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HomeMy WebLinkAboutCT 80-09; The Meadows La Costa; Soils Report Addendum No. 1; 1981-08-10ADDENDUM NO. 1 TO REPORT OF SLOPE STABILITY ANALYSIS PROPOSED 'THE MEADOWS, LA COSTA SUBDIVISION' CARLSBAD, CALIFORNIA" .- (C.T.80-9), DATED AUGUST 10, 1981 PREPARED FOR: The Woodward Company 5100 Campus Drive Newport Beach, California 92660 PREPARED BY: SHEPARDSON ENGINEERING ASSOCIATES, INC. 1083 North Cuyamaca Street El Cajon, California 92020 The Woodward Company 5100 Campus Drive Newport Beach, California 92660 3HEPARPlON ENOINEERINO A33OClATEB INC. 1033 N. CUVAMACA STREET 3L CAJON, CA. EROEO TELE 448-9330 October 22, 1981 S.E.A. 010153 ATTENTION: Mr. Scott Woodward SUBJECT: .e Addendum No. 1 to “Report of Slope Stability Analysis, Proposed ‘The Meadows, La Costa’ Subdivision, Carlsbad, California,” (C.T.80-9), dated August 10, 1981. Gentlemen: In accordance with your request, we herewith submit the subject addendum. The intent of this addendum is to: 1) Address the requirements set forth in the correspondence from Mr. Richard H. Allen, Jr., R.C.E., to your office dated September 11, 1981 regarding slopes steeper than 2: 1; 2) 3) Transmit the results of additional laboratory testing; Present the results of additional slope stability calculations which incorporate laboratory tests performed on the granitic soils to be utilized in the recommended facial buttress; 4) Present the results of a literature review regarding the use of subsurface drainage systems and the location of projects on which we have utilized shallow drainage systems to improve surficial slope stability. DISCUSSION The September 11, 1981 correspondence from Mr. Allen refers to two methods of analyzing surficial slope stability. We concur with .Mr. : Allen’s statement that “the conservative approach would be to use ‘the lower factor of safety of the two methods”. The two methods referred to are briefly described as follows: October 22, 1981 -2- S.E.A. 010153 1) Infinite Slope, aka Skempton Method As described in our August 10, 1981 report, this is the method most commonly used to analyze surficial stability. We do have a major concern regarding the test procedures utilized to obtain shear strength data which is utilized in the formula presented in our August 10, 1981 report. The shear strength data is normally obtained from direct shear tests performed at confining pressures which are more applicable to deep-seated stability analysis. The use of the higher confining pressures normally produces test results which indicate a lower angle of internal friction and a higher cohesion intercept. It is ours opinion that the results obtained from the test utilizing the higher confining pressures do normally produce an over estimation of the factor of safety for surficial stability, as determined by this method. It was further our opinion that the use of direct shear test results performed on samples remolded to 90% of maximum dry density will, in certain soil conditions, produce an over estimation of the factor of safety. We have mitigated this concern, with respect to the subject project by requiring the use of non-expansive granitic soils and the recommendations regarding near surface relative compaction as described hereinafter. 2) Finite Slope Method The basic principle of this method of analysis is to install a subdrain system which will affectively limit the possibility of developing parallel seepage forces within vertical heights greater than 15 feet. It is our opinion that the factor of safety for slopes of finite height should be determined through the application strength or force formulas. We do, however, concur with the statements made by Mr. Owen in his correspondence dated September 9, 1981 regarding the possibility of developing parallel seepage forces within the 15 foot vertical height. It is our opinion that the results of calculations performed, utilizing the soil strength parameters of the decomposed granite and the infinite height formula as described in method #l above, will properly evaluate the potential for parallel seepage and surface sloughing within the 15 foot vertical ,height of slope. In accordance with the statement made by Mr. Richard Allen, we have performed calculations to determine the factor of safety produced by both of the above described methods. LABORATORY TEST RESULTS A review of the laboratory test results presented in then preliminary soil investigations performed by Benton Engineering and the supplemental geotechnical investigation performed by our office dated November 6, 1980, will indicate that the direct shear test results presented therein October 22, 1981 -3- S.E.A. 010153 were obtained on the De1 Mar Formation soils. To provide actual laboratory data regarding the soil strength parameters of the decomposed granite, an additional boring was extended in the area of Lot No. 21, Unit No. 2. Representative samples obtained from this boring were tested to determine shear strength characteristics under various confining pressures and at various densities. The results of these laboratory tests are presented on attached Plate No. 1. SLOPE STABILITY The slope stability analysis presented herein, was conducted using the two methods described above. The analysis of the finite slope involved three popularly used methods of analysis. The results of these compu- tations are summarized for your review. METHOD NO. 1 - INFINITE SLOPE aka SKEMPTON METHOD - Our analysis shows that the factor of safety utilizing this method is 2.98 for soil parameters with a phi angle equal to 42’ and apparent cohesion equal to 400 psf as determined by A.S.T.M. D3080 with confining pressures varying from 57L to 2,300 psf referred to herinafter as “normal” confining pressures. The least factor of safety for the proposed slopes using this methood of analysis would be 1.85 for soil parameters of a phi angle of 42 and an apparent cohesion intercept of 180 psf as determined by direct shear test analysis using confining pressures varying from 72 to 287 psf referred to hereinafter as “low” confining pressures on a sample remolded to 80% of maximum dry density, as shown on Plate Nos. 1, 2 and 3. METHOD NO. 2A - FINITE SLOPE METHOD This method is sometimes referred to as the Fellenius method or Swedish Slip Surface Method. This is a “force method” of analysis and assumes that the soil strength is mobilized over the entire slip surface area at the same time before failure takes place. Our analysis indicates that the minimum factor of safety for this analysis would be 2.5, if it was assumed that water penetrated to a depth of 5 feet into the slope. A more realistic estimate of the factor of safety would be 3.56, assuming that water penetration would be limited by the subsurface drainage to not more than 3 feet. METHOD NO. 28: This method is the same as Method No. 2A except the assumed slide surface has been changed from a circular configuration to a wedge configuration. The minimum factor of safety calculated for this condition is 3.33 using low confining pressure soil test data for the upper wedge and normal confining pressure test data remolded to 85% for the lower wedge. METHOD NO. 2C: This method is generally called the “wedge method” of analysis and may use a graphic analytical approach for determining the minimum factor of safety in accordance with the soil strength. It is categorized as a “strength method” of analysis and is used to calculate the apparent cohesion and shear required for stability. The factor of October 22, 1981 -4- S.E.A. 010153 safety is then determined by the ratio between the strength requirements for stability and the actual soil parameter strength as determined by test. This is generally considered to be a more accurate method of analysis than the “Swedish Circle” for non circular slide surfaces. Our findings indicate that the minimum factor of safety, based on this method of analysis, with drainage, using low confining pressure soil test parameters for the upper wedge, and normal confining pressure test data remolded to 85% maximum dry density for the lower wedge is 3.5. LITERATURE REVIEW Since earliest times the adverse effects of water on soil strength has been known. It has long been a standard of practice to include drainage behind retaining walls in order to reduce the likelihood of wall failure. In the Southern California area, surficial slope stability has not been a major concern to many engineers because of the relatively dry seasons we have experienced in the past. .-~ The above average rainfall during the years of 1977 thru 1980 has resulted in a significant increase in the use of subdrain (aka underdrain I systems in Southern California. We anticipate that the recent advancement in the use of subdrain systems will result in an increase in technical literature regarding this subject. The results of our limited search is briefly summarized for your review. In the recently released book “Construction & Geotechnical Engineering Using Synthetic Fabrics”, by Robert M. Keener and J.P. Welsh, John Wiley Publishers (1980), on Page No. 130, in quoting case histories, referred to the Healey and Long System. It is described as a combined fabric interceptor and wrapped under drain which was introduced in the early 1970’s. It states the method has been field tested with two of the six case histories sited being specifically for the purpose of stabilizing slopes. For additional information, read the article C.R.COLL.lNT.SOLS. Text, 1977, Volume 2, Page 237-241. The title t: “Fabric Filters on Prefabricated Underdrains” by R. Long and K. Healey. In addition to the articles sited, we have included, for your review, copies of the article which appeared in the A.S.C.E. publication of Civil Engineer in April of 1974, starting on Page 50 and entitled “Prefabri- cated Fin Underdrain Promises Faster Soil Drainage”. This article was written by the same Kent A. Healey and Richard P. Long, Associate Professors of Civil Engineering at the University of Conneticut. Possible Applications on Page 52 It is stated “fin drains could also drain slopes along highways to prevent sloughing”. On Page 53 under the subheading Fin Drain Performance at Field Sites it states regarding slope subdrains “Two lines of underdrains were installed, one near the top of the slope and the other at the base. These drains are intended to collect groundwater before it seeps to the surface. Sand was used to backfill the lower trench to help control surface water. These drains have weathered four winters- with no adverse affects due to frost. The slope has been stabilized.” . . October 22, 1981 -5- S.E.A. 010153 This literature search, though limited in scope, does provide us with the confidence that the proposed system as set forth in our report dated August 10, 1981, will contribute substantially to increasing the stability of the proposed slopes by reducing the likelihood of parallel seepage patterns developing. In our original report, it was our intent to show the minimum soil strength parameters required to provide a factor of safety of 1.5 for a slope drain system based upon stability analysis. A slope drain system would prevent parallel seepage from developing, so the use of a parallel seepage analysis using these minimum design parameters is incorrect. As shown in the sections entitled “Laboratory Test Results” and “Slope Stability”, the current analysis has been enlarged to incorporate within the scope of its consideration, the subject site materials and no longer represents minimum criteria but rather site specific criteria for design. In all cases, even the most conservative analytic approach gives a factor of safety in excess of 1.5. Local Case Histories .L In Appendix “B”, under the heading “Table 2”, we have included four local case histories which were installed at our recommendation and which had been inspected by us within the last four weeks. All of these installations were made on 1.5:1 slopes which had experienced surficial sloughing in the past. All of them have been subjected to at least one or more severe wet winters, higher than average for this area, without failure. Only one of these, namely Job No. 910189, Casa de la Mesa Apartments, was installed under our supervision and specifically in accordance with our recommendations. The others were installed without our supervision but according to our design recommendations. For instance, Job No. 810236, the Thorn Residence, presently shows settlement in the trench area indicating the trench was dug and a drain is present. The slope, having originally been failed by over-watering of the adjacent lawn in the month of July 1978, is still standing without any evidence of surficial sloughing. Our Senior Engineer saw personally the trench excavation for the proposed subdrain system while unofficially inspecting the installation. Based on these case histories and our confidence in the analytical measures employed to determine the probable stability of the slopes on the subject site, we have the highest degree of confidence that the recommendations set forth herein are conservative and the calculated factors of safety are realistic. CONCLUSIONS & RECOMMENDATIONS - A review of the data presented herein and the stability calculations which were submitted directly to Owen & Associates for their further review does, in our opinion, substantiate the conclusion that the factor of safety against deep-seated and shallow failures within fill slopes constructed in accordance with the recommendations presented in our August 10, 1980 report will be in excess of 1.5. We further conclude . . October 22, 1981 -6- S.E.A. 010153 that the installation of subdrain systems, as recommended in the above referenced report, will significantly increase the factor of safety against shallow failures. It should be noted that the factor of safety under a condition of parallel seepage is in excess of 1.5. Based on the above described conclusions, we recommend that the fill slope areas to be constructed at inclinations of 1.75:1, horizontal to vertical units, be constructed from the decomposed granitic soils preva- lent in the western portion of the subject site and incorporate the subdrain systems as described in our report dated August 10, 1981. We further recommend that the provisions for future maintenance of these slopes by the Homeowners Association, in the incorporation of land- scaping as recommended in our August 10, 1981 report, be incorporated. Although the recommendations regarding subdrains and future main- tenance of the slopes are not required, as a result of the stability calculations, it is our opinion that the benefits derived from these recommendations will significantly exceed the cost of implementation. We further recommend that the grading contractor be advised that tests to confirm the 90 percent relative compaction will be performed within 12 inches of the finish slope surface. Please do not hesitate to contact the undersigned, if you have any questions regarding the discussion, conclusions, or recommendations pre- sented herein. Respectfully submitted, SHEPARDSON ENGINEERING ASSOCIATES, INC. Senior Engineer DES: jgr V cc: (3) Submitted (3) City of Carlsbad Attn: Mr. Richard H. Allen. Jr., R.C.E. (2) Owen & Associates Attn: Mr. Martin Owen Enclosed: Slope Stab. Calcs. -z. TABLE 1 \ '~ 1 Maximum Date Confining Relative Density No. Tested Pressures t Compaction M.C. DEG #In c2 - .: " 1 9l24lai Normal 87 131D.5 41 325 2 g/29/01 L.C. 290 13118.5 48 327 ': .. 3 9/29/81 L.C. 390/Min. 13118.5 44.5 230 4 10/5/01 L.C. a5 13118.5 42 280 5 10/5/81 L.C. 85/Min. 131l8.5 43 170 6 10/5/Bl L.C. 80 13118.5 42 180 7 lol7fal Normal 90 13118.5 42 400 1. Confining Pressure: A) Normal: N(l) = 574 C/n2 N(2) = 1150 i/n2 N(3) = 2300 C/R2 B) Low Confining (L.C.): N(l) = 72'#Dt2 N(2) = 144 ?mt2 N(3) - 287 b/Et2 1 2. Test results represent peak strengths. 3. Tests represent strength at horizontal deflection of 0.200 inches in lieu of peak shear strength. ~'~ THE MEADOWS, LA COSTA SHEPAFUXGNENGINEEXING ASSOCIATlifS. Inc. BY NMJ DATE 10/22/81 J08No 010153 PLATE NO. 1 ‘.EQlJATION I FS=C+(S’ - s ;I, 2 sinp Cos$ Where: j? = Soil Friction Strength Parameter C = Soil Cohesion Z = Depth of Slide Mass p = Slope Angle Test data from direct shear test in accordance with A.S.T.M. Standard D-3080-72 Data Used: d = 42O c = 400 #Ilk2 b”, = 150 #/Ft3 (saturated) 2 = 3 Feet / - tan -' 1 = 29.7 DEG 1.75 FS _ 400,+4178.5 - 2.98 THE MEADOWS, LA COSTA SHEPARDSGN ENGINEERING AssGcIA!rEs, Inc. 1 BY DATE NMJ 10/22/81 .kxrm 010153 PLATE NO. 2 I . . .~ EQUATION 2 .. '. FS=C+(d sAw) Z Cosstan 8’ j, Z sinj3 Co+ ‘. Where: $ = Soil Friction Strength Parameter C = Soil Cohesion Z = Depth of Slide Mass p = Slope Angle Test data from low confining pressure direct shear test Data Used: ji- = 42' C = 180 i/Ft? r i - 150 #/Ft3 (saturated) - 3 Feet 1 = tan -' 1 - 29.7 DEG 1.75 FS - 180 + 178 _ , 85 194 * THE MEADOWS, LA COSTA SHEPARDSGN ENGINEEZWUG ASSOCIATES. Inc. BY NMJ IaTE 10/22/81 JOBNO. 010153 PLATE NO. 3 APPENDIX "A" Prefabricated, fin undhain - promises ‘laster soil drainage or ““d ,d “0”” m UC m&p b Ihe tq 8” pea mdr.dr.in p,. ,m-. sm.4 pmid” IW * aan emaS& w nc4 IO mud I,. IN ky cm! of .mllwdi”~ ,c4, -.ms mo I* pmf”mUd *pr. A, t* “me ,imc. ““d “va me4 b m mmI* OI ‘-nding Dil lill drm’“” md dq lk drunr DIDID-man 1 ?ic&7-” hns-“- I. n”m.lmfa *LU-. C. .m.-l* r__ -- .I_ ‘-: ’ 1, I i ‘V’ o”oo~o , 1,) it?%%* 00 00 I ’ 9MI*II1- n , * ., n. -a.- liccz%- e2.h EzzX-- “CpDlaa --*I -a- 0”WN.lS.R --W-W... -..“-.I# Pae.Ear.“n w-&T m .m! cs.s.,n Firld4ul.d mmderdr.i. wn* - al* sr Imni”C he “da, Im h.4. kmghdin8l llol i” l . i. 110 cm, 6un. dminyp: a”d rn&q bmh ppr .nd I” i” Ip’WC In. mh dalh. IJaded%” wdrm nn c.?mald by up mnyin&l m lh, pK”n?” ntdi, ” kid rn m,i,* h 1.h in ail rnh ,,rp - w.. w.rmP Ihwl w., “4 - APPENDIX "B" / 1 MEADO”’ LA COSTA 0 3ATE JOB NO. NAME ADDRESS REPAIRED 910189 Casa de La Mesa Apts. 5515 Shasta Lane La ksa, CA 92041 l/O0 ; 010220 DUnCan 7483 Comet View Ct 5/79 San Diego, CA 92120 810236 Thorn 6913 Newall Drive 12178 San Diego, CA 92112 SCTL Rancho Los Arbeles Pepper Drive/GreenfieId 1374 DATE INSPECTED SLOPE GRAD I ENT lo/e1 1.5:1 1 O/B1 1.5:1 9/81 1.5:l lO/Bl 1.5:1 Table 2 SLOPE HEIGHT 22 20 18 APPENDIX “8”