Loading...
HomeMy WebLinkAbout20200717110934385S. E. Medall & Associates, Inc. Consultants in the Earth Sciences r 3030"South Bundy Drive, Los Angeles, California 90066 . 213/390.4079 2168 South Hathaway Street, Santa Ana, California 92705. 7141546.6602 March 23, 1978 W. O. 605D County of Los Angeles Department of County Engineer Soils Engineering Section 550 South Vermont Los Angeles, California 90020 Attention Mr. Charles F. Ruggles Subject: Addendum Comments Regarding, Soils Engineering Report, dated February 20, 1978, Tracts 34160 and 34161, Diamond Bar Area, County of Los Angeles. References: 1. Report by S. E. Medall & Associates, Inc., dated February 20, 1978. 2. Report by S. E. Medall & Associates, Inc., dated December 19, 1977. Gentlemen: In our recent telephone conversation we discussed the buttress design as presented in the referenced No. 1 report. This buttress design actually reflected a redesign of a previous buttress present- ed in the referenced No. 2 report. This redesign was dictated by the county due to the fact that in one area over which two material types would be anticipated, a single shear strength was selected which represented the higher shear strength for the two materials. County of Los Angeles Page 2 March 23, 1978 W. O. 605D Our redesign included a single shear strength also; however, the lowest shear strength of the two materials was utilized. This redesign increased the key depth requirement by 2 feet. Shear strength parameters along planar conditions were assigned values in accordance with generally accepted county criteria. Shear strength parameters across bedrock were estimated from the results of direct shear strength testing. Within our aforementioned conversation, we were informed that when the county applied a computer evaluation to our graphicalvector analysis, their force diagram did not balance; thus indicating a factor of safety slightly less than the required 1.1. It is our immediate opinion in this regard, that this application may be an unwarranted refinement of an evaluation that by nature must contain certain simplified assumptions. For example; the along bedding shear strength criteria utilized were in accordance with generally accepted county criteria. A review of our plan for this section which accompanied the referenced No. 2 report, however, will indicate that strictly planar conditions do not exist. Typically, the onsite bedding conditions were found to be extremely undulatory, locally faulted, and folded. The 13 degree overall planar condition assumed for our buttress design is considered a generalized representation' S. E. Medall & Associates, Inc. County of Los Angeles March 23, 1978 Page 3 W. O. 605D of the planar features in the area under consideration. To these planar features, the aforementioned shear strength was applied without increases in consideration of the undulatory nature of the bedding. It should be further noted that in accordance with the general standard practice of design, shear strengths were further reduced by the assumed factors of safety to determine or estimate mobilized shear strength conditions. At this point, a brief discussion of the limitations of the pseudostatic evaluation appears in order. Although the pseudostatic calculations are extremely simple to perform,,which most likely accounts for their common usage, the method is also recognized to, contain many limitations which significantly:.effect its general applicability. First of all, the origin of the pseudostatic analytical procedure is unknown and accordingly, no known relation- ship exists between analytical evaluations of performance and proven site conditions. It is the opinion of this consultant, that the only real justification for the use of the pseudostatic evaluation is its simplistic approach and perhaps the precedent established by utilization of others. It is our understanding that no known documentation exists that would demonstrate a slope would be more stable given the fact a pseudostatic evaluation has been performed S. E. Medall &Associates, Inc. County of Los- Angeles Page 4 March 23, 1978 W. O. 605D than if the pseudostatic evaluation had been omitted. A pseudostatic factor of safety of less than one would not indicate that a slope would not be expected to perform successfully under seismic loading conditions. On the contrary, a seismic factor of safety greater than one should not be considered to suggest that a slope will perform successfully under seismic loading conditions. The pseudostatic evaluation would appear to be merely a pencil and paper vehicle to satisfy the earthquake consciousness of our, increasingly technical society. Actually, there is no known relation- ship between the magnitude of the assumed pseudostatic force and the variable nature of actual seismic loading conditions. Similarly, there is no known relationship between the assumed constant application of the seismic force and the transient nature of actual seismic loadings. The limitations of the pseudostatic evaluation can be further recognized by a review of the county criteria for the application of the pseudostatic approach. Under Item 11 of the minimum standards of slope stability analyses adopted by the County of Los Angeles, November 1, 1977, it is indicated that "In the design of slope support, bedding planes flatter than 12 degrees from the horizontal may not be normally considered in a pseudostatic analysis." S. E. Madall & Associates, Inc. County of Los Angeles Page 5 March 23, 1978 W. O. 605D This criteria was dictated as it was readily recognized that when bedding plane conditions became flatter,the pseudostatic evaluation became the critical criteria, even though this was not supported by proven performance of slopes. It should be readily recognized in this regard, that had we selected an overall planar condition of just one degree flatter,the pseudostatic evaluation would not have been a requirement. And a static factor of safety of 1.5 or greater would have been the minimum design criteria. A review of our analysis indicates a static factor of safety of 1.9, considerably greater than the minimum required static factor of safety. It should be further noted that planar condition shear strengths were utilized even though it is recognized that undulatory conditions exist. Locally, favorable geologic conditions also exist. Thus, actual shear strengths along the assumed plane of weakness are likely to be considerably greater than those assumed in the evaluation. Furthermore, in accordance with standard of practice, the shear strength utilized was further reduced by the factor of safety to determine mobilized shear strength to be used in the balancing of the vector system of analysis. In summary, it must be stated that to make thebuttress design previously submitted a reasonably determinant evaluation, certain S. E. Medall & Associates, Inc. County of Los Angeles Page 6 March 23, 1978 W. O. 605D simplifying assumptions must be made. These assumptions include boundary conditions, planar configuration, shear strength character- istics, as well as the method of analysis itself. All of these assumptions tend to be conservative, and thus, in the.opinion of the consultant, further refinement of the analysis previously submitted is not warranted. It is our opinion that the buttress configuration as recommended is appropriately conservative, and if in the opinion of the Department of the County Engineer a deeper or wider key is required to satisfy the letter of their design criteria or their computer application of our graphical vector evaluation, we would respectfully request that they make such a modification and formally recommend the adoption. of their design as a condition of project approval. Also discussed in our aforementioned conversation with the county was the fact that in the referenced No. 1 report, no signature by a certified engineering geologist was presented, and thus, a resubmittal or equivalent would be required with such a signature. In this regard, we must comment on the fact that the conditions addressed in the referenced No. 1 report were directed by the Soils Engineering Section of the Department of the County Engineer. S. E. Medall & Associates, Inc. County of Los Angeles Page 7 March 23, 1978 W. O. 605D This -direction or request for additional information came in the form of a review sheet dated December 19, 1977 signed by two registered civil engineers. The items addressed in our response were directed to geotechnical conditions the significance of which was entirely engineering oriented. We strongly suggest that a review and a signature by our staff geologist may suggest approval of conditions outside the geologic area of expertise. In regard to the counties concerns, however, the signature hereon of a member of our geologic staff is intended to indicate that the referenced report No. 1 has been reviewed along with the information herein and no misrepresentations of items of geologic significance have been made in the engineering discussions presented. Respectfully submitted, S. E. MEDALL & ASSOCIATES, INC. GREGORY /XTEN, R.C.E. 26098 Director of Engineering Services GWA:DRE:1s c: (3) Addressee 3) Weatherfield Homes, Attn: Executive Vice President Mr. Al Blunt S. E. Medals & Associates, Inc.