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HomeMy WebLinkAboutMORNINGSTAR ASSISTED LIVING & MEMORY CARE - PDP130024 - SUBMITTAL DOCUMENTS - ROUND 1 - GEOTECHNICAL (SOILS) REPORTTABLE OF CONTENTS SUMMARY ................................................................................................................................ 1 PURPOSE AND SCOPE OF STUDY ......................................................................................... 2 PROPOSED DEVELOPMENT ................................................................................................... 2 SITE CONDITIONS ................................................................................................................... 3 SUBSURFACE CONDITIONS ................................................................................................... 3 WATER SOLUBLE SULFATES ................................................................................................. 4 GEOTECHNICAL ENGINEERING CONSIDERATIONS ............................................................ 5 SITE GRADING ......................................................................................................................... 7 PRELIMINARY PAVEMENT DESIGN .......................................................................................10 LIMITATIONS ...........................................................................................................................12 FIG. 1 – LOCATIONS OF EXPLORATORY BORINGS FIG. 2 – LOGS OF EXPLORATORY BORINGS FIG. 3 – LEGEND AND NOTES FIGS. 4 AND 5 – SWELL-CONSOLIDATION TEST RESULTS TABLE I – SUMMARY OF LABORATORY TEST RESULTS 1 SUMMARY 1. Beneath a thin layer of topsoil, the borings encountered man-placed fill consisting of clayey sand, fat to lean clay, lean clay with sand, sandy lean clay, and occasional gravels to depths ranging from approximately 8 feet to 14 feet. The sampler penetration blow counts in some of the fill were relatively low and suggests the fill may not have been placed in a controlled condition. Beneath the fill, relatively thin layers of native clayey sand and lean clay were encountered in two of the borings. The fill and native soil in Borings 1 through 4 and Boring P-1 were underlain by firm to hard claystone bedrock at depths of approximately 8.5 to 16 feet and extended to the maximum depth explored which ranged from approximately 10 to 25 feet. The upper several feet of the claystone was weathered in Borings 1, 2 and P-1. Groundwater was encountered in Borings 1, 2 and 3 at the time of drilling at depths ranging from approximately 10 to 12 feet. When subsequently checked 9 days after drilling, groundwater was encountered in Borings 1 through 4 at depths ranging from approximately 5.5 to 15 feet below the ground surface. Water levels may fluctuate with time, and may fluctuate upward in response to precipitation and after landscape irrigation is implemented. 2. The geotechnical conditions which will significantly impact the develop include the potentially uncontrolled fill, the shallow groundwater level and claystone bedrock with a moderate swell potential. Recommendations for mitigation of the concerns are provided herein. 3. In is our opinion that shallow spread footing foundations will not be feasible for this site due to the uncontrolled clay fill and shallow groundwater level. Drilled shaft/helical pier foundations and structurally supported floors over a crawl space may be considered for the lightly to moderately loaded structure. For preliminary design purposes, straight- shaft piers drilled into the underlying bedrock can normally be designed for an allowable end bearing pressure in the range of 15,000 psf to 20,000 psf. An allowable skin friction of 10% of the allowable end bearing can be anticipated for the portion of pier penetrating bedrock. 4. Because of the uncontrolled clay fill, we anticipate the exterior concrete flatwork and pavement sections will require overexcavation and replacement or recompaction of the upper 3 to 5 feet of the clay fill subgrade to provide some bridging over the uncontrolled and potentially soft, wet material. Geogrid or coarse granular fill may be required to stabilize areas of wet, unstable material if encountered in the excavations. Kumar & Associates, Inc. 2 PURPOSE AND SCOPE OF STUDY This report presents the results of a preliminary geotechnical engineering study and preliminary pavement thickness design for the proposed MVG Morning Star senior housing facility to be located at the northwest corner of Horsetooth Road and Lochwood Drive in Fort Collins, Colorado. The general site layout and boring locations are shown on Fig. 1. Based on the site plan provided on January 27, 2012, we understand that the proposed building and pavement layout changed after we had performed our subsurface exploration; therefore, our borings do not fall entirely within the existing footprint of the structure. The study was conducted to characterize the general site subsurface conditions and to provide preliminary geotechnical engineering recommendations to be used for planning and to provide conceptual discussion regarding suitable foundation type or types, depths and allowable bearing pressures. This study was performed in general accordance with our Proposal No. P-12-1-641 dated December 18, 2012. This report has been prepared to summarize the data obtained during this study and to present our conclusions and preliminary recommendations based the subsurface conditions encountered. The information and conclusions presented herein are based on data obtained from widely-spaced exploratory borings drilled for this study in and around the proposed building site. PROPOSED DEVELOPMENT Based on the site plan provided to us, we understand the approximately 4.9 acre parcel will be developed as a senior housing facility. The facility will consist of a two-story building with approximate footprint of 30,500 sq. ft. Asphalt-paved parking and drive areas will be constructed to the east and northeastern sides of the proposed building. Based on the proposed construction, we assume the foundation loads will be light to moderate, typical for such structures. A grading plan was not available at the time of this study, but we anticipate the development will occur at or near the existing grades. If the proposed development varies significantly from that generally described above or depicted throughout this report, we should be notified to reevaluate the recommendations provided herein. Kumar & Associates, Inc. 3 SITE CONDITIONS The subject site is bound to the east by the asphalt-paved Lochwood Drive, to the south by the asphalt-paved Horsetooth Road, to the west by a multi-family apartment buildings consisting of two- and three-story structures, to the northwest by single-family homes, and to the northeast by another multi-family apartment building development consisting of two-story buildings. In the vicinity of the subject site, we observed several small ponds south of Horsetooth Road; approximately 1,000 feet south of the property is the larger Warren Lake. At the time of drilling, the site was vacant of structures although an apparent 5- to 8-foot high soil stockpile with approximately plan dimensions of 125X125 feet was observed on the central portion of the property, partially occupying the footprint of the proposed structure. During our visit to the site, snow covered the ground surface obscuring much of the site. The topography at the site generally slope down gently to moderately to the north and west from the road grades along Lochwood Drive and Horsetooth Road. The total elevation difference across the site is estimated to be on the order of 12 feet. A concrete-paved drainage pan was observed along the western property boundary and contained 2-3 inches of water during our visit. The drainage pan sloped to the northeast. Vegetation on the site generally consisted of grasses and weeds. SUBSURFACE CONDITIONS Information on the subsurface conditions was obtained by drilling a total of six (6) exploratory borings within the footprint of the proposed structure and pavement as shown on the plans dated December 8, 2012. We understand that the site plan has changed after we drilled our borings, the location of the proposed structure has moved to the south and west and the pavements have shifted to the east as compared to the site plan we had been provided. Based on the plan provided to us on January 27, 2013, as shown on Fig. 1, three borings are within the footprint of the proposed structure, one boring is within the pavement area and two borings are approximately located near the building or pavement. Graphic logs of the borings are presented on Fig. 2. A legend and notes describing the soils encountered are presented on Fig. 3. Beneath a thin layer of topsoil, the borings encountered man-placed fill consisting of clayey sand, fat to lean clay, lean clay with sand, sandy lean clay, and occasional gravels to depths ranging from approximately 8 feet to 14 feet. The sampler penetration blow counts in some of the fill were relatively low and suggests the fill may not have been placed in a controlled Kumar & Associates, Inc. 4 condition. The swell-consolidation test results shown on Figs. 4 and 5, indicate that the tested samples of clayey fill have a low to negligible swell potential when wetted under a 1 ksf surcharge. The moisture content of the fill was highly variable and ranged from approximately 4.6% to 25.7% in the tested samples. Native medium dense clayey sand and medium stiff lean clay soil was encountered beneath the fill in Borings 4 and P-2. The lean clay was encountered at depths of approximately 14 to 16 feet in Boring 4, and the clayey sand was encountered at a depth of approximately 8 feet in Boring P-2 and extended to the maximum 10-foot depth explored in that boring. The fill and native soil in Borings 1 through 4 and in Boring P-1 were underlain by firm to hard claystone bedrock at depths of approximately 8.5 to 16 feet and extended to the maximum depth explored which ranged from approximately 10 to 25 feet. The upper several feet of the claystone was weathered in Borings 1, 2 and P-1. Swell-consolidation test results shown on Fig. 4, indicate the tested sample of claystone had a moderate swell potential when wetted under a 1 ksf surcharge. Groundwater was encountered in Borings 1, 2 and 3 at the time of drilling at depths ranging from approximately 10 to 12 feet. When subsequently checked 9 days after drilling, groundwater was encountered in Borings 1 through 4 at depths ranging from approximately 5.5 to 15 feet below the ground surface. Water levels may fluctuate with time, and may fluctuate upward in response to precipitation and after landscape irrigation is implemented. WATER SOLUBLE SULFATES The concentration of water soluble sulfates measured in samples obtained from the exploratory borings ranged from 0.02% to 0.48%. This concentration of water soluble sulfates represents a Class 0 to Class 2 level of severity for exposure in accordance with the guidelines presented by the American Concrete Institute (ACI). The guidelines have severity levels for potential exposure of Class 0 through Class 3 as provided in Section 601 of the Colorado Department of Transportation (CDOT) Standard Specifications for Road and Bridge Construction, 2011. Considering the potential for Class 2 severity sulfate exposure, we recommend that all concrete exposed to the on-site materials meet the sulfate resistance requirements provided in Section 601-04 of the CDOT specifications manual. Kumar & Associates, Inc. 5 GEOTECHNICAL ENGINEERING CONSIDERATIONS As indicated, man-placed fill is present on the site. We reviewed historical topographic maps of the area for additional information on the potential presence of fill and found that topography presented on the 1960 USGS “Fort Collins Quadrangle, Colorado 7.5-minute Series” topographic map suggested a pronounced drainage trending to the northeast once occupied the site. The topography on the 2010 update of the USGS map presented more subdued topography in the area of the suspected drainage. We interpret the differences in the topography to be related to potential filling of an old broad drainage which is consistent with the subsurface conditions encountered in our borings. Although sampler penetration blow counts suggest that portions of the fill are relatively compact, some relatively noncompact portions of the fill were encountered and the in-situ moisture contents are variable. It is our opinion that the fill be considered unsuitable for support of the proposed building. Significant drying of the fill will be required for excavations below the water table. Foundations: The existing overlot grading poses risks of significant total and differential settlement. As a result of the presence of potentially uncontrolled clayey fill and shallow groundwater across the southern and central portions of the site, including the proposed building footprint, we believe that deep foundations such as drilled piers or helical piers will be the most economical foundation type for the structure. We believe the use of shallowly founded spread footings would require the complete removal and replacement of the existing fill and potentially additional soft unstable native soils to provide a stable foundation for the placement of new structural fill and the foundation. It is our opinion that some amount of dewatering would be required to perform the overexcavation and replacement due to the groundwater elevation measured in our exploratory borings. We anticipate that drilled piers would be designed for allowable end-bearing pressures of 15,000 to 20,000 psf in the claystone bedrock, with allowable side shear equal to 10% of the end bearing pressure for the portion of the pier in bedrock. Minimum pier depths on the order of 25 to 30 feet are anticipated to be recommended due to the depth of bedrock and the presence of weathered bedrock. Piers should also be designed for minimum dead load pressures between 5,000 and 10,000 psf. Considering the groundwater and soil conditions, pier holes will likely require casing and/or dewatering. Kumar & Associates, Inc. 6 Helical piers bearing in the claystone bedrock could be considered as an alternative to drilled piers. Helical piers would have the advantage that casing and dewatering would not be required for the construction of this foundation system unlike drilled piers. The helical piers have the disadvantage that they may not be able to penetrate significantly into the claystone bedrock and therefore would likely have a bearing capacity on the lower end of the range estimated for the drilled piers. Seismic Design Considerations: The subject site is located in a low seismic activity area. Existing clayey fill and overburden soils classify as International Building Code (IBC) Site Class D, and the weathered to unweathered claystone bedrock generally classifies as IBC Site Class C or D. For design purposes, we recommend that IBC Site Class C be used. Based on the clayey nature of the subsurface profile and site seismicity, liquefaction is not anticipated to be a design consideration. Floor Slabs: Floor slabs present a problem where settlement prone materials exist below the floor slab elevation. It is our opinion that complete removal and replacement of the existing fill and replacement with properly compacted nonexpansive structural fill should be performed for the use of slab on grade floors to mitigate the potential of settlement related to the uncontrolled clayey fill. As an alternative, if some risk of movement of the floor slabs could be tolerated and the risk for distress associated to slab settlement was understood by the owner, we would recommend partial overexcavation and replacement of the fill on the order of 8 to 10 feet leaving 5 to 7 feet of the fill below the new structural fill. However, considering the shallow groundwater, significant excavation below the water level may not be economically feasible. It is our opinion the most positive option to mitigate the potential movement and distress caused by settlement of the fill is for the floor slabs to be structurally supported over a well-ventilated crawl space. Considering the presence of relatively shallow groundwater and the high moisture content of some of the fill, we recommend a vapor barrier be placed over the floor of the crawl space and be sealed to the foundation walls to mitigate the buildup of moisture in the crawl space. We also recommend the crawl space be protected by a perimeter underdrain system as discussed in the “Underdrain System” section below. Underdrain Systems: Because of the groundwater elevation encountered at the site and our experience that elevated or perched groundwater conditions could develop after development Kumar & Associates, Inc. 7 has occurred, the crawl space of the structure should be protected by a perimeter underdrain system. The underdrain system should consist of an interior perimeter drain that extends up to the ground surface of the crawl space, with a minimum 4-inch diameter perforated pipe placed in the bottom of a trench and surrounded above the invert level with free-draining gravel. This free-draining gravel should extend up to the ground surface of the crawl space, and should be surrounded with filter fabric. Free-draining gravel used in the drain system should contain less than 5% passing the No. 200 sieve, less than 30% passing the No. 4 sieve and have a maximum size of 2 inches. The invert of the drain lines should be placed at least 12 inches below the crawlspace ground surface, and graded to a gravity outlet or sump at a minimum 1% slope. Details of the underdrain design should be completed by the building designers with input from the geotechnical design engineer. Design of sumps and pumping equipment should consider possible sump inflow rates and the disposition of sump discharge. SITE GRADING Temporary Excavations: For temporary excavations that occur during site grading, most of the on-site materials classify as Type C according to OSHA criteria. All excavations greater than 4 feet and less than 20 feet in depth should be constructed in accordance with the applicable OSHA guidelines. OSHA requires excavations or trenching over 20 feet deep be designed by a registered professional engineer. The OSHA criteria are appropriate for soils above the groundwater level, excavations below the water level may need to be sloped more gently or dewatering may need to occur prior to excavations, and should be evaluated by a qualified engineer. Cut/Fill Slopes: Permanent unretained cuts in the clayey overburden soils less than 10 feet in height should be sloped to 3 horizontal to 1 vertical, although flatter slopes may be desired due to erosion and revegetation considerations. The risk of slope instability will be significantly increased if seepage is encountered in cuts. If seepage is encountered in permanent excavations, an investigation should be conducted to determine if the seepage will adversely affect the cut stability. Cuts exceeding 10 feet or cuts that may approach the groundwater level should be evaluated further once the grading plan is developed. The placement of additional fill over the existing potentially uncontrolled fill poses the risk of inducing additional settlement of the clayey fill. The settlement may occur over a relatively long period as the pore pressures of the wet clay fill adjusts to the additional load. It is difficult to estimate the total additional settlement of the fill and would require additional sampling and Kumar & Associates, Inc. 8 laboratory testing and more information regarding the proposed grading. Overexcavation and replacement of the existing fill with new properly compacted structural fill could help reduce the potential magnitude of settlement or differential settlement; however, considering the groundwater level in the borings we anticipate that post construction wetting would occur in much of the fill which could induce further settlement even in properly compacted new fill. New fill slopes should be evaluated once grading has been established. For the preliminary design, we anticipate that new fill slopes up to 10 feet in height may be used if the fill slopes do not exceed 3 horizontal to 1 vertical and the fills are properly compacted and drained. Good surface drainage should be provided around all permanent cuts and fills to direct surface runoff away from the slope faces. Fill slopes, cut slopes and other stripped areas should be protected against erosion by revegetation or other methods. No formal stability analyses were performed to evaluate the slopes recommended above. Published literature and our experience with similar cuts and fills indicate the recommended slopes should have adequate factors of safety. If a detailed stability analysis is required, we should be notified. Compaction Requirements: Prior to placement of fill, the ground surface should be stripped to remove all topsoil and organics. All soft materials should be excavated and removed. The subgrade should be scarified to a depth of 8 inches and recompacted to at least 95% of the standard Proctor (ASTM D 698) maximum dry density at a moisture content of 1 percentage point below to 3 percentage points above optimum. Fill used within the building footprint should be placed in uniform lifts not exceeding 8 inches and compacted to at least 98% of the standard Proctor maximum dry density. The moisture content of cohesive soils should be within 1 percentage point below to 3 percentage points above optimum, and a range within 2 percentage points of optimum for granular soils. Fill used for site grading purposes outside the building footprint should be recompacted to at least 95% of the standard Proctor (ASTM D 698) maximum dry density at the same moisture requirement indicated above. Exterior Flatwork: Because of the presence of clay fill with a questionable placement history, we believe the existing fill has the potential for significant differential and total settlement. Considering that the proposed structure will probably bear on a deep foundation, settlement of Kumar & Associates, Inc. 9 the structure is expected to be marginal compared to potential settlement of the surrounding fill. Exterior flatwork like sidewalks or patios could have significant settlement as compared to the structure which could pose hazards at the entrances of the proposed structure, or due to uneven surface of the flatwork itself. The most positive method to control settlement would be to completely remove the fill and replace it with properly compacted structural fill; however as discussed above in the “Foundation Recommendations” section this may not be feasible. Considering the condition of the fill and the relatively light loading that will occur in areas of the exterior flatwork, it is our it is our opinion a majority of the existing fill may be left in place, provided that the owner accepts that there will be an increased potential for settlement, associated distress and maintenance that could result from consolidation of any zones of poorly compacted fill. It is our opinion that overexcavation and replacement of the upper 3 to 5 feet of the subgrade will help span over areas of non-uniform compaction. Utility Trench and Backfill: The utility trenches should be excavated in accordance with all OSHA requirements, and other applicable local and state requirements. OSHA soil classifications based on the materials encountered in our exploratory borings are presented in the “Temporary Excavations” section above. In our opinion, the overburden soils should be excavatable with conventional excavation equipment. Groundwater was encountered at depths in the range of 5.5 to 15 feet; therefore, we anticipate that water levels may be near or above the assumed utility trench level on portions of the site. Dewatering may be required during the project. Based on laboratory classification testing, it appears that the subsurface fill may have relatively low to moderate permeability and that ground water will likely have a slow flow rate into relatively narrow excavations, although conduits through subsurface materials may allow localized high flow rates into the excavations. The requirements and type of dewatering effort will be based upon the depth of the excavation and the level of groundwater at the specific location. In areas of clay soils, we believe that dewatering of the trench may generally be achieved with gravel-filled sumps within the excavations. If free water is not encountered in the excavations but the existing soil surface is too soft to maintain acceptable working conditions, it may be possible to over-excavate the bottom of the excavation and place a layer of clean crushed rock, or geogrid reinforcement overlain by crushed rock, to achieve a stable work platform. Excavation below the water level will significantly reduce excavation sidewall stability, and those excavations should either be adequately shored or dewatered. Kumar & Associates, Inc. 10 Backfill placed above the pipe-zone materials to the surface should consist of suitable on-site soil obtained from the pipeline excavation. Suitable soils should have a maximum size of 3 inches, a plasticity index of less than 25, and a liquid limit of less than 50, and should be free of organics, wood, or other deleterious material that could decay over time. Most of the soils encountered in the exploratory borings satisfy the material requirements based on laboratory testing of selected samples. The backfill should be compacted to at least 95% of the standard Proctor (ASTM D 698) maximum dry density at a moisture content within 2 percentage points of optimum for granular soils and within -1 to +3 percentage points of optimum for clay soils. Highly variable moisture contents were indicated for the soils encountered in the exploratory borings. The contractor should anticipate that the material excavated from the trench may require processing such as the addition of water, or allowing time for material to dry out. PRELIMINARY PAVEMENT DESIGN A pavement section is a layered system designed to distribute concentrated traffic loads to the subgrade. Performance of the pavement structure is directly related to the physical properties of the subgrade soils and traffic loadings. Soils are represented for pavement design purposes by means of a soil support value for flexible pavements and a modulus of subgrade reaction for rigid pavements. Both values are empirically related to strength. Subgrade Materials: Based on the results of the field exploration and laboratory testing programs, the majority of the clayey fill anticipated to be the pavement subgrade material encountered at the subject site generally classify as A-6 and A-7-6 soils with group indices between 3 and 30, in accordance with the AASHTO soil classification system. A-6 and A-7-6 materials are generally considered to provide poor subgrade support. For preliminary design purposes, an R-value of 5 was selected for the existing clay fill, a resilient modulus value of 3,025 psi was selected for flexible pavements and a modulus of subgrade reaction of 70 pci was selected for rigid pavements bearing on the onsite overburden soils or new structural fill. Design Traffic: Because anticipated traffic loading information was not available at the time of report preparation, an equivalent 18-kip daily load application (EDLA) of 5 was assumed for areas restricted to automobile traffic areas and an EDLA of 10 was assumed for combined automobile and truck traffic areas, driveways, loading and delivery areas and fire lanes. Kumar & Associates, Inc. 11 If it is determined that actual traffic is significantly different from that estimated, we should be contacted to reevaluate the pavement thickness design. Preliminary Pavement Thickness Design: Preliminary asphalt and concrete pavement sections were determined in accordance with the 1993 AASHTO pavement design procedure. Based on this procedure, we believe that pavement thickness in the range of 6 to 7 inches of full-depth asphalt pavement, or a composite pavement section consisting of 4 to 5 inches of asphalt over 6 to 8 inches of compacted aggregate base course material. In lieu of an asphalt pavement section, a 6.0-inch Portland cement concrete pavement section may be used. Concrete pavement should contain sawed or formed joints to ¼ of the depth of the slab at a maximum distance of 12 to 15 feet on center. Concrete slabs used in delivery or trash collection areas should also be at least 6 inches in thickness. Subgrade Preparation: Based on the subsurface conditions encountered in our exploratory borings, the pavement subgrade will generally consist of be the clayey fill present on the site. The results of swell-consolidation tests suggest the existing fill possesses a low swell potential, but the fill has a questionable history and may be prone to settlement. As discussed in “Geotechnical Engineering Considerations,” there is a risk of settlement associated with leaving existing fills with questionable placement history in place. However, we assume it would be cost-prohibitive to remove and replace the entire thickness of the fill. Considering the condition of the fill and the relatively light loading that will occur in pavement areas, it is our opinion a majority of the existing fill may be left in place in pavement areas, provided that the owner accepts that there will be an increased potential for settlement and the associated distress that could result from consolidation of any zones of poorly compacted fill. It is our opinion that overexcavation and replacement of the upper 3 to 5 feet of the subgrade will help span over areas of non-uniform compaction. If very soft or noncompact materials are present, further overexcavation and replacement may be needed in some areas. The use of one or more layers of biaxial or triaxial geogrids may be required to stabilize subgrade due to the shallow groundwater and the proposed depth of overexcavation and replacement. As an alternative, coarse granular material could be placed over the potentially unstable ground to bridge the poor or wet material. Prior to placing the pavement section, the entire subgrade area should be thoroughly scarified and well mixed to a depth of at least 12 inches, adjusted to a moisture content within 0 to plus 3 percentage points of the optimum and compacted to 95% of the standard Proctor maximum dry Kumar & Associates, Inc. 12 density (ASTM D 698). The moisture content may need to be near the lower end of the moisture range to provide for stability. The pavement subgrade should be proofrolled with a heavily loaded pneumatic-tired vehicle. Pavement design procedures assume a stable subgrade. Areas which deform excessively under heave wheel loads are not stable and should be removed and replaced to achieve a stable subgrade prior to paving. The on-site cohesive soils may be unstable under construction traffic when moisture conditioned to the range indicated above. Alternatives for chemical stabilization associated with providing a stable paving platform as well as contribution to the pavement substructure section could be considered. Drainage: The collection and diversion of surface drainage away from paved areas is extremely important to the satisfactory performance of pavement. Drainage design should provide for the removal of water from paved areas and prevent the wetting of the subgrade soils. LIMITATIONS This report has been prepared in accordance with generally accepted geotechnical engineering practices in this area for use by the client for preliminary design and planning purposes. The preliminary conclusions and recommendations submitted in this report are based upon the data obtained from the widely spaced exploratory borings drilled at the locations indicated on the exploratory boring plan. Additional investigation must be conducted once building locations and floor elevations have been determined to provide final recommendations. We recommend on- site observation of site grading by a representative of the geotechnical engineer. CAJ/jw Rev. by JAN cc: file, book Kumar & Associates, Inc.