Date: September 27, 2018 Project No.:

Transcription

1 Date: September 27, 2018 Project No.: Prepared For: Mr. Jonathan Stone PROMETHEUS REAL ESTATE GROUP 1900 S. Norfolk Street, Suite 150 San Mateo, California Re: Preliminary Shoring and Foundation Recommendations 303 Baldwin Avenue Retail, Office, and Residential Building 303 Baldwin Avenue San Mateo, California Dear Mr. Stone: As requested, this letter presents our preliminary geotechnical recommendations for shoring and foundations for the above referenced project. As you know we previously prepared a draft preliminary geotechnical report for the site and are currently performing our design-level geotechnical investigation for the above referenced project. Our services were performed in accordance with our agreement dated August 27, 2018; revised September 11, Project Description The project site is located at 303 Baldwin Avenue in San Mateo, California. We have discussed the site with you, reviewed relatively recent aerial photographs of the site, reviewed set of 100% schematic design plans, and visited the site. The site is currently occupied by a one-story supermarket building and surrounding asphalt concrete parking lot and landscaping areas. We understand that a new mixed-use retail/office/residential building is currently planned for the site. The new structure will include four levels above-grade with a fifth-level penthouse located on the northwest side of the building and a four-level (i.e. 45 to 50 feet deep) below-grade garage. We anticipate the planned development will be of concrete and wood construction, and utilities, landscaping and other improvements necessary for site development will also be part of the development. The site is bounded by residential and commercial development to the north, North B Street to the east, Baldwin Avenue to the south, and North Ellsworth Avenue to the west. Structural loads are not available at the time of our proposal; however, structural loads are expected to be representative of this type of structure. We assume column loads on the order of 900 to 1200 kips per column, or 16 to 18 kips per lineal foot along the perimeter wall. We assume average bearing pressures under the mat foundation on the order of 1,200 to 1,300 psf. In addition, based on the plans provided to us, we understand the tie-downs will need to support 1000 kips per column or 500 kips per 30 linear feet of perimeter wall. The building will likely be supported on a mat slab foundation with tie-downs to resist hydrostatic uplift. We anticipate cuts on the order of 45 to 50 feet being required for the four-level below-grade garage.

2 Preliminary Recommendations SUBSURFACE CONDITIONS BASED ON PREVIOUS EXPLORATIONS Below the surface pavements, our previous exploratory borings (EB-1 and paired CPT-2) generally encountered very stiff lean clay with varying amounts of sand to a depth of approximately 17 feet. Beneath the lean clay, our boring encountered medium dense silty sand to a depth of 20 feet underlain by dense poorly graded sand with silt and gravel to a depth of approximately 25½ feet. Beneath the sand, our boring encountered medium stiff to stiff lean clay with sand to a depth of 30 feet underlain by medium dense clayey sand to a depth of 34½ feet. Beneath the medium dense clayey sand our boring encountered stiff to very stiff lean clay with varying amounts of sand to a depth of 42½ feet underlain by dense to very dense clayey sand to the terminal boring depth of 50 feet. On a preliminary basis, our Boring EB-2 generally encountered stiff to hard lean clays to a depth of approximately 22 feet underlain by a layer of loose clayey sand to a depth of approximately 27 feet. beneath the clayey sand, Boring EB-2 encountered medium dense poorly graded sand to a depth of 32 feet underlain by interbedded layers of stiff to hard lean clays and medium dense to very dense clayey sands to the maximum boring depth of 120 feet. CGS (2018) has located the site at the edge of a liquefaction hazard zone. This will be discussed in detail in our report. From a practical viewpoint the soils below the proposed basement depth are not liquefiable. GROUNDWATER Groundwater was encountered in some of our previous explorations (Boring EB-1 and CPT-2) at depths of approximately 16 to 19 feet below current grades and in our recent exploration (Boring EB-2) at a depth of approximately 22 feet below current grades. All measurements were taken at the time of drilling and may not represent the stabilized levels that can be higher than the initial levels encountered. Based on our previous explorations in the area and groundwater data reported on GeoTracker, we estimate stabilized levels of groundwater are estimated to be on the order of 14 to 15 feet below existing grade. Mapping by CG (2018) indicates the historic high groundwater in the area of the site is estimated to be between 12 to 20 feet; however, the data provided is very difficult to interpolate. We recommend a high groundwater level of 14 feet be used for design. We note this value does not include any additional free board to account for potential future fluctuations of the groundwater depth. To account for any future fluctuations in the water table we recommend a high groundwater level of 12 feet be used for design. Fluctuations in groundwater levels occur due to many factors including seasonal fluctuation, underground drainage patterns, regional fluctuations, and other factors. BELOW-GRADE EXCAVATIONS On a preliminary basis, temporary shoring may support the planned cuts up to 45 feet. We have provided preliminary geotechnical parameters for shoring design in the section below. Our preliminary recommendations are based on a design groundwater level of 12 to 14 feet below existing grades as discussed above. The choice of shoring method should be left to the Project No Page 2 September 27, 2018

3 contractor s judgment based on experience, economic considerations and adjacent improvements such as utilities, pavements, and foundation loads. Temporary shoring should support adjacent improvements without distress and should be the contractor s responsibility. A pre-condition survey including photographs and installation of monitoring points for existing site improvements should be included in the contractor s scope. We should be provided the opportunity to review the geotechnical parameters of the shoring design prior to implementation; the project structural engineer should be consulted regarding support of adjacent structures. The below parameters should be confirmed during our design-level geotechnical investigation. Temporary Shoring Based on the site conditions encountered during our preliminary investigation, the cuts may be supported by soldier beams and tie-backs, braced excavations, soil nailing, or potentially other methods. Where shoring will extend more than about 10 to 15 feet, restrained shoring will most likely be required to limit detrimental lateral deflections and settlement behind the shoring. In addition to soil earth pressures, the shoring system will need to support adjacent loads such as construction vehicles and incidental loading, existing structure foundation loads, and street loading. We recommend that heavy construction loads (cranes, etc.) and material stockpiles be kept at least 15 feet behind the shoring. Where this loading cannot be set back, the shoring will need to be designed to support the loading. The shoring designer should provide for timely and uniform mobilization of soil pressures that will not result in excessive lateral deflections. Minimum suggested preliminary geotechnical parameters for shoring design are provided in the table below. Our preliminary shoring design parameters are based on encountering primarily stiff clays and clayey sands below a depth of approximately 45 to 50 feet and a design groundwater depth of 12 to 14 feet below current grades. Preliminary Suggested Temporary Shoring Design Parameters Design Parameter Minimum Lateral Wall Surcharge (upper 5 feet) Design Value 120 psf Cantilever Wall Triangular Earth Pressure 45 pcf (2) Restrained Wall Trapezoidal Earth Pressure Increasing from 0 to 20H (1)(3) Passive Pressure Starting below the bottom of the adjacent excavation (2)(3). This includes a factor of safety of 2.0 3,500 psf maximum uniform pressure (1) H equals the height of the excavation; passive pressures are assumed to act over 2.5 times the soldier pile diameter. (2) The cantilever and restrained pressures are for drained designs with dewatering. If undrained shoring is designed, an additional 40 pcf should be added for hydrostatic pressures below the water table. (3) Bottom of adjacent excavation is bottom or mass excavation of bottom of mat foundation excavation, whichever is deeper directly adjacent to the shoring element. The restrained earth pressure is estimated for the soft to medium clay case shown on Figure 23C of the FHWA Circular NO. 4 Ground Anchors and Anchored Systems. If shotcrete lagging is used for the shoring facing, the permanent retaining wall drainage materials, as discussed in the Wall Drainage section of this preliminary design memo, will need to be installed during temporary shoring construction. At a minimum, 2-foot-wide vertical panels should be placed between soil nails or tiebacks, and soldier beams, that are spaced at 6- Project No Page 3 September 27, 2018

4 to 8-foot centers. A horizontal strip drain connecting the vertical panels should be provided, or pass-through connections should be included for each vertical panel. We performed our previous borings with hollow-stem auger drilling equipment and as such were not able to evaluate the potential for caving soils, which can create difficult conditions during soldier beam, tie-back, or soil nail installation; caving soils can also be problematic during excavation and lagging placement. The contractor is responsible for evaluating excavation difficulties prior to construction. Where relatively clean sands (especially encountered below ground water) or difficult drilling or cobble conditions were encountered during our exploration, pilot holes performed by the contractor may be desired to further evaluate these conditions prior to the finalization of the shoring budget. Based on our preliminary explorations, we encountered a layer of loose to medium dense sands between approximately 17 and 32 feet below current grades. And some shoring methods such as the use of wooden lagging may be problematic for installation because of the water seepage and potential flowing sands and may not be feasible below the water table. Additional dewatering recommendations are provided in the section below. In addition to anticipated deflection of the shoring system, other factors such as voids created by soil sloughing, and erosion of granular layers due to perched water conditions can create adverse ground subsidence and deflections. The contractor should attempt to cut the excavation as close to neat lines as possible; where voids are created they should be backfilled as soon as possible with tamped sand, or grouted. As previously mentioned, we recommend that a monitoring program be developed and implemented to evaluate the effects of the shoring on adjacent improvements. All sensitive improvements should be located and monitored for horizontal and vertical deflections and distress cracking based on a pre-construction survey. For multi-level excavations, the installation of inclinometers at critical areas may be desired for more detailed deflection monitoring. The monitoring frequency should be established and agreed to by the project team prior to start of shoring construction. Detailed recommendations for monitoring of excavations will be included in our design-level geotechnical report. The above preliminary recommendations are for the use of the design team; the contractor in conjunction with input from the shoring designer should perform additional subsurface exploration they deem necessary to design the chosen shoring system. A California-licensed civil or structural engineer must design and be in responsible charge of the temporary shoring design. The contractor is responsible for means and methods of construction, as well as site safety. Construction Dewatering Groundwater levels are expected to be about 30 to 35 feet above the planned excavation bottom; therefore temporary dewatering will be necessary during construction. Our previous explorations encountered groundwater at depths of 16 to 22 feet below grade and recommend a design groundwater level of 12 to 14 feet based on previous explorations in the area. Design, selection of the equipment and dewatering method, and construction of temporary dewatering should be the responsibility of the contractor. Modifications to the dewatering system are often required in layered alluvial soils and should be anticipated by the contractor. The dewatering plan, including planned dewatering well filter pack materials, should be forwarded to our office for review prior to implementation. Project No Page 4 September 27, 2018

5 The dewatering design should maintain ground water at least 5 feet below the bottom of the mass excavation, and at least 2 feet below localized excavations such as deepened footings, elevator shafts, and utilities. If the dewatering system was to shut down for an extended period of time, destabilization and/or heave of the excavation bottom requiring over-excavation and stabilization, flooding and softening, and/or shoring failures could occur; therefore, we recommend that a backup power source be considered. Temporary draw down of the ground water table can cause the subsidence outside the excavation area, causing settlement of adjacent improvements. Evaluation of the potential deflection and settlement of adjacent structures and improvements will be provided in our forthcoming design level Geotechnical report. Depending on the ground water quality and previous environmental impacts to the site and surrounding area, settlement and storage tanks, particulate filtration, and environmental testing may be required prior to discharge, either into storm or sanitary, or trucked to an off-site facility. Below-grade Wall Drainage Miradrain, AmerDrain or other equivalent drainage matting should be used for wall drainage where below-grade walls are temporarily shored and the shoring will be flush with the back of the permanent walls. The drainage panel should be connected at the base of the wall by a horizontal drainage strip and closed or through-wall system such as the TotalDrain system from AmerDrain. Sections of horizontal drainage strips should be connected with either the manufacturer s connector pieces or by pulling back the filter fabric, overlapping the panel dimples, and replacing the filter fabric over the connection. At corners, a corner guard, corner connection insert, or a section of crushed rock covered with filter fabric must be used to maintain the drainage path. In addition, where drainage panels will connect from a horizontal application for at-grade areas to vertical basement wall drainage panels, the drainage path must be maintained. We are not aware of manufactured corner protection suitable for this situation; therefore, we recommend that a section of crushed rock be placed at the transitions. The crushed rock should be at least 3 inches thick, extend at least 12 inches horizontally over the top of the basement roof and 12 inches down from the top of the basement wall, and have a layer of filter fabric covering the crushed rock. Drainage panels should terminate 18 to 24 inches from final exterior grade unless capped by hardscape. The drainage panel filter fabric should be extended over the top of and behind the panel to protect it from intrusion of the adjacent soil. PRELIMINARY FOUNDATION RECOMMENDATIONS On a preliminary basis, in our opinion, the proposed structures may be supported on a mat foundation provided the recommendations in the Earthwork section of our forthcoming geotechnical report and the sections below are followed. Reinforced Concrete Mat Foundations The mixed-use structure may be supported on a mat foundation bearing on natural soil or engineered fill prepared in accordance with the Earthwork section of this report, and designed Project No Page 5 September 27, 2018

6 in accordance with the recommendations below. Reinforced concrete mat foundations should be designed in accordance with the 2016 California Building Code. For our preliminary analysis, we assumed maximum average areal bearing pressures of 1,200 to 1,300 psf for dead plus live loads across the mat; the maximum allowable localized bearing pressure should be limited to 4,500 psf at wall or column load locations. When evaluating wind and seismic conditions, allowable bearing pressures may be increased by one-third. These pressures are net values; the weight of the mat may be neglected for the portion of the mat extending below grade. Top and bottom mats of reinforcing steel should be included as required to help span irregularities and differential settlement. If the assumed weight (average areal bearing pressure) is higher than assumed, or there are other aspects of design not accounted for in this report, please notify us so that we may revise our recommendations. As described below, once contact pressures are available for review, please forward a copy for our final analysis. Mat Modulus of Soil Subgrade Reaction The modulus of soil subgrade reaction is a model element that represents the response to a specific loading condition, including the magnitude, rate, and shape of loading, given the subsurface conditions at that location. Design experts recommend using a variable modulus of soil subgrade reaction to provide a more accurate soil response and prediction of shears and moments in the mats. This will require at least one iteration between our soil model and the structural SAFE (or similar) analysis for the mat. As discussed above, we estimated an average areal mat pressure of 1,200 psf to 1,300 psf within the structure. Based on this assumed pressure, we calculated a preliminary modulus of subgrade reaction value for the mat foundation. For preliminary SAFE runs (or equivalent analysis), we recommend an initial modulus of soil subgrade reaction of 8 to 20 pounds per cubic inch (pci) for the mat foundation. As discussed above, the modulus of soil subgrade reaction is intended for use in the first iteration of the structural SAFE analysis for the mat design. Once the initial structural analysis is complete, please forward a color plot of contact pressures for the mat (to scale) so that we can provide a revised plan with updated contours of equal modulus of soil subgrade reaction values. Hydrostatic Uplift and Waterproofing Where portions of the structures extend below the design ground water level, including bottoms of slabs-on-grade and mat foundations, they should be designed to resist potential hydrostatic uplift pressures. Retaining walls extending below design ground water should be waterproofed and designed to resist hydrostatic pressure for the full wall height. Where portions of the walls extend above the design ground water level, a drainage system may be added as discussed in the Retaining Wall section of our forthcoming design-level geotechnical report. In addition, the portions of the structures extending below design ground water should be waterproofed to limit moisture infiltration, including mat foundation/thickened slab areas, all construction joints, and any retaining walls. We recommend that a waterproof specialist design the waterproofing system. Project No Page 6 September 27, 2018

7 Lateral Loading On a preliminary basis, lateral loads may be resisted by friction between the bottom of mat foundation and the supporting subgrade, and also by passive pressures generated against deepened mat edges. An ultimate frictional resistance of 0.40 applied to the mat dead load, and an ultimate passive pressure based on a uniform pressure of 7,000 psf may be used in design. The structural engineer should apply an appropriate factor of safety (such as 12.0) to the ultimate values above. The upper 12 inches of soil should be neglected when determining passive pressure capacity. Mat Foundation Construction Considerations Prior to placement of any vapor retarder or waterproofing and mat construction, the subgrade should be proof-rolled and visually observed by a Cornerstone representative to confirm stable subgrade conditions. As the planned basement excavation will extend below the current ground water level, we recommend that the contractor plan for stabilization of the excavation bottom to provide a working platform upon which to construct the foundation. This may include excavating an additional 12 to 18 inches below subgrade, placing a layer of stabilization fabric (Mirafi 500X or approved equivalent) at the bottom, and backfilling with clean, crushed rock. The crushed rock should be consolidated in place with vibratory equipment. Rubber tired and heavy track equipment should not be allowed to operate on the exposed subgrade; the crushed rock should be stockpiled and pushed out over the stabilization fabric. Because of the water table, we anticipated that chemically treating the bottom with lime treatment may not be feasible due to the concern of additional water inflow during the time frame needed for the mixing, curing and compaction. The pad moisture should also be checked at least 24 hours prior to waterproofing, or mat reinforcement placement to confirm that the soil has a moisture content of at least 1 percent over optimum in the upper 12 inches. MICRO-PILES FOR TENSION AND COMPRESSION LOADS We understand micro-piles will be implemented to resist the hydrostatic uplift forces. The depth, spacing and number of micro-piles depends on the strength of the soil, the geometry of the building foundation, and the required capacity. The design of the micro-piles should be performed in accordance with CBC Section including , Seismic Requirements and PTI Recommendations for Prestressed Rock and Soil Anchors (PTI, 2004). For design and project budgeting purposes, the following criteria can be used micro-piles design: Reinforcement should have Class I corrosion protection; steel casing should have at least 1/16 inch of corrosion allowance. Minimum Borehole Diameter = 6 inches, Minimum Spacing = 30 inches. The capacity of micro piles with 24-inch spacing should be reduced by 10 percent. Minimum Un-bonded Length of Pile = 10 feet. Minimum Factor of Safety for Tension and Compression = 2.0 Project No Page 7 September 27, 2018

8 Preliminary Recommended Ground Anchor Ultimate Capacities: Anchor Length Below Mat Foundation (feet) Ultimate Uplift Capacity (kips) 6-inch Diameter 8-inch Diameter During initial installation, the micro-piles may be gravity-grouted (i.e. no minimum grout pressure requirement) and minimum post-grout pressure of 250 psi should be used. At least two post-grout tubes should be installed with each micro-pile. Micro-piles should be grouted as soon as possible after drilling. Performance test(s) should be at a minimum of 2.0 times the design loads. Proof test(s) should be performed at a minimum of 1.33 times the design loads. The first two load tests should be performance tests and performance tests should be ran on 2 percent of the remaining production piles. The balance of the production piles should be proof tested. No creep testing is recommended because the micro-piles will not have a lockoff load greater than 5 to 10 kips. If lock-off loads are higher than 10 kips, we should be contacted to provide criteria for creep testing. Lock-off load or seating load of 5 to 10 kips, or as recommended by the structural engineer. Micro-piles may be drilled with rotary wash, or other suitable method to address installation below the groundwater table or a combination of these methods. Due to the soil conditions, the contractor is responsible for selecting appropriate drilling equipment capable of drilling the anchors for this project. The Geotechnical Engineer s representative should observe drilling of the micro-piles and actively participate in the testing of the micro-piles in accordance with project requirements. Performance and Proof Tests should be performed in general accordance with the PTI (2004) recommendations. Micro-Pile Construction Considerations The excavation of all drilled shafts should be observed by a Cornerstone representative to confirm the soil profile and that the micro-piles are constructed in accordance with our recommendations and project requirements. The drilled micro-piles should be straight, dry, and relatively free of loose material before grout and reinforcing steel is placed. If ground water cannot be removed from the excavations prior to grout placement, casing or drilling slurry may be required to stabilize the shaft and the grout should be placed using a tremie pipe, keeping the tremie pipe below the surface of the grout to avoid entrapment of water or drilling slurry in the grout. Project No Page 8 September 27, 2018

9 Due to the loose nature of the cleaner sand layers (fines content between 4 and 10 percent) documented in the previous borings, the use of casing, drilling slurry, or other methods to stabilize the hole of each drilled shaft may be required. Closure We hope this provides the information you need at this time. Recommendations presented in this letter are preliminary and have been prepared for the sole use of Prometheus Real Estate Group specifically for the property at 303 Baldwin Avenue in San Mateo, California. The recommendations contained in this letter may be revised once we complete our supplemental borings and complete our design-level report. Our professional services were performed, our findings obtained, and our preliminary recommendations prepared in accordance with generally accepted geotechnical engineering principles and practices at this time and location. No warranties are either expressed or implied. If you have any questions or need any additional information from us, please call and we will be glad to discuss them with you. Sincerely, Cornerstone Earth Group, Inc. Maura F. Ruffatto, P.E. Project Engineer Scott E. Fitinghoff, P.E., G.E. Principal Engineer 2020 MFR: SEF Copies: Addressee (1 by ) Project No Page 9 September 27, 2018