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HomeMy WebLinkAboutCT 80-09; The Meadows La Costa; Soils Report; 1981-08-10CTSO-q *4 REPORT OF STABILITY ANALYSIS OF THE MEADOWS LA COSTA FOR THE WOODWARD COMPANY PREPARED FOR: THE WCQDWARD COMPANIES 5100 Campus Drive Newport Beach, California 92660 PREPARED BY: SHEPARDSON ENGINEERING ASSOCIATES, INC. 1083 North Cuyamaca Street El Cajon, California 92020 QEOTECHNICAL ENQINEERS EHEPARDEON ENOINEERINQ AEEOCIATE~ INC. 1083 N. CUYAMACA ETREET EL CAJON, CA. eEOE0 TELE 449-8830 August 10, 1981 The Woodward Companies 5100 Campus Drive Newport Beach, California 92660 ATTENTION: Mr. Scott Woodward S.E.A. 010153 , SUBJECT: Slope Stability Analysis, Proposed “The Meadows, La Costa” Subdivision, Melrose Avenue, Carlsbad, California. Gentlemen: In accordance with your request, we have completed the subject analysis. The intent of this analysis was to determine if certain slopes within the subject subdivision will possess an adequate factor of safety against deep seated and near surface failures when constructed at an inclination of 1.75 units horizontal to 1.0 unit vertical. It was further our intent to address the concerns expressed by Mr. Richard H. Allen, Jr., Principle Civil Engineer, City of Carlsbad, in his correspondence dated July 23, 1981. In consideration of the requirements set forth in the City of Carlsbad Grading Ordinance and the current standard of practice, we are aware that the utilization of slope inclinations steeper than 2.0 units horizontal to 1.0 unit vertical does mandate that detailed stability analyses be performed to properly analyze both the near surface and deep seated stability of these slopes. The recent advance- ments in the state-of-the-art within the geotechnical engineering pro- fession does, in our opinion, permit the performance of this type of an analysis. The results of our analysis, as presented herein, indicates that cut-off drains will be required in all fill slopes in excess of 15 feet in height. A review of the stability calculation summary as presented in Table 1, on Plate No. 2 of this report will show that the installation of the cut-off drains and the use of the on-site decomposed granite as a facial buttress will produce a near surface factor of safety in excess of 1.5. We have included additional recommendations in the body of this report which will further increase the factor of safety of slopes inclined at 1.75 units horizontal to 1.0 unit vertical. August 10, 1981 -2- S.E.A. 010153 Please do not hesitate to contact this office, if you have any questions regarding this report. Respectfully submitted, SHEPARDSON ENGINEERING ASSOCIATES, INC. DES: NMJ: jgr I cc: (4) Submitted (2) Rick Engineering (2) City of Carlsbad Attn: Mr. Richard Allen, Jr. REPORT OF STABILITY ANALYSIS OF THE MEADOWS LA COSTA FOR THE WOODWARD COMPANY INTENT The intent of this analysis has been to apply current state-of-the-art procedures to the analysis of deep seated and near surface stability of certain slopes within the subject subdivision. It was further our intent to provide recommendations which will further enhance the stability of slopes which are designed at inclinations steeper than 2 units horizontal to 1 unit vertical. SCOPE The scope of our analysis has consisted of reviewing a grading plan prepared by Rick Engineering Company dated August 4, 1981, and stamped “Preliminary Not For Construction”. A copy of this grading plan is enclosed herein as Plate No. 1. We have further reviewed the previous geotechnical reports prepared for the subject site. These reports are briefly summarized as follows: 1. Benton Engineering, Inc. Project No. 79-7-18BF, dated August 15, 1979. 2. Shepardson Engineering Associates, Inc. Project NO. S.E.A. 010153, dated November 6, 1980. 3. Shepardson Engineering Associates, Inc. Project No. S.E.A. 010153, dated January 27, 1981. August 10, 1981 -2- S.E.A. 010153 Additionally, we have conducted a review and analysis of current state-of-the-art procedures for the performance of laboratory testing to determine near surface shear strength characteristics and methods of performing stability analysis. DISCUSSION Surface Slope Stability In the Southern California area, the increase in annual rainfall has led to the surface failure of many artifically produced slopes. These surface type failures are to be distinguished from the more dangerous deep failures and do not affect adjacent facilities or lands beyond the top of the fill slope. Surface slips require a combination of three conditions: l)a mantle of less dense soil; 2)a slope with sufficient steepness where; 3)soil moisture is equal to or greater than the liquid limit of the remolded soil. The soil moisture is the result of artificial irrigation and seasonal rainfall, and experience has shown that a .25 inch/hour intensity apparently represents the minimum rate at which surface infiltration exceeds subsoil drainage for many soils. Mechanism Needed to Produce Shallow Sloughing Failures When the rate of rainfall infiltration into and through the upper layers is equal to or greater than the capacity of the interior materials to remove it by deep percolation, a temporary perched water table is formed, as shown on Figure No. 1. If the surface infiltration rate continues to exceed the deep soil percolation rate, then complete perched saturation will occur and, also, a downslope parallel seepage pattern will develop. Unfavorable groundwater and seepage conditions, as described, are among the most frequent causes of surface slides. The presence of water lowers the stability and contributes to slope failures in the following ways: 1. BY reducing or eliminating chemical and/or capillary tension which provide cohesion. August 10, 1981 -3- S.E.A. 010153 2. By producing neutral pore pressures which reduce effective stress thereby lowering shear strength. 3. By producing horizontal seepage forces parallel to the potential failure plane. Current Analytic Procedures Currently, the standard of practice regarding surface slope stability recommends that a slope be viewed as infinite in height and analyzed in accordance with the formula developed by Skemton & DeLory (1957), for the condition where the groundwater flow is parallel to the slope at shallow depth. It may be written in the form: FS=C+(cl/ - s>w) 2 Cos$tan 8’ (JS Z sir+ Cosp Where: $ = Soil Friction Strength Parameter C = Soil Cohesion 2 = Depth of Slide Mass p = Slope Angle Experience has shown that the utilisation of this formula allows for high factors of safety to be assigned to slopes whose surfaces have failed. Therefore, it becomes apparent that a different analytic technique is needed if successful analysis is to be accomplished. This is supported by a recent paper submitted by Leighton & Associates to the Environ- mental Management Agency, Orange County, dated October 27, 1980, which indicates that a study of 11 different failed slopes of that area shows that adjustments to the soil strength parameters are necessary. As pointed out in that report, the laboratory method for determining the erroneous design parameters is generally referred to as the “direct shear test”. This test method does not allow for measurement of pore water pressure to be taken during the test, resulting in inaccurate soil parameters for soils which are not free draining. Usually the test over estimates the cohesion intercept of the soil and under estimates the August 10, 1981 -4- S.E.A. 010153 angle of internal friction. Since deep slides are more sensitive to soil friction then cohesion, this test results in conservative evaluation of deep seated failures. Therefore, the test has been used successfully throughout the Southern Calfiornia area for evaluating the potential of deep seated failures. However, since surface slides are more sensitive to changes in cohesion, the same test result would naturally over estimate the stability of a slope surface. Additionally, there is significant evidence that tests conducted at low confining pressures produce higher angles of internal friction then tests conducted on the same material at higher confining pressures, as indicated by Figure No. 3, on Plate No. 2. This supports the conclusion that shear tests conducted to determine the soil strength parameters to be used in the analysis of surface stability should be obtained using low confining pressures as shown on Figure 4 of Plate No. 2. Surface Slope Stability, Design Recommendations Therefore, we recommend that slopes within the subject site which are designed at inclinations steeper than 2~1 be designed in accordance with the following procedures. 1. Since parallel seepage is recognized as being the prime cause of slope failures, we recommend that a slope drain be installed at a depth equal to the tension crack potential for the slope material or 3 feet, whichever is greater, as shown on Figure No. 1, on Plate No. 2. The maximum vertical interval of the slope drains must be based on the stability calculations described in Item No. 3. 2. The direct shear test for determining the soil strength parameters should be conducted at low confining pressures, i.e. less than 800 psf, on materials which have been remolded and allowed to ex‘pand and dry through at least three (3) cycles. August 10, 1981 -5- S.E.A. 010153 3. The surface analysis should be performed using accepted slope stability techniques considering an upper wedge of saturated soil being buttressed by a lower wedge of unsaturated soil situated down stream from the proposed slope drain, as shown on Figure No. 1, Plate No. 2. Deep Seated Stability The results of our analysis to determine the critical height of a cut or fill slope within the project inclined at a ratio of 1.75 units horizontal to 1.0 unit vertical indicate that slopes to a maixmum height of 50 feet will possess a factor of safety in excess of 1.5. The results of our analysis of deep seated stability are consistant with the recommendations presented by Benton Engineering, Inc. in their report dated August 15, 1979. The implementation of these recommendations will require that the select fill material placed within the zones indicated on attached Plate Nos. 1 and 3 possess the above described shear strength characteristics. The results of our previous investigations indicate that the decomposed granite, which will be obtained from the major cut areas adjacent to the western boundary of the subject subdivision, will meet the above described shear strength characteristics. Although our previous investi- gations have indicated that portions of the De1 Mar Formation soils will meet the above described shear strength characteristics, we recommend that these soils not be utilized in the select fill areas due to the presence of localised stratas of low strength clays in the De1 Mar Formation. Reference is made to the proposed cut in the area of cross-section C-C. We understand that this cut slope will not be constructed until the final phases. of the proposed site grading. The results of our geologic mapping’ indicate that the De1 Mar Formation may be in this cut slope. August 10, 1981 -6- S.E.A. 010153 The approximate surface contact between the granitic and De1 Mar Formations are shown on attached Plate No. 4. In the event the proposed cut slope in this area does expose the De1 Mar Formation soils, we will recommend that they be removed and a facial buttress, equivalent to that shown for section B-B on Plate No. 3, be installed. In the event the facial buttress exceeds a height of 15 feet, the installation of a cut-off drain will be required. RECOMMENDATIONS Based on the above analytic technique, we therefore recommend that the slope drain be installed at a height interval of not less than 15 feet in accordance with the recommendations set forth on Figure No. 2 of Plate No. 2, and that soil strength show test results of a minimum of 100 pounds cohesion and 38’ angle of internal friction, when tested at low confining pressures. Slope surfaces, analyzed in accordance with these recommendations, shall possess a factor of safety against surficial failure in excess of 1.5. We feel this is a conservative analysis for the following reasons: 1. We have given no consideration to boundary conditions which would include the tensile properties of the drain system. 2. The method of analyses used is based on the Swedish Circle and considered conservative for this type application. 3. We have reduced the cohesion along the potential slip surface by a factor of safety of 4. Additional Mitigating Measures Although the results of our calculations, performed in accordance with the current state-of-the-art, do indicate that the proposed 1.75 units horizontal to 1.0 unit vertical slopes will possess a factor of safety in excess of 1.5, it is our opinion that the concerns expressed by Mr. August 10, 1981 -7- S.E.A. 010153 Richard H. Allen, Jr., in his July 23, 1981 correspondence do dictate the need for additional mitigating measures. We, therefore, recommend that the following landscaping and irrigation measures be implemented on the proposed 1.75:l.O slopes. 1. The landscaping plans should include the installation of either an acacia or eucalyptus tree at intervals of approximately 100 feet. Although the trees can be placed at variable heights on the slopes, we recommend that they, generally, be placed at or above the mid-point of the slope. I 2. The irrigation systems proposed for the above referenced slopes should be a common system for each slope and the control of these systems maintained by the Homeowners Association. We note that a slope in the area of Lot Nos. 30 thru 39 will be in a proposed open space area. It must be noted that the near surface stability analysis presumes that a perched water table will produce a saturated soil condition within the upper 3 feet in those portions of the slope above the cut-off drains. Our analysis accomodates this extreme condition favorably and should successfully mitigate the concerns expressed by Mr. Allen in his July 23, 1981 correspondence. Please do not hesitate to contact the undersigned, if you have any questions regarding the opinions or recommendations presented herein. Respectfully submitted, SHEPARDSON ENGINEERING ASSOCIATES, INC. D.E. Shepar&&, R.C.E. President SLOPE DESIGN Statistical analysis of 255 trial circles reveals that uee of a factor of safety of 1.89 and Taylor's charts is not significantly different from the use of a factor of safety of 1.5 and a seismic load of O.lG .The chart below,is based on factors of safety of 1.5 and 1.89 and Taylor's chart. Enter the chart from the bottom left -Slope Ratio ~‘b’ - SloGheight without. Seismic * S, f*su.m n.r- ., J