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HomeMy WebLinkAboutBLD2005-00179 Geotechnical Report , • 0 Geotechnical Engineering Design Study .... ',1.' ft t Proposed Hazlehurst Residence . 4 ' - 153 Marina Drive .. Port Townsend, Washington Prepared for L._.--- 71..% Hamilton Hazlehurst August 11, 2004 17102-00 - - - • , , ma i'• ' ,, • ", ,,' „1-?-''' irsit, • ,v. 1 . z\ '' 44, 4. , i t.,, ,•,...,...,, ,,,,,t,„ i ,./ r---- - 1 . ta. •f'.- '42 4 1:- ,Y,,,,. ‘ I 1-', r, '' ",, 1 --., [, , • u . --• ,.- .,-0,,,- ,---=--,,,,- -...-,:, ,-..,-- ',--..---,i,-.5-.?3,,,,,s,-,;.! “,!' '-i,' .....1_f_PT OF C01,,;tvli.i'vl',`I uEVELOPML-0, ..i.. ..., .41P 111111 vill•%. , .:', IlUt wi; IFIIIIRTCROWSER • ;j • 811111 al - - HARTCROWSER „PT or Geotechnical Engineering Design Study Proposed Hazlehurst Residence 153 Marina Drive Port Townsend, Washington Prepared for Hamilton Hazlehurst August 11, 2004 17102-00 Prepared by Hart Crowser, Inc. 1104, ‘3- W . 0 A, V'Vl'A V,t. v4-d- ' "VIC '". R lay EXPIRES 1 0,03/o Garry Horvitz, P.E. Senior Principal • • Li CONTENTS Pa e Cu INTRODUCTION ��pt OF�,, t PURPOSE AND SCOPE OF THIS STUDY Purpose Scope 1 1 PROJECT UNDERSTANDING 1 FIELD EXPLORATIONS 2 SUBSURFACE CONDITIONS 2 Soil Groundwater 2 3 ENGINEERING ANALYSES AND RECOMMENDATIONS 3 Slope Stability Considerations Seismic Design 3 Temporary Excavation Slopes 4 Earth Pressures and Drainage Recommendations 4 Footing Design 4 Foundation Settlement S Additional Recommendations for Foundation Construction 6 Slabs-on-Grade 6 Site Preparation 7 Drainage Recommendations 7 Structural Fill 8 8 RECOMMENDATIONS FOR ADDITIONAL GEOTECHNICAL SERVICES 9 LIMITATIONS 10 FIGURE 1 Site and Exploration Plan Hart Crowser Page i 17102-00 August 11,2004 • • IT 17: f I GEOTECHNICAL ENGINEERING DESIGN STUDY t ' PROPOSED HAZLEHURST RESIDENCE PORT TOWNSEND, WASHINGTON INTRODUCTION This report presents the results of our geotechnical engineering design study for the proposed Hazlehurst Residence in Port Townsend, Washington, as shown on Figure 1. We have organized this report into several distinct sections. The main body of the report presents the results of our study organized into discussions of Slope Stability, Site Preparation, Grading, Subsurface Structures and Foundation Design. PURPOSE AND SCOPE OF THIS STUDY Purpose The purposes of this study were to: • Assess subsurface site conditions; and • Provide geotechnical recommendations related to design and construction. Scope The scope of this study included: • Field explorations at the site; ■ Identification and analysis of the geotechnical engineering considerations; and • Preparation of this report. PROJECT UNDERSTANDING We understand that the project will consist of the construction of a new residential structure at 153 Marina Drive in the Cape George Colony in Port Townsend. The site is located on Marina Drive. Figure 1 is a topographic map Hart Crowser 17102-00 August 11,2004 Page 1 • that shows the layout of the site and the configuration of the proposed structure on the site. The new house will be three stories with an attached garage structure and deck. The site is located adjacent to a steep slope on the west side. From the west property line to the street below, the elevation difference is approximately 30 feet. Some minor cutting will be required to set the house into the gently sloping upper portion of the site. FIELD EXPLORATIONS • Our subsurface exploration program consists of a visual reconnaissance by an engineering geologist/geotechnical engineer from our firm who conducted surface test trenches at the site. The locations of these trenches are shown on Figure 1. SUBSURFACE CONDITIONS Soil The Washington State Department of Ecology's Coastal Zone Atlas for Jefferson County maps this site as Advance Outwash. Based on the results of our reconnaissance and test trenches, it appears that the site is indeed underlain by advance outwash soils. These soils consist of a thin surficial zone of looser sandy soils underlain by what appears to be dense sand and gravel. The material ranges in consistency from fine to coarse Sand with gravel to slightly silty, fine to medium Sand with some gravel. No signs of groundwater seepage or spring activity were noted anywhere on the site. We observed the site for any signs of historical instability. A series of old madrona trees are present at different elevations with one old tree present within the steep slope portion of the site. This would suggest that there has not been any major instability manifested at the site in the recent past. A "toe buttress"of angular boulders has been placed (at some time in the past) at the toe of the slope as a buttress fill or"rockery." This material was apparently placed to protect the toe of the slope from erosional-type failures. • Hart Crowser 17102-00 August 11,2004 Page 2 t • • Groundwater Groundwater was not observed at the site during our visit. There were no signs, from vegetation or gullying, that water is present at the site above the elevation of the adjacent roadway. ENGINEERING ANALYSES AND RECOMMENDATIONS Slope Stability Considerations The Coastal Zone Atlas for Jefferson County maps this site as being potentially unstable along with this entire portion of the coastal Port Townsend area. In addition to the site of the proposed residence, we also observed slope conditions below the adjacent roadway to the north and west. These slopes also appear to consist of sandy outwash soils. We did not observe the presence of the possible underlying glacially consolidate lacustrine silts that are typically encountered below the advance outwash soils in areas where instability is prevalent. Slope instability within the bluffs adjoining Puget Sound is usually associated with the contact between outwash sands above and glacially consolidated silts below. Water will tend to accumulate within the base of the outwash sands over the low permeability silts and clays. As the hydrostatic pressures associated with this perched water table increases (typically during the wet winter months) the potential for slope instability increases. There is no evidence of this geologic stratigraphy at this site and no signs of flowing groundwater other than some minor surficial erosional features. As is indicated on Figure 1, there is a large tree along the west slope that has been undercut by erosion of material from below the tree at that point. The nature of the site soils and the presence of these steep slopes suggests that the site soils are susceptible to erosion over time. This is likely the reason for the construction of the rock toe buttress/rockery observed along the northern edge of the site adjacent to the road. It is our opinion that the potential for deep-seated slope failures at this site is low based on the geologic conditions encountered. The potential for increased erosion of the clean sandy soils is significant in our opinion and care should be taken to maintain the slopes over the life of the project. We recommend that the Hazlehursts work closely with the owners of the right-of-way adjacent to the road where erosion was noted to repair this erosion and keep it from getting worse. An approach similar to what has been used along the north edge would Hart Crowser Page 3 17102-00 August 11,2004 • • be the most effective means of accomplishing this enhanced resistance to erosion. The use of quarry spalls placed against a filtering geotextile would be effective in reducing the potential for erosion in the area where the large madrona tree has been undercut and gullying has been noted below the tree. The quarry spalls blanket should be supported by heavier riprap at the base to act as a buttress. Additional design recommendations can be formulated once discussions with the owner of the right-of-way has been contacted. The most likely form of slope instability would be from small and thin erosional slides. To protect footings for the house and deck, we recommend that footings be embedded deeply enough into the slope such that a horizontal setback from the edge of the footing to the edge of the slope is maintained. That horizontal setback distance should be a minimum of 10 feet for deck footings and 15 feet for house or garage footings. Seismic Design We interpret the on-site soils conditions to correspond with a seismic profile type Sc as defined by Table 16-J of the 1997 Uniform Building Code. Soils located below groundwater are not susceptible to liquefaction (loss of strength) under the design earthquake. Temporary Excavation Slopes Consistent with conventional practice, actual temporary excavation slopes should be made the responsibility of the contractor. All temporary excavation slopes should be accomplished in accordance with local, state, and federal safety regulations. All soils encountered within the anticipated utility depths are classified as Type C soils according to OSHA and WISHA. Considering the actual subsurface conditions, we anticipate that flatter side slopes might be required. The project specifications should be written in such a way as to make the contractor responsible for preserving the integrity of the base and sides of the excavation. Earth Pressures and Drainage Recommendations Our recommendations for lateral earth pressures on the subgrade wall are presented below. • Active earth pressure: use an equivalent fluid density of 40 pcf. If the wall is. not free to rotate, an at rest pressure should be used corresponding to an equivalent fluid weight of 55 pcf; • Hart Crowser Page 4 17102-00 August 11,2004 • • ■ Surcharge loading: lateral earth pressures (rectangular distribution) due to surcharge loading=(0.33*q) psf, where q is the vertical surcharge in psf; • Seismic loading: lateral earth pressure (rectangular distribution) due to seismic loading= (6.5 * H) psf, where H is the wall height in feet; • Passive pressure in front of the wall can be calculated using an ultimate equivalent passive fluid pressure of 350 pcf; • Use an ultimate coefficient of friction of 0.50 for cast in-place concrete bearing on native soils; and • Use a factor of safety equal to at least 1.5 when calculating resistance to lateral loads. Wall drainage should consist of 12 inches of free-draining well graded sand and gravel (less than 3 percent fines based on the minus 3/4-inch fraction) and drains installed at the base of the subgrade wall. The drains should be at least 4-inch- diameter, perforated pipe placed on a bed of, and surrounded by, 6 inches of clean well graded sand and gravel. The drains should be sloped to carry the water to a sump or other suitable discharge location. Footing Design The following is a summary of our recommendations for the design and construction of shallow footings: • Shallow footings may be founded on competent native soils: • All footings should be embedded deeply enough to provide a horizontal setback from the leading edge of the footing to the slope. For footings for the house and garage, the setback distance should be at least 15 feet. For the deck to setback distance should be at least 10 feet. • Some overexcavation may be required to remove overlying softened and weathered soils to expose the dense glacial soils. The lateral extent of the overexcavation is defined by a plane extending outward and downward from the edge of the footing to the top of the bearing layer at an angle no steeper than 1 H:1 V. • Design footings using a maximum allowable bearing pressure of 2 ksf. This value is applicable for footings supported on competent native sod or on imported structural fill. • Allow an increase in the allowable soil bearing pressure of up to one-third for loads of short duration, such as those caused by wind or seismic forces.. Hart Crowser Page 5 17102-00 August 11,2004 • ■ Isolated spread and continuous footings should have minimum widths of 24 inches. ■ Place the base of all footings at least 18 inches below the lowest adjacent finished grade for consideration of frost penetration. ■ Footings should be founded outside of an imaginary 1 H:1 V plane projected upward from the bottom edge of adjacent footings or utility trenches. • For resistance to lateral loads, use an equivalent fluid density to represent the passive resistance of the soil. For a typical footing use an ultimate passive equivalent fluid density o 350 pounds poured 'per cubic nst gfoolt (pcf). ■ Use an ultimate coefficient of friction to resist sliding equal to 0.50 for footings poured neat against imported structural fill or competent native soils ■ Use a minimum factor of safety equal to 1.5 when computing the resistance of footings to lateral loads. • A qualified geotechnical engineer or geologist should document footing and backfill subgrade conditions during construction, prior to placement of the footings or structural fill. Foundation Settlement For foundations designed and constructed as described above, we estimate that the total settlement of individual footings will be less than 1 inch. We estimate that differential settlement between adjacent footings will be about one-half to three-fourths of the total settlement. We anticipate that settlements will be essentially elastic and will occur as loads are applied. Additional Recommendations for Foundation Construction The foundation settlements estimated above assume that careful preparation and protection of the exposed subgrade will occur prior to concrete placement. Any loosening of the materials during construction or the presence of loose material beneath footings could result in larger settlements than those estimated herein. It is important that all foundation excavations be cleaned of loose or disturbed soil prior to placing any concrete and that there be no standing water in any foundation excavation. Hart Crowser Page 6 17102-00 August 11,2004 Slabs-on-Grade • • In our opinion, the proposed floor slabs may be designed and constructed as slabs-on-grade. Due to the potentially variable nature of the existing loose sands at the ground surface as discussed above, there is a somewhat increased risk that there could be some differential settlement of the floor slab-on-grade if constructed over existing loose material that is present. To reduce this risk, we recommend that the floor slabs be underlain by a minimum 2-foot-thick densely compacted soil or the underlying dense sand whichever is shallower. The following is a summary of recommendations for the design and construction of the slabs-on-grade: • Support slabs-on-grade on competent native soils or provide a minimum of 2 feet of compacted structural fill beneath slabs-on-grade. The structural fill should meet the requirements presented in the Structural Fill section. ■ Provide a minimum of 4-inch thickness of compacted capillary break layer beneath the slabs-on-grade. This layer should consist of well graded sand and gravel with less than 3 percent by weight passing the No. 200 sieve (based on the minus 3/4-inch fraction). This layer may be considered part of the 2-foot-thick structural fill. ■ Any soil that is to be considered for this capillary break material should be submitted to Hart Crowser for gradational analysis and acceptance prior to placement. • For design of the floor slab, use a modulus of subgrade reaction of 150 pounds per cubic inch (pci), as measured on a 1-foot-square plate. This assumes that the construction is accomplished as described above and that the capillary break layer is underlain by densely compacted structural fill. • Provide a vapor barrier beneath the slabs-on-grade. • Have a qualified geotechnical engineer or geologist observe the exposed slab subgrade prior to placement of the drainage layer or structural fill to verify suitable bearing surfaces. Site Preparation In preparation for construction, we recommend the following: • Remove all vegetation and debris from the site. r M Hart Crowser 17102-00 August 11,2004 Page 7 • • ■ Proof roll the exposed ground surface to identify any soft, wet, organic, or otherwise unsuitable soils that cannot be compacted in-place to a dense condition. Remove and replace such material with on-site or structural fill depending on the location. All surfaces should be compacted to a non- yielding condition. Drainage Recommendations Slab and Footing Drainage • We recommend that a perimeter footing drain be installed around the proposed residence. The drains should consist of minimum flinch-diameter perforated pipe surrounded by free-draining fill material for 6 inches in all directions. • All slabs should be underlain by a minimum 4-inch-thick capillary break layer, with perimeter footing drains. At this time, we do not anticipate the need for cross drains beneath the structure given our understanding of the site and our expectation that building grades will be near existing surface grades. Site Drainage We recommend the following: ■ Grade the site in such a way that surface water will not pond near the structure; ■ Slope all roof drains to a suitable outlet away from the building; and ■ Do not connect the roof drains into the foundation or wall drains. Structural Fill Selection of Imported Structural Fill For imported structural fill, we recommend using clean, well graded sand and gravel with less than 5 percent by weight passing the No. 200 mesh sieve (based on the minus 3/4-inch fraction). Where drainage material or capillary break is required, we recommend importing a well graded, free-draining sand and gravel with less than 3 percent Hart Crowser Page 8 17102-00 August 11,2004 • • i t by weight passing the No. 200 mesh sieve (based on the minus 3/4-inch fraction). Placement and Compaction of Structural Fill Before fill control can begin, the compaction characteristics of the fill material must be determined from representative samples of the structural and drainage fill. Samples should be obtained as soon as possible prior to placement of the fill. A proper study of compaction characteristics will include determination of the optimum moisture content and the moisture content of these soils at the time of placement. We make the following recommendations: ■ Control the moisture content of the fill to within 2 percent of the optimum moisture. Optimum moisture is the moisture content corresponding to the maximum Proctor dry density; • Place and compact all structural fill in lifts with a loose thickness no greater than 10 inches. If small, hand-operated compaction equipment is used to compact structural fill, fill lifts should not exceed 8 inches in loose thickness; ■ Compact structural fill to 95 percent of the maximum dry density as determined by the modified Proctor(ASTM D 1557) test procedure beneath all structural elements and pavement sections; and ■ Field test the compacted densities of all lifts to verify compaction. RECOMMENDATIONS FOR ADDITIONAL GEOTECHNICAL SERVICES • Hart Crowser should review the final specifications and plans to verify that our geotechnical recommendations have been properly interpreted and implemented in the design. • Consult with Hart Crowser during the remainder of the design so that we can refine or confirm our recommendations as more information about the project requirements becomes available, or in the event of design changes. • During the construction phase of the project, retain Hart Crowser to observe: • Site preparation activities; • Placement and compaction of structural fill including field testing of compacted fill and assessment of subgrades prior to placement of the fill; Hart Crowser Page 9 17102-00 August 11,2004 • • • Footing subgrades; • Slab-on-grade subgrades; • Installation of drainage materials and systems; and • Other geotechnical considerations that may arise during the course of construction. The purpose of these observations is to determine compliance with the design concepts, specifications, or recommendations and to allow design changes in the event that subsurface condition differ from those anticipated prior to the start of construction. LIMITATIONS We completed this work in general accordance with our agreement with the Hazlehursts. Our report is for the exclusive use of the Hazlehursts for specific application to the subject project and site. We completed this study in accordance with generally accepted geotechnical practices for the nature and conditions of the work completed in the same or similar localities, at the time the work was performed. We make no other warranty, express or implied. F:\Docs\Jobs\1710200\Hazlehurst Residence Report.doc { � Ira}ri .1* • !-" 1 i ;L • :PT. Or C�:;i i;;: E:.O`H.n_!, Hart Crowser 17102-00 August 11,2004 Page 10 4 � • • ,t �. 3 March 2005 Drainage and Erosion Control Program for Beach Access Stairs Reference: MLA05-00-102, BLD05-00096 Location: 1081 Griffith Point Rd. Project Description: Construction of a wood stairway for beach access covering 30 square feet of area and involving the excavation of one cubic yard of soil for concrete footings. The stairway is elevated above existing grade and will not alter the natural drainage. The construction will necessitate the removal of one tree. Element 1: Mark clearing limits Prior to construction mark location of stairway including locations of drilled anchors in existing rock at base and on the slope. Mark location of concrete foundations at top. Element 2: Establish construction access Use existing path. Carry materials to site by hand. Element 3: Control flow rates N/A—existing flow rates will not change. Element 4: Install sediment controls Excavate only in dry weather. Excavate ten 12"diameter holes for concrete foundations using hand tools. Remove approximately a total of one cubic yard of soil from holes and stockpile. Prevent any sediment- laden storm water runoff by installation of silt fence of hay-bale barrier if required. Element 5: Stabilize soils Cover exposed soil with plastic. At completion of construction,cover exposed soils with mulch. Element 6: Protect slopes Provide construction access to beach by ladder. Element 7: Protect drain inlets N/A—no drain inlets exist or are proposed in the area. run 1'R Element 8: Stabilize channels and outlets pp mg N/A—storm water will not be concentrated nor will natural flows be disturbed or changed. CO. O Element 9: Control pollutants Provide plastic covering as required to prevent construction pollutants from spilling onto the ground. p WZ W Element 10: Control de-watering -".1( CO � Excavate only in dry weather to prevent need for de-watering. O O Element 11: Maintain BMP's O Maintain any BMP's used during construction to ensure satisfactory operation during entire construction project. Element 12: Manage the project Perform construction activities only in dry weather. '' MAR 1 4 2005 y • • 3 March 2005 Re-vegetation Plan for Beach Access Stairs Reference: MLA05-00-05, BLD05-00096 Location: 1081 Griffith Point Rd. Project Description: Construction of a wood stairway for beach access covering 30 square feet of area and involving the excavation of one cubic yard of soil for concrete footings. The stairway is elevated above existing grade and will not alter the natural drainage. The construction will necessitate the removal of one tree. . Plan: At the completion of construction any disturbed areas at the top of the bank or on the slope will be replanted with a combination of Oregon Grape,Salal and Sedum as consistent with the existing vegetation. Any Madrone seedlings in the construction area will be transplanted to the northern boundary of the property before construction commences. • II NAA 14 2005