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HomeMy WebLinkAboutSubmitted by Felicia Penn Geotechnical Report 1 Geotechnical Report Proposed Residential Development Hyannis, Massachusetts LGCI Project No. 2026-Rev. 2 2.4 Subsurface Conditions The subsurface description in this report is based on a limited number of test pits and one (1) boring and is intended to highlight the major soil strata encountered during our test pits and boring. The subsurface conditions are known only at the actual test pit and boring locations. Variations may occur and should be expected between test pit and boring locations. The test pit and boring logs represent conditions that we observed at the time of our test pits and boring, and were edited, as appropriate, based on the results of the laboratory test data and inspection of the soil samples in the laboratory. The strata boundaries shown in our test pit and boring logs are based on our interpretations and the actual transitions may be gradual. Graphic soil symbols are for illustration only. The soil strata encountered in the test pits and boring were as follows, starting at the ground surface. Topsoil – A layer of topsoil was encountered at the ground surface in all explorations and extended to depths ranging between 0.5 and 1.7 feet beneath the ground surface. Subsoil – A layer of subsoil was encountered in boring B-4 and in test pit TP-1, and extended to depths of 3 and 4.5 feet beneath the ground surface, respectively. The samples in the subsoil were described as silty sand or poorly graded sand. The fines content ranged in the subsoil up to 30 percent. The subsoil contained between 10 and 25 percent fine to coarse gravel. The subsoil contained traces of organic soil and roots. Fill – Fill was encountered beneath the topsoil or subsoil in all explorations except in test pit TP- 1 and boring B-4, and extended to depths ranging 3.5 and 4.5 feet beneath the ground surface. The samples in the fill were mostly described as poorly graded sand or well graded sand. In one (1) test pit, the fill was described as silty sand. The fines content ranged in the fill up to15 percent and the gravel content ranged between 10 and 40 percent. The fill contained traces of organic soil and roots. The fill may be deeper at locations not explored by LGCI. Sand – A layer of sand was encountered beneath the topsoil or subsoil in all test pits and in the boring, and extended to the test pit and boring termination depths. The samples in this layer were described as poorly graded sand with up to 10 percent fines and up to 15 percent gravel. The standard penetration tests SPT N-values in this layer ranged between 9 and 27 blows per foot (bpf) with most values higher than 11 bpf, indicating mostly medium dense sand. 2.5 Groundwater Groundwater was not encountered in the test pits, and was encountered at a depth of 14 feet during drilling and 19.9 feet beneath the ground surface at the end of drilling in boring B-4 as shown in Table 2 and in the boring log. Geotechnical Report Proposed Residential Development Hyannis, Massachusetts LGCI Project No. 2026-Rev. 2 7 The groundwater information reported herein is based on observations made during or shortly after the completion of drilling and excavation, and may not represent the actual groundwater 2 conditions, as additional time may be required for the groundwater levels to stabilize. The groundwater information presented in this report only represents the conditions encountered at the time and location of the explorations. Seasonal fluctuation should be anticipated. 2.6 Laboratory Test Data LGCI submitted two (2) soil samples collected from the test pits for grain-size analysis. The results of the grain-size analysis are provided in the test data sheets included in Appendix D and are summarized in the table below. Grain-Size Analysis Test Results Test Pit No. Sample No. Stratum Sample depth (ft.) Percent Gravel Percent Sand Percent Fines TP-1 TP-9 Grab Grab Subsoil Fill 0.8 – 4.5 1.4 – 4.5 14.5 28.0 60.3 60.3 25.2 11.7 Geotechnical Report Proposed Residential Development Hyannis, Massachusetts LGCI Project No. 2026-Rev. 2 8 3. EVALUATION AND RECOMMENDATIONS 3.1 General Based on our understanding of the proposed residential development, our observation of the explorations, and the results of our laboratory testing, there are a few issues that we would like to highlight for consideration and discussion. 3.1.1 Surficial Topsoil and Subsoil The surficial topsoil and subsoil are not suitable to support the proposed 3 buildings and should be entirely removed from under the proposed building footprints. The removal should extend over an area extending beyond the zone of influence of the footings and at a minimum 2 feet outside the proposed building footprint, whichever is greater. The zone of influence is defined as the zone beneath a line starting at the bottom outer edge of the footings and extending outward and downward at a slope of 1H:1V. In paved areas, we recommend entirely removing the surficial topsoil from within the proposed paved areas. We recommend removing the subsoil to the top of the natural sand, to the top of the existing fill, or to a minimum depth of 18 inches beneath the bottom of the proposed pavement, whichever occurs first. Where the subsoil extends to depths greater than 18 inches beneath the bottom of the proposed pavement, the subsoil deeper than 18 inches beneath the bottom of the proposed pavement may remain in place provided that it is improved in accordance with the recommendations in Section 4.1. The removal should extend 5 feet outside the limits of improvement areas. 3.1.2 Existing Fill The existing fill is not suitable to support the proposed buildings an d should be entirely removed from within the proposed building footprints. We anticipate that the removal will extend to depths of about 4.5 feet beneath the ground surface. The removal should extend over an area extending beyond the zone of influence of the footings and at a minimum 2 feet outside the proposed building footprint, whichever is greater. The zone of influence is defined as the zone beneath a line starting at the bottom outer edge of the footings and extending outward and downward at a slope of 1H:1V. The fill may be deeper at locations not explored by LGCI, especially near the brooks. We recommend engaging LGCI to perform additional explorations at the site to further delineate the limits and thickness of the existing fill. The existing fill may remain in place within the proposed parking lots and driveways after it is improved in accordance with the recommendation in Section 4.1. Geotechnical Report Proposed Residential Development Hyannis, Massachusetts LGCI Project No. 2026-Rev. 2 4 9 3.1.3 Shallow Foundations and Slab-on-grade After the surficial topsoil, subsoil, and existing fill are entirely removed from within the proposed building footprint, the proposed building may be supported on shallow footings bearing in the natural sand. Due to the susceptibility of the natural sand to disturbance, we recommend placing footings on minimum of 6 inches of Structural Fill. The proposed slab may be designed as a slab-on-grade supported on Structural Fill placed directly on top of the natural sand. Our recommendation for footing design and slab-on-grade are presented in Section 3.2 and 3.3, respectively. 3.1.4 Reuse of Onsite Materials The subsoil is too silty and may not be used as backfill may be used in landscaped areas; however, the contractor is cautioned that when wet, the subsoil will become very soft and difficult to handle. l under roadways and buildings. It The natural sand is generally poorly graded and while it may be used as Ordinary Fill (see Section 4.3), it will require to be wetted and will require significant effort to achieve the required relative compaction. Additional recommendations for fill materials and reuse, including amendment/improvement, of onsite materials are presented in Sections 4.3 and 4.4. 3.2 Foundation Recommendations 3.2.1 Footing Design • For footings supported on a minimum of 6 inches of Structural Fill placed directly over the natural sand after removing the surficial topsoil, the subsoil, and the existing fill, we recommend a net allowable bearing pressure of 4 kips per square foot (ksf). • Footing subgrades should be prepared in accordance with the recommendations in Section 4.1. • All foundations should be designed in accordance with The Commonwealth of Massachusetts State Building Code 780 CMR, ninth Edition (MSBC 9th Edition). • Exterior footings and footings in unheated areas should be placed at a minimum depth of 4 feet below the final exterior grade to provide adequate frost protection. Interior footings in heated areas may be designed and constructed at a minimum depth of 2 feet below finished floor grades. • Wall footings should be designed and constructed with continuous, longitudinal steel reinforcement for greater bending strength to span across small areas of loose or soft soils that may go undetected during construction. Geotechnical Report Proposed Residential Development Hyannis, Massachusetts LGCI Project No. 2026-Rev. 2 10 • A representative of LGCI should be engaged to observe that the subgrade has been prepared in accordance with our recommendations. 3.2.2 Settlement Estimate 5 For footings designed using the net allowable bearing pressure recommended above, we anticipate that the settlement will be about 1 inch and that the differential settlement of the footings will be 3/4 inch or less, over 25 feet. Total and differential settlements of these magnitudes are usually considered tolerable for the anticipated construction. As the design progresses and the settlement estimates are refined, the tolerance of the proposed structure to the predicted total and differential settlements should be assessed by the structural engineer. 3.3 Concrete Slab Considerations • Floor slabs can be constructed as slabs-on-grade bearing on a minimum of 12 inches of Structural Fill placed directly on top of the natural sand. The subgrade of the slabs should be prepared as described in Section 4.1. • To reduce the potential for dampness in the proposed floor slabs, the project architect may consider placing a vapor barrier beneath the floor slabs. The vapor barrier should be protected from puncture during construction of the slabs. • For the design of the floor slabs bearing on the materials described above, we recommend using a modulus of subgrade reaction, ks1, of 80 tons per cubic foot (tcf). Please note that the values of ks1 are for a 1 x 1 square foot area. These values should be adjusted for larger areas using the following expression: where: ks = Coefficient of vertical subgrade reaction for loaded area, ks1 = Coefficient of vertical subgrade reaction for 1 x 1 square foot area, and B = Width of area loaded, in feet. Please note that cracking of slabs-on-grade can occur as a result of heaving or compression of the underlying soil, but also as a result of concrete curing stresses. To reduce the potential for cracking, the precautions listed below should be closely followed for construction of all slabs-on- grade: Geotechnical Report Proposed Residential Development Hyannis, Massachusetts LGCI Project No. 2026-Rev. 2 11 • Construction joints should be provided between the floor slab and the walls and columns in accordance with the American Concrete Institute (ACI) requirements, or other applicable code. • Backfill in interior utility trenches should be properly compacted. • In order for the movement of exterior slabs not to be transmitted to new foundations or superstructures, exterior slabs such as approach slabs and sidewalks, should be isolated from the superstructure. 3.4 Under-slab Drains Based on the current groundwater levels observed in the explorations, we anticipate that under- slab drainage systems will not be required under the proposed buildings. 3.5 Seismic Design In accordance with Section 1613 of MSBC 9th Edition and International Building Code (2015 6 IBC) and based on the boring data, the seismic criteria for the site are as follows: • Site Class: D • Spectral Response Acceleration at short period (Ss): 0.152g • Spectral Response Acceleration at 1 sec. (S1): 0.055g • Site Coefficient Fa (Table 1613.5.3(1)): 1.6 • Site Coefficient Fv (Table 1613.5.3(2): 2.4 • Adjusted spectral response SMS: 0.242 g • Adjusted spectral responses SM1: 0.132 g Based on the boring information, we believe the site soils are not susceptible to liquefaction. 3.6 Lateral Pressures for Wall Design 3.6.1 Lateral Earth Pressures Lateral earth pressures recommended for design of below grade building walls, if any, or site retaining walls are provided below. Coefficient of Active Earth Pressure, KA: 0.33 Coefficient of At-Rest Earth Pressure, Ko: 0.50 Coefficient of Passive Earth Pressure, Kp: 3.0 Total Unit Weight : 125 pcf Note: The values in the table are based on a friction angle for the backfill of 30 degrees and neglecting friction between the backfill and the wall. The design active and passive coefficients are based on horizontal surfaces (non-sloping backfill) on both the active and passive sides, and a vertical wall face. Geotechnical Report Proposed Residential Development Hyannis, Massachusetts LGCI Project No. 2026-Rev. 2 12 • Exterior walls of below ground spaces, and retaining walls braced at the top to restrain movement/rotation, should be designed using the “at-rest” pressure coefficient. • We recommend placing free-draining material within the 3 feet immediately behind retaining walls. We recommend providing weep holes in site walls to promote drainage where possible, or a pipe should be placed at the base of the wall to collect the groundwater. Groundwater collected by the wall drains should be discharged in a lower area if gravity flow is possible. • Passive earth pressures should only be used at the toe of the wall where special measures or provisions are taken to prevent disturbance or future removal of the soil on the passive side of the wall, or in areas where the wall design includes a key. In any case, the passive pressures should be neglected in the top 2 feet. • Where a permanent vertical uniform load will be applied on the active side immediately 7 adjacent to the wall, a horizontal surcharge load equal to half of the uniform vertical load should be applied over the height of the wall. At a minimum, a temporary construction surcharge of 100 psf should be applied uniformly over the height of the wall. • We recommend using an ultimate friction factor of 0.45 between the natural sand and the bottom of the wall. Below grade walls should be designed for minimum factors of safety of 1.5 for sliding and 2.0 for overturning. 3.6.2 Seismic Pressures In accordance with MSBC 9th Edition, Section 1610, a lateral earthquake force equal to 0.100*(Ss)*(Fa)**H2 should be included in the design of walls (for horizontal backfill), where Ss is the maximum considered earthquake spectral response acceleration (defined in Section 3.5), Fa is the site coefficient (defined in Section 3.5),  is the total unit weight of the soil backfill, and H is the height of the wall. The earthquake force should be distributed as an inverted triangle over the height of the wall. In accordance with MSBC 9th Edition, Section 1610.2, a load factor of 1.43 shall be applied to the earthquake force for wall strength design. Temporary surcharges should not be included when designing for earthquake loads. Surcharge loads applied for extended periods of time shall be included in the total static lateral soil pressure and their earthquake lateral force shall be computed and added to the force determined above. Geotechnical Report Proposed Residential Development Hyannis, Massachusetts LGCI Project No. 2026-Rev. 2 13 3.6.3 Perimeter Drains • We recommend that free-draining material be placed within 3 feet of the below grade spaces, if any. To reduce the potential for dampness in below-ground spaces, perimeter walls of the proposed below-ground spaces, if any, should be damp- proofed. • We recommend that drains be provided behind the exterior of walls of below-ground spaces, and behind site retaining walls, if any. The drains should consist of 6-inch perforated PVC pipes installed with the slots facing down. Perimeter drains should be installed at the bottom of the wall in 18 inches of crushed stone wrapped in a geotextile fabric for separation and filtration. Site retaining walls may be designed 8 with weep holes discharging near the bottom of the face of the walls. • Groundwater collected by the wall drains could be discharged in a lower area if gravity flow is possible. Alternatively, it should be discharged into the street drains. A permit would be required for discharge into street drains. 3.7 Pavement Considerations 3.7.1 General The subsurface conditions encountered at the site are generally suitable to support the proposed driveways, parking lots, and sidewalks after preparation of the subgrade as described in Section 4.1. • We recommend entirely removing the topsoil from within the footprint of the proposed driveways and parking lots. • The subsoil should be removed in accordance with the recommendations in Sections 3.1.1 and 4.1. • The existing fill should be improved in accordance with the recommendations in Section 4.1. • Cobbles and boulders should be removed to at least 18 inches below the bottom of the pavement. 3.7.2 Sidewalks Sidewalks should be placed on a minimum of 12 inches of Structural Fill with less than 5 percent fines. To reduce the potential for heave caused by surface water penetrating under the sidewalk, the joints between the sidewalk concrete sections should be sealed with a waterproof compound. The sidewalks should be sloped away from the building or other vertical surfaces to promote flow of water. To the extent possible, roof leaders should not discharge onto sidewalk surfaces. 4.1 Subgrade Preparation • The surficial topsoil, subsoil, existing fill, and other deleterious matter should be entirely removed from within the proposed building footprint before the start of foundation work. • Tree stumps, root balls, and roots larger than ½ inch in diameter should be removed and the cavities filled with suitable material and compacted per Section 4.3 of this report. • Topsoil, root balls, and other deleterious material should be entirely removed from within the proposed paved areas. • Cobbles and boulders should be removed at least 6 inches from beneath footings, and 24 inches beneath the bottom of proposed slabs and paved areas. The resulting excavations should be backfilled with compacted Structural Fill under the building and with Ordinary Fill under the subbase of paved areas. • Due to the high susceptibility of the natural soil for disturbance under foot and vehicular traffic, we recommend placing a minimum of 6 inches of Structural Fill at the bottom of the excavation or 4 inches of lean concrete to serve as a working mat. • The base of the footing excavations in granular soil should be compacted with a dynamic vibratory compactor weighing at least 200 pounds and imparting a minimum of 4 kips of force to the subgrade before placing the required 6 inches of Structural Fill. • The subgrade of the slabs should be compacted using a vibratory roller compactor imparting a minimum of 40 kips of force to the subgrade before placing Structural Fill. 9 • Where soft zones are revealed during the preparation of the subgrade, the soft materials or buried organic soil should be removed and replaced with Structural Fill within the building footprint and with Ordinary Fill beneath the subbase of paved areas. • To reduce the potential of increasing lateral pressures on the retaining walls, fill placed within 3 feet of the walls, if any, should be compacted using a small plate compactor imparting a maximum dynamic effort of 4 kips. The fill within 3 feet of the walls should be placed in maximum 8-inch loose lifts. • After the surficial topsoil is entirely removed and after the subsoil is removed from within the proposed paved areas in accordance with the recommendations in Section 3.1.1, the existing subsoil deeper than 18 inches beneath the bottom of the proposed pavement and/or the existing fill should be improved by compacting the exposed surface with at least six (6) passes of a vibratory roller compactor imparting a dynamic effort of at least 40 kips. Where Geotechnical Report Proposed Residential Development Hyannis, Massachusetts LGCI Project No. 2026-Rev. 2 16 soft zones of soil are observed, the soft soil should be removed, and the grade should be restored using Ordinary Fill to the bottom of the proposed subbase layer. • Fill placed within the footprint of the proposed building should meet the gradation and compaction requirements of Structural Fill shown in Section 4.3. • Fill placed under the subbase of paved areas, should meet the gradation and compaction requirements of Ordinary Fill shown in Section 4.3. • Fill placed in the top 12 inches beneath sidewalks and exterior slabs should consist of Structural Fill with less than 5 percent fines. • When crushed stone is required in the drawings or it is used for the convenience of the contractor, it should be wrapped in a geotextile fabric for separation. The geotextile fabric should not be used under retaining walls as it promotes a plane of sliding. • An LGCI geotechnical representative should observe the removal of the existing fill and the subgrades of footings and slabs prior to fill and concrete placement to verify that the exposed bearing materials are suitable for the design soil bearing pressure. If soft or loose pockets are encountered in the footing excavations, the soft or loose materials should be removed, and the bottom of the footing should be placed at a lower elevation on firm soil, or the resulting excavation should be backfilled with Structural Fill or crushed stone wrapped in geotextile for separation. The LGCI representative should also observe the improvement of the existing subsoil and/or fill within the proposed paved areas. 4.4 Reuse of Onsite Materials The subsoil is silty and cannot be used as Ordinary or Structural Fill. The subsoil can be used in landscaped areas that do not support structures, pavements, sidewalks, or ramps. Based on our field observations and the results of the grain-size analyses, we anticipate some of the natural soil may be used as Ordinary Fill. The natural soil and subsoil free of organic matter may be amended/improved by blending with 10 crushed stone to produce Ordinary and Structural Fill. Suitable imported material and amended/improved materials should be stockpiled separately from unimproved onsite soils. Should reusable materials be encountered during excavation, they should be excavated and stockpiled separately for compliance testing. Soils with 20 percent or greater fines content are generally very sensitive to moisture content variations and are susceptible to frost. Such soils are very difficult to compact at moisture contents that are much higher or much lower than the optimum moisture content determined from the laboratory compaction test. Therefore, strict moisture control should be implemented during compaction of onsite soils with fines contents of 20 percent or greater. The contractor should be prepared to remove and replace such soils if pumping occurs. All materials to be used as fill, including blended materials, should first be tested for compliance with the applicable gradation specifications. 4.5 Groundwater Control Procedures Based on the groundwater levels encountered in our explorations, we do not anticipate that groundwater control procedures will be needed during the excavations to remove the topsoil and subsoil, and the existing fill, and in utility trenches. We anticipate that filtered sump pumps installed in a series of sump pits located at least three feet below the bottom of the proposed excavations may be sufficient to handle surface runoff that may enter the excavations during wet weather. Please note that the natural soil was fairly permeable. Accordingly, the site contractor should be prepared to use multiple sump pumps during wet weather. The contractor should be permitted to employ whatever commonly accepted means and practices are necessary to maintain the groundwater level below the bottom of the excavations, and to maintain a dry excavation during wet weather. Groundwater levels should be maintained at a minimum of 1-foot below the bottom of excavations during construction. Placement of reinforcing steel or concrete in standing water should not be permitted. To reduce the potential for sinkholes developing over sump pump pits after the sump pumps are removed, the crushed stone placed in the sump pump pits should be wrapped in a geotextile Geotechnical Report Proposed Residential Development Hyannis, Massachusetts LGCI Project No. 2026-Rev. 2 19 fabric. Alternatively, the crushed stone should be entirely removed after the sump pump is no longer in use and the sump pump pit should be restored with suitable backfill. 4.6 Temporary Excavations All excavations to receive human traffic should be constructed in accordance with the OSHA guidelines. The site soils should generally be considered Type “C” and should have a maximum allowable 11 slope of 1.5 Horizontal to 1 Vertical (1.5H:1V) for excavations less than 20 feet deep. Deeper excavations, if needed, should have shoring designed by a professional engineer. The contractor is solely responsible for designing and constructing stable, temporary excavations and should shore, slope, or bench the sides of the excavations as required to maintain stability of the excavation sides and bottom and to protect existing structures. Test Pits show poorly graded sand, some silt, down to 11-12’