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HomeMy WebLinkAbout; Slope Failure South of Las Olas Court; Preliminary Geotechnical Findings; 2005-02-174 Leighton andAssociates, Inc. A LEIGHTON GROUP COMPANY February 17, 2005 MM Project No. 971146-006 To: Attention: Subject: References: Introduction Concordia Homes 1903 Wright Place, Suite 120 Carlsbad, Califomia 92009 Mr. Dale Wilson Preliminary Geotechnical Findings, Slope Failare South of Las Olas Court, Villagio, Carlsbad Tract No. 98-02, Carlsbad, Califomia Leighton and Associates, 2000, Geotechnical Update Report, Colina Roble, Carlsbad Tract No. 98-02, Carlsbad, Califomia, Project No. 4971146-002, dated March 24,2000 , 2001a, As-Graded Report of Rough and Fine Grading, Lots 6 tlirough 28, Colina Roble, Carlsbad Tract No, 98-02. Carlsbad, Califomia, Project No. 4971146-004, dated March 16,2001 -, 2001b, As-graded Geotechnical Report of Post-Grading Operations, Colina Roble, Carlsbad Tract No. 98-02, Carlsbad, Califomia, Project No. 971146- 004, dated September 18,2001 In accordance with your request, Leighton and Associates, Inc. (Leighton) has performed a limited preliminary evaluation of the recent slope failure on the south side of Las Olas Court within the Villagio project (Carlsbad Tract No. 98-02), located in Carlsbad, Califomia. The purpose of our evaluation was to geologically map the slope failure and the general vicinity; evaluate potential future movement and impacts to adjacent residential properties (both below the failure area and beyond the ridgeline to the south); and to provide preliminary recommendations relative to the mitigation of the slope failure. 3934 Murphy Canyon Road, Suite B205 • San Diego, CA 92123-4425 858.292.8030" Fax 858.292.0771 •www.leiglntongeo.com 971146-006 Site Description and Existing Geotechnical Conditions The Villagio project (formerly known as Colina Roble) was rough and fme graded between June and December 2000 (Leighton, 2001a) while the post grading operations were performed in 2000 and 2001 (Leighton, 2001b). Rough grading of the site in the general vicinity of the slope failure included the removal of potentiaUy compressible soils (including alluvium, colluvium, and weathered formational material), placement of compacted fill, and the constmction of a stability fill along the south side of Las Olas Court. Based on our review of the as-graded geotechnical conditions (Leighton, 2001a), the Delmar Formation claystone is present beneath the compacted fill along Las Olas Court and on the lower hillside to the south below an approximate elevation of 170 feet mean sea level (msl). The upper portion of the hillside (above an approximate elevation of 170 feet msl) consists of sandstone. Colluvial soils are present in the drainage on the east side of the recent slope failure. The approximate limits of the geologic units are presented on the Geologic Map (Figure 1). The Geotechnical map from the as-graded report for the project was used as the base map for the figure. Geologic mapping of the slope on the south side of Las Olas Court during the rough grading operations (Leighton, 2001a) indicated the presence of a clayseam within the bedrock material. Due to the clayseam and blocky nature of the claystone in this area, a stability fill was recommended and constmcted during the rough grading operations in this area. The stability fill (consisting of compacted fill) was approximately 15 feet wide at the toe-of-slope and extended at least 5 feet below the toe-of-slope elevation. Due to the presence of sensitive habitat (i.e. Coastal Sage) above the top of the slope (that was off-limits and could not be entered), the stability fill extended less than 5 feet above (or behind the top-of-slope). A subdrain system was instaiied along the bottom of the buttress and stability fill backcut and supposedly outletted into the storm drain inlet stmcture located on the south side of Las Olas Court at approximate Station No. 11+25. Also during the rough grading operations, a landslide was encountered and geologically mapped to the west of the recent slope failure. A buttress was designed and constmcted to stabilize the landslide to the south of Lots 24, 25, and 28 (Leighton, 2001a). The approximate location of the stability fill and buttress key is shown on Figure 1. The eastem end of the buttt:ess was located approximately at the west side of Lot 28. Findings At the beginning of February 2005, we were notified that one of the slopes at the site was moving. On Febmary 4, 2005, a geologist from our office made a site visit to observe the reported slope movement. Based on our site visit, it was evident that the slope east of Lot 28 was failing in a classic rotational fashion where the toe of the slope was moving out and upwards whiie the upper portion of the landslide was moving downwards. Geologic recoimaissance indicated the slope -2- Leighton 971146-006 failure extend from a point approximately 75 feet east of the storm drain inlet at approxunate Las Olas Court Station No. 11+25 to approximately 30 feet west ofthe Lot 28 property line (i.e. across from Lots 10 through 12). The slope failure was estimated to extend approximately 100 feet behind the top of the slope along the south side of Las Olas Court. At the time of our first visit, the toe of the failure extended up to and slightly into the street just beyond the curb and gutter. Subsequent movement of the slope failure has resulted in a bulge ofthe toe and additionai movement into the street (on the order of 5 feet or less) as of Febmary 11, 2005. A row of sandbags was placed in front ofthe toe and observations as of Febmary 15, 2005 indicate the failure is still moving but only on the order of a few inches per day. The lower portion of the failure (i.e. below the concrete drainage ditch at the top of the slope) was covered with plastic to minimize the infiltration of water into the landslide mass. Due to the movement ofthe slope failure, the down drain for the concrete drainage ditch at the top of the slope was damaged causing water to enter the backfill around the damaged storm drain pipe. Provisions were made to minimize the inflltration of the surface water by covering the area with plastic. During our site visit on Febmary 10, 2005 that occurred during a rain storm, a significant amount of flowing water was noted in the concrete drainage ditch on the westem side of the recent slope failure and at the top ofthe slope behind Lot 28. Observations indicated the water was coming a from a previously unknown drain outlet from the residential development uphill and the south of the Villagio project. The storm drain water was flowing out of the storm drain outlet stmcture (within the city of Encinitas) and along the ground surface and ultimately into the top-of-slope concrete drainage ditch. Due to the damaged down drain pipe connecting the concrete drainage ditch to the storm drain system in the street, the water in the top-of-slope concrete drainage ditch behind Lot 28 and on the westem portion of the recent siope failure has been directed on to the plastic on the slope and into the gutter in the street. Based on our reviews performed to date, we understand that there is at least one more storm drain outlet located above and to the south of the Villagio project. On Febmary 15, 2005, the limits of the slope failure were surveyed with a GPS unit and the limits shown on the Geologic Map (Figure 1). A definite back scarp is evident on the eastem and central portions of the failure; however, the back scarp is less evident on the westem side of the failure. The back scarp is on the order of 10 to 15 feet in height. On the westem side of the recent slope failure, only some discontinuous cracks (or fractures) are present indicating the extent of the movement in this area. It appears that the buttress on Lot 28 has partially stabilized the recent slope failure and minimized the amoimt of down slope movement. Movement of the slope has also caused the buckling of the metal fence along the southeastem and eastem sides of Lot 28, as well as, a portion of the wooden fence on Lot 28 along Las Olas Court. During our site visits we also observed the hillside on either side and above the recent slope failure. No indications of any significant slope movement were observed outside the limits of the recent slope failure with the exception of some minor slope creep on some of the steeper portions ofthe hillside southeast of the recent failure. These slope creep areas appear to be less than 1 foot thick and limited in extent. -3- Leighton 971146-006 During our GPS mapping of the recent slope failure, we also installed 6 survey monuments that can be used to evaluate any fiiture slope movement. Five of the monuments were instaiied on the ridgeline south (and uphill of the recent slope failure); the sixth monument was installed in the middle of the landslide. Due to the lack of accuracy of the GPS survey unit, precise horizontal and vertical measurements of the monuments were not made. A civil engineer/surveyor shouid be retained to monitor the monuments on a weekly basis. The approximate locations of the survey monuments are presented on the Geologic Map (Figure 1). Slope Stabilitv Analysis Based on the mapped configuration of the recent slope failure (along Cross-Section B-B' - Figure 2) and data from our prior slope stability analysis for the project (that was obtained during the design of the buttress on Lots 24, 25, and 28), we performed a preliminary slope analysis of the recent slope failure. Based on our assumed landslide geometry, soil strengths, and relatively high ground water we calculated a factor of safety of I.O for the current conditions (which matches the assumption that in order to get movement, the factor of safety has to be less than 1.0). Additional preliminary analysis indicates that a buttt-ess with a key width of approximately 60 to 70 feet can be designed to stabilize the landslide. However, temporary stability analysis of the backcut (which will be within landslide deposits) indicated the temporary stability of the backcut will be less than acceptable. As a result, our preliminary slope stability analysis has indicted that in order to stabilize the slope failure; the entire landslide may need to be removed. It should be noted that additional analysis (assuming that the removal of the slope failure is done in sections) might result in a buttress design that can leave some of the landslide deposits in place. As an altemative to the large buttress stabilization design, constmction of a smaller buttress and the installation of shear pins may be able to be performed to stabilize the slope failure. Additional data (such as the actual elevation and inclination of the mpture surface), preparation of additional cross- sections, and more analysis will be needed to determine the feasibility of a shear pin design. Conclusions Regarding Potential Impacts to Adiacent Properties Based on our geologic mapping, slope stability analysis, and professional experience with similar conditions, it is our professional opinion that the risk of fijrther adverse impacts to the residential stmcture on Lot 28 and the residential properties above and below the recent slope failure is low. Geologic observations of the hillside above the recent slope failure, indicated there is no evidence of adverse geotechnical conditions that could impact the residential properties on the ridgeline south of the recent slope failure. In addition, it is our professional opinion that the negative impacts of the recent slope failure to the residential stmcture on Lot 28 or to the residential properties on the north side of Las Olas Court are considered low. Our professional experience has indicated that slope failures similar to the one that occurred at the Villagio project tend to reach equilibrium over time as the outward and downward movement -4- Leighton 971146-006 of the slope failure reduces the driving force that caused the movement. Our experience has also indicated that the movement of these types of slope failures is relatively slow (i.e. similar to what has already occurred) and that catastrophic movement is highly unlikely. As long as nothing is added to the upper portion of the landslide and nothing is removed from the toe, the equilibrium of the landslide will not likely change and no significant additional movement should occur. However, the only significant additional factor that may change the equilibrium is adding water (in the form of precipitation) to the landslide mass. The addition of water will increase the weight of the landslide (effectively increasing the driving force) while at the same time the subsurface would impart additional destabilization forces. Since the entire slope failure area cannot be covered with plastic, future precipitation events may result in addhional movement of the slope failure. However, we do not anticipate that any future movement will be significant. We recommend that the material at the toe of the slope failure not be removed unless it impacts the movement of vehicular traffic on Las Olas Court. Should the toe impact vehicular traffic, the material should be removed and placed back on the toe of the slope failure. If it is not acceptable for soil to be left on the street and placement of the material on the slope south of the street is not feasible or is not effective in decreasing the outward movement of the toe, the material may be hauled off-site. It should be noted that as indicated above, removing the material from the toe will change the equilibrium and may cause the continued movement of the toe; however, the movement should be relatively slow (i.e. inches to a few feet per day). Although we anticipate that the fiorther impacts to the residential properties above and below the slope failure are considered low, the oversteepened back scarp may be susceptible to minor slumps and other instability conditions. While these instability conditions are not anticipated to be significant, there is a possibility of additional failures above the existing slope failure. As a result, we recommend that the measures to stabilize the slope failure (i.e. the constmction of the buttress) be performed as soon as practical in order to minimize additional failures. Conclusions and Recommendations Concerning the Mitigation ofthe Slope Failure Additional slope stability analysis will be required to determine the most practical and cost- effective design to stabilize the slope failure. However, the most likely design will be essentially the complete removal of the slope failure and the constmction of a relatively large buttress (that is constmcted in sections). The main purpose of the additional analysis will be to determine the lateral extent of the buttress and what size the sections should be limited too. For planning purposes, we anticipate that the first thing that should be done during the constmction of the buttress is the removal of the upper portion of the slope failure mass to an elevation of 130 to 140 feet msl. The material removed from the upper portion of the slope failure can either be stockpiled in the street or hauled off-site. Then the lower portion of the slope failure can be removed in 3 or more sections and the material stockpiled and/or flip-flopped and placed as fill in the adjacent section. -5- Leighton 971146-006 In addition, the following recommendations should be implemented: • Slope Survev Monuments - The six survey monuments recentiy placed on the hillside should be surveyed by a civil engineer/surveyor on a weekly basis to monitor any potential movement. In order to establish a baseline, we recommend that two readings be performed within 1 to 2 days. Subsequent reading can be performed on a weekly basis. The results should be provided to us so that we can evaluate the movement (if any). If no appreciable movement is measured, the frequency of the readings can be spaced out to every two weeks or greater. If movement is measured, recommendations to increase the frequency of the readings may be made. • Existing Buttress/Stabilitv Subdrain - The only outiet location for the buttress/stability fill subdrain is into the storm drain inlet stmcture on the south side of Las Olas Court at approximate Station No. 11+25. We recommend that the outlet location be observed to detennine if the subdrain is properiy outietted. If it does not appear to be outietted, we will need to evaluate the impact of the potential build-up of ground water in the subdrain. The subdrain at either end of the recent slope failure may need to be dug up and temporary outletted until the storm drain system is repaired. It should be noted that the subdrain system would likely be removed during the constmction of the recommended buttress. • Minimizing Surface Water Percolation into the Slope Failure - We recommend that provisions be made to ensure that the water in the concrete drainage ditch at the top of the slope is outletted into the storm drain system or street curb and gutter and not allowed to percolate into the slope failure. Off-Site Storm Drain Outlet Flow - The surface flow from the off-site storm drain outlet located southwest of the recent slope failure (and within the City of Encinitas) should be picked up and placed in some type of drainage device (i.e. pipe or open concrete drainage swale). The water should not be allowed to percolate into the ground especially since it will increase the amount of water in the mapped landslide south of Lots 24, 25, and 28. As a temporary fix, the erosional channel that currently exists should be lined with plastic to minimize infiltration of the water flow. In addition, other storm drain outiets that are present above (and to the south of the Villagio development) should be evaluated to determine if those outlets need to be picked -up and extended by hard drainage facilities (i.e. pipe or open concrete drainage swale). -6- Leighton 971146-006 Ifyou have any questions regarding this letter, please do not hesitate to contact the undersigned. We appreciate this opportunity to be of continued service. Respectfully submitted, LEIGHTON AND ASSOCIATES, INC. Randall K. Wagner, CEG 1612 Senior Associate Michael R. Stewart, CEG 1349 Principal GeologistA^ice President Attachments: Figure 1 - Geologic Map Figure 2 - Geologic Cross-Section B-B' -7-Leighton NSERT MAP HERE