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Home My WebLink AboutEIR 83-05; ALANDA; LIMITED GEOLOGIC INVESTATION; 1983-08-111.· I I I I I I I I I I I I I I - I I I • -,( LIMITED GEOLOGIC INVESTIGATION FOR ALANDA PROPERTY CARLSBAD, CALIFORNIA For LAND SPECIALISTS, INCORPORATED San Diego, California By GEOCON, INCORPORATED San Diego, California August, 1983 X ! 1 r ) GEOCON I N C O R p O R A T E D ENGINEERS AND GEOLOGISTS • CONSULTANTS IN THE APPLIED EARTH SCIENCES File No. D-2981-M02 August 11, 1983 Land Specialists, Incorporated 3878 Old Town Avenue, Suite 202 San Diego, California 92110 Subject: ALANDA PROPERTY CARLSBAD, CALIFORNIA LIMITED GEOLOGIC INVESTIGATION Gentlemen: In accordance with your request, we have performed a limited geologic investigation within the northern half of the subject property. The purpose of our investigation was to determine if a previously reported large-scale landslide exists on the site. If this slide was confirmed, we were to evaluate its potential impact on the proposed development. Our investigation consisted of geologic mapping and the drilling of two large-diameter exploratory borings at the approximate locations shown on Figure 1, Geologic Map (pocket). The depths of the borings vary from 60 to 95 feet below existing ground level. No laboratory testing was performed as part of this investigation. The subject site is underlain by the Eocene-aged Santiago Formation, late Quaternary landslide and mudflow debris, colluvium, alluvium, topsoils, and fill soils. With the exception of the topsoils and fill soils, each of these units is discussed below. ■ 9530 DOWDY DRIVE • SAN DIEGO, CALIFORNIA 92126 • PHONE {714) 695-2880 I j I· l. ; • .,; File No. D-2981-tt02 August 11, 1983 The Santiago Formation consists of interbedded sandstones, siltstones, subordinate claystone. Slope stabilization measures are often required and for the clay-rich portions of this sequence such as encount.ered in the borings at approximately 18o+ feet in elevation. This clay bed may be exposed on the 80-foot-high cut slope proposed along the future Elm Avenue. The majority of the landslides encountered at the site belong to a particular type commonly defined as mudflows. These features are relatively shallow, without a distinct and continuous slip plane and are typically confined within ravines or canyon bottoms. The landslide debris consists of highly disturbed clayey sands, clay, cobble and large angular sandstone or siltstone fragments removed from the underlying Santiago Formation . Stabilization of mudflows usually requires less extensive methods than the large-scale rotational landslides. The colluvial deposits consist of very loose, unconsolidated silty sands. The depth of these soils may exceed 15 feet and remedial grading will be required if settlement sensitive improvements are proposed for the areas of their occurrence. Typically, the colluvial deposits are well developed within the depressed areas at the upper portions of the property. These depressed areas essentially represent zones where the mudflows were originated. Alluvial soils composed of loose clayey sands occur along the property's eastern limit. Grading within these soils could be seriously hampered due -2- GEOCON INCORPORATED L ! I I-" ,. I !: ,. I ' i i I I I I I I I j 1 I I ) ) I l J j File No. D-2981-M02 August 11, 1983 to their saturated condition, therefore, removal and recompaction of these deposits should be scheduled for the end of summer when the water table should be the lowest. The geologic structure at the site is that of a homocline dipping gently to the west or northwest. Relatively heavy seepage was encountered in Boring No. 2 within the Santiago Formation sandstones. It is our opinion that the primary sources of groundwater at the site is landscape irrigation within the neighboring subdivision. In conclusion, it is our opinion that the property is not underlain by the suspected large-scale rotational or translational landslide shown on our previous report of geologic reconnaissance for this property dated July 13, 1983. Several ancient mudflows were discovered within the property limtis as a result of this study, however, it is our opinion that this adverse condition can be mitigated with common grading procedures. On the attached Geologic Map, Figure 1, we have only shown the approximate extent of the most significant mudflow encountered within the north half of the property. We suspect that two smaller mudflows as well as several additional small landslides are are present within the property's southern portion. These suspected features are not shown on Figure 1. -3- GEOCON INCORPORATED I I I I I I f 1 j I ,. File No. D-2981-M02 August 11, 1983 We anticipate that stabilization measures may be required for some sections of the proposed cut slopes where weak clay beds will be encountered • . Significant removal and recompaction will be required in the areas where loose colluvial deposits and mudflow debris are present if settlement sensitive improvements are proposed for the subject areas. In order to provide specific recommendations with regard to landslide mitigation, slope stability, foundations, drainage provision, etc., it is our opinion that further investigative work will be required when the ,___.. ... ... ------- grading plans are more fully developed. If you have any questions regarding this report or if we may be of further service, please do not hesitate to contact the undersigned. Very truly yours, ~ORPORATED Michael W. Hart ~~/A. Andrei E. Farcas RG 3738 CEG 706 AEF:MWH:lm (2) addressee (4) HCH and Associates Attn: Mr. Tony Ambrose -4- GEOCON INCORPORATED I I I I I I I I I I • File No. D-2981-NOl August 11, 1983 DEPTH SMl/11.E LOG a ~,.., DESCRl,.TION IN Nf/lllllER IPCIITION Rw,- FEET OF 61~ 0 .. . 2 . .. - "" 4. "" . .. 6 . "" . "" 8- "" . .. 10· i. 12. .. .. 14 16 .. . 18 • i. 20 .. 22 SAMPLE BORING 1 TOPSOIL Soft, moist, dark brown, Silty CLAY SANTIAGO FORMATION ...... '. .. . Dense, humid, grayish-tan, medium grained, weakly cemented SANDSTONE . ... __ becomes coarse, well graded, weakly . ·._·. / cemented SANDSTONE wiht SILTSTONE . . I rip-up clasts . ' • • I • : . _. • I . • . . I ·.·.. Ir-stiff, humid, light garyish-brown, • :_ .. ': 11 Silty CLAY STONE bed, attitude : : ,·.,·.-. / r approximately horizontal ·1 .. . U I . : •.• ,___ I .•• •. I • : . ·.-.. 1 . • I : • , I:_"-, I • • • ', 1, r-grades into whitish-gray, weakly ·tx··1· 1:· .>I.Jr •. •. ✓ 1 cemented, well graded SANDSTONE : : • : I_·. v/ I ... •• ,' .... . : : I .. .. . ~ I •• . • .. I I • • .. . I . • . I • I ... -. ,.. ___ u ..... -- IN-PLACE DltY //1O/STlllfE DENSITY ctwTENT 11.c-.f % dr,y wt .. . . ... [Very stiff, massive, humid, light gray, 24 • . ""26 • .. . 28 • . 30 . .. . . . . . / Sandy SILTSTONE Figure A-1, Log of Test Boring 1 Continued next page GEOCON INCORPORATED I I 1 ] ] ] ] ] ] ] ] ] J File No. D-2981-M02 August 11, 1983 DE1'TH SAMPLE LOG a ~-DC SCRIPT/ON IN NCJMIIER iu)cATION ..,.._ FEET OF a-.,,,, 30 . 32- -. .. 34. . • 36 .. . 38· -. 40. . .. 42 .. . 44. .. 46 - 48' .. 50 52- . • 54 . 56" .. 58. . 60 SMIPLE ::. : : ;: ; i ..... : •· ... . . .. .. \ \ BORING 1 CONTINUED Stiff, moist, fractured, dark gray CLAYSTONE with shiny parting surfaces . : . : . \...._ grades into very stiff, moist, purplish-gray massive SILTSTONE ::f·{f: ~-;I~_--,::,\ L ________________ _ , ·-.. \)i( .... :<:· .. Very dense, humid, massive tanish-light gray, very fine Silty SANDSTONE ::JJI:: r-grades into very stiff, massive, J humid, light grayish-brown SILTSTONE, jJL·:· f bedding attitude horizontal I • •• I I••••• .·.·..... I ·111 · Ir-dense, humid, light gray SANDSTONE :: :: ----Ui interbed ., •: I :· :. : ,-_-:_:: r---...__ I / .·-:·:··:. >U /·_:<:.'·_:i-,-,.........-/very dense, humid, massive~ weakly ::j-J:(:: I cemented, light grayish-tan, fine, ;:.:-.:._. Silty SANDSTONE, bedding attitude :ti::i:-; Nl0°W/6°w • • .-:•:. / •-becomes coarse •. . .. I :··1:it·. I }IF I Ir stratigraphic disconformity ill}:: I ,Ir-very stiff' humid, purplish-brown-'::j--"j::(:' 1/1 gray, fractured SILTSTONE/CLAYSTONE, ::< :-.:; '1/ slightly sheared in limited discon-.:1:1: I·:. ht tinuous areas, orientation of shearing :,1·:1·:1·;:,_ __ Ul random : •• : ·. . II ': ·:·,· II 1Jn:_: ~-:--UI / __ V_e_r_y_s-tiff, hard, humid, massive, light r I % 1,, .... :::->~ gray SILTSTONE 117 - IN-PLACE DRr IIOISTUltE DENSITY Ct:NTENT A t:.I " ti'.>' wf Figure A-2, Log of Test Boring 1 Continued Continued next page GEOCON INCORPORATED ' ' .. j I j I I I I I 1 I 1 I I I File No. D-2981-M02 August 11, 1983 DEPTH SAM~ LOG a ~.,,- IN MMIIElf ILoc,tTtON ,-,,_ l'EET OF a,-.,,,, SAMPLE IN-PLACE 0£SCRIPTION DlfY MOISTI.IRE DENSITY CCW1ENT 11.c.f " ~ry llff BORING 1 CONTINUED 60-1-----1--~~~--1-----------------s-~---1t-----t----l . 62- -- .. 64. lo • .. 66 . .. 68 • .. . 72. .. . 74. 76 .. . 1o 78 .. "'80 .. . .. 82. . 88 lo : 90 • . 1o 92 . -. 94 .. : : .• .. . . . • • 1 r' f • , . .... I • •• o I• I : •, • .... . .. ·'. . . . · .· •.. • ·.: .. . . Very dense, gray, massive, very fine, Sandy SILTSTONE t-!-~·~ .. .,_'~---Ii----------------------Very hard, massive, well cemented SILTSTONE .,__ __ .___ grades into very stiff, massive, light grayish-brown SILTSTONE stiff, humid, purplish-gray, fractured CLAYSTONE interbed with shiny parting surfaces and numerous black manganese stains Very dense, humid, grayish-tan, very fine Silty SANDSTONE Break in log very har1, very slow drilling BORING TERMINATED AT 95.0 FEET Figure A-3, Log of Test Boring 1 Continued GEOCON INCORPORATED I I I I I I I File No. D-2981-M02 August 11, 1983 OE1'rH SAMPLC LOG a F'MMrttf-D£SCRtl'TION lN NUMBER /..OC,tTION NN,- l'EET OF' 111-.11'1 • 0 . . 2- -- ... 4 - . - 6 - -- 8 - -- '"10-. ... 12. . .. 14. .. - -. --18 • . . 20. . - 22 .. . . 24 • . "26 • . 28 . .. . 30 SAMPLE :i:r.i:: BORING 2 SLOPEWASH Very loose, dry to humid, dark reddish brown, cohesionless, Silty, very fine J:-'i!( . .·. SAND -~1_:!X. ------grades into moist to wet, orange-red, medium-grained, Silty cohesionless SAND J 1:r: . . . : . :flt· \\(. : i~ 1·:i":: f(t::· :1-J1·: ; j'·:, ~ i ·,; . ~ . . /\\ ·.·:;/: ··;,-•. / .. • .. ~ ·.·; , • : /. . .. '/. : . ·/, •. % ,. ' .. • ::-; :-. /.;'_ ·/ .. , 1/. • ••• ---- LANDSLIDE (MUDFLOW) DEBRIS Medium dense, wet to saturated, disturbed, weakly cemented to cohesionless, Clayey, medium-grained SAND r-becomes very coarse, / tan, Silty SAND with I zones, rare pebbles moist, grayish rare Silty/Clayey I J : u;0 -Q/. jSoft to loose, moist, light brown-tan, ;-tl.:I~'. I Silty SAND and gray, slightly sheared • • .•• :, CLAY bl•~Y.: 0<1.::p:· I /Very fine bedded, loose to medium dense, : _I:: . ,). / fine, light gray SAND, typical flow ··r Jl / pattern with attitude of bedding .-.; .::: l contorted, highly irregular with ~f_'.l~/~~-~j;_~~---~~1/ numerous clay lenses, dips approximately ~;-~=:i..------10° to 15° toward North ~ --- IN-PLACE DlrY MOISTllffE DENSITY CONTENT ,.c., " ~,,,., figure A-4, Log of Test Boring 2 Continued next page GEOCON INCORPORATED J j 1 File No. D-2981-M02 August ll, 1983 DO'rH SAMl't.£ l.06 II IA,,,,,,o,,.., IN NUMBER iU)CArtON Rn,- FEEr OF 8IOW$1W SAMPLE • DESCRIPTION BORING 2 CONTINUED 30+----+=~jt:===-+------___;~~~;;.;;..;;..;;.;~~----:----t-----1t----t ~£ -l Highly disturbed CLAY/SAND mixture• 32- -- -34. -- -36- .. - • 38· -- 40_ . 42- - 44- . - .. 46- - '"48 • 50 -- 52 - . - . 56 _ • . 58. . 60 ·r1!~"50· 1 \ with numerous pebbles and cobbles i:,(~::<? t~t~t·" L_ ______________ _ :">~1~f.: ''--Shear zone, highly sheared, disturbed, ~,:::i.... /"h light gray-tan, Clayey SAND and light ~~-i7_;/ / I gray CLAY with numerous cobbles and :: .. 1/::. ~ I 4!-,.i;,. _::=:==-ii pebbles within the sheared mass. ··:O-~V I General attitude N65° /45°NE .. . .. . I ·:H:·( 11 (:f:}( \\ :t\( l~- ·lF1:: 1 •• ·-· • I :..!l.:J.J:·· -I .... •.. L_ large SILTSTONE fragments exceeding l' in diameter in a coarse Clayey Sandy matrix shear zone agproximately l" thick, attitude N75 E/4°S \"·: .. ·( I SANTIAGO FORMATION • ·:. · :·: \ Very dense, moist, whitish-gray, :: _ _-_·:.: ·:~ __ \ Silty SANDSTONE /tf L :::.-~?\ \ Very dense, saturated, coarse, weakly :( :-':: •: ----~ cemented, whitish-gray SANDSTONE, ·: ::1i.?.' \ \ general slight seepage l~i,,'. •.. I : ••. •· I •◄1w. ~~-: IL---------------------. . I Very dense, saturated, weakly to medium : .. "'iw. :: I cemented, very coarse SANDSTONE with ·.: . •• ---11 rare pebbles, general moderate seepage, ----1\\ flow increasing with depth, no caving hL_ II II 1\-li. II 11 II stratigraphic disconformity, light grayish-brown, fractured SILTSTONE/ CLAYSTONE, slightly sheared in limited discontinuous areas; heavy seepage along the contact IL--very dense, wet, light gray-tan, weakly \ cemented, medium-grained SANDSTONE,cavirg I Figure· A-5, Log of Test Boring 2, Continued Continued next page GEOCON INCORPORATED I I I I I J I I I I t I t I I File No. D-2981-M02 August 11, 1983 D£PTH SAMPLE LOG II ,,.,,.,,_,,,,, IN N(Jll8ER IIXATION --FEET OF s,_,.,,,, SMIPLE 60 I .. I I . I I L_ --.. -.. . -... -.. -- .. . --. .. . . . -.. . - . - -. . .. . • DESCRll'TION BORING 2 CONTINUED . light gray, very stiff SILTSTONE BORING TERMINATED AT 60.0 FEET Figure A-6, Log of Test Boring 2 Continued IN-l'LACE DltY MOIS7lllfE OENSITY CONTENT P.C.f ".,,,,, GEOCON INCORPORATED . .. : ... SIZE REDUCTION PREPARED BY -LAND SPECIALISTS, INC. FROM DRAWINGS NO"S 0826A & D-2981-M01 BY GEOCON INC. -·-•