Loading...
HomeMy WebLinkAboutSDP 02-16; COASTLINE COMMUNITY CHURCH; MSE RETAINING WALL DESIGN RECOMMENDATIONS; 2019-06-12GEOCON INCORPORATED G E 0 1 E C N N I CAL. ENVIRONMENTAL. MATERIALS Project No. G1968-11-02 June 12, 2019 Coastline Community Church % Grant General Contractors 5051 Avenida Encinas Carlsbad, California 92008 Attention: Mr. Jim Grant Subject: SERETMNINGWAbL-DESIGNRECOMMENDATIONS-- CC5ASTLINECOMMUNITYtHURCH EXPANSION- 2215 CALLE BARCELONA CARLSBAD, CALIFORNIA Reference: Update Geotechnical Report, Coastline Community Church Expansion, 2215 Calle Barcelona, Carlsbad, California, prepared by Geocon Incorporated, dated October 12, 2018 (Project No. G1968-11-02). Dear Mr. Grant: In accordance with the request of Mr. Eric Armstrong with Fuscoe Engineering, we prepared this letter to provide mechanically stabilized earth (MSE) retaining wall recommendations at the Coastline Community Church Expansion project located on Calle Barcelona in Carlsbad, California. We understand that tiered MSE retaining walls are being considered south of the proposed new ministry building and turf area playground. The proposed walls will be constructed at the toe of the existing roughly 15 to 20 foot high ascending fill slope that supports a driveway and parking stalls. The slope was constructed as part of a buttress fill in 1997. Once an MSE wall design has been determined, Geocon Incorporated will need to perform a global slope stability analysis to verify the buttress's factor of safety has not been reduced below 1.5. MSE retaining walls are alternative walls that consist of modular block facing units with geogrid reinforced earth behind the block. The reinforcement grid attaches to the block units and is typically placed at specified vertical intervals and embedment lengths. The grid length and spacing will be determined by the wall designer. The geotechnical parameters listed in Table 1 can be used for preliminary design of the MSE walls. TABLE I GEOTECHNICAL PARAMETERS FOR MSE WALLS Geologic Unit Parameter Reinforced Zone Retained Zone Foundation Zone Existing Fill Soils Angle of Internal Friction 30 degrees 30 degrees 30 degrees I Cohesion 1 200 psf 200 psf 200 psf Wet Unit Density 130 pcf 130 pcf 130 pcf 6960 Flanders Drive 0 San Diego, California 92121-2974 • Telephone 858.558.6900 0 Fax 858.558.6159 The soil parameters presented in Table 1 are based on our experience and direct shear-strength tests performed during the prior grading of the site and represents some of the on-site materials. The wet unit density values presented in Table 1 can be used for design but actual in-place densities may range from approximately 90 to 135 pounds per cubic foot. Geocon has no way of knowing which materials will actually be used as backfill behind the wall during construction. It is up to the wall designers to use their judgment in selection of the design parameters. As such, once backfill materials have been selected and/or stockpiled, sufficient shear tests should be conducted on samples of the proposed backfill materials to check that they conform to actual design values. Results should be provided to the designer to re-evaluate stability of the walls. Dependent on test results, the designer may require modifications to the original wall design (e.g., longer reinforcement embedment lengths and/or steel reinforcement). The soil excavated from the existing fill materials would likely provide the required soil parameters as requested from the MSE retaining wall designer. We should perform additional laboratory testing when the soil is excavated to evaluate if the on-site fill soils possess the requested parameters to meet the MSE wall backfill requirements determined by the MSE retaining wall designer. Wall foundations having a minimum depth and width of one foot may be designed for an allowable soil bearing pressure of 2,000 psf. This soil pressure may be increased by 300 psf and 500 psf for each additional foot of foundation width and depth, respectively, up to a maximum allowable soil bearing pressure of 3,000 psf. retaining wall foundations should be deepened such that the bottom outside edge of the footing is at least 7 feet horizontally from the face of the slope. Backfill materials within the reinforced zone should be compacted to a dry density of at least 90 percent of the laboratory maximum dry density near to slightly above optimum moisture content in accordance with ASTM D 1557. This is applicable to the entire embedment width of the reinforcement. Some wall designers specify heavy compaction equipment should not be allowed within 3 feet of the face of the wall. However, smaller equipment (e.g., walk-behind, self-driven compactors or hand whackers) can be used to compact the materials without causing deformation of the wall. If the designer specifies no compactive effort for this zone, the materials are essentially not properly compacted and the reinforcement grid within the uncompacted zone should not be relied upon for reinforcement, and overall embedment lengths will have to be increased to account for the difference. The wall should be provided with a drainage system sufficient to prevent excessive seepage through the wall and the base of the wall, thus preventing hydrostatic pressures behind the wall. Geosynthetic reinforcement must elongate to develop full tensile resistance. This elongation generally results in movement at the top of the wall. The amount of movement is dependent upon the height of the wall (e.g., higher walls rotate more) and the type of reinforcing grid used. In addition, over time the reinforcement grid has been known to exhibit creep (sometimes as much as 5 percent) and can undergo additional movement. Given this condition, the owner should be aware that structures and pavement placed within the reinforced and retained zones of the wall may undergo movement. The MSE wall contractor/designer should evaluate the estimated deformation of wall and adjacent ground in associated with wall construction. The estimated movements should be provided to the project structural engineer to determine if the planned improvements located within the influence of the MSE retaining wall can tolerate the expected movements. The MSE wall designer/contractor should review this letter and the referenced geotechnical investigation report, including the slope stability requirements, and incorporate our recommendations as presented herein. We should be provided the plans for the MSE walls to check if they are in conformance with our recommendations prior to issuance of a permit and construction. Project No. G1968-11-02 -2- June 12, 2019 Should you have any questions regarding this correspondence, or if we may be of further service, please contact the undersigned at your convenience. Very truly yours, GEOCON iNCORPORATED e Hoobs CEG 1524 JH: SFW:dmc (e-mail) Addressee ONAL JOHN 0 o HOOBS No. 1524 0 CERTIFIED * ENGINEERING * GEOLOGIST OPC &a4e Wff GE 2714 U No. 2714 IM- Project N.. G1968-11-02 -3- June 12,2019