HomeMy WebLinkAboutB930113(1) TERRA ASSOCIATES, Inc.
Consultants in Geotechnical Engineering, Geology
and
Environmental Earth Sciences
July 29, 1993
Project No. T-2364
Mr. Shepard Cutler, AIA
21821 - 96th Avenue SE
Snohomish, Washington 98290
Subject: Geotechnical Engineering Study
Enterprise Rent-A-Car Site
South Puget Drive and Benson Road South
Renton, Washington
Dear Mr. Cutler:
As requested, we completed the geotechnical engineering study for the Enterprise Rent-a-Car project in
Renton, Washington. The purpose of our work was to evaluate the subsurface soil conditions at the site in
order to provide recommendations for site grading, pavement, and retaining wall design.
PROJECT DESCRIPTION
The site is located northeast of the intersection of Benson Road South and South Puget Drive in Renton,
Washington. The site location is shown on the attached Vicinity Map, Figure 1. The site is situated on a
terraced, west-facing hillside. It is planned to regrade the site to provide paved parking for a rental car lot.
Cuts and/or retaining walls will be required in order to maximize the usable area on this sloping site.
SUBSURFACE EXPLORATION
On June 28, 1993, we explored the site using a subcontracted Ford 555 backhoe. Subsurface soil conditions
at the site were explored by excavating six test pits to depths of 5 to 11 feet below existing grade. The test
pit locations are shown on the attached Exploration Location Plan, Figure 2. Test pit locations were
determined by pace and compass measurements from the northeast property corner stake. Elevations were
interpolated from the site plan.
A geologist from our firm conducted the field exploration, classified the soil conditions encountered,
maintained a log of each test pit, obtained representative soil samples, and observed pertinent site features.
All soil samples were visually classified in accordance with the Unified Soil Classification System described
on the attached Figure 3. The Test Pit Logs are presented on the attached Figures 4 throu .6. I
r E RNTV E
DEC 0
12525 Willows Road, Suite 101, Kirkland, Washington 98034 • Phone (206 8'�(UFNG DIVISION
Mr. Shepard Cutler
July 29, 1993
Representative soil samples obtained from the test pits were placed in closed containers and taken to our
laboratory for further examination and testing. The moisture content of each sample was measured, and is
reported on the attached Test Pit Logs, Figures 4 through 6.
SITE CONDITIONS
Surface
The site is situated on a west-facing hillside. Existing grades drop 56 feet across the site from Elev. 324 near
the southeast corner to Elev. 268 at the northwest corner. The site is accessed by a fire access driveway
ramping onto the site from Puget Drive South. The driveway continues north along the east margin of the
site, and provides fire access to the apartment buildings adjacent to the east.
The eastern 100 feet of the site is a rough-cut terrace with ten feet of relief. The terrace overlooks a 25 to 35
foot high, cut slope. The cut slope is graded to a 2:1 (Horizontal:Vertical) inclination, which drops down to
an existing retail center adjacent to the west.
Subsurface
The site is underlain at shallow depths by very dense glacial till soils. The till consists of strongly cemented,
silty fine sands. The till is predominantly fine-grained, with some scattered gravels. However, numerous
small boulders and fragments of sandstone bedrock are incorporated into the till. These soils were difficult to
excavate using our subcontracted Ford 555 backhoe.
A layer of fill soils averaging four feet thick blankets the site, thickening under the fire access driveway. Fill
depths also thicken below the north end of the site. A test pit excavated near the northeast corner of the site
encountered 11 feet of old fills. The fills primarily consisted of loose, wet, silty sands with gravel.
We did not encounter any groundwater seepage in any of our test pits. No evidence of seepage zones was
encountered on the western cut slope. We did not encounter any surface water on the site.
COAL MINE HAZARDS
The site is underlain by the abandoned workings of the Renton Mine #3 coal seam. We reviewed the
Engineering Investigation for the Renton, Washington Area, prepared in January 1985 by Morrison Knudsen
for the Office of Surface Mining. In this report, the overburden thickness above the workings is mapped as
being on the order of 300 feet. The coal seam had a reported thickness on the order of ten feet. The
overburden consists of sandstone.
Project No. T-2364
Page No. 2
Mr. Shepard Cutler
July 29, 1993
Mining activities ceased in the 1930's. The final condition of the mine and pillar geometrics is not known.
A review of the historic problems related to the abandoned coal mines in the Renton area indicates that
ground subsidence is generally limited to cave-ins at old air shafts or openings, and some roof collapse in
areas where the coal workings were close to the surface. None of these problems have been identified in the
immediate vicinity of the site. Therefore, we consider the risk of ground movements affecting surface
improvements to be minimal.
DISCUSSION AND RECOMMENDATIONS
The primary geotechnical factors which will affect project design are space limitations imposed by
topographic relief and adjacent developments. In order to maximize the usable space on the site, engineered
slopes will be required. These may consist of graded slopes, engineered keystone or criblock walls, or
engineered retaining walls.
It should be noted that the native till soils are difficult to excavate, and that both the native and fill soils on-
site are moisture-sensitive.
Site Preparation
The construction area should be stripped of vegetation and any deleterious material. The strippings are not
suitable for use as structural fill,but may be reserved for use in landscaping areas.
Depending on the final grades selected, project grading will occur as cuts and/or fills. On average, there is a
four foot thick layer of old fills blanketing the upper terrace of the site. The proposed parking lot may be
constructed above existing fills, provided that a structural fill mat is constructed between the old fills and the
new pavement section. The structural fill mat should be at least 18 inches thick. Therefore, preparations for
constructing a structural fill mat will include some overexcavation where subgrades are underlain by old fills.
General recommendations for cuts and fills are discussed in the following paragraphs.
Excavations and cut slopes should conform to the limits specified in local, state, and national government
safety regulations. In accordance with current OSHA standards, temporary cuts up to four feet high may be
nearly vertical. For excavations greater than four feet, the native till soils should be laid back at an
inclination no greater than 3/4:1. Cuts in fill soils should be no steeper than 1.5:1.
Structural fills and backfill should be placed in uniform loose layers not exceeding 12 inches, and compacted
to a minimum of 95 percent of the soils' maximum dry density as determined by the Standard Proctor, ASTM
D-698. It will be easiest to compact these materials when the moisture content of the fill is within two
percent of the soils' optimum moisture content, as determined by the same referenced standard.
Project No. T-2364
Page No. 3
Mr. Shepard Cutler
July 29, 1993
The site soils are moisture-sensitive and are difficult to excavate. Much of the old fill soils are silty and
loose, and have become wet over time. The underlying native soils are very dense and contain scattered
boulders, making them difficult to excavate. However, once loosened by excavation, they can easily become
wet and unsuitable for use as structural fill.
If grading operations take place during the winter, or if they are initiated in summer and extend into winter
conditions, the owner should be prepared to import structural fill. For this purpose, we recommend using a
free-draining granular material that has a maximum aggregate size of six inches, a minimum retained on the
No. 4 sieve of 25 percent, and a maximum of five percent passing the No. 200 sieve, based on the 3/4 inch
minus fraction.
Retaining Walls
Alternatives to graded slopes include engineered retaining walls and earth retaining systems, such as
keystone or criblock walls. Depending on the final design grades selected, these options can provide more
usable space by permitting steeper permanent slopes. For example, the existing western 2:1 slope may be
reconstructed at a steeper grade by constructing and backfilling a new keystone or criblock wall.
New retaining structures should be founded on dense glacial till soils and may be proportioned for maximum
bearing stresses of 5,000 pounds per square foot (pso, considering both dead and live loads. The calculated
friction resistance at the base of the foundations should not exceed 40 percent of the foundation's dead load.
We anticipate that glacial till soils are present at shallow depths at the toe of the western cut. However, old
fills are present on the upper terrace of the site. Till soils are generally present within five feet of existing
grades along most of the eastern side of the site. However, the till horizon drops off to the north. Depending
on the final grades selected, foundations may need to be overexcavated for an eastern wall.
Retaining systems should be designed to resist lateral earth pressures imposed by the soils they retain. Walls
maybe designed to resist active earth pressures equivalent to a fluid weighing 35 pounds per cubic foot(pcf).
This pressure assumes that no hydrostatic forces, adjacent high footings, surcharge loads, or sloping backfill
will occur above the walls. If any of these are expected, we should be contacted for the appropriate design
criteria.
Pavement
The design pavement section will be dependent, in part, on the traffic loads and volumes applied, and on the
supporting capability of the subgrade. All subgrades should be prepared in accordance with
recommendations provided in the Site Preparation section of this report. Regardless of the compaction
achieved, the subgrade must be in a stable and non-yielding condition prior to paving.
Project No. T-2364
Page No. 4
Mr. Shepard Cutler
July 29, 1993
With the subgrade prepared in this manner, the following pavement sections are recommended for the
anticipated light traffic use:
o Two inches of asphalt concrete (AC)over four inches of crushed rock base(CRB)
o Two inches of AC over three inches of asphalt treated base (ATB)
In areas where heavier traffic is anticipated, we recommend using:
o Three inches of AC over six inches of CRB
Pavement drainage is an important aspect of pavement performance. A poorly drained pavement section
allowing the ponding of water will be subject to premature failures. We suggest drainage gradients on the
pavement surface be no less then 1.5 percent. Also, if pavement subgrades will be exposed to winter weather
conditions prior to final paving, we strongly suggest you consider using ATB to protect the subgrade from
disturbance by construction activity.
ADDITIONAL SERVICES
We recommend that Terra Associates, Inc. be provided the opportunity to review the final design and
earthwork specifications in order to verify that our recommendations have been properly interpreted in the
project design.
We also recommend retaining Terra Associates, Inc. to provide geotechnical services during construction to
observe compliance with the design concepts, specifications, and recommendations, and to allow expedient
design changes in the event subsurface conditions differ from those anticipated. These services should
include:
1) Observing earthwork operations and the placement and compaction of any structural fill that
may be required
2) Observing foundation preparation in order to verify that footings bear on till soils
3) Performing field density tests on compacted fills and backfills
4) Observing and testing the subgrade under pavement
LIMITATIONS
The recommendations submitted herein are based on data obtained from our test pits. However, actual
subsurface conditions may differ from those observed in the test pits. The nature and extent of such
variations may not become evident until construction.
Project No. T-2364
Page No. 5
Mr. Shepard Cutler
July 29, 1993
If variations are observed during construction, Terra Associates, Inc. should be requested to evaluate the
actual site conditions and to review the recommendations in this report prior to proceeding with construction.
We prepared this report specifically for the Enterprise Rent-a-Car project. It is the property of Terra
Associates, Inc. and is intended for the exclusive use of Enterprise Rent-a-Car and their representatives. We
do not guarantee project performance in any respect. Our work meets normal standards of professional care.
No other warranty, expressed or implied, is provided.
We trust this information is sufficient for your current needs. Please call if you have any questions or need
additional information.
Sincerely Yours,
TERRA ASSOCIATES, INC.
Theodore J. Sch per, P.E.
-----
Principal Engineer exPJfles 6/18/95
DHG/TJS:tm
Encl: Vicinity Map, Figure 1
Exploration Location Plan, Figure 2
Soil Classification System, Figure 3
Test Pit Logs, Figures 4 through 6
Project No. T-2364
Page No. 6
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Vicinity Map
TERRA Enterprise Rent-A-Car
ASSOCIATES Renton, Washington
Geotechnical,Consultants Pro?. No. 2364 Date 7/93 Figure 1
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tr'c" .-. .. �. jd
Na9'16'5Co"W =_ 402.65'
o = 03'00'53° : , G:,� = 63 a TPLEGEND
R = 2a24.93'
L I48.64'Ad � \
TP-2 \ ❑TP-1 APPROXIMATE TEST PIT LOCATION
TP 1
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E R RA Exploration Location Plan
Enterprise Rent-A-Car
r'• • a ASSOCIATES
Renton, Washington
Ref- Plan by David Evans and Associates,Inc., dated 7/93. f--' J/ Geotechniral Consultants Proj. No.2364 Date 7/93 Figure 2
MAJOR DIVISIONS LETTER GRAPH TYPICAL DESCRIPTION
SYMBOL SYMBOL
GRAVELS Clean GW Q • O . Well-graded gravels, gravel-sand mixtures, little
Gravels •.�•p• or no fines.
tiro (less than • • • • • • Poorlygraded gravels, gravel-sand mixtures, little
a) More than 5% fines) GP ' • • • ' ' or no fines.
N 50
O -T• % of coarse ' •
N an fraction is GM 3 : ' n Silty gravels, gravel-sand-silt mixtures, no -
G a> > larger than No. Gravels plastic fines.
Z4 sieve. with fines GC Clayey gravels, gravel-sand-clay mixtures, plastic
:R o O . . c O/N fines.
CS SANDS Clean Well graded sands, gravelly sands, little or
LO Sands SW no fines.
W o >% <
(Al ca Z (less than
M /� `�
ore than / Poorly graded sands or gravelly sands, little
Q 50% of coarse 5% fines) SP � or no fines.
O fraction is
V o smaller than SM Silty sands, sand-silt mixtures, non-plastic fines.
2 No. 4 sieve. Sands
with fines
SC Clayey sands, sand-clay mixtures, plastic fines.
SILTS AND CLAYS ML Inorganic sifts and very fine sands, rock flour, silty or
Jclayey fine sands or clayey silts with slight plasticity.
O `0 > Inorganic clays of low to medium plasticity, gravelly
N m•� Liquid limit IS less than 50% CL' clays, sandy clays, silty clays, lean clays.
W e,0 QL i i i i i Organic silts and organic clays of low plasticity.
Z_ 0 N ���� �� ����
Q `n o
oZ SILTS AND CLAYS MH Inorganic silts, micaceous or diatomaceous fine
O C sandy or silty soils, elastic.
. cc
W o:5
Z Liquid limit is greater than 50% CH Inorganic clays of high plasticity, fat clays.
E OH 1 1 1 1 1 1 1 Organic clays of medium to high plasticity,
I I I I I I 1 1
I I I I , I , I I organic silts.
HIGHLY ORGANIC SOILS PT Peat and other highly organic soils.
DEFINITION OF TERMS AND SYMBOLS
Standard Penetration 2" OUTSIDE DIAMETER SPLIT
W Density Resistance in Blows/Foot T SPOON SAMPLER
4 2.4" INSIDE DIAMETER RING SAMPLER
Very loose 0-4
OR SHELBY TUBE SAMPLER
Loose 4
° Medium dense 10-30 P SAMPLER PUSHED
z Dense 30.50 * SAMPLE NOT RECOVERED
N Very dense >50 S7 WATER LEVEL (DATE)
b WATER OBSERVATION STANDPIPE
Standard Penetration C TORVANE READINGS, tsf
gDensity Resistance in Blows/Foot qu PENETROMETER READING, tsf
V Very soft 0 2 W MOISTURE, percent of dry weight
`o Soft 2-4 pcf DRY DENSITY, pounds per cubic foot
F; Medium stiff 4-8 LL LIQUID LIMIT, percent
d Stiff 8-16
N Very stiff 16-32 PI PLASTIC INDEX
Hard >32 N STANDARD PENETRATION, blows per foot
SOIL CLASSIFICATION SYSTEM
TERRA Enterprise Rent-A-Car
ASSOCIATES Renton, Washington
Geotechnical Consultants
Proj. No. T-2364 Date 7/93 Figure 3
Test Pit No. TP-1
Logged by: DHG
Approximate Elev. +296
Date: 6-28-93
Depth .
(ft.) Graph Soil Description W
SMG FILL-Tan, silty SAND,wet, loose.
SM Blue-orange mottled, silty SAND, pockets of organics, wet, loose.
13
Blue-gray, strongly cemented, very silty , fine SAND, with g
SM' trace gravel, one boulder, moist to wet, very dense.
5
XX
(Glacial Till) 8
Blue gray, strongly cemented, sandy SILT, with trace
ML pebbles, moist to wet, very hard.
SM Blue gray, silty SAND with gravel, wet, very dense (Glacial Till). 17
10
Test pit terminated at 10 feet.
No groundwater seepage encountered.
15
Test Pit No. TP-2
Logged by: DHG
Approximate Elev. +292
Date: 6-28-93
Depth
(ft.) USCS/ Soil Description W
0 Graph /o
SM FILL Light brown, silty SAND with gravel, wei, medium dense. 12
16
Blue ray, clayey SAND, rooty,wet, loose.
5 a
* '
XBlue-gray, strongly cemented, silty, fine SAND, with trace
SMt> gravel, moist to wet, very dense (Glacial Till).
10 Test pit terminated at 9 feet.
No groundwater seepage encountered.
15
TEST PIT LOGS
TERRA Enterprise Rent-A-Car
Renton, Washington
ASSOCIATES
Geotechnical Consultants
Proj. No. T-2364 Date 7/93 Fgure 4
Test Pit No. TP-3
Logged by: DHG -
Approximate Elev. +294
Date: 6-28-93
Depth
(ft.) Graph Soil Description W
inches .IL
Gray-tan, strongly cemented, silty, fine SAND, some gravel, 7
small cobbles and scattered fragments of sandstone
bedrock, moist, very dense (Glacial Till).
Large boulder encountered at 4.5 feet.
5
8
Test pit terminated at 5 feet due to refusal.
No groundwater seepage encountered.
10
15
Test Pit No. TP-4
Logged by: DHG
Approximate Elev. +303
Date: 6-28-93
Depth
�ft Graph Soil Description W
0
'S:"` FILL-Gray, silty SAND, with some gravel, wet, loose
to medium dense.
11
(Boulder at 4') 9
5 Tan, strongly cemented, very silty, fine SAND, with some
gravel and boulders, moist to wet, dense to very dense.
SM: ;:: (Boulder at 7')
Many sandstone bedrock fragments below 8 feet.
10
Test pit terminated at 10 feet.
No groundwater seepage encountered.
15
TEST PIT LOGS
TERRAEnterprise Rent-A-Car
Renton, Washington
::.::.........
ASSOCIATES
Geotechnical Consultants
Proj. No. T-2364 Date 7/93 Figure 5
Test Pit No. TP-5
Logged by: DHG
Approximate Elev. +304
Date: 6-28-93
Depth
(ft.) Graph Soil Description W
0
FILL Light brown, silty SAND with gravel, wet, loose to medium 8
dense.
P'%>`<% FILL- Brown, gravelly SAND and topsoil, wet, loose.
5 SM FILL- Brown, silty SAND with gravel,wet, loose.
U. sM FILL- Blue-gray, very silty, fine SAND, wet, loose.
10
SM FILL- Brown, silty SAND, wet, loose.
10 [J, 19
Test pit terminated in fill at 11 feet.
No groundwater seepage encountered.
No caving of sidewalls.
15
Test Pit No. TP-6
Logged by: DHG
Approximate Elev. +305
Date: 6-28-93
Depth
(ft ) Graph Soil Description W
0
g FILL Blue-gray, silty SAND with gravel, moist to wet, very dense
to loose. 9
S Loose, caving below about 3 feet.
5
9
S Tan, silty SAND, moist, dense to very dense.
Blue-gray, strongly cemented, silty SAND, moist, very dense. 9
10 Test pit terminated at 9 feet.
No groundwater seepage encountered.
Slight caving from 3 to 6 feet.
15
TEST PIT LOGS
TERRA Enterprise Rent-A-Car
7 1,717Renton, Washington
ASSOCIATES
Geotechnical Consultants
Proj. No. T-2364 Date 7/93 Figure 6