SCH Number 2021020227

Project Info

Title

Atria Park of Lafayette – Retrofit of Existing Bridge Retaining Walls & Culvert; County File #CDDP20-03005

Description

Background: In 1991, a Development Plan was approved to allow a congregate care facility (County File #CDDP88-03007) to be constructed and operated on the subject property. The facility was constructed in 2003 and is currently in operation as the Atria Park of Lafayette assisted living facility. The facility is accessed from Pleasant Hill Road near the intersection of Taylor Boulevard via a two-lane, approximately 250-foot-long driveway. The driveway is supported by two block-type retaining walls that cross a ravine and tributary to “Murderers Creek,” which flows through an arch culvert constructed within the retaining wall (bridge) structure. There is evidence of differential displacement of the walls which is causing the bridge to sink which in turn is causing major roadway rutting and potholing. In addition, due to heavy rainfall in the 2022/23 rain season, increased creek flows through the channel caused scouring and significant erosion of the open-bed culvert. Thus, there is now a potential for the culvert footings to be undermined in such a way that they may collapse and cause additional damage to the bridge structure and roadway above. An Initial Study and Draft Mitigated Negative Declaration was published for public comment in January of 2021, after which the applicant proposed a substantial design change and a new methodology for retrofitting the bridge structure including the open-bed channel of the culvert. As such, the Initial Study and Draft Mitigated Negative Declaration for the project has been revised to address the proposed changes as described below. Project Description: The applicant is requesting approval of a Development Plan modification of County File #CDDP88-03007 to allow construction of a retrofit of two existing retaining walls that form a bridge supporting the primary driveway to a congregate care facility currently operated as the Atria Park of Lafayette assisted living facility, and repairs to an existing culvert within the bridge structure on the subject property. The project consists of the following elements: 1. Injection of a “Uretek” polymer into the soils under the culvert footings, along the base of both retaining walls, and from the road surface extending approximately 30 feet into the existing engineered walls; 2. Bridge: • Excavation and grading (approximately 50 cubic yards, cut) at the base of the walls to set pads for scaffolding; • Installation of soil nails and tie rods throughout the surface and underlying soils of both retaining walls; • Application of shotcrete to cover the tie rods and soil nails; and, • Replacement of the existing roadway surface; 3. Culvert: • Excavation and grading (approximately 450 cubic yards, cut) within the culvert; • Construction of new footings, • Construction of a concrete apron the length of the culvert; • Placement of native gravels within the culvert to match the creek flowline; and, • Placement of up to 12 inches of rip rap upstream and downstream of the culvert for erosion control. The applicant estimates that the bridge will need to be closed to non-construction traffic for approximately 6 months due to crane and concrete pump equipment on the roadway during retrofitting work and repaving of the roadway. Thus, the project seeks approval of a temporary modification of COA #4B of file #CDDP88-03007 to allow public access to be detoured to a restricted access driveway via Diablo View Road during construction activities. Approximately 8,095 square feet of the approximately 16,945-square-foot project area is located within a restricted development area (Grant Deed of Development Rights, 13693OR414, recorded June 8, 1987), therefore, the applicant seeks approval of the County to perform work within the restricted development area. The applicant also requests approval of a tree permit to allow removal of up to ten (10) code-protected trees (two coast live oak and one valley oak, three coast redwood, one walnut, two blue gum eucalyptus, and one deodar cedar) due to grading and construction activities or poor health and condition, and to allow work within the driplines of approximately seven (7) code-protected trees (three coast live oak, two blue gum eucalyptus, one white alder, and one salix) for required grading and construction activities. If approved, the retrofitted retaining walls will allow the driveway continued to be used for safe access to the facility and the repaired and retrofitted culvert bed will prevent further erosion of the existing culvert walls and scouring of the culvert apron. No changes to the buildings or congregate care use of the facility are proposed.

2 documents in project