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HomeMy WebLinkAbout; Carlsbad Lot, APN 210120-30; Geotechnical Update Bluff and Lot Conditions; 2004-04-23C.J. HANDLE, P.E. CJ^/^ ^ 5858 MT. ALIFAN DR., SUITE 235 SAN DIEGO, CA 92111 April 23, 2004 Mr. .Ion .lensen 451 South Escondido Blvd. Escondido, California 92025 SUB.IECT: GEOTECHNICAL UPDATE Bluff and lot conditions Carlsbad Lot, APN 210120-30 5319 Carlsbad Blvd. Carlsbad, Ca. 92008 INTRODUCTION In accordance with a request from the City of Carlsbad, we have completed an on-site geolechnical study and update of the conditions at the subject site. Specific to concerns ofthe city, the purpose of our work was to see if the conditions of the bluff have essentially remained the same since the latest geotechnical update, and make appropriate recommendations regarding the bluff following our investigations, if necessary. SCOPE OF WORK The scope of our work has included the following: a. To review geological reports and data pertinent to the subject site, including review of subsurface exploration, and Laboratory testing of prevailing soils. b. To review past photographs of the subject site and compare features of such prior photographs to present photographs and present conditions. c. To review known features ofthe site observed in prior site visits and investigations and coin pare such features with the present site conditions in the field. d. To review the existence of planting on the subject site and surrounding bluff areas and determine its impact on the subject site and adjacent sites. e. To conduct field or other testing on the bluff as may be appropriate. f To conduct field inspections of the bluff and adjacent areas using the above data and determine if surface and subsurface conditions at the site have essentially remained the same since our latest geolechnical update. g. To make any necessary recommendation based on our review and investigation. HISTORY OF BLUFF SITE ON THE SUBJECT PROPERTY The subject site was originally part ofa larger 200 foot wide property. The property is located between the ocean on the west and Carlsbad Blvd on the east. Tlie west side of the property consists ofa bluff that goes down to the sand adjacent to the ocean and the east side ofthe property is bordered by Carlsbad Blvd. The construction of site improvements occuiTcd before the property was divided and the subject site created. This occurred when the owner of the larger 200 foot property contracted for beach stairs to be built on a 10 foot wide easement at the south end of the property. The stairs were built for the purpose of providing beach access from the top of the bluff to the sand in front ofthe cove area adjacent to the ocean. This beach access is used by the public for access from the top of the hluff to the sand and area in front ofthe cove adjacent to the ocean . The next historical event occurred when the original 200 foot wide parcel was divided into three parcels. The parcels were numbered Parcels 1,2, and 3. Parce! 1, which is the subject parcel, is approximately 70 feet wide and at the north end ofthe original property. Parcel 2 is approximately 60 feet wide and is the middle parcel, and Parcel 3 is approximately 70 feet wide, which footage includes the 10 foot wide easement and is located on the south end of the original 200 foot property. Following the division of the 200 foot parcel into three lots, the next historic event was the construction ofthe residence on Parce! I, the subject property. The next histonc event was the construction ofthe residence on Parcel 2, which is just south of Parcel 1. The final historic event occurred when construction of a residence was commenced on Parcel 3, which is located south of both Parcels 1 and 2 and north ofthe beach stairs. To the south ofthe beach stairs the land is State of California property. Tothe north of Parcel 1 the property consists of series of other coastal lots. The lot north of the subject property is presently being prepared for the construction of another residence. At various times excavation and other equipment, including heavy equipment during the construction of the stairs, have been used on all ofthe lots within the original 200 foot parcel and such equipment has been used on the parcel to the norih which is in the beginning stages of construction. As staled above, the subject property is approximately 70 feet wide and contains a bluff and ocean to the west and Carlsbad Blvd to the east. The bluffs in front ofeach of the lots in the original 200 foot property, however, are each different from the other. The bluffs in front ofand a part of Parcels 2 and 3 have greater and more pronounced vertical portion to such bluffs than the bluff of the subject property. The subject property also has had a historic path from the top ofthe bluff to the Santiago Deposit to the beach due to its more gently sloping configuration. As a result of the path down the bluff in front of Parcel 1, the subject properly, this property -2- was used in such a manner in the past that it became the most convenient path for the owners ofthe properly, and unfortunately, for individuals who desired to trespass onto the land to get to the beach and ocean from the top ofthe bluff As a result, paths and routes over and down the bluff of the subject property were used before the construction of the beach stairs to the south on Parcel 3. This action was done although the prior owners initialiy posted "no trespassing^' signs on fencing surrounding the original property and although trespassers cut though such fencing and ignored the posted signs. The parcel north ofthe subject property also has a historic connection to the subject property and such parcel is benefitted by a recorded easement which allows for the owners of such parcel to use the easement to go to the beach. As a result ofthe previous use ofthe bluff in front ofthe subject property, the subject property has a well defined established path from the top ofthe bluff to the Santiago Formation shelf above the sand. In orderto stop the illegal use ofthe bluff, and in orderto provide beach access, the stairs were suggested, recommended and constructed as previously discussed. When and after the stairs were constructed on parce! 3, the bluff path on parcel I, the subject property, was no longer used by trespassers and use of the paths in front ofthe subject property was substantially reduced. At the present time, the stairs located on Parcel 3 are used and are open to the pubiic. The soil conditions ofthe bluff in front of the subject property is comprised of different formations. The lowest formation has a prominent high shelf of dense Santiago deposit bedrock. The elevation ofthe sandstone shelf platform is above 13.5 feet. The massive sandstone ofthe Santiago Formation is the most erosion resistant material at the site. This formation extends beyond the subject property on both sides and rounds into the cove to the south. The Santiago Formation is a natural formation that acts as a naturally occurring sea wall protecting the subject property. The remaining soil above the high shelf consists of Pleistocene terrace deposits, overlying the Santiago formation bedrock. The slope ofthe face of the bluff varies as the elevation increases upward toward the top of the hluff At some points, the bluff is gently sloping, and at others the bluff is steeper. Near the cove, the Santiago deposits extend beyond the shelf and step down lo the sand. At the top ofthe Santiago deposit, the bluff elevation slopes up the bluff to the top of the property where the residence is localed. As noted from prior Geotechnical investigations by others and as noted from our prior site visits prior to the stair construction on the larger site, vegetation at the site, including the bluff, consisted of ice plant and grass. The property north and next to the subject properly has a significant amount of ice plant on the face ofthe bluff as do other bluffs in the area including privately owned property, slate properly, and local municipality property. PREVIOUS WORK Several site specific geotechnical investigations and other investigations have previously been conducted on this site. Some of the previous geotechnical investigations, and other -3- investigations, testing, and reports which have been prepared for the property have previously concluded in written report form, which reports have previously been reviewed by governmental agencies and incorporated into their respective permit process, include the following: * "No geotechnical conditions were apparent during investigation which would preclude the site development as planned". * '^No faults or other regionally penetrating disconfinuities appear to cut the bedrock at the site." * 'A shallow rock reef extending out from the beach provides protecfion against violent wave attack on the bluffs at the site." * 'The bedrock platform to the north ofthe cove acts as a natural seawall." * "It is recommended that measures be taken to prevent the erosion ofthe bluff face. Such measures could include the channeling of surface drainage away from the bluff face, construction ofa sea wall and or covering the bluff face with an erosion resistant material, ie. rip rap, or gunite." * "All permanent finished slopes should be protected from erosion, particularly from wave effects. Finished slopes not directly exposed to wave forces may be protected from erosion, as appropriate, using deep rooting ground cover planting, cement, or a combination of these procedures." * "In our opinion, this protection measure should provide suitable protection against occasional splashes from waves, adequate control of surface runoff on the slope faces. Positive slope drainage, including drains, if needed should be provided." * "Aiternative coastal protection measures for the site were considered and included a rock revetment aiong the subject shoreline or a retaining (sea) wall along portions of the shoreline. Ultimately, a retaining wall along the northern half of the shoreline founded on the existing bedrock platform .... were selected." * "In our opinion, the on-site soils and bedrock material may be excavated with conventional griiding equipment" SUMMARY OF REVIEW AND INVESTIGATION in our investigation, we conducted all of the actions set out in the portions ofthis report entitled Scope of Work and there were instances where we exceeded the scope ofwork actually staled. We began by again reviewing all prior Geolechnical and geological reports, studies, and their supporting information, and reports, including subsurface exploration, and Laboratory testing and soils reports. We consider the scientific provisions in these reports and Laboratory results to be accurate and highly reliable. We also reviewed several photographs which show maintenance being conducted on the bluff with hand labor, and before and after photographs from various angles taken over different -4- years on the subject site. In our review of photographs we generally consider before and after photographs from identical angles, under similar time and lighting conditions to be reliable. Absent these conditions, photographs from different angles, and under different time and lighting conditions, or ones taken with biased angles, are less helpful and are not sufficiently reliable to form an opinion absent other actions, investigations, site visits, and a familiarity with the site. Fortunately, we have the personal, on site, prior experience and familiarity with the site and bluff from our work in the past in the area. In a specific attempt to be accurate, we went beyond the photographs and plotted points of reference from various sources for the subject site, and we plotted points of reference from prior site inspections and visits made by us. This information was then compared to present existing conditions. We considered our professional experience wilh the site and our professional knowledge ofthe site to be not only invaluable, but highly reliable. We were able to confirm that the same land forms ofthe bluff existed in prior investigations that exist presently and we did not find any land form alteration had occurred on the site when we compared our prior data and investigations ofthe site lo present site data and investigations of the site. During our investigation, we probed locations to determine and contirm surface reference points and we also investigated their respecfive densities. Each probe confirmed the existence of a historic known land form and each density was more than adequate. We also determined that depth of soil over more dense and stable formations is not excessive and is suitable. Finally, we analyzed and confirmed the soil to be unchanged at the subject site when present soil conditions were compared to the information and prior reports for the site. After concluding our invesfigation, we determined that the surface conditions ofthe bluff on the subject site have essentially remained the same, including the pathways to the Santiago Formation. The bluff and soil conditions on the bluff are stable and have nol been effected by the most recent niaintenance work recently conducted at the site, nor will they be significantly or adversely affected by (1) the existence of the ice plant or any other deep rooted plant on the bluff at this site; or (2) by the undertaking of future routine maintenance; or (3) by walking on the paths. After reviewing all ofthe prior geotechnical information we have also concluded that our findings are consistent with prior findings, investigations, reports, and recommendations by others. In our inspection, we did not determine any actions that were inconsistent with proper and appropriate site maintenance, nor did we determine that the actions and work done on the bluff previously exceeded normai and customary maintenance ofthe subject site. After comparing soil information with prior findings of others, we specifically did not find any existing soil inconsistent wilh prior findings by others or inconsistent with the soil conditions on the bluff for the subject site as compared lo the sites located to the north or soulh. We did not find that grading, fill, or earth excavation had been performed on the site, and the subject site has been well maintained. The vegetation on the subject site is self sustaining due to natural coastal precipitation with little future mainlenance required. As stated in the historic section of this report, the bluff contains historic paths on the subject site and also contains a shelf at the Santiago Formation. Both the shelf and the path are actually -5- conlributingbenefitsto the subject site. This is because both are flatter in elevation than surrounding slopes and both act as benches that would reduce the likelihood of any natural occurring dislodgement above these elevations from going further down the bluff than the path or Santiago Formation shelf respectively. Therefore, these pre-existing topography features assist in benefitting tlie subject site, to tlie degree possible, in a manner similar to benches engineered for slopes. As stated by other consultants, and as observed by us, the bluff at the subject site has previously supported grass, and ice plant. The property and parcel to the north of the subject property also contains ice plant on most of the bluff face. While the variety of ice plant may vary, and while different varieties may have different effects on their ability to reduce erosion, the ice plant existing on the subject site should actually perform desired erosion control better than other choices which have been made along the coast. How plants look has no engineering significance, but the positive effect of controlling surface water and reducing erosion does. Upon our site inspection, we determined that all ofthe surface water from the top portion ofthe lot, where the residence is located, drains away from the bluff. This is consistent with prior recommendations of others and of our firm. Since the control of surface water on the upper lot has been accomplished, the only remaining surface water issue to consider could arise from precipitation falling on the face of the bluff. The choices of mitigation measures to handle these occurrences has been discussed in prior reports by us and others. For example, and as stated in prior reports by others, retaining walls, sea walls, rip rap, and or surface gunnite are alternatives. In view of their appearance, however, planting is usually highly ranked as an alternative to reduce erosion and the effect of surface water within the priority of alternatives. The condition ofthe subject site in the manner that it has been maintained including the benefits of the existence of vegetation has assisted the subject site and will continue to assist the subject site in the future. Both the subject site and the property to the north of the subject site have both benefitted by ground cover which are possible by their bluff formations including their Santiago Formafion as compared to parcels withinthe cove. With existing vegetation they are much less likely to form gullies where concentrated surface water can occur. Simply put, vegetation at this site reduces the possibility of the development of gullies. Eliminating and or reducing gullies reduces the possibility of damage to the bluff by reducing drainage into concentrated gullies. The absence of concentrated uncontrolled surface water draining inlo concentrated gullies benefits this site and the site to the north of the subject site. If this were nol done al this site and if one ofthe above choices lo control erosion did not exist, gullies would certainly develop which would ultimately get deeper over fime and incise into the bluff, requiring more invasive procedures to correct. Therefore, the existence of the vegetation, or deep rooted species on the bluff and slopes at this subject site and the site to the north, is highly beneficial and recommended and is consistent with our recommendations and the recommendations of prior firms. As stated in prior reports and studies, there are other methods of protecfion ofthe bluff, or portions of the bluff at this subject site. These methods include the recommendafions by others for the placement of covering the bluff face with an erosion resistant material, ie. rip rap, gunite, a retaining wali localed and founded upon the exisfing bedrock or Sanfiago Formation, a sea wall, or -6- a rock revetment, At the present time, and due to our examination ofthe subject site and its present configuration and due to the existence ofa stable existing bluff, our best recommendation is to continue lo use the existing vegetation and routine landscape mainlenance as one ofthe preferred methods of protecfing the bluff at the subject site. If an episode occurred, that endangered a preferred method of protection, then our recommendation would be to proceed with other methods suggested by others, such as a retaining wali or sea wall. Within our evaluation and investigafion, we have concluded that the placement of a retaining wall or sea wall would allow for proiection ofthe site from an ocean episode and there is nothing in prior findings or our findings that would preclude the construction of a retaining wall or sea wall, including the prior findings by others that on-site soils and bedrock material may be excavated with conventional grading equipment during construction. However, since the prior findings have also concluded that the Santiago Formation in front of the property acts as a naturai sea wall, we believe that the subject site, its bluff configurafion, and the current existence of vegetafion, should presently remain unchanged and should actually be promoted. C..1. Randle. P.E. RCE 22096CA -7-