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Home My WebLink AboutTract 29053, 34160, 34161 (Geotechnical)1 1 1 1 1 1 1 1 1 1 1 1 i 1 1 S. E. Medall & Associates, Inc. Consultants in the Earth Sciences 3030 South Bundy Drive, Los Angeles, California 90066. 2131390.4079 2168 South Hathaway Street, Santa Ana, California 92705. 7141546.6602 RESPONSE .TO THE GEOLOGY APPEAL BOARD FOR THE COUNTY OF LOS ANGELES TRACTS 29053, 34160 AND 34161 DIAMOND BAR, CALIFORNIA for COUNTY OF LOS ANGELES W. 0.. 605D 0 March 20, 1978 J 'G S. E. Medall & Associates, Inc, Consultants in the Earth Sciences Angeles, CaSouthBundyDrive,Street, California 927021714,546.66 2188SouthHathawayStreet, Santa Ana, California 92705.714l5 8.8802 2168March 20, 1978 W. 0. 605D County of Los Angeles Engineering Geology Section 550 S.'Vermont Avenue Los Angeles, California 90029 Attention: Mr. Dave Popplar Subject: Response to the Geology Appeal Board for the County of Los Angeles, Tracts 29053, 34160 and 34161, Diamond Bar, California. Gentlemen: The following letter is in response to the recommendation by the Geology Appeal Board for the County of Los Angeles regarding the appeal on the subject property. To paraphase the request by the Board, we were to summarize our findings as presented at the appeal on March 16, 1978 and submit the 40-scale grading plan with geologic data collected subseqeunt to our original submittal. The subsequent data includes 7 borings and 12 bulldozer trenches, the information from which appeared on the 100-scale topographic map submitted to the County on March 7, 1978. Additionally, the Board wished a re -statement of our opinion regarding the absence of a major landslide as speculated by the County Geologist. 1 County of Los Angeles March 20, 1978 Page 2 W. O. 605D At our appeal, we presented evidence regarding topography, colluvial deposits, open fractures and slickensided beds and their relationship to our findings in contrast to the County's suposition. Firstly, the topography does reveal fault related breaks in slope, a condition proven by subsurface exploration. Addition- ally, there is differential erosion between the harder sandstone Soquel Member of the Puente Formation) and the softer siltstone and shale deposits.(La Vida Member). The faults were encountered in the exploratory trenches as.well as in borings placed at critical locations. Definite delinea- tions can be seen in the topography from stereoscopic view of the aerial photographs we examined. These features carry offsite and are recognizable. If a slide were to have occurred, steeply dipping faults would have been obliterated, unless the faults developed subsequent to a landslide. The drainage pattern on the site is in accordance with.the drain-- age pattern of the undisturbed portions of the surrounding jnounr tains, that is intermittent streams drain normal to the.ridge- line that generally forms the eastern boundary of the property., A more radial drainage configuration would be expected 'if. a landslide were to have occurred. offset canyons controlled by lJ I S. E. Medall & Associates, Inc. U County of Los Angeles March 20, 1978 Page.3 W. 0. 605D faulting, deeply eroded canyons and ridges all support our con- clusion that the topography is not suggestive of a landslide. Scattered areas of thickened soil mapped as colluvium were noted in the upper reaches of the property. These deposits can be explained by their proximity to faults and faulted siltstone and shale materials which are more readily weathered than the surrounding sandstone. The colluvial accumulations are generally in saddles or flattened highlands where erosion is minimized. It should be restated that there is a subjective element of interpretation to this material as some may identify this adeposit as weathered bedrock in the process of becoming soil. Open fractures were noted in the bedrock. Most were less than 1/2 inch, which is not abnormal throughout the entire Puente Hills, formed as a tectonic expression within the more brittle sedimentary layers. In two of our borings, open fractures were recorded that were greater than 2 inches in width. These frac- tures were in some cases filled with softer sandstone derived from superjacent soft sand layers. The wider fractures occurred in brittle sandstone beds adjacent to fault zones. Additionally, we noted in subsequent downhole inspection that the fractures widened near the borings and pinched inward away from them. It is probable that there had been some 1 S. E. Medall & Associates, Inca eCounty of Los Angeles Page 4 March 20, 1978 W. O. 605D disturbance of these materials by the drilling process itself and some of the more extensive opening were likely affected by the drilling procedure. Later borings in the vicinity of the borings where the larger open fractures were recorded did not reveal any major voids in the rock. In any case, the fractures were considered minor, consistent with that which prevails within the Puente Hills and will not cause settlement of overlying materials. Slickensided clay beds, referred to as gouge in Boring 1 of our December 19,.1977 report, represent plastic beds interlayer- ed within the sedimentary sequence. These }:eds,.have responded to the tectonic influences exerted on the region through fold- ing and faulting. Particularly, folding would cause movement of beds above and below these plastic layers and slickensides indicative of this movement would be found. We noted that these beds are in conformance with the surrounding layers rather than cutting through them. This evidence has led to our conclusion that the slickensides are a tectonic phenomenon, not related to a slide plane and should not be confused with fault gouge. The slipping of one bed across another during folding is called flexural slip folding. Q S. E. Medall & Associates, Inc. County of Los Angeles Page 5 March 20, 1978 W. O. 605D The features described above cannot be taken in themselves as being indicative of a landslide. Detailed downhole inspection 1 has revealed that a landslide does not exist on the property to the extent of a massive feature encompassing most of the site as had been speculated by the County. Rather, the features found are tectonic in origin with reasonable explanations. 1 Additionally, slopes that display disadvantageously oriented bedding planes, areas that have thickened colluvium or have slopes offsite that are subjacent to the proposed development 1 will all be supported in a manner consistent with the standard 1 of the profession as illustrated in our previous reports. in our opinion, the development will be safe as proposed. 1 Enclosed are additional cross -sections and the grading plan 1 as mentioned above with all current field data plotted. Respectfully submitted, S. ALL & ASSOC TES, INC. 1 SHELDON E. MEDALL C.E.G. 51 SEM:dk c: (2) Addressee 2) Weatherfield Homes, Attention: Mr. Glen Brengle Enclosures: Grading Plan Cross -sections I 1 S. E. Medall & Associates, Inc. a 760 N53W a I— w a Wa Q 0I— W J Cl. N0 Co Cr o - w----- 77 EXISTING GROUND PROPOSED GRADE SECTION A -AI 840 EXISTING PROPOSED - GROUND --- GRADE ---- 800 --- ---- TP 7 6 0 I, 1 N70E SECTION D-DI w H- a a a 3a ac J WN @ I 2 H CL ia_ Cr — uj9 840 N 4: T EXISTING U D P _ -` GROUND3=- 23 \ .\ 760 PROPOSE GRADE \ w H U W D Q. 3 0 -- a J a) PROPOSED -------- 800 GRADE EXISTING GROUND-'--- - -- - -__-7_ 760 TV 720 N5E SECTION E-EI a. o: w 840 o a- cr co 800 o PROPOSED Col GRADE-- EXISTING GROUND) -- 5- 7 2 0 N70W SECTION G-GI N75W SECTION B-B' 9404 a C COI w -- PROPOSED- NN N GRADE 1 EXISTING 900 GROUND 1 I - N71W SECTION C- CI 840 F- a w a ® o Gr 800 N PROPOSED GRADE ol 760 -- J - EXISTING GROUND--_9_ PASADO DRIVE 720 9 If D - N69W SECTION F-FI F- v v a^ a co a:ar w , w .0 0 0 ON o p a PROPOSED a - I Z GRADE EXISTING -- 840 GROUNDI 15— f - 31— Boo Tp 760 - N77w SECTION H-HI i 4 SMEAR 1 PLATE MA S. I- Medatl 6 AssociM" Inc. Consultants in the Earth Sclancss aey Sc. • ..u. •. _. 9_lei ON: 405 YOy S. E. MedaN 3 Associates, Inc. 6wis Olts /n the Earth semmw acm sa ..w. a, o a.w... c. s°oee a• swo» ee s.. ne.....• sc. s.... ww •. s-oe . es...6ox Mrs, 720 t mm e Mel s mme 740 N38W O ANCIENT LANDSLIDE'S jz m AS HYPOTHESIZED BY L. A. COUNTY T EXISTING GROUND — f .. • •" ' is 2. 5` s PROPOSED N GRADE 0 - m 9 T. C. 47' I APPARENT DIPS SHOWN HERE; TRUE DIP DIRECTION IS TOWARD VIEWER O I PROPOSED GRADE \ EXISTING. GROUND - Tp 3 \ N64W O EXISTING - GROUND m N74W 13_ a _ v U i V W • 0 N 1 O n yPROPOSED GRADE coil 12TO 1 SECTION T- TI SECTION Q — QI SECTION p - PI n W a In a Ir w 3 N Q O J 0 I ti Tp cr W co Q 3 J a u x d N N O EXISTING PROPOSED °D O n GROUND GRADE \R 1040 -- - o 840 Tp_ J _ 1000 - 800 N2W SECTION R- RI U NO 1W n Q a _ d 040 U W O N o n m PROPOSED EXISTING----- GRADEL i GROUND\ 4 1000 33 T - - 880 W c Q 5 a I m_ j a. a a w N tr o w 0 I N1O n DATE 12 - 14 -77 1 BY ,JJR/ WFR SCALE I°= 40' W.O. 605 r) REVISED 3-24-78 PROPOSED GRADE EISTING COUND Tp N45N SECTION S- SI I CD r 2. 5 13 EXISTING PROPOSED _ 840 _ GROUND GRADE T — 9— TPA - 800 =--_ ---- - --- --- — N37W SECTION N72W SECTION U- UI V-VI PLATE MC 1020 M•; Col 940 - t i I13 4 4 --! IS— N61W 7601 7204 EXISTING GROUND y - E- a x W N o n PROPOSED a GRADE N Col 0 Q01 w G — SECTION W-W, a Ud W O N O a n" 1040 -- -5=-- a a PROPOSED GRADE W 1000 Tp --- — 960 EXISTING GROUND N50E DOZER PIT I i a /I T- fW PROPOSED /I1 oGRADE y---------------- _ 1/ EXISTING GROUND ' ' _ jam' TpFE I N15W SECTION Z-ZI a_ a cr W cr N W 0 N 0 0 0 880 m -- EXISTING 1040 PROPOSED LOT 45 GROU 1 GRADE 840 - _ - ` P 1000 aif5 N42W SECTION BB -BB' N- S i e iEXISTINGGROUND TV — SECTION CC-CCI SECTION X-XI a cr W o Col W O 760 o PROPOSED I I GRADE 1 f' EXISTING p GROUND 720 680 N64W SECTION Y-YI co' PROPOSED 980 - - - - - - - - GRADE' EXISTING 940 - GROUND _ - ' T' r' 900 N35W SECTION AA-AAI 860 PROPOSED GRADE Qal 820 \\ T\\ N 35 W EXISTING I \ GROUND \ = Tn SECTION DD-DDI REVISED 3-24-78 PLATE MD S. E. Medan a A"odstu, im Conaattenrs in the Earl" Sciences a,ae Se. ' r.w.r. a,. s..w •w. ca aaaoe, .>Mt wawa '. DATE 12-19-77 BY JJB/WFR SCALE 1" = 40' W.O. 60 5 D r.Me 1M 1M 740 N85W SECTION EE-EEI 700 m N60W R PROPOSED EXISTING GRADE GROUND QIS SECTION GG-GGI 720 e E-W SECTION FF-FFI a w EXISTING EXISTING WATER 880 o GROUND — TANK PROPOSE) GRADE DOZER PIT 14 4 800 - - - -- N82W SECTION HH -HH' REVISED 3-24-78 a v z 0 PLATE DIE E. Me"ll A Associates, Ina _ Consuitoft in Me Eatth Sciences 7601 7204 II EXISTVNG GRADE PROPOSED GRADE I. Qal TD. 46 LANDSLIDE SECTION II -II PLATE IF II1