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HomeMy WebLinkAboutCT 79-05; La Costa Greens; Soils Report; 1979-10-02- I BENTON ENGINEERING, INC. APPLIED 801L MECH*NIC* - FcwNO*TION* lsB*o RUl=FlN RO*o SAN DIEGO. EILIFORNIa 92123 October 2, 1979 PHILIP HENKING BENTON PIEP,DINT 61YIL IUGINCEI Frederick J. Meyer, A. I.A. Architect 8, Associates 2348 liarcourt Drive Son Diega, California 92123 TLLLP”0.I (71.1 *6s-t0ss Gentlemen: This is ta transmit ta yw seven copies of our report of Project No. 79-8-23A, entitled “Limited Sails Investigation, Proposed Alta Verde Condominiums, Lots 6 and 7 of La Costa Greens Subdivision, North of La Costa Avenue, Carl&ad, California,“dated October 2, 1979. If you should have any questions concerning any of the data presented in this report, please contact us. Very truly yours, BENTON ENGINEERING, INC. PHB/jr - SlOlO .17.77 LIMITED SOILS INVESTIGATION -. - PROPOSED ALTA VERDE CONDOMINIUMS LOTS 6 AND 7 OF LA COSTA GREENS SUBDIVISION NORTH OF LA COSTA AVENUE CARLSBAD, CALIFORNIA - - - - - - - - - - Prepared far FREDERICK J. MEYER A. I. A. by BENTON ENGINEERING, INC. PROJECT NO. 79-8-23A OCTOBER 2, 1979 TABLE OF CONTENTS Page Nos. LIMITED SOILS INVESTIGATION - Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Field Investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 and 2 Laboratory Tests . . . . . . . , . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 and 3 Conclusions and Recommendations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3, 4, 5, 6 and 7 - APPENDICES - Sampling, etc.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B - BENTON ENGINEERING. INC. APPLIED SOIL YECHA.NlCS - FO”NO*TIONs *s*o RUFFlN ROaD **N DIEGO. CALIFORNIA 9*17.3 TELe.P”c.NL ,714, sss.less PHILIP “ENKING BENTON PllESIDEHI CIYIL INaIHclz” Introduction LIMITED SOILS INVESTIGATION This is ta present the results of a limited sails investigation conducted at the proposed Alta Verde Condominium site, which is legally known as Lots 6 and 7 of La Costa Greens Subdivision, in Carl&ad, California. At the site, a compacted fill ground, varying from 7 ta 20 feet in thickness, was placed between 1969 and 1970. The canpacted fill ground was known to be placed aver variable thicknesses of cmpressible natural sails in the north half of the property. In the south half of the property, the compacted fill ground was placed m firm natural ground. The objectives of this investigation were to determine the existing subsurface cmditims at me location in the northern portion of the site, and certain physical properties of the sails, so that appropriate foundation design recammendatims could be developed and presented. In order ta accomplish these objectives, me boring was drilled at the site, and both undisturbed and lacse soil samples were obtained for laboratory testing. This investigation was based m the 1” = 10’ Site Plan prepared by the Project Architect, Frederick J. Meyer, of San Diega, California. Field Investigation The baring was drilled, 5.5 inches in diameter, with a truck-mounted auger type drill rig at approximately 62 feet southerly and 82 feet easterly of the northwest comer of the lot. The baring was drilled ta a depth of 65 feet below the existing ground surface. A continuous log of the soils encountered in the boring was recorded at the time of drilling and is described m the fallowing p09e. -2- Baring 1: Approximately 62 feet southerly and 82 feet easterly of the northwest corner of the lat. The approximate elevation of existing grwnd surface is Eiev. 21.5 feet. Depth Bel aw Existing Ground Surface, in Feet Sail Description 0 to 5.5 Fill, Silty fine to medium sand with same asphaltic concrete fragments, Tbme ta medium compact, dry, light brown ta light gray brawn. 5.5 ta 19.3 - Fill , Sandy clay and clayey fine ta medium sand, compact ta very - compact, moist, light brawn and gray brown, lenses of dark gray sandy clay soils with scattered organic materials between 14.0 feet and 16.0 feet. 19.3 ta 23.0 Fine sandy clay, medium firm, saturated, dark gray - 23.0 ta 26.0 Clayey fine sand with packets of fine sandy clay and scattered raatlets, medium firm, saturated, light gray and light alive gmy. - 26.0 to 57.0 Fine sandy clay, with packets and lenses of clayey fine sand, medium firm to 43 feet; firm ta 49.0 feet and very firm below. - Saturated, light gray, light alive gray, dark blue gray and dark gray brawn. - 57.0 to 64.3 Silty clay with small chunks of claystone and siltsbne, very firm, saturated, alive gray and gray . - 64.3 ta65.0 Gravelly fine to caarse sand, very firm, saturated, gray br>wn. End of baring at 65.0 feet. - Undisturbed samples were obtained at frequent intervals, where possible, in the sails Q = ahead of the drilling. The drop weight used far driving the sampling tube into the sails was a ) steel rammer which weighs 140 pounds, and the average drop was 30 inches. ke general K -. ! procedures used In field sampling are described under “Sampling” in Appendix B. -Tj Laboratory Tests -i Laboratory tests were performed an all undisturbed samples of the sails in order ta determine - the dry density, moisture content, and shearing strength. The results of these tests and the expended energy ta drive the sampling tube far a vertical distance of 12 inches are presented an the following - iwe. llLNTON LNQINLL”ING. INC. -3- - - - - - - - 5 K 5 - - - Baring Sample * No. 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Depth, Feet 3.0 6.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 55.0 60.0 65.0 Drive Dry Energy Density Ft-kips/ft lb/cu ft 7.7 100.2 12.6 118.4 12.3 103.4 10.2 110.6 7.0 98.3 4.9 104.2 3.9 103.7 4.2 106.2 5.3 99.7 6.3 100.9 8.8 101.5 10.5 97.8 21.0 100.9 9. 1 98.9 Moisture searing Content Resistance % dry soil wt. kips/sq ft 4.5 0.61 12.3 4.28 18. 1 1. 16 17.6 2.32 24.8 1.56 19.5 1.32 17.6 1. 16 21.3 2.41 25. 1 1.11 23.9 0.37 22.9 I.77 27.0 2.23 24.7 3.23 24.8 3. 16 Consalidatim tests were performed on representative samples in order to determine the Icad-settlement chamcteristiw of the sails and the results of these tests are presented below: Cmsalidatim Under Surcharge Loads (Percent of Original Sail Sample Height) Surcharge Loads Baring: 1 Sample: 6 kips/sq ft. Depth: 25.0 Feet 0.2 and added water 0. 10 1.0 0. 98 2.0 1.45 4.0 2. 18 8.0 3. ii 0.2 2.23 Cmclusims and Recommendations Surcharge Loads Baring: 1 Sample: 7 kips/sq ft. Depth: 30.0 Feet 0.2 0.72 1.0 1.60 2.0 2.69 Added Water 2.66 4.0 4.20 8.0 6.29 0.2 4.32 It is concluded from the results of field investigation and laboratory tests that: 1. The existing fill sails below 5.5 feet and the underlying medium firm ta very firm clayey natuml soils have acceptable strength and load-settlement characteristics. It is recommended that all existing upper fills, less than 90 percent of maximum dry density be removed and be recompacted under continuous engineering inspection and testing ta at least 90 percent of maximum dry density. rnLNTON LH(IWCLIINO. WC -4- The proposed building foundations may then be founded on the fill soils uniformly compacted to at least 90 76 of maximum dry density if the bearing fill soils under all future buildings arc similar to those found below 5.5 feet at Boring 1. 2. Due to placement of compacted fill ground ct~ both compressible ground and firm natural ground, and in order to minimize possible adverse effects of Img-term settlement of saturated natural clayey soils in the northern portion of the site on the performance of future building foundations, - it is recommended that continuous interconnected reinforced concrete footings be used to support each of the proposed buildings. The recommended bearing value for sizing footings - is 1,000 pounds per square foot provided that the footings are, (a) at least 1.0 Foot in width, (b) founded at least 2.0 feet below the undisturbed final ground surface, (c) Founded in the uniformly compacted filled ground, and (d) I ocated at least 8 feet inside the undisturbed sloping - ground surface. The allowable bearing value is for dead plus live lmdr only and may be increased one - third for combined dead, live and seismic loodings. - The anticipated total settlements of a cmtinuous footing lmded to a unit foundation pressure of 1,000 pounds per sqwre Foot may vary From 0. 1 to 5.8 inches if the footing is less than 2.0 - Feet in width and is Founded at the recommended minimum depth of 2.0 feet below the undisturbed 4 3 final ground surface. , r; The settlements ore expected to be attributed mainly to the long-term ccnsolidatia, of the i I various thicknesses of the compressible soil layers existing beneath the compacted filled ground. ,.g T Since the thickness of the compressible soil layers increases northwardly, it is anticipated that the settlements of comprcted filled ground will also increase northwardly. The rate of settlement - increase was estimated to be approximately one inch vertical in 15 feet horizontal distance, or - ,- 1 to 180 ratio over a 30 year period during our soils investigation for Lot 8 of La Costa Greens Subdivision. Elevation readings taken on Lot 8 show that as much OS 5 3/4 inches of total mENION l!NOINLLRINQ. INC. - - - - - - -5- settlement has occurred since construction. Therefore, it would be best to construct the walls on the north portion of this site on a batter of approximately l/2 inch horizontal to 8 feet vertical to the south, so that when the walls deflect northward at a ratio of 1 to 180, the movement will not be apparent. Likewise, if interconnected reinforced cmcrete Footings and concrete slabs are poured m an upward slope of l/2 inch vertical per 15 feet horizontal distance during cmstructicn toward the north, it is unlikely that the tilting will be visually detected. Also, this procedure will allow the slab to rotate downward toward the north with the same beneficial effects. As an alternative, considerations may also be given to the possibility of separating the units structurally and designing the units with interconnected reinforced concrete Footings with raised wocd floors bolted with extra long anchor bolts at the northerly ends. The initial footings could be poured in an upward slope toward the north. As the long term downward settlement toward the north occurs, the anchor bolt nuts could be released and wood wedges or shims driven beneath the floor beams and the mud sill to relevel each unit. The adjustments could be accomplished whenever needed. Special consideration should also be given to buildings spanning the line of demarcation between Fill placed over Firm natural ground and fill placed over soft compessible soils. It wwld be best to provide complete structural separation and a horizcmtal separation of a few Feet between buildings supported on the two different soil conditions in order to reduce potential adverse differential settlements within any one building. The demarcation line can be located by connecting the two points across the property in an east-west direction. The first point lies a, the west lot line and is located approximately 125 Feet north of thesouthwest corner point of the property. The second point is located on the east lot line and is measured approximately 140 Feet north of the southeast corner of the lot. North of this line, compressible natural soils exist, and south of this line, Firm natural soils exist. The special foundation design that was used successfully cn Lot 8 oF La Costa Greens - .ENTON LNOma!l!“INO. INC - - - - - - . with a similar soil condition is recommended for this development or some such similar design. It is also anticipated that potentially expansive soils may be exposed after Fwndation trench excavation or exist beneath the bottom of the concrete slab m grade unless all building areas are Filled with 3 feet of non-expansive soils under cmtinuous engineering inspection during site grading. In wder to minimize possible adverse effects of these expansive scilr on the performance of Future building Fwndatims and concrete slabs m gmde, special design and precautimary measures should also be taken to design the Footings and the cmcrete slabs m grade. The suggested minimum measures to accomplish this goal are as follows: 1. Isolated interior piers should not be used. Continuous Footings should be used throughout, and these should be placed at a minimum depth of two feet below the lowest odjocent exterior final ground surface. 2. Reinforce and intercmnect continuously with steel bars all interior and exterior Footings with at least me #5 bar at 3 inches above the bottom of all Footings and at least me #5 bar placed 1 l/2 inches below the top of the stems of the Footings. Additional reinforcing steel may be required to reduce the adverse effects of pcesible differential settlements similar to that used m Lot 8. 3. Use raised wood floors that span between continuous Footings, or reinforce all concrete slabs with at least 6 x 6 = lO/lO woven wire Fabric and provide o minimum of 4 inches of 3/4 gravel, then a moisture vapor barrier covered with 2 inches of sand below slabs under living area. 4. Separate garage slabs From perimeter footings by l/2 inch thickness of constructim felt or equivalent, to allow independent movement of garage slabs relative to perimeter Footings. - - mv4TON CNOINLLIINQ. INC. - 5. Provide positive drainage oway From all perimeter Footings to a horizontal distance of at least 6 Feet outside the footing walls. It is recommended that the bearing soils exposed at the bottom of foundotion trench excavations be inspected by a soils engineer before placing steel reinforcement and pouring concrete. Respectfully submitted, BENTON ENGINEERING, INC. By &#A~-~ ~_ &E ‘No. Uf Civil Engineer 19913 - - Reviewed by WGe , Civil Engineer 4 RCE No. 10332 - - Distributiiwl: (7) Frederick J. Meyer, A. I.A. - SHS/PHB/c - - m?JKnd LNOINIL”INO. INC - - .- - - - - - - - - _. -- BENTON ENGINEERING. INC. APPLIED *o,L MEC”ANICS - FO”NDATION. SB.0 RU,FIN ROID s*r4 D,eGo. CIL,CORNII **I23 PHILIP HENKING BENTON C”s*ID.N,. ClYlL ‘I1aIWes” Tl!LIC”OYI ,7,., SaLless APPENDIX B Sampling The undisturbed soil samples are obtained by forcing a special sampling tube into the undisturbed soils at the bottom of the boring, at frequent intervals below the ground surface. The sampling tube consists of a steel barrel 3.0 inches outside diameter, with a special cutting tip on one end and o double ball valve on the other, and with a lining of twelve thin brass rings, each one inch long by 2.42 inches inside diameter. The sampler, connected to a twelve inch long waste barrel, is either pushed or dl-iven approximately 18 inches into the soil and a six inch section of the center portion of the sample is taken. for labratory tests, the soil being still confined in the brass rings, after extraction from the sampler tube. The samples are taken to the laboratory in close Fitting waterproof containers in order to retain the field moisture until completion of the tests. The driving energy is calculated as the average energy in Foot-kips required to force the sampling tube through one Foot of soil at the depth at which the sample is obtained. Shear Tests The shear tests are run using a direct shear machine of the strain control type in which the rate of deformation is approximately 0.05 inch per minute. The machine is so designed that the tests are made without removing the samples from the brass liner rings in which they are secured. Each sample is sheared under a normal load equivalent to the weight of the soil above the point of sampling. In some instances, samples are sheared under various normal loads in order to obtain the internal angle of friction and cohesion. Where considered necessary, samples are saturated and drained before shearing in order to simulate extreme field moisture conditions. Consolidation Tests The apparatus used For the consolidation tests is designed to receive one of the one inch high rings of soil as it comes From the Field. Loads are applied in several increments to the upper surface of the test specimen and the resulting deformations are recorded at selected time intervals for each increment. Generally, each increment of load is maintained on the sample until the rate of deformation is equal to or less than l/l0000 inch per hour. Porous stones are placed in contact with the top and bottom of each specimen to permit the ready addition or release of water. Expansion Tests One inch high samples confined In the brass rings are permitted to air dry at 105’ F for at least 48 hours prior to placing into the expansion apparatus. A unit load of 500 pounds par sqquare Foot is then applied to the upper porous stone in contact with the top of each sample. Water is permitted to contact both the top and bottom of each somple through porous stoner. Continuous observations are made until downword movement stops. The dial reading is recorded and expansion is recorded until the rate of upward movement is less than l/10000 inch par hour.