the crib wall. No pile support is provided. Based on the review of the available geotechnical information, our field observations during trenching, and our underatanding of the construction sequence for the crib walls summarized above, our recommendations for the maximum temporary construction slopes in the Santiago Formation in various Wall Segments is as follows: Wall Seement 1 (Station 58+15 to Station 63+47.5) The maximum height of crib wall in this Wan Segment is 34 feet above the foundation elevation and 31 feet above the finished grade Type I Wall will be constructed between Stations 60+22.5 and 62+92.5. The maximum distance between the top of piles and the top of crib wall is about 16 feet. In areas where piles are not provided (Type II Walls) the maximum wall height is about 20 feet at Station 60+10. t'O/O d 960 V6Z STP SaIldloossu saor IS: Pt 866t-e-Nflf Trenches were excavated at Stations 61+50 and 62+40. The rock exposed at these locations was weakly cemented and could be excavated quite easily with the excavator. We recommend that temporary construction slopes for both Type 1 and Type II Walls in this Wall Segment be no steeper than ½ (H) to 1(V). Wall Segment 2 (Station 72+15 to Station 76+62.5) The maximum height of the crib walls in this Wall Segment is 28 feet above the foundation elevation and 26 feet above finished subgrade. Type I Wall will be used between stations 72+45 and 75+82.5. The maximum distance between top of piles and top of crib wall is about 13 feet. The maximum height of wall where Type I Wall will be constructed is about 18 feet at Station 75+95. Trenches were excavated at Station 72+70 and 73+20 for rock quality evaluation. The rock exposed at these locations was found to be well cemented. Excavation was hard and required a large crowding force to peel the rock- We recommend that temporary construction slopes for both Type land Type II Walls in this Wall Segment beno steeper than ¼(H)tol (V). Wall Seament 3 (Station 85+77.5 to Station 92+00) This entire Wall Segment will be constructed using Type II Walls. The maximum height of wall is about 19 feet above the finished grade. Trenches were excavated at station 89+00 and 90+50. The Santiago Formation exposed at Station 89+00 was found to be well cemented and more competent than the rock exposed at station 90+50. We recommend that a slopes no steeper than ½(H) to 1(V) be used generally for this Wall Segment The slopes may be steepened to no more to Va (H) to 1 (V) in those reaches where well cemented, competent rock (such as was exposed at Station 89+00) is present. Our recommendations for the sloped excavations are based on review of available geotechnical information and inspection of in-situ rock in six trenches. It is possible that the actual rock conditions in some reaches may be different than those encountered during field exploration. In the interest of safety, we recommend the following: t'O'd 960 t16C St' S31UIOSS Sa03tsr TS:Pt 866t—ø--Nflf t'd id101 All cut slopes be inspected at the beginning and end of each work shift to check for signs of instability, such as the development of tension cracks at the top of slope. Jacobs Associates shall be informed immediately if any unexpected conditions are encountered during construction. Revised slope recommendations will then be provided based on the conditions exposed. I trust that this letter provides the information you require. Please call me at your convenience if you have additional questions. Sincerely, JACOBS ASSOCIATES T •AO-TIL I. Kumar Singh P.E., G.E. Associate I:W3,sbi4oc Pø/t'Od 969 V62 S11' S:Pt 866t-ø-Nflf