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HomeMy WebLinkAboutCT 84-25; BATIQUITOS BLUFF; GEOTECHNICAL INVESTIGATION; 1984-07-31• • • • • • , Woodward-Clyde Consultants GEOTECHNICAL INVESTIGATION FOR THE PROPOSED BATIQUITOS BLUFF CARLSBAD, CALIFORNIA Prepared for Sammis Properties 5030 Camino de la Siesta San Diego, California 92108 Jon D. Briggs Vice President PROPERTIES 2650 Camino Del Rio North #100 • San Diego, CA 92108 (619) 298-7112 • • • • • • • • • • • Project No. 53173K-SI01 Woodward-Clyde Consultants GEOTECHNICAL INVESTIGATION FOR THE PROPOSED BATIQUITOS POINTE CARLSBAD, CALIFORNIA PURPOSE AND SCOPE OF INVESTIGATION This report presents the results of our geotechnical investigation at the site of the proposed Batiquitos Bluff project . The site is located on the north side of Batiquitos Lagoon, east of the A. T. &S. F. railway, west of Interstate 5 Freeway, and south of Ponto Drive in Carlsbad, California . This report has been prepared exclusively for Sammis Properties and their consultants for use in evaluating the property and in project design. This report presents our conclusions and/or recommendations regarding: 0 0 0 0 0 0 0 0 The geologic setting of the site Potential geologic hazards General subsurface soil conditions General extent of existing fill soils Conditions of areas to receive fill Characteristics of proposed fill material Expansive soils Depth to water (if within the depths of our subsurface investigation) • • • • • • • • • • • Project No. 53173K-SI01 Woodward-Clyde Consultants 0 Stability of proposed cut and fill slopes 0 Grading and earthwork 0 Types and depths of foundations 0 Allowable soil bearing pressures 0 Design pressures for retaining walls In addition, we are evaluating the erosion potential of the on-site soils and the bluff-face fronting the lagoon to aid the developer's compliance with Section 30253 of the 1976 Coastal Act. The results of that portion of our study will be reported under separate cover. DESCRIPTION OF THE PROJECT For our study, we have discussed the proposed project with Mr. John Briggs of Sammis Properties and Mr. Patrick O'Day of O'Day Consultants. We have been provided a copy of the "Tentative Map of Batiquitos Bluff, Carlsbad Tract No. CT-84-25/P. U .D. 72" prepared by O'Day Consultants dated June 4, 1984. Finish floor elevations of the buildings and spot design elevations of streets are shown on these plans. Additionally, we have reviewed work performed by our firm on Batiquitos Pointe a similar site, approximately 1000 feet to the west, on the west side of the AT & SF Railway. We understand that the proposed project will include subdividing the parcel into 4 lots for single family and multi-unit residential structures. Site grading will create cut and fill slopes, constructed at maximum inclinations of 2 :1 (horizontal to vertical), to heights of less than 25 feet. Site improvements will consist of 1 to 3 story structures of wood frame and masonry construction and underground parking. Swimming pools and lined 2 • • • • • • • • • • • Project No. 53173K-SI01 Woodward-Clyde Consultants shallow ponds are also planned. The planned location and layout of the major project structures are shown on the Site Plan (Figure 1). FIELD INVESTIGATIONS Our field investigation included making a visual reconnaissance of the existing surface conditions, made observations of the erosion of the existing bluffs, making ten test borings on May 13 and 14, 1984 and obtaining representative soil samples. The test borings were advanced to depths ranging from 16 to 33 feet. The locations of the borings are shown on Figure 1 . A Key to Logs is presented in Appendix A as Figure A-1. Final logs of the borings are presented in Appendix A as Figures A-2 through A-11. The descriptions on the logs are based on field logs and sample inspection . SITE CONDITIONS Geologic Setting The site lies on a generally west sloping, Quaternary age, marine terrace surface. The relatively horizontally bedded terrace materials have been deposited on a wavecut platform on the underlying, of the Tertiary age, Santiago Formation . Surface Conditions The surface of the site which appears to be little changed from the natural state, is partly under cultivation for flowers and is subject to daily irrigation. Lot 4 covers the terrace surface above the southeast and west facing 60-foot high bluff slopes . An existing 66-inch storm-drain extends north-south across Lot 3 into the desiltation basin which has been constructed in the open space easement 3 • • • • • • • • • • • Project No. 53173K-SI01 Woodward-Clyde Consultants west of the site; an existing 8-inch water line (to be abandoned) extends east-west across the north end of Lot 1. Soil is currently being borrowed from the hilltop at the east property line, in conjunction with the construction of the desiltation basin. This cut- grading is removing an irrigation water storage pond embankment in that area. Minor man-made features on the site include irrigation pipe, fencing and roads. No fill soils were observed in our test borings, however, cultivation has resulted in a few scattered shallow fills and piles of organic debris across the site and along the bluff top. An undocumented "sliver" fill has been placed along the top of the west-facing bluff slope above the desiltation basin . Subsurface Soils The site is underlain by 2 to 3 feet of slightly porous loose to medium dense, red brown, silty medium to fine sandy topsoil. This overburden is underlain by 2 to 30 feet of dense to very dense light brown, silty to clayey sands of the Lindavista Formation of Quaternary age. The upper part of this unit (within the proposed grading depth in some areas) contains some clay and may be moderately expansive. The Lindavista Formation is underlain by the Santiago Formation of Tertiary age, which consists of relatively horizontally bedded dense, light gray clayey sands and hard siltstones and claystones . Structure Exposures of Quaternary and Tertiary age sediments on and near the site exhibit horizontal or nearly horizontal bedding attitudes; however, actual regional bedding attitudes are difficult to evaluate due to cross-bedding in these units. No evidence of faulting was observed on or near the site during our field work . 4 • • • • • • • • • • • Project No. 53173K-SI01 Woodward-Clyde Consultants Landslides Our field investigation and review of aerial photographs and available geologic literature disclosed no evidence of landsliding on the site area . Furthermore, no landslides have been mapped on or adjacent to the site. Ground Water Ground water was encountered in four test borings at the following depths: Test Boring 5 6 8 10 No. Depth to Water (Feet) 6 13 14 23 Other borings did not exhibit ground water at the time of drilling. No evidence of marshy areas or ground water seepage was noted during our reconnaissance. DISCUSSIONS, CONCLUSIONS AND RECOMMENDATIONS The discussions, conclusions, and recommendations presented in this report are based on the results of our field and laboratory studies, analyses, and professional judgment . Potential Geologic Hazards Faulting and Ground Breakage No faulting or evidence of faulting is known for the subject site. A few minor faults have been mapped in the seacliffs south of the site in the Leucadia area, and in the bluffs above Batiquitos Lagoon, but the closest 5 • • • • • • • • • • • Project No. 53173K-SI01 Woodward-Clyde Consultants fault of significant mapped length is the postulated offshore extension of the Rose Canyon zone, approximately 5 miles to the southwest. The closest faulting to the site with recorded earthquakes of magnitude 4 or greater is associate with the Elsinore Fault zone about 25 miles northeast of the site and the San Clemente Fault zone about 50 miles southwest of the site. No magnitude 4 or larger earthquakes are associated with the Rose Canyon Fault or other faults in the general San Diego-Oceanside coastal area . Although no known faulting is indicated on the site, it is possible that splinter faults or ground fractures may be encountered in the proposed excavations. Cut areas should be inspected by a geologist during grading and if such features are encountered, some remedial work or adjustment to the construction procedure may be required. Landslide Potential In our opinion, there are no apparent landslides on the site. Liquefaction The geologic units at the site are not recognized as having a potential for liquefaction. Ground Water Our investigation indicates that the "perched" water at various levels in the test borings may have resulted in part from surface irrigation as well as migration from the land area generally north of the site . We recommend carefully observing cut slopes during grading. If water seeps are encountered during grading, we recommend that drains be designed when actual slope conditions are exposed . 6 • • • • • • • • • • • Project No. 53173K-SI01 Woodward-Clyde Consultants Slope Stability Analysis The available plans indicate that the proposed cut and fill slopes on the order of 25 feet high ( or less) at an inclination of 2: 1 (horizontal to vertical) are proposed . For this condition, we have performed stability analyses by the Janbu method using parameters developed from the results of plasticity tests and direct shear tests performed on representative samples of similar soils from nearby areas, and our experience in the general site area. The results of our analyses indicate that 2: 1 inclined cut and/ or fill slopes having maximum heights of 25 feet have calculated factors of safety for deep-seated failure in excess of 1. 5 for static conditions. Stability analyses require using parameters selected from a range of possible values; thus there is a finite possibility that slopes having calculated factors of safety as indicated above could become unstable. In our opinion, the probability of the slopes becoming unstable is low, and it is our professional judgment that the slopes can be constructed as indicated above . We recommend that an engineering geologist from our firm examine all cut slopes for possible adverse conditions during grading . Fill slopes, particularly those constructed at inclinations steeper than 2: 1, are susceptible to shallow slope sloughing in periods of rainfall, heavy irrigation, required . and/ or upslope runoff. Periodic slope maintenance may be Sloughing can be reduced by backrolling slopes at frequent intervals. As a minimum, we recommend that fill slopes be backrolled at maximum 4-foot height intervals. Additionally, we recommend that all fill slopes be trackwalked so that a dozer trnck covers all surfaces at least twice . 7 • ) Project No. 53173K-SI01 Woodward-Clyde Consultants Cut slopes in the central part of the site may encounter Santiago Formstion mudstones at the toe. This condition may require buttressing for stability. We recommend inspection of all cut slopes during g!ading by the engineering geologist. Recommendations for buttressing can be presented on an addendum report during grading if required. Site Preparation We recommend that the building areas and all fill areas be cleared of any existing vegetation, trash, and any debris or rubble. The undesirable materials generated during the clearing should be disposed of off the site. We recommend that all porous topsoils, and other natural loose soils or existing fills within the building areas or proposed fill areas not removed by planned grading be excavated or scarified as required, replaced, watered, and then recompacted prior to placing fill or structures. Based on our field investigation, we anticipate that up to 3 feet of topsoil may have to be removed. We recommend that the soil engineer evaluate the actual depth of excavation in the field at the time of grading. Building areas are generally defined as the building limits plus a horizontal distance of 5 feet beyond all settlement-sensitive portions of the building. If building locations are not known, we recommend that the recompacted zone extend over the entire level portion of the lots. We recommend that the upper 2 feet of materials in the fill areas be composed of nonexpansive soils. Nonexpansive soils are defined as granular soils that have • a potential swell of less than 3 percent when recompacted to 90 percent of maximum laboratory density at optimum moisture content, placed under an axial load of 160 psf, and soaked in water. We recommend that all fill soils be placed between optimum moisture content and 3 percent above optimum moisture content. 8 • • • • • • • • • • • Project No. 53173K-SI01 Woodward-Clyde Consultants In our opinion, the formation soils on site should generally be suitable as select structural fill material and are considered to be nonexpansive or low to moderately expansive in nature; however, potentially expansive clay layers may be encountered within the sands of the Lindavista formation, or in the Santiago formation. Where these expansive soils are encountered at finish grade they should be undercut to a minimum depth of 2 feet and replaced with properly compacted nonexpansive soils . We recommend that all earthwork be done in accordance with the attached Specifications for Controlled Fill (Appendix B). Excavation Characteristics Based on the results of the test borings and on our experience with similar soils, it is our opinion that most of the on-site soils can generally be excavated by medium ripping with conventional excavation equipment. It is anticipated, however, that heavy ripping will be required for excavating the hard cemented zones that are usually encountered in the Lindavista formation . Drainage We recommend that positive measures be taken to properly finish grade each pad after structures and other improvements are completed so that drainage waters from the pads and adjacent properties are directed off the pads and away from foundations, floor slabs, and slope tops. Even when these measures have been taken, experience has shown that a shallow ground-water or surface-water condition can and may develop in areas where no such water condition existed prior to site development; this is particularly true where tbe graded surface is near the Lindavista formation contact, and where a substantial increase in surface-water infiltration results from landscaping irrigation . We recommend that all finished grading be inspected to evaluate the possible need for subsurface drains. 9 I I , , Project No. 53173K-SI01 Woodward-Clyde Consultants Foundations Most of the proposed buildings, as presently located, will be supported on relatively unyielding formation al soils; however, some buildings will be supported partially on formational and partially on compacted fill. In our opinion, the proposed structures can be supported on conventional spread or continuous strip foundations founded on either formational soils, properly compacted fill soils, or on a combination of both. We recommend that spread or continuous footings placed on either compacted fill or a combination of compacted fill and formational soil be designed using a maximum allowable bearing capacity of 2,500 psf. Footings founded in only formational material should be designed using a maximum allowable bearing capacity of 5,000 psf. Continuous footings should have a minimum width of 18 inches and spread footings should have a minimum width of 2 feet. All footings should be at least 18 inches deep measured from rough finish grade, or in the case of interior footings, from finish grade. A one-third increase in allowable soil bearing pressure may be used for design of footings to resist total load including wind and seismic forces. We recommend that all continuous footings be reinforced top and bottom with at least one No. 4 reinforcing bar. We recommend that additional steel be placed in the footings at cut-fill locations. Footings should not be located within 8 feet of the top of a fill slope or 5 feet of the top of a cut slope. Footings located closer than 8 feet (or 5 feet) from the top of a slope should be extended in depth until the outer bottom edge of the footing is the required distance (8 or 5 feet) horizontally from the outside face of the slope. 10 • • a • • • • • • Project No. 53173K-SI01 Woodward-Clyde Consultants Floor Slabs We recommend th.at the slab-on-grade floors have a minimum thickness of 4 inches and that they be nominally reinforced with 6 x 6, 10/10 wire mesh placed at the midpoint of the slab. Concrete slabs should be underlain by 4 inches of sand and a plastic vapor barrier in those areas where floor coverings are sensitive to moisture. Retaining Walls and Lateral Loads We recommend that retaining walls not restrained from movement at the top and required to support lateral earth pressures due to differential soil height be designed for an equivalent fluid pressure of 35 pcf. Retaining walls restrained from movement at the top, such as basement walls, should be designed for an equivalent fluid pressure of 35 pcf plus a uniform lateral pressure of lOH psf (H = the height of retained earth in feet). These pressures are based on horizontal backfill surfaces, the use of on-site granular materials for backfilling the walls, and adequate drainage to prevent buildup of hydrostatic pressure behind the wall. If other conditions and/or particular loads, such as sloping backfill, adjacent footings or vehicle surcharge loads, are to be considered in the vicinity of retaining walls, we should be advised so that additional recommendations can be given as required . To provide resistance for lateral loads, we recommend that passive pressure be assumed equivalent to a fluid pressure of 300 pcf for footings and shear keys poured neat against cut formational soils or properly compacted fill soils. The upper 12 inches of material in areas not protected by floor slabs or pavements should not be included in design for passive resistance to lateral loads. This lateral pressure is based on the assumption that the ground surface adjacent to the footing is nearly horizontal for a minimum distance of 10 feet from the face of the footing or three times the height of the surface generating passive pressure, whichever is greater. 11 • • • • • • • • • • • Project No. 53173K-SI01 Woodward-Clyde Consultants In calculation of frictional resistance to lateral loads, we recommend using a value of O. 4 as the allowable coefficient of sliding friction between concrete and the underlying soil. If combined frictional and passive lateral resistance are utilized in design, we recommend using a frictional resistance of O. 3. We recommend that samples of representative soils encountered on site during grading be tested in the laboratory for varification of their strength parameters and of the above recommended values based for design. UNCERTAINTY AND LIMITATIONS We have observed only a small portion of the pertinent soil and ground-water conditions. The recommendations made herein are based on the assumption that soil conditions do not deviate appreciably from those found during our field investigation. We recommend that Woodward-Clyde Consultants review the foundation and grading plans to verify that the intent of the recommendations presented herein has been properly interpreted and incorporated into the contract documents. We further recommend that Woodward-Clyde Consultants observe the site grading, sub grade preparation under concrete slabs and paved areas, and foundation excavations. If the plans for site development are changed, or if variations or undesirable geotechnical conditions are encountered during construction, the geotechnical consultant should be consulted for further recommendations. This report is intended for design purposes only and may not be sufficient to prepare an accurate bid. California, including San Diego, is an area of high seismic risk. It is generally considered economically unfeasible to build a totally earthquake-resistant project; it is therefore possible that a large or nearby earthquake could cause damage at the site . Geotechnical engineering and the geologic sciences are characterized by uncertainty. Professional judgments presented herein are based partly on 12 • • • • • • • • • • Project No. 53173K-SI01 Woodward-Clyde Consultants our evaluations of the technical information gathered, partly on our understanding of the proposed construction, and partly on our general experience. Our engineering work and judgments rendered meet current professional standards; we do not guarantee the performance of the project in any respect. Inspection services allow the testing of only a very small percentage of the fill placed at the site. Contractual arrangements with the grading contractor should contain the provision that he is responsible for excavating, placing, and compacting fill in accordance with the project specifications. Inspection by the geotechnical engineer during grading should not relieve the grading contractor of his primary responsibility to perform all work in accordance with the specifications. This firm does not practice or consult in the field of safety engineering. We do not direct the contractor's operations, and we cannot be responsible for the safety of personnel other than our own on the site; the safety of others is the responsibility of the contractor. The contractor should notify the owner if he considers any of the recommended actions presented herein to be unsafe . 13 • • • • • • • • • • Project No. 53173K-SI01 Woodward-Clyde Consultants APPENDIX A FIELD INVESTIGATION Ten exploratory test borings were advanced at the approximate locations shown on the Site Plan (Figure 1). The drilling was performed on May 13 and 14, 1983 using a 6-inch diameter, continuous-flight power auger. In addition, we made a visual inspection of the existing bluffs to help evaluate the erosion potential of the near surface soils. Samples of the subsurface materials were obtained from the borings using a modified California drive sampler (2-inch inside diameter and 2½-inch outside diameter) with thin brass liners. The sampler was generally driven 18 inches into the material at tbe bottom of the hole by a 140-pound hammer falling 30 inches . The location of each boring and the elevation of the ground surface at each location were estimated from the grading plans provided us . A-1 • • • • • • • • • Boring 8 Approximate El. 77' DEPTH .,__T..,E,..S_T_D_A .... T_A_--1*0THER SAMPLE F~~T *MC *DD •ec TESTS NUMBER SOIL DESCRIPTION 5- - - - - - - 15- -8-1 20 - - - - 30- - - 35- - - - - 40- *For description of symbols, see Figure A-1 I I ~ltl~lll~! ·•·•·•·•·• =~:;~~~~=~ ii~ I iijj ..... ..... ..... lll~l11 ❖:-:❖ Loose, moist to wet, red brown silty fine sand (SM) CULTIVATED TOPSOIL Dense, moist to wet, red brown silty fine sand (SM) TERRACE DEPOSITS Dense, moist to wet, light brown to gray brown silty fine sand (SM) TERRACE DEPOSITS Perched water; boring caved to 14' Becomes saturated; rounded gravels Very dense, saturated, light gray fine sand (SM-SP) SANTIAGO FORMATION Becomes moist to wet at 17' Bottom of Hole LOG OF TEST BORING.8 BATIQUITOS BLUFF DRAWN BY: mkc I CHECKED BY: lhJ:'.I PROJECT NO: 531-73K-SI01 I DATE: 7-25-84 I FIGURE NO: A-9 WOODWARD-CL YOE CONSULTANTS • • • • • • • • > ) Project No. 53173K-SI01 Woodward-Clyde Consultants APPENDIX B GUIDE SPECIFICATIONS FOR CONTROLLED FILL I. GENERAL These specifications cover preparation of existing surfaces to receive fills, the type of soil suitable for use in fills, the control of compaction, and the methods of testing compacted fills. It shall be the contractor's responsibility to place, spread, water and compact the fill in strict accordance with these specifications. A soil engineer shall be the owner's representative to observe the construction of fills. Excavation and the placing of fill shall be under the direct observation of the soil engineer, and he shall give written notice of conformance with the specifications upon completion of grading. Deviations from these specifications will be permitted only upon written authorization from the soil engineer. A soil investigation has been made for this project; any recommendations made in the report of the soil investigation or subsequent reports shall become an addendum to these specifications . II. SCOPE The placement of controlled fill by the contractor shall include all clearing and grubbing, removal of existing unsatisfactory material, preparation of the areas to be filled, spreading and compaction of fill in the areas to be filled, and all other work necessary to complete the grading of the filled areas. III. MATERIALS 1. Materials for compacted fill shall consist of any material imported or excavated from the cut areas that, in the opinion of the soil engineer, is suitable for use in constructing fills. The material shall contain no rocks or hard lumps greater than 24 inches in size and shall contain at least B--1 • • • • • • • • • • • Project No. 53173K-SI01 Woodward-Clyde Consultants 40% of material smaller than ! inch in size. (Materials greater than 6 inches in size shall be placed by the contractor so that they are surrounded by compacted fines; no nesting of rocks shall be permitted.) No material of a perishable, spongy, or otherwise improper nature shall be used in fills. 2. Material placed within 24 inches of rough grade shall be select material that contains no rocks or hard lumps greater than 6 inches in size and that swells less than 3% when compacted as hereinafter specified for compacted fill and soaked under an axial pressure of 160 psf. 3. Representative samples of material to be used for fill shall be tested in the laboratory by the soil engineer in order to determine the maximum density, optimum moisture content, and classification of the soil. In addition, the soil engineer shall determine the approximate bearing value of a recompacted, saturated sample by direct shear tests or other tests applicable to the particular soils. 4. During grading operations, soil types other than those analyzed in the report of the soil investigation may be encountered by the contractor. The soil engineer shall be consulted to determine the suitability of these soils. IV. COMPACTED FILLS 1. (a) General Unless otherwise specified, fill material shall be compacted by the contractor while at a moisture content near the optimum moisture content and to a density that is not less than 90% of the maximum dry density determined in accordance with ASTM Test No. Dl557-78, or other density test methods that will obtain equivalent results . (b) Potentially expansive soils may be used in fills below a depth of 24 inches and shall be compacted at a moisture content greater than the optimum moisture content for the material. B-2 • • • • • • • ) Project No. 53173K-SI01 Woodward-Clyde Consultants 2. Clearing and Preparing Areas to be Filled (8) All trees, brush, grass, and other objectional material shall be collected, piled, and burned or otherwise disposed of by the contractor so as to leave the areas that have been cleared with a neat and finished appearance free from unsightly debris. (b) (c) All vegetable matter and objectionable material shall be removed by the contractor from the surface upon which the fill is to be placed, and any loose or porous soils shall be removed or compacted to the depth shown on the plans. The surface shall then be plowed or scarified to a minimum depth of 6 inches until the surface is free from uneven features that would tend to prevent uniform compaction by the equipment to be used. Where fills are constructed on hillsides or slopes, the slope of the original ground on which the fill is to be placed shall be stepped or keyed by the contractor as shown on the figure included in these specifications. The steps shall extend completely through the soil mantle and into the underlying formational materials . (d) After the foundation for the fill has been cleared, plowed or scarified, it shall be disced or bladed by the contractor until it is uniform and free from large clods, brought to the proper moisture content, and compacted as specified for fill. 3. (a) Placing, Spreading, and Compaction of Fill Material The fill material shall be placed by the contractor in .layers that, when compacted, shall not exceed 6 inches. Each layer shall be spread evenly and shall be thoroughly mixed during the spreading to obtain uniformity of material in each layer. B--3 • • • • • • .. ) Project No. 53173K-SI01 Woodward-Clyde Consultants (b) When the moisture content of the fill material is below that specified by the soil engineer, water shall be added by the contractor until the moisture content is as specified. (c) When the moisture content of the fill material is above that specified by the soil engineer, the fill material shall be aerated by the contractor by blading, mixing, or other satisfactory methods until the moisture content is as specified . (d) (e) V. After each layer has been placed, mixed, and spread evenly, it shall be thoroughly compacted by the contractor to the specified density. Compaction shall be accomplished by sheepsfoot rollers, vibratory rollers, multiple-wheel pneumatic-tired rollers, or other types of acceptable grading equipment. Equipment shall be of such a design that it will be able to compact the fill to the specified density. Compaction shall be continuous over the entire area, and the equipment shall make sufficient trips so that the desired density has been obtained throughout the entire fill. The surface of fill slopes shall be compacted and there shall be no excess loose soil on the slopes. INSPECTION 1. Observation and compaction tests shall be made by the soil engineer during the filling and compacting operations so that he can state his opinion that the fill was constructed in accordance with the specifications. 2. The soil engineer shall make field density tests in accordance with ASTM Test No. Dl556-64. Density tests shall be made in the compacted materials below the surface where the surface is disturbed. When these tests indicate that the density of any layer of fill or portion thereof is below the specified density, the particular layer or portion shall be reworked until the specified density has been obtained. B--4