GEOTECHNICAL ENGINEERING REPORT

Transcription

1 GEOTECHNICAL ENGINEERING REPORT Project: NW Bucklin Hill at Silverdale Way NW Project Number: Prepared for: Barber Development P.O. Box 473 Redmond, WA Prepared by: South Sound Geotechnical Consulting P.O. Box Lakewood, Washington Date:

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3 Table of Contents EXECUTIVE SUMMARY... i PROJECT INFORMATION... 1 SITE CONDITIONS... 1 SUBSURFACE CONDITIONS... 1 Soil Conditions... 1 Groundwater Conditions... 1 Geologic Setting... 1 GEOTECHNICAL DESIGN CONSIDERATIONS... 2 Site Preparation... 2 Subgrade Preparation... 2 Grading and Drainage... 3 Structural Fill Materials... 3 Structural Fill Placement... 4 Earthwork Procedures... 4 Foundations... 5 Foundation Construction Considerations... 6 On-Grade Floor Slabs... 6 Seismic Considerations... 7 Liquefaction... 7 REPORT CONDITIONS... 7 ATTACHMENTS Figure 1: Site Plan Appendix A: Field Exploration Procedures and Boring Logs B-1 and B-2 Appendix B: Unified Soil Classification Chart

4 EXECUTIVE SUMMARY South Sound Geotechnical Consulting () has completed a geotechnical evaluation that included two (2) borings advanced at the site on August 22, The site is considered suitable for the planned development based on subsurface conditions observed in the borings. The following information is intended only as a summary of geotechnical considerations for development at this site: Site Conditions: The planned retail building footprint is currently in an asphalt paved parking area. The site is level with a landscaped area between the building site and NW Bucklin Hill Road. Subsurface Conditions: Asphalt pavement had a thickness of 2.5 inches at the boring B-2 location with about 6 inches of mixed fill (sand, gravel and bricks) below. Native soils were observed below the surface in boring B-1 and the pavement section fill in boring B-2. Native soils consisted of principally silty sand to sand with variable silt and gravel in a medium dense to very dense condition. Groundwater: Perched groundwater was observed in sand seams at about 8 feet below the surface at the time of drilling. The permanent groundwater table was observed at a depth of about 26.5 feet. Foundations: Conventional spread footings on the dense to very dense native soils should provide adequate support for foundations. Allowable bearing pressures of 3,000 psf may be used for foundations supported on properly prepared subgrades. Floor Slabs: Floor slabs may be supported on properly prepared native subgrades. Floor slab design should include a 4-inch minimum thickness capillary break. Seismic Considerations: Site Class C would be used to classify the site per Table of the 2009 International Building Code (IBC). This executive summary should not be used for design and/or construction purposes. Specific details are not included or fully explained in this executive summary. The entire report must be read for a comprehensive understanding of the information and recommendations presented. i

5 PROJECT INFORMATION We understand that the planned development consists of a new retail building with a footprint of 6,760 square feet located in the southeastern portion of the pavement area southeast of the Firestone Tire Store on Silverdale Way NW. The retail site is on the north side of NW Bucklin Hill Road. We understand the building will be a one-story, wood- or steel- framed structure with a concrete slab-on-grade floor. We further understand that the existing pavement surrounding the store footprint will be left in place for future parking. SITE CONDITIONS The site is currently asphalt paved and generally level. A landscape area of grass and isolated trees separates the planned building site from NW Bucklin Hill Road. Based on the USGS 7.5 Minute Poulsbo Quadrangle, Washington map, site elevation is on the order of around 20 feet above sea level. SUBSURFACE CONDITIONS Subsurface conditions were explored by drilling two (2) borings within the building footprint on August 22, The borings were advanced to depths ranging from 13 to 28.5 feet below surface grades. Approximate boring locations are shown on Figure 1 (Site Plan). A summary description of observed subgrade soils is provided below, while detailed logs of the borings are provided in Appendix A. Soil Conditions Asphalt thickness at the boring B-2 location was about 2.5 inches. Below the asphalt was mixed sand and gravel with bricks on the order of 4 to 6 inches thick. Native soils below the pavement section in boring B-2, and the ground surface in boring B-1, consisted of silty sand to sand with silt and variable gravel. Native soils were in a medium dense to very dense condition. Groundwater Conditions Groundwater was observed at the time of drilling of the borings. Perched groundwater was observed in saturated sand seams at about 8 feet below the surface in the borings at the time of drilling. The permanent groundwater table was observed in boring B-1 at a depth of about 26.5 feet at the time of drilling. This lower water table will likely be influenced by tides. The upper perched groundwater may be influenced by precipitation and seasonal trends, as well as site development and off-site impacts. We would anticipate that perched groundwater will be at higher elevations during the rainy seasons of the year. Geologic Setting Soils on the site have been surveyed and classified by the U.S. Soil Conservation Service. The report of this survey is presented in the "Soil Survey of Kitsap County Area, Washington" issued in The soil survey map of this report indicates that the soil at the site consist of Kapowsin gravelly loam. 1

6 Kapowsin gravelly loam formed in glacial till on broad uplands. Permeability is reported as moderate in the upper portions and very slow with depth. Surface runoff is slow with a reported slight hazard to water erosion. Native soils observed in the upper portion of the borings appear to conform to the mapped soil type. GEOTECHNICAL DESIGN CONSIDERATIONS The site is considered suitable for the planned development. Spread footings and floor slabs can be supported on the medium dense to very dense native soil, or on structural fill placed on firm native soils. Recommendations presented in the following sections are based upon the subsurface conditions observed in the borings and our current understanding of project plans. However, it should be noted that subsurface conditions across the site may vary from those depicted on the boring logs, particularly since the site is developed, and can change with time. Therefore, proper site preparation will depend upon the weather and soil conditions encountered at the time of construction. We recommend that review final plans and assess subgrade conditions for foundations, slabs, and pavements at the time of construction. Site Preparation Preparation for site grading and earthwork should include procedures intended to drain ponded water and control surface water runoff. Grading the site without adequate drainage control measures may negatively impact the amount of site soil available for use, increase the amount of export soil and import fill materials, and potentially increase the cost of the earthwork and subgrade preparation phases of the project. Site grading should include removal (stripping) of asphalt and any deleterious fill encountered within the new building area. Stripping depths are anticipated to average about ½ to 1 foot across the site, but may be locally deeper. Subgrade Preparation Following stripping, exposed subgrades should consist of the native silty sand to sand with silt and gravel. We recommend that exposed subgrades are proofrolled using a large roller or loaded dump truck to assess subgrade conditions following stripping. Proofrolling efforts should result in the upper 1 foot of subgrade soils achieving at least 95 percent of the maximum dry density (MDD) per the ASTM D1557 test method. Wet, loose, or soft subgrade soils should be compacted or removed and replaced with structural fill. A representative of should be present to assess subgrade conditions during proofrolling. 2

7 Grading and Drainage Positive drainage should be provided during construction and maintained throughout the life of the development. Allowing surface water into utility trenches or foundation excavations should be prevented during construction. Downspouts, roof drains or scuppers should discharge into splash blocks or extensions when the ground surface is not protected by exterior slabs or paving. Structural Fill Materials The suitability of soil for use as structural fill depends primarily on the gradation and moisture content of the soil when it is placed. As the amount of fines (soil fraction passing the U.S. No. 200 sieve) increases, soils can become increasingly sensitive to small changes in moisture content. It can become difficult to impossible to achieve adequate compaction if soil moisture is outside of optimum condition for soils that contain more than about 5 percent fines. Site Soils: Native site soils consist of sand with silt to silty sand with variable gravel. These soils should be suitable for use as structural fill during the drier months of the year when moisture contents can be conditioned (dried or wetted) to optimal ranges. In general, optimum moisture is within about +/- 2 percent of the moisture content required to achieve the maximum density per the ASTM D-1557 test method. If natural moisture contents are higher than optimum, soils would need to be dried prior to placement as structural fill. Import Fill Materials: We recommend that import structural fill placed during extended dry weather periods consist of material which meets the specifications for Gravel Borrow as described in Section (1) of the 2010 Washington State Department of Transportation (WSDOT) Specifications for Road, Bridge, and Municipal Construction (Publication M 41-10).. Gravel Borrow fill must be protected from disturbance if exposed to wet conditions after placement. During wet weather, or for backfill on a wet subgrade or in areas exposed to surface water flow, import soil suitable for compaction in wet conditions should be provided. Imported fill for use in wet conditions should generally conform to specifications for Select Borrow as described in Section (2), or Crushed Surfacing per Section (3) of the 2010 WSDOT M-41 manual, with the modification that a maximum of 5 percent by weight shall pass the U.S. No. 200 sieve for these soil types. It should be noted that the placement of structural fill is often weather-dependent. Delays due to inclement weather are common, even when using select granular fill. We recommend that site grading and earthwork be scheduled for the drier months of the year, if possible. 3

8 Structural Fill Placement We recommend that all structural fill be placed in lifts not exceeding 10 inches in loose measure. Each lift must be conditioned to the proper moisture content and uniformly compacted to a firm, unyielding condition using mechanical equipment. Structural fill should be compacted to a minimum of 95 percent of the maximum dry density per the ASTM D-1557 test method in foundation, floor slab and pavement areas, and in the upper two (2) feet of the surface in utility trenches. Utility trench backfill below two feet should be compacted to at least 92 percent of the referenced test method. Trench backfill within about 2 feet of the utility line should not be over compacted to reduce the risk of damage to the utility line. Fill should be compacted to at least 90 percent of the maximum dry density in utility trenches that are outside of foundation or pavement areas, or in non-structural (e.g. landscape) areas. We recommend that all fill procedures include maintaining grades that promote drainage and do not allow for ponding within the fill area. The contractor should protect compacted fill subgrades from disturbance during wet weather. In the event of rain during structural fill placement, the exposed fill surface should be allowed to dry prior to placement of additional fill. Alternatively, the wet soil can be removed. We recommend that consideration be given to protecting haul routes and other high traffic areas with freedraining granular fill material (i.e. sand and gravel containing less than 5 percent fines) or quarry spalls to reduce the potential for disturbance to the subgrade or fill during inclement weather. Structural fill should not consist of frozen material. Earthwork Procedures Conventional earthmoving equipment should be suitable for earthwork at this site. However, earthwork may be difficult during periods of wet weather or if elevated soil moisture is present. Excavated site soils may not be suitable as structural fill depending on the soil moisture content and weather conditions at the time of earthwork. If soils are stockpiled for future reuse and wet weather is anticipated, the stockpile should be protected with securely anchored plastic sheeting. If on-site soils become unusable, it may become necessary to import clean, granular soils to complete wet weather site work. Subgrade soils that become disturbed due to elevated moisture conditions should be overexcavated to expose firm, non-yielding, non-organic soils and backfilled with compacted structural fill. We recommend that the earthwork portion of this project be completed during extended periods of dry weather. If earthwork is completed during the wet season (typically November through May) it may be necessary to take extra precautionary measures to protect subgrade soils. Wet season earthwork may require additional mitigative measures beyond that which would be expected during the drier months of the year. If earthwork takes place during freezing conditions, we recommend that the exposed subgrade be allowed to thaw and be recompacted prior to placing subsequent lifts of structural fill. The contractor is responsible for designing and constructing stable, temporary excavations (including utility trenches) as required to maintain stability of both the excavation sides and bottom. Excavations 4

9 should be sloped or shored in the interest of safety following local and federal regulations, including current OSHA excavation and trench safety standards. Perched groundwater was observed at a depth of around 8 feet below the surface at the time of drilling. These perched zones could be encountered at shallower depths depending on the time of year and precipitation patterns, as well as from on- and off-site impacts. The contractor should be prepared to dewater and/or shore deeper excavations (such as sewer lines) that extend below a depth of 4 feet. A qualified geotechnical engineer should be retained during the construction phase of the project to observe earthwork operations and to perform necessary tests and observations during subgrade preparation, placement and compaction of structural fill, backfilling of excavations, and prior to construction of foundations. Foundations Foundations should be placed on the medium dense to very dense native silty sand that has been prepared as described above. The following recommendations have been prepared for conventional spread footing foundations. Bearing Capacity (net allowable): 3000 pounds per square foot (psf) for footings supported on native medium dense to very dense soil or structural fill prepared as described in this report. Footing Dimensions (Minimum): Strip: 18 inches Column: 24 inches Embedment Depth (Frost Protection): Exterior Footings: 18 inches Interior Footings: 12 inches Settlement: Total: < 1 inch Differential: < 3/4 inch over 40 feet Allowable Lateral Resistance: 300 psf/ft * (below 18 inches) Allowable Coefficient of Friction: 0.40 * * These values include a factor of safety of approximately 1.5 The net allowable bearing pressures presented above may be increased by one-third to resist transient, dynamic loads such as wind or seismic forces. Please note that lateral resistance to footings should be ignored in the upper 12-inches from exterior finish grade. 5

10 Foundation Construction Considerations All foundation subgrades should be free of water and loose soil prior to placing concrete, and should be prepared as recommended in this report. Concrete should be placed soon after excavating and compaction to reduce disturbance to bearing soils. Should soils at bearing level become excessively dry, disturbed, saturated, or frozen, the affected soil should be removed prior to placing concrete. We recommend that observed all foundation subgrades prior to placement of concrete. We recommend that footing drains are installed around the new building due to the overall impermeable nature of the underlying native soils. Below grade footings should include a minimum 4-inch diameter perforated rigid plastic or metal drain line installed at the base of the footing. The drain line should be sloped to provide positive gravity drainage to an approved storm drain receptor. The drain line should be surrounded by a zone of clean, free-draining granular material having less than 5 percent passing the No. 200 sieve or meeting the requirements of section (2) Gravel Backfill for Walls in the 2010 WSDOT Standard Specifications for Road, Bridge, and Municipal Construction manual (M41-10). The free-draining aggregate zone should be at least 18 inches wide. The granular fill should extend to within 6 inches of final grade where it should be capped with compacted fill containing sufficient fines to reduce infiltration of surface water into the wall drains. Alternately, the ground surface can be paved with asphalt or concrete. Cleanouts are recommended for maintenance of the drain system. On-Grade Floor Slabs On-grade floor slabs should be placed on subgrades and/or structural fill prepared as described in this report. We recommend a modulus subgrade reaction of 250 pounds per square inch per inch (psi/in) for native subgrades prepared as discussed in this report, and 150 psi/in for point loading conditions on structural fill. We recommend that a capillary break is provided between the prepared subgrade and bottom of slab. Capillary break material should be a minimum of 4 inches thick and consist of compacted clean, freedraining, well graded course sand and gravel. The capillary break material should contain less than 5 percent fines, based on that soil fraction passing the U.S. No. 4 sieve. Alternatively, a clean angular gravel such as No. 7 aggregate per Section (4) C of the 2010 WSDOT (M41-10) manual could be used for this purpose. We recommend that positive separations and/or isolation joints are provided between slabs and foundations, columns or utility lines to allow independent movement where needed. Backfill in interior trenches beneath slabs should be compacted in accordance with recommendations presented in this report. A vapor retarder should be considered beneath concrete slabs that will be covered with moisture sensitive or impervious coverings (such as tile, wood, etc.), or when the slab will support equipment sensitive to moisture. We recommend that the slab designer refer to ACI 302 and/or ACI 360 for procedures and limitations regarding the use and placement of vapor retarders. 6

11 Seismic Considerations The following seismic parameters and values are recommended based on the 2009 International Building Code (IBC). PARAMETER VALUE 2009 International Building Code (IBC) Site Classification 1 C 2 Site Latitude N Site Longitude W S s Spectral Acceleration for a Short Period 1.229g S 1 Spectral Acceleration for a 1-Second Period 0.462g F a Site Coefficient for a Short Period 1.00 F v Site Coefficient for a 1-Second Period Note: In general accordance with 2009 International Building Code, Table IBC Site Class is based on the average characteristics of the upper 100 feet of the subsurface profile. S s, S 1, F a, and F v values based on USGS Java Ground Motion Parameter Calculator using referenced site latitude and longitude. 2 Note: The 2009 IBC requires a site soil profile determination extending to a depth of 100 feet for seismic site classification. The borings drilled on the site did not include the required 100 foot soil profile determination. The recommended seismic site class considers that a stiff soil profile continues below the maximum depth of the borings based on the referenced maps in this report and other geologic information in the area. Liquefaction Soil liquefaction is a condition where loose, typically granular soils located below groundwater lose strength during ground shaking, and is often associated with earthquakes. Native soils in the borings consist of glacial till in a dense to very dense condition at depth. Based on these conditions, the risk of liquefaction is considered low for the design level earthquake at this site. REPORT CONDITIONS This report has been prepared for the exclusive use of Barber Development for specific application to the referenced project as discussed and has been prepared in accordance with generally accepted geotechnical engineering practices in the area. No warranties, either express or implied, are intended or made. Site safety and earthwork construction procedures are the responsibility of others. In the event that changes in the nature, design, or location of the project as outlined in this report are planned, the conclusions and recommendations contained in this report shall not be considered valid unless reviews the changes and either verifies or modifies the conclusions of this report in writing. 7

12 The analysis and recommendations presented in this report are based upon the data obtained from the subsurface explorations completed at the indicated locations and from other information discussed. This report does not reflect variations that may occur between explorations, across the site, or due to the modifying effects of construction or weather. The nature and extent of such variations may not become evident until during or after construction. If variations appear, we should be immediately notified so that further evaluation and supplemental recommendations can be provided. 8

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14 Appendix A Field Exploration Procedures and Boring Logs

15 Field Exploration Procedures Our field exploration for this project included two (2) borings completed on August 22, The approximate exploration locations are shown on the Site Plan (Figure 1). The exploration locations were determined in the field via taping or pacing. Ground surface elevations referenced on the boring logs were inferred from the USGS 7.5 Minute Quadrangle map of the area. The location of the borings and elevations should be considered accurate only to the degree implied by the means and methods used. An independent driller working under subcontract to drilled the borings. A trailer mounted hollow-stem auger drill rig was used to advance the borings. Standard penetration test (SPT) samples were collected at 2.5 to 5 foot intervals during drilling. A 140 pound hammer dropped 30 inches was used to advance the sampler. Soil samples were stored in moisture tight containers and transported to our lab for further visual identification. Upon completion of the borings, the driller was responsible for backfilling the borings in compliance with Washington State laws. The boring logs indicate the observed lithology of soils and other materials observed in the borings. Where a soil contact was observed to be gradational, our log indicates the average contact depth. Our logs also indicate the approximate depths of groundwater observed, along with sample numbers and approximate sample depths. Classification of the soils indicated on the logs is in general accordance with the Unified Soil Classification System. A-1

16 Depth (ft) Sample Interval Sample Number Ground Water N-values Testing PROJECT: Bucklin Hill Retail Location: JOB NO BORING B-1 PAGE 1 OF 2 Approximate Elevation: 20 feet Soil Description Penetration Resistance Standard Blows per foot Other inches topsoil oversilty SAND with gravel: Dense, brown, moist. S Silty SAND with trace gravel: very dense, gray, wet. Sand seams, saturated with perched groundwater S-2 ATD 50 for 6" 10 SAND with silt and trace gravel to Silty SAND with trace gravel : Very dense, gray, wet. S S S-5 50 for 6" 25 Explanation Monitoring Well Key 2-inch O.D. split spoon sample Clean Sand Moisture Content 3-inch I.D Shelby tube sample Cuttings Plastic Limit Natural Liquid Limit ATD No Recovery Groundwater level at time of drilling or date of measurement South Sound Geotechnical Consulting Bentonite Grout Screened Casing BORING LOG Boring B-1 Date Drilled: 8/22/12 Logged By: THR

17 Depth (ft) Sample Interval Sample Number Ground Water N-values Testing PROJECT: Bucklin Hill Retail Location: JOB NO BORING B-1 PAGE 2 OF 2 Approximate Elevation: 20 feet Soil Description Penetration Resistance Standard Blows per foot Other SAND with silt and trace gravel: Very dense, gray, wet. SAND with trace silt and gravel: Very dense, gray, saturated ATD S-6 50 for 6" 30 Boring completed at 28.5 feet on 8/22/12. Groundwater table at approximately 26.5 feet at time of drilling Explanation Monitoring Well Key 2-inch O.D. split spoon sample Clean Sand Moisture Content 3-inch I.D Shelby tube sample Cuttings Plastic Limit Natural Liquid Limit ATD No Recovery Groundwater level at time of drilling or date of measurement South Sound Geotechnical Consulting Bentonite Grout Screened Casing BORING LOG Boring B-1 Date Drilled: 8/22/12 Logged By: THR

18 Depth (ft) Sample Interval Sample Number Ground Water N-values Testing PROJECT: Bucklin Hill Retail Location: JOB NO BORING B-2 PAGE 1 OF 2 Approximate Elevation: 20 feet Soil Description Penetration Resistance Standard Blows per foot Other 2.5 inches asphalt over sand, gravel and rubble pieces to 6-inch diameter (FILL) Silty SAND with gravel: Very dense, brown, moist SAND with silt and trace gravel: Medium dense, gray, moist to wet. Possible rock in tip while driving. S-1 50 S-2 30 Perched groundwater in saturated sand seams ATD S Silty SAND with gravel: Dense to very dense, gray, wet S-4 41 S-4 50 for 6" 15 Boring completed at 13 feet on 8/22/12. Perched groundwater observed at about 8 feet at time of drilling for 6" 25 Explanation Monitoring Well Key 2-inch O.D. split spoon sample Clean Sand Moisture Content 3-inch I.D Shelby tube sample Cuttings Plastic Limit Natural Liquid Limit ATD No Recovery Groundwater level at time of drilling or date of measurement South Sound Geotechnical Consulting Bentonite Grout Screened Casing BORING LOG Boring B-2 Date Drilled: 8/22/12 Logged By: THR

19 Appendix B Unified Soil Classification Chart

20 UNIFIED SOIL CLASSIFICATION SYSTEM Criteria for Assigning Group Symbols and Group Names Using Laboratory Tests A Coarse Grained Soils: More than 50% retained on No. 200 sieve Fine-Grained Soils: 50% or more passes the No. 200 sieve Gravels: More than 50% of coarse fraction retained on No. 4 sieve Sands: 50% or more of coarse fraction passes No. 4 sieve Silts and Clays: Liquid limit less than 50 Silts and Clays: Liquid limit 50 or more Group Symbol Soil Classification Group Name B Clean Gravels: Cu 4 and 1 Cc 3 E GW Well-graded gravel F Less than 5% fines C Cu 4 and/or 1 Cc 3 E GP Poorly graded gravel F Gravels with Fines: Fines classify as ML or MH GM Silty gravel F,G,H More than 12% fines C Fines classify as CL or CH GC Clayey gravel F,G,H Clean Sands: Cu 6 and 1 Cc 3 E SW Well-graded sand I Less than 5% fines D Cu 6 and/or 1 Cc 3 E SP Poorly graded sand I Sands with Fines: Fines classify as ML or MH SM Silty sand G,H,I More than 12% fines D Fines classify as CL or CH SC Clayey sand G,H,I Inorganic: Organic: Inorganic: Organic: PI 7 and plots on or above A line J CL Lean clay K,L,M PI 4 or plots below A line J ML Silt K,L,M Liquid limit - oven dried Organic clay K,L,M,N 0.75 OL Liquid limit - not dried Organic silt K,L,M,O PI plots on or above A line CH Fat clay K,L,M PI plots below A line MH Elastic Silt K,L,M Liquid limit - oven dried Liquid limit - not dried Highly organic soils: Primarily organic matter, dark in color, and organic odor PT Peat 0.75 OH Organic clay K,L,M,P Organic silt K,L,M,Q A Based on the material passing the 3-in. (75-mm) sieve B If field sample contained cobbles or boulders, or both, add with cobbles or boulders, or both to group name. C Gravels with 5 to 12% fines require dual symbols: GW-GM well-graded gravel with silt, GW-GC well-graded gravel with clay, GP-GM poorly graded gravel with silt, GP-GC poorly graded gravel with clay. D Sands with 5 to 12% fines require dual symbols: SW-SM well-graded sand with silt, SW-SC well-graded sand with clay, SP-SM poorly graded sand with silt, SP- SC poorly graded sand with clay (D ) E 30 Cu = D 60/D 10 Cc = D x D F If soil contains 15% sand, add with sand to group name. G If fines classify as CL-ML, use dual symbol GC-GM, or SC-SM. H If fines are organic, add with organic fines to group name. I If soil contains 15% gravel, add with gravel to group name. J If Atterberg limits plot in shaded area, soil is a CL-ML, silty clay. K If soil contains 15 to 29% plus No. 200, add with sand or with gravel, whichever is predominant. L If soil contains 30% plus No. 200 predominantly sand, add sandy to group name. M If soil contains 30% plus No. 200, predominantly gravel, add gravelly to group name. N PI 4 and plots on or above A line. O PI 4 or plots below A line. P PI plots on or above A line. Q PI plots below A line.