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Home My WebLink AboutSWP272159(1) report of ICAL. INVESTIGATION- for GRADY j Ka,m :�: '--'NIT APARTMENT ' April 29, 1993 ' Mr. Lawrence M. Campbell Architect 10024 Southeast 240th Street, Suite 102 Kent, Washington 98031 ' Re: Grady/Kemp Apartment Complex ' Dear Mr. Campbell: You are designing eight apartment units to be constructed in Renton and you asked me to ' investigate subsurface conditions and to make recommendations for structural design. We met last week, discussed the project, and reviewed work-in-progress on several plansheets. I subsequently had a discussion with the structural engineer, Mr. Codispoti. Presented herein are descriptions of the project, the site, the investigative procedure, the subgrade conditions, and recommendations for foundation and retaining wall design. Introduction ' Page five of this report was prepared from your yet-undated Sheet A-1. The project will consist of two nearly identical four-story fourplex units as shown. The east end of the site will accommodate off-alley parking. First floor slabs on grade will have elevations about ' eight feet below existing grade at the deepest. A north-south retaining wall is planned along the east side of the combined buildings. Slab- supporting fill will be placed east of the wall. Initially it was proposed that the entirety of the parking slab be poured on grade. As this report is being prepared, options are being considered for structurally supporting part of the slab and reducing the height of the ' retaining wall. Lot 3 is now occupied by a frame house at 512 Mill Avenue; the garage to that house is ' along the alley of Lot 4. There are no other significant improvements and it appears that there will be no interference between old and new because in the process of demolition and rough site grading, old structures, and any disturbed ground will be removed. ' C S V SG ; JUN 3 ��: *A# ti JAMES EATON, PE " GEOTECHNICAL SERVICES (206) 682-6942 '�� Box 126 • Hobart, WA 98025 Mr. Lawrence M. Campbell Page 2 April 29, 1993 Subsurface Conditions A search of m own job files found a eotechnical report for the apartment house at Y J g 430 Mill Avenue, 100 yards to the north of this project. The files also contain information on several other sites slightly farther away. In preparing this report, I also observed a highway project excavation immediately across Mill Avenue and downslope. From these efforts it was correctly anticipated that your client's property would be underlain by sandstone bedrock and that the rock would be covered with a residual or transitional layer of soil. The sandstone is part of the Puget group of sediments common to the greater ' Renton area. Several options for exploring the soils were considered. Existing utilization and terrain ' make the site poorly accessible to most types of digging or drilling equipment. After having had the locations of existing underground utilities marked on the ground, two test borings were made. These were drilled by a truck-mounted, 24-inch Altec boom auger provided by UDL, Inc. Locations of the two test holes are shown on page fove. The holes were carried to depths of 11 and 14 feet, respectively. Similar conditions were found at each location. Both holes were backfilled shortly after digging. The bottoms of the two holes represent the approximate depths at which the material grades from soil weathered from rock to sandstone. Though it was not practical to dig deeper with the equipment at hand, much deeper penetration would have been difficult. No groundwater was observed in either test boring but, at location 2, the cuttings were very moist near the bottom, which I take to mean that intermittent and/or local saturation is to be expected over the bedrock surface. Thickness of the soil layer is greater than at other measured or observed locations in the neighborhood. The transitional horizon of soil exhibits no dramatic improvement in strength with depth until the approximate bottoms of the test holes. rLog summaries of conditions found at each of the two test locations are presented on page six. Conclusions As you would expect, the sandstone bedrock has bearing strength and slope stability far in excess of adequate for light construction as proposed. Above the bedrock, strength, compressibility, and slope stability vary over a wide range, depending on the degree to which the soil has weathered. i ' Mr. Lawrence M. Campbell Page 3 April 29, 1993 Temporary shoring will not be necessary to protect adjacent properties or the alley. ' From the standpoints of total settlement, differential settlement, lateral stability, and durability under earthquake conditions, there is good reason to found the buildings on bedrock even though doing so would require excavating several feet deeper than might be adequate under ordinary conditions. It would not be possible by ordinary construction methods to penetrate piles sufficiently far into bedrock to be of lateral benefit. Instead, it ' would be preferable to key or trench into rock at structurally critical locations. Recommendations Initial site g g radin will be no different than if conventional shallow footings were to be poured. Excluding floor slabs, all structural loads will be transmitted to bedrock through concrete-filled trenches. All trenches would be aligned east-west with minimum width of 18 inches. The trenches would be notched into sandstone. The concept is illustrated schematically on page seven; the number and spacing of trenches will be determined by the ' structural engineer. Sandstone—even severely weathered sandstone—can be assumed to have allowable bearing capacity of 10 ksf; this should not be taken as justification for narrowing ' the trenches below 18 inches. There would be logistical and quality control problems with making narrow trenches. ' Floor slabs toward the west end of each building and, perhaps, part of the parking slab will rest on compacted fill. Free-draining sand or pit-run sand-gravel mixture should be used for that purpose. Onsite borrow is unsuitable for any use in the project. I recommend that the contractor coordinate with me about equipment and procedure for compacting select imported borrow for beneath the slabs. Parts of the basement and parking slabs will be supported by undisturbed soil below the topsoil; although this soil is substandard for ' foundation support, it is adequate for slabs and incidental improvements that are not heavily loaded and for which settlement is not critical. I recommend that a layer at least 4 inches thick of clean sand or pea gravel immediately underlay all interior slabs and their plastic ' membranes; this is to help minimize floor moisture. Footing drains will be needed around the north, south, and east sides of each building. Inverts for these should be at least one full foot below the elevation of the finished floor. The drains are especially needed along the east ends of the buildings, where there will be no actual footings. Perhaps there will be grade beams or false stemwalls. ' For purposes u of structural design of the retaining wall, lateral resistance will be developed P rP g from friction and passive pressure on the backfilled slurry trenches and to some degree ' through the stairwell structure. Friction on the walls and walls and bottoms of the slurry trenches can be taken as 0.3, where the unit weight of the soil can be taken as 110 pcf. i 1 ' Mr. Lawrence M. Campbell Page 4 April 29, 1993 Passive pressure of the soil west of the slurry trenches can be taken as 250 pcf equivalent ' fluid. Active pressure for the yieldable case will depend on the nature of imported backfill and on the backslope angle. With free-draining, engineer-approved granular material at a backslope angle not steeper than 1-1/2:1 (H to V), I recommend an assumed Pe of 60 pcf. ' Significantly lower figures could be assumed for flatter backslope angles. Mr. Codispoti should coordinate with me about details as the design becomes more refined. ' My final recommendation is that you, your clients, or the contractor arrange for me to be present during critical parts of site excavation. This would be to verify continuity of conditions as anticipated and to verify construction compliance with the recommendations herein. t ry truly aton, P. ' 4 copies submitted JE/mer/701 ' cc: Codispoti Company Consulting 611 Market St., Suite 15 Kirkland, WA 98033 S N. E'I ' �P of wAs,yti O c'o � 2 11917 <: 0'; 9FGlsTEp�° �C� FON AL EXPIRE -Zy' 71 — - EDGE OF PAVEMENT -- 7 r CENTERLINE MILL AVENUE RIGHT - OF - WAY / AS-BUILT CENTERLINE O \ / O 20-0" sozssozs 20_0" Zo pr ' ._ s-a" I t , i i 1 - ----- ---- ------ 142 1 I \� — -- ----- -- \ ` -Soo0•o of t� -So. oas .� _ Soo 00 of"E �� so.o25�.� _ 7. ri i '/sow \\ • • -W- _�'- � ' ```/Rr� cn 80 ts -o"eco r o,00 ' - - - ---�• -w .a —_.30=o m \ - - It {► - ----_- _ �� r 4 e.a o zz w m ice__ `� - - - - - ,o: , o -- � `• _ -H-1 - At- -a oa"oo o/"W So.015=' @ v oo'oo'o/"w \i so.ozs � � N i s o" s o -o" ---_,-_2a o"-- /3.025 - "/-o2s' TEST BORINGS ' Boring 1 0' - ' Dark brown loam topsoil Brown to reddish brown sandy silt with occasional gravel and cobbles (residual) (grades hard below 8') 9' - Tan to brown weathered sandstone 11' - Completed and backfilled 4/23/93. No groundwater encountered. t Boring 2 0' - Dark brown loam topsoil ' 0.9' - Brown to reddish brown sandy silt with occasional gravel, cobbles, and a small boulder (residual) (grades hard below 12') ' 13' - Tan weathered sandstone (very moist) ' 14' - Completed and backfilled 4/23/93. High moisture below 13 feet may be indicative of perched water above impervious layer. C/`aL -77�/,r?WP 7/v0j o ! LOST, 3 a I I \ I I 0 II I I I y I 6 0"134 - "Ek, I a - N I LOT 4 -- I S 90° 00' 00"d //S.o(I �@ \ �d9 I I 1