RFP_Attach_4_Geotech_Report_by_CTL_Thompson_from_PPR_Appendix_A.pdf

1971 West 12th Avenue | Denver, Colorado 80204 | Phone: 303-825-0777 | Fax: 303-825-4252

GEOTECHNICAL INVESTIGATION

NIST BUILDING ONE

RENOVATION PROJECT

325 BROADWAY

BOULDER, COLORADO

Prepared For:

IRON HORSE ARCHITECTS

1701 Wynkoop Street, Suite 200

Denver, Colorado 80202

Attention: Victoria David, AIA

Project No. DN45,673-125

August 5, 2011

TABLE OF CONTENTS

SCOPE

SUMMARY

SITE CONDITIONS

PROPOSED CONSTRUCTION

PREVIOUS INVESTIGATION

INVESTIGATION

SUBSURFACE CONDITIONS

Fill Native Clay, Sand and Gravel Bedrock Ground Water

GEOLOGIC HAZARDS

Undocumented Fil Expansive Soil and Bedrock Seismicity Radioactivity

SITE DEVELOPMENT

Demolition Existing Fill Excavation

Bracing Fill and Backfill

DEWATERING

Construction Dewatering Permanent Dewatering

FOUNDATIONS

Laterally Loaded Piers

Closely-Spaced Pier Reduction Factors

BELOW-GRADE CONSTRUCTION

Lateral Earth Loads

Foundation Drain

FLOOR SYSTEMS

Exterior Flatwork

RETAINING WALLS

CONCRETE

SURFACE DRAINAGE

DESIGN CONSULTATION AND CONSTRUCTION OBSERVATIONS

GEOTECHNICAL RISK

LIMITATIONS

IRON HORSE ARCHITECTS

NIST BUILDING ONE RENOVATION PROJECT

CTL | T PROJECT NO. DN45,673-125

S:\PROJECTS\45600\DN45673.000\125\2. Reports\R1\DN45673-125-R1.doc

NIST BUILDING ONE RENOVATION PROJECT

CTL | T PROJECT NO. DN45,673-125

S:\PROJECTS\45600\DN45673.000\125\2. Reports\R1\DN45673-125-R1.doc

TABLE OF CONTENTS

(Continued)

FIG. 1 – LOCATIONS OF EXPLORATORY BORINGS

FIG. 2 – ESTIMATED ELEVATIONS OF COMPARATIVELY UNWEATHERED BEDROCK

SURFACE

FIG. 3 – APPROXIMATE GROUND WATER ELEVATIONS AND BORING LOCATIONS

FIG. 4 – RECOMMENDED LATERAL EARTH PRESSURE DISTRIBUTION

FIG. 5 – SURCHARGE LOADS

FIGS. 6 AND 7 – FOUNDATION WALL DRAIN DETAILS

FIG. 8 – TYPICAL EARTH RETAINING WALL DRAIN

APPENDIX A – SUMMARY LOGS OF EXPLORATORY BORINGS

APPENDIX B – LABORATORY TEST RESULTS

SCOPE

This report presents the results of our Geotechnical Investigation for the Building One Renovation (B1R) Project at the National Institute of Standards and Technology (NIST) campus in Boulder, Colorado (Fig. 1). The purpose of our investigation was to evaluate the subsurface conditions and provide geotechnical design and construction criteria for the project. The scope of this investigation was described in our Service Agreement (No. DN 11- 0318) dated June 21, 2011. Evaluation of the property for the possible presence of potentially hazardous materials (Environmental Site Assessment) was not included in the scope of this investigation.

This report is based on conditions found in our exploratory borings, results of laboratory tests, engineering analysis of field and laboratory data, previous investigations on the NIST campus, and our experience with similar conditions and projects. The report contains descriptions of the subsoil and ground water conditions found in the exploratory borings and geotechnical design and construction criteria for site development, dewatering, foundations, lateral earth pressures, floor systems, concrete, and surface and subsurface drainage. The recommendations presented in the report are based on the project as currently planned. If the proposed construction will differ significantly from descriptions herein, we should be contacted to review our recommendations and determine if revisions are necessary. A brief summary of our conclusions and recommendations follows, with more detailed discussion and design criteria presented within the report.

SUMMARY

1. Subsoils found in our borings generally consisted of nil to about 23 feet of fill with variable amounts of gravel, sand, silt, and clay over native clay, sand, and gravel. Weathered and comparatively unweathered claystone bedrock was found underlying the fill and/or native soils at depths of about 12 to 36 feet.

The clay and claystone bedrock are generally low swelling, the sand is non-expansive or has low swell potential, and the gravel is considered non-expansive.

2. Ground water was encountered during drilling at depths between about 16 and

25 feet. When the holes were checked after drilling, water was measured at depths of about 9 to 22 feet (elevations 5396 to 5416). We understand a basement is being considered for the corridor addition adjacent to the south/west side of Wing 5 and anticipate the floor level will match the existing

IRON HORSE ARCHITECTS

NIST BUILDING ONE RENOVATION PROJECT

CTL | T PROJECT NO. DN45,673-125

basement floor at elevation 5420, as much as 2 feet below measured ground water levels. Water levels may rise in response to seasonal fluctuation, precipitation, and landscape irrigation. Temporary and permanent dewatering systems or water-tight foundations may be required to construct the potential basement.

3. We encountered up to about 23 feet of fill in our exploratory borings. Records of fill compaction were not provided and we do not know whether the fill was placed under controlled conditions. Field penetration tests indicate the fill has variable consistency. We assume fill depths vary widely; deeper near basement foundation walls and shallower away. Additional discussion is provided in the report.

4. Considering the risk of differential movement of a shallow foundation system, the variable characteristics of the subsoils, and our experience, we recommend the utility corridor additions be constructed on drilled piers bottomed in comparatively unweathered bedrock. The additions may apply increased loading to the existing belled piers; allowable capacity and potential settlement of these existing piers should be evaluated prior to construction.

Connections between the additions and existing building should be detailed to allow for movement to reduce potential damage. Design criteria, construction details, and further discussion are presented in the report.

5. The soils below the anticipated floor levels will likely consist of variably thick fill and/or native clay, sand, and gravel. We estimate potential heave or settlement of about 1 inch is likely. Greater risk of load-induced settlement is possible for slabs constructed on undocumented fill. Provided this risk is acceptable, slab-on-grade floors can be used. Structurally supported floors are the conservative choice and should be provided if significant floor movement cannot be tolerated.

6. Surface drainage should be designed, constructed, and maintained to provide rapid removal of surface water runoff away from the existing building, proposed corridor additions, and pavements. Water should not be allowed to pond adjacent to the building, additions, or on pavements and conservative irrigation practices should be followed to minimize excessive wetting.

Excessive wetting of foundation soils can cause movement and distress to floor slabs and concrete flatwork.

SITE CONDITIONS

The National Institute of Standards and Technology (NIST) Campus is located at 325 Broadway in Boulder, Colorado (Fig. 1). Building One consists of six wings with a central spine extending from the northeast side of the campus near Broadway to the southwest for about 650 feet. The surrounding ground surface and building floor levels rise approximately 25 feet across the building from the northeast to southwest. The building is constructed into the hillside with ground floors generally “day-lighting” to the northeast. Wilson and Marconi

IRON HORSE ARCHITECTS

NIST BUILDING ONE RENOVATION PROJECT

CTL | T PROJECT NO. DN45,673-125

Roads as well as surface parking lots surround the building. The existing asphalt pavement appears to be in fair condition with some cracks and patch work. Landscaping consisting of grass, trees, and bushes is generally present between the wings.

We understand the existing building was constructed on belled piers bottomed in bedrock. We assume ground floors are slabs-on-grade. To our knowledge, Building One has performed well from a soils and foundations perspective. The finished floor level of Wings 1 and 2 are at elevation 5408. Wings 3 and 4 have a finished floor level at elevation 5420. No basements exist within Wings 1 through 4. The lowest floor level of Wing 5 is at an elevation of 5420 and serves as the main level at the north/east side of the wing and a basement along the south/west side. The main floor level along the south/west side is at elevation 5432. Wing 6 has a basement with a floor level of 5420 beneath the last 60 to 70 feet of the southeast end. The main floor level is at an elevation of 5432.

PROPOSED CONSTRUCTION

Plans are preliminary at this time. Information provided by Iron Horse Architects and Martin/Martin indicates the project will consist of utility corridors constructed on the south/west side of Wing 3 and both sides of Wings 5 and 6. We understand the corridors will mainly be used to re-locate existing utilities to provide more office and laboratory space in the existing building. Some new utilities may be constructed in the corridors. The utility corridors adjacent to Wings 3 and 6 will have finished floor elevations of 5420 and 5432, respectively. The floor levels for the corridors adjacent to Wing 5 are unknown at this time.

We anticipate the corridor on the north/east side will match the existing floor level at elevation 5420. The floor for the corridor on the south/west side may be constructed at elevation 5420 or 5432, depending on whether a basement is selected. The maximum anticipated column loads for the current design include a dead load of 150 kips and a live load of 30 kips.

PREVIOUS INVESTIGATION

Woodward-Clyde Consultants, Inc. (WCC) performed a Geotechnical Study for the Building 1 Expansion (B1E) Project (WCC Project No. 23518; report dated February 1995).

The project was never constructed and CTL | Thompson, Inc. was retained in 2007 to

IRON HORSE ARCHITECTS

NIST BUILDING ONE RENOVATION PROJECT

CTL | T PROJECT NO. DN45,673-125

perform a new Geotechnical Investigation for the B1E Project (Project No. DN43,264-125;

report dated January 9, 2008 and revised March 25, 2008). Data from these studies were reviewed and considered in preparation of this report.

INVESTIGATION

Subsurface conditions at the site were investigated by drilling sixteen exploratory borings at the approximate locations shown on Fig. 1. Prior to drilling, we contacted the Utility Notification Center of Colorado and local sewer and water districts to clear locations of buried utilities. We also retained a subcontractor to locate private utilities. The borings were advanced to depths between 9 and 45 feet using 4-inch diameter, continuous-flight auger and a truck-mounted drill rig or 3-inch diameter, continuous-flight auger and a limited-access drill rig. Samples of the soils and bedrock were obtained at 5 to 15-foot intervals using 2.0-inch diameter (O.D.) Standard split-spoon samplers and 2.5-inch diameter (O.D.)

modified California barrel samplers driven by blows from a 140-pound hammer falling 30 inches. In three separate attempts at and around boring TH-13, practical auger drilling refusal occurred on cobbles or boulders at depths of about 12.5, 13, and 17.5 feet. Practical auger drilling refusal occurred on an unmarked electrical conduit at a depth of about 9 feet in six different locations at and around boring TH-14. One boring was eliminated because construction operations for the B1E Project blocked access.

The drilling operations were observed by our field representative who logged the soils and bedrock encountered in the borings and obtained samples for laboratory testing.

Upon completion of drilling, 1.25-inch diameter, hand-slotted PVC pipe was installed in the borings to facilitate delayed ground water measurements. One hole was cold patched with asphalt. At the time delayed ground water measurements were obtained, representatives of CTL | Thompson, Inc. surveyed the elevations of the borings relative to finished floor levels of Wings 3, 4, 5 and 6 (Fig. 1). Summary logs of the soils and bedrock encountered, results of the field penetration resistance values, and a portion of the laboratory data are presented in Appendix A.

Samples were returned to our laboratory where they were examined by our engineers and laboratory testing was assigned. Laboratory tests included dry density, moisture content, gradation, percent fines (silt and clay-sized particles passing the No. 200 sieve), IRON HORSE ARCHITECTS

NIST BUILDING ONE RENOVATION PROJECT

CTL | T PROJECT NO. DN45,673-125

Atterberg limits, swell-consolidation, unconfined compression, and water-soluble sulfate concentration. Swell-consolidation tests were performed using confining pressures that approximated the overburden pressures (the weight of the overlying soils). Load-back analysis was performed on select samples to obtain approximate swell pressures. Results of laboratory tests are presented in Appendix B and summarized in Table B-I.

SUBSURFACE CONDITIONS

Subsoils encountered in our exploratory borings generally consisted of nil to about 23 feet of fill with variable amounts of gravel, sand, silt, and clay over native clay, sand, and gravel. Weathered and comparatively unweathered claystone bedrock was found underlying the fill and/or native soils at depths of about 12 to 36 feet and extended to the maximum explored depth of 45 feet. Ground water was measured at about 9 to 22 feet. The following sections describe the subsurface conditions in more detail.

Fill

Fill composed of variable amounts of gravel, sand, silt, and clay was found at the ground surface or below the asphalt pavement in thirteen of the sixteen borings. The depth of fill was based upon drilling observations and examination of the soil samples. Field penetration resistance tests indicate the fill is stiff to very stiff or loose to very dense. We assume the fill was placed during construction of the existing building and parking lots. Ten fill samples exhibited nil to 1.4 percent swell after wetting under approximate overburden pressures; one sample compressed 0.1 percent when tested similarly. Swell pressures determined by load-back analysis were between 1,300 and 2,300 pounds per square foot (psf). One fill sample from a depth of 19 feet in TH-12 exhibited an unconfined compressive strength of 3,300 psf. Fill samples selected for gradation analysis contained 11 to 62 percent gravel (material retained on the No. 4 sieve), 34 to 37 percent sand, and 4 to 52 percent fines (material passing the No. 200 sieve). One gravelly fill sample exhibited low plasticity with a liquid limit of 25 percent and a plasticity index of 6 percent.

CTL | T PROJECT NO. DN45,673-125

The existing building wings have below-grade construction with backfill placed against the below-grade walls. The extent and compaction of the wall backfill is unknown.

There is risk of uneven settlement or heave if loads from the new construction are to be supported by existing fill.

Native Clay, Sand, and Gravel

Native clay, sand, and gravel were found at the ground surface or below the fill in ten of the borings. Clay was predominant and ranged from about 4 to 17 feet thick. Interlayered sand and clay about 6 to 31 feet thick was found in three borings. Clayey sand and gravel were penetrated in four borings; the thickness varied between about 4 and 15 feet. Based on field penetration resistance tests, the clay is medium stiff to very stiff and the sand and gravel are loose to very dense.

Eight clay samples did not swell or swelled 0.1 to 0.8 percent when wetted under approximate overburden pressures. Swell pressures determined by load-back analysis on two samples were 1,900 and 2,100 psf. Unconfined compressive strengths of 900 to 2,200 psf were measured in three samples. One sample contained 58 percent fines and exhibited moderate plasticity with a liquid limit of 44 percent and a plasticity index of 58 percent. We judge the clay is generally low swelling.

Two sand samples either did not swell or swelled 0.3 percent after wetting. Four sand samples contained 30 to 41 percent fines. Two of these samples exhibited low to moderate plasticity with liquid limits of 26 and 41 percent and plasticity indices of 12 and 25 percent.

Sand and gravel samples selected for gradation analysis contained 2 to 40 percent gravel, 39 to 73 percent sand, and 21 to 39 percent fines. The sand is generally non-expansive or has low swell potential. The gravel is considered non-expansive.

Bedrock

Claystone bedrock was found in fourteen of the sixteen borings. Bedrock was not found in TH-13 and TH-14 where practical drill rig refusal occurred. Based on field penetration resistance tests, the bedrock is highly weathered to very hard. The weathered zone was about 3 to 16 feet thick in seven borings. The surface of the comparatively

IRON HORSE ARCHITECTS

NIST BUILDING ONE RENOVATION PROJECT

CTL | T PROJECT NO. DN45,673-125

unweathered bedrock was at depths of about 18 to 39 feet (elevations 5389 to 5405).

Estimated elevations of the comparatively unweathered bedrock surface are shown on Fig.

2.

Weathered and comparatively unweathered claystone samples swelled 0.1 to 2.0 percent; two samples compressed 0.1 and 0.2 percent. Swell pressures ranged from 2,300 to 14,000 psf for six samples. Unconfined compressive strengths of 2,000 and 8,500 psf were measured in weathered and comparatively unweathered claystone samples, respectively.

Three claystone samples contained 94 to 96 percent fines and exhibited high plasticity with liquid limits of 51 to 56 percent and plasticity indices of 29 to 39 percent.

Ground Water

Ground water was encountered during drilling in twelve of the sixteen borings at depths between 16 and 25 feet (elevations 5396 to 5416). When the holes were checked after drilling on July 26, 2011, water was measured in fourteen of the sixteen borings at depths of about 9 to 22 feet (elevations 5406 to 5422). Approximate ground water elevations at each boring location are presented on Fig. 3. Relatively shallow ground water was measured in boring TH-11 at about 2 feet above the basement floor level. We were informed by NIST they have not noticed any ground water problems in the existing basement of Wing 5. It is not known what type of basement foundation drain system exists under Wing 5. It is possible the shallow ground water could be a result of surface water intrusion, the new construction to the south, and/or localized perched water.

Ground water may affect below-grade construction for the proposed corridor on the south/west side of Wing 5 and will likely be encountered during installation of drilled piers everywhere. Temporary dewatering systems may be required to construct the potential basement and a permanent dewatering system will likely be necessary to maintain water levels below the anticipated basement floor elevation. Water-tight construction may also be considered. Water levels may rise in response to seasonal fluctuation, precipitation, and landscape irrigation. Our experience indicates ground water levels are typically highest in late summer and fluctuations of 1 to 2 feet are not uncommon.

CTL | T PROJECT NO. DN45,673-125

GEOLOGIC HAZARDS

Our study identified conditions on the site that constitute potential geologic hazards.

These hazards include:

Undocumented, variable fill;

Expansive soil and bedrock; and Regional issues of seismicity and radioactivity.

These hazards and conceptual mitigation methods are discussed in the following sections.

Undocumented Fill

We encountered up to about 23 feet of existing fill in our exploratory borings.

Foundation wall backfill adjacent to the existing building is present. Records of fill and backfill compaction were not provided. Existing fill may not be suitable to support improvements. If risk due to settlement and/or heave of the fill cannot be tolerated, then the fill should be completely removed and re-processed, or a structural floor should be provided. More detailed discussion is provided in later sections of this report.

Expansive Soil and Bedrock

Many sites in the Denver area and Front Range of Colorado contain expansive soil and bedrock, our most common geologic hazard. Expansive clay and bedrock are present at this site. The presence of expansive soil and bedrock, collectively referred to as expansive or swelling soils, constitutes a geologic hazard. When comparatively dry expansive soils are wetted, they absorb water, increase in volume (swell), and shear strength may decrease.

Swell pressures can exceed the applied structural load of a footing or slab-on-grade to the subgrade soils. Therefore, heave that occurs upon wetting can damage improvements.

Our investigation indicates there is low risk that ground heave will damage foundations and slabs at this site. The risks associated with swelling soils can be mitigated, but not eliminated, by careful design, construction, and maintenance procedures.

IRON HORSE ARCHITECTS

NIST BUILDING ONE RENOVATION PROJECT

CTL | T PROJECT NO. DN45,673-125

Seismicity

The soil and bedrock are not expected to respond unusually to seismic activity.

According to the 2009 International Building Code (IBC), and based upon the results of our investigation, we judge the site classifies as Site Class D. The subsurface and ground water conditions indicate low susceptibility to liquefaction. Only minor damage to relatively new, properly designed and constructed buildings would be expected with a major seismic event.

Wind loads typically govern dynamic structural design in this area.

Radioactivity

It is normal in the Front Range of Colorado and nearby eastern plains to measure radon gas in poorly ventilated spaces in contact with soil or bedrock. Radon 222 gas is considered a health hazard and is one of several radioactive products in the chain of the natural decay of uranium into stable lead. Radioactive nuclides are common in igneous and metamorphic rock in this area and in the soils and sedimentary rocks underlying the subject site. Because these sources exist on the site, there is potential for radon gas accumulation in poorly ventilated spaces. The amount of soil gas that can accumulate is a function of many factors, including the radio-nuclide activity of the soil and bedrock, construction methods and materials, pathways for soil gas, and existence of poorly-ventilated accumulation areas. Typical mitigation methods consist of sealing soil gas entry areas and periodic ventilation of below-grade spaces and perimeter drain systems. Radon rarely accumulates to significant levels in above-grade spaces. The only reliable method to determine the concentration of radon is to perform testing after construction.

SITE DEVELOPMENT

Demolition

After demolition of the existing asphalt pavement, flatwork, and structures (if any) in the area of the new utility corridors, all remnants of foundation elements, utilities, pavements, flatwork, debris, tree roots, vegetation and topsoil should be removed from the area. If soft or loose soils are encountered during construction, they can be removed and replaced with suitable material as discussed in the Fill and Backfill section or stabilized in

IRON HORSE ARCHITECTS

NIST BUILDING ONE RENOVATION PROJECT

CTL | T PROJECT NO. DN45,673-125

place by crowding crushed rock into the subsoils until the base of the excavation does not deform more than about 1 inch when heavy compactive effort is applied. Acceptable rock materials include, but are not limited to, No. 2 and No. 57 rock, or 1 to 3 inch recycled concrete. Crushed rock on a layer of geosynthetic grid or woven fabric can also be used and will likely require less rock to stabilize the grade.

Existing Fill

About 23 feet of fill was encountered in TH-12 drilled on the south/west side of Wing

5. Fill compaction records were not provided. We anticipate the fill was placed during construction of the existing Building One. We do not know if the fill was placed under controlled conditions. Because the fill was likely placed over 50 years ago, it is difficult to predict the amount of consolidation that has occurred. Load-induced movement may occur as a result of floor pressures from the utility corridors. If the owner cannot tolerate the risk of movement and associated damage, the fill should be completely removed and re-processed, and replaced as moisture conditioned, properly compacted fill. Considering the new utility corridors will be constructed adjacent to the existing building, complete removal of the fill may not be a feasible alternative because underpinning the existing structure may be required to protect the existing floors. A structural floor should be provided for the new corridors if movement of a slab-on-grade cannot be accommodated.

Excavation

Excavations will likely be sloped where practical. Areas that do not permit sloped excavations will require an earth retention system. We believe the soils penetrated by our exploratory borings can be excavated with typical heavy-duty construction equipment. The existing utility concrete conduits may cause difficulties for excavation and pier drilling. We recommend the owner and contractor become familiar with applicable local, state, and federal safety regulations, including the current Occupational Safety and Health Administration (OSHA) Excavation and Trench Safety Standards. Based on our investigation and OSHA standards, we anticipate the fill and native soils will classify as Type B or C soil.

Based on OSHA regulations, maximum slope inclinations of 1:1 (horizontal to vertical) for Type B soil and 1.5:1 for Type C are required for temporary excavations in dry conditions.

IRON HORSE ARCHITECTS

NIST BUILDING ONE RENOVATION PROJECT

CTL | T PROJECT NO. DN45,673-125

Flatter slopes should be anticipated below ground water or where seepage is present. If adequate slopes cannot be maintained, the excavation should be braced.

Excavation slopes specified by OSHA are dependent upon soil types and ground water conditions encountered. The contractor’s “competent person” should identify the soils in the excavations and refer to OSHA standards to determine appropriate slopes.

Stockpiles of soil and construction equipment should not be placed within a horizontal distance equal to one-half the excavation depth, from the edge of the excavation. A professional engineer should design excavations deeper than 20 feet.

Bracing

We anticipate excavations on the order of about 10 to 12 feet will be required for basement construction being considered on the south/west end of Wing 5. Areas that do not allow for properly sloped excavations will require braced or shored excavations. The sand, gravel, and boulders will be prone to caving or raveling when exposed. Potential alternatives for excavation bracing, if necessary, are anticipated to include soldier piles or piers and lagging with tie-backs or soil nails. Continuous drilled piers are also an option, but may not be economical given the subsoil conditions and proposed construction. The cost of these methods varies with their complexity.

Most large excavations in the Denver area are accomplished using soldier piles and lagging tied back with various anchor systems. The type of system chosen will depend on many factors. The use of tie-backs or soil nails may require easements below the surrounding streets. The availability of easements for tie-backs or soil nails could affect bracing design. The owner, design team, and contractor should verify the location, depth, and size of the existing utilities early in the design process. This information will need to be considered as part of the bracing design and potential locations of tie-backs or soil nails.

All bracing systems undergo some movement. The degree of movement depends on the type of system and anchors utilized. These potential movements should be estimated and considered in design. Settlement and distress of adjacent streets, parking lots, and sidewalks may occur and repair or replacement may be needed. The costs associated with

IRON HORSE ARCHITECTS

NIST BUILDING ONE RENOVATION PROJECT

CTL | T PROJECT NO. DN45,673-125

minimum ground movement may be substantially higher than a more flexible system. An experienced specialty contractor should be consulted for relative costs.

Recommended design lateral earth pressures for various bracing conditions are presented on Fig. 4. The earth pressures presented do not include hydrostatic loads or surcharge from traffic or adjacent structures. Figure 5 presents methods for evaluating additional pressures due to traffic surcharge and line and uniform loading conditions.

The excavation and bracing plan should incorporate a monitoring program to document existing sidewalk, street, and parking lot conditions and elevations before, during, and after construction.

Fill and Backfill

Based on the construction as currently planned, we anticipate less than about 2 to 3 feet of fill will be required for most of the site. Deeper foundation wall backfill will be necessary if a basement is selected for the utility corridor on the south/west side of Wing 5.

On-site soils are suitable for re-use as fill provided debris, vegetation, organics, and other deleterious materials are removed prior to placement. Excavated claystone or cuttings from pier excavations should not be used for fill. Existing asphalt pavement should be removed and wasted. If desired, the demolished pavement materials can be processed and re-used if they meet the import fill guidelines. If import fill is necessary, it should consist of granular soil with a maximum particle size of 3 inches, less than 50 percent passing the No. 200 sieve, a liquid limit less than 30 percent, and a plasticity index less than 10 percent. Import fill materials should be submitted to our office for testing and approval prior to stockpiling on the site.

Prior to fill placement, debris, trees and vegetation, organics, and deleterious matter should be substantially removed from areas to receive fill. Areas to receive fill should be scarified to a depth of at least 8 inches prior to fill placement, moisture conditioned to between 0 and 3 percent above optimum moisture content for clay and to within 2 percent of optimum for sand, and compacted to at least 95 percent of standard Proctor maximum dry density (ASTM D 698). All subsequent fill should be placed in thin, loose lifts of 8 inches or less and moisture conditioned and compacted similarly. Placement and compaction of fill

IRON HORSE ARCHITECTS

NIST BUILDING ONE RENOVATION PROJECT

CTL | T PROJECT NO. DN45,673-125

and backfill should be observed and tested by a representative of our firm during construction.

Water and sewer lines are often constructed beneath slabs and pavements.

Compaction of utility trench backfill can have a significant effect on the life and serviceability of floor slabs and exterior flatwork. We recommend utility trench backfill be placed in thin, loose lifts and moisture conditioned and compacted according to the criteria presented above. Our experience indicates use of a self-propelled compactor results in more reliable performance compared to fill “compacted” by a sheepsfoot wheel attachment on a backhoe or trackhoe. The placement and compaction of utility trench backfill should be observed and tested by a representative of our firm during construction.

Our experience indicates fill and backfill will settle, even if properly compacted to the criteria provided above. Some of the factors that influence the amount of settlement are depth of fill, soil type, degree of compaction, and time. The length of time for the compression to occur can be a few weeks to several years. The degree of compression of fill under its own weight will likely range from low for granular soils (1 percent or less), to moderate for clay/sand mixtures (0.7 to 1.5 percent), to high for highly plastic clay (1 to 2 percent or more). Improvements placed over backfill should be designed to accommodate movement. As discussed in the following paragraphs, special precautions should be taken for backfill placed adjacent to the basement foundation walls.

DEWATERING

Ground water was measured in our borings at depths of about 9 to 22 feet (elevations 5406 to 5422). If a basement is selected for the corridor adjacent to the south/west side of Wing 5, we anticipate the floor level will match the existing basement floor at elevation 5420.

Excavations may be about 2 feet below current ground water levels at the northwest end of Wing 5. Ground water levels may rise in response to seasonal fluctuation, precipitation, and landscape irrigation. Approximate ground water elevations at each boring location are presented on Fig. 3.

CTL | T PROJECT NO. DN45,673-125

We anticipate subsoils exposed in basement excavations along the south/west side of Wing 5 will consist of fill with variable amounts of gravel, sand, silt, and clay. The hydraulic conductivity (permeability) of the clayey fill is comparatively less than fill containing significant amounts of sand and gravel. We anticipate higher flow rates into the excavation where sand and gravel are present. Considering the isolated area of comparatively shallow ground water at TH-11 and the erratic subsoil conditions, accurately estimating dewatering quantities is difficult. Based on our experience in the area, dewatering quantities of 50 to 100 gallons per minute (gpm) should be considered for preliminary planning and design purposes.

Construction Dewatering

If a basement is selected for the corridor adjacent to the south/west side of Wing 5, temporary dewatering may be necessary during construction. Dewatering can likely be accomplished by intercepting ground water with drainage trenches around the perimeter of the excavation and sloping the trenches to sumps where water can be removed by pumping.

Ditches and sumps should be at least three feet below excavation grade so water is pumped down through the soils, which should enhance their stability compared to controlling water at the excavation surface. Well points or cased wells are another alternative to lower ground water in granular materials. We recommend experienced dewatering/excavation contractors be contacted to develop alternatives early in the design process.

If soft, wet soils are exposed in excavations, the bottom can be stabilized by crowding crushed rock into the subsoils until the base of the excavation does not deform more than about 1 inch when heavy compactive effort is applied. Acceptable rock materials include, but are not limited to, No. 2 and No. 57 rock, or 1 to 3 inch recycled concrete.

Crushed rock on a layer of geosynthetic grid or woven fabric can also be used and will likely require less rock.

Dewatering permits will likely be required by the City of Boulder. Our experience indicates periodic environmental testing may be required with these permits, with reporting.

Permitting requirements may also influence the construction schedule. We recommend researching these requirements and permit processing times well before dewatering begins.

CTL | T PROJECT NO. DN45,673-125

Permanent Dewatering

Current ground water conditions indicate a permanent dewatering system will likely be necessary if a basement is included for the corridor adjacent to the south/west side of Wing 5. The dewatering system will need to control ground water to a long-term, steady-state condition. There are significant costs associated with permanent dewatering. In addition to the drains and collector trenches, multiple sumps, pumps, backup pumps, and alarm systems will be required. Permanent dewatering will likely require discharge permits as well as periodic environmental testing. To reduce the risk of failure during power outages, backup power sources will be required.

The dewatering system should consist of foundation wall drains and lateral collectors along with sumps, pumps, clean-outs, alarm systems, and back-up pumps and electrical supply. Hydrostatic relief valves should also be installed in the floor slab as a back-up to electrical failure. The drain should consist of 4-inch, perforated rigid PVC pipe encased in at least 18 inches of free-draining gravel. Any buried pipe should be strong enough to resist the weight of the overlying soil. The drain should slope to a positive gravity outlet or sump at a minimum of 0.5 percent. A 6-inch thick underslab gravel drainage layer should be provided. Typical drain details are presented on Figs. 6 and 7.

As discussed above, dewatering quantities may be on the order of 50 to 100 gpm. We recommend considering an initial capacity of at least 100 gpm. The permanent dewatering system should be capable of accommodating at least twice the flow experienced at the initiation of construction. Conditions encountered during construction, including dewatering rates and/or areas of seepage, should be monitored to determine if revisions are required.

An in-situ permeability test (pump test) and further analysis can be performed to evaluate actual permeabilities for this site and provide more accurate flow rates. We can provide this service.

Elevator cores (if any) that extend below adjacent floor levels will likely require separate drains and dedicated sump and pump or they should be designed and constructed water-tight. The effects of buoyancy and hydrostatic pressures need to be considered.

Drains below elevator pits can consist of a 6-inch thick layer of clean gravel with connection to a sump and pump.

IRON HORSE ARCHITECTS

NIST BUILDING ONE RENOVATION PROJECT

CTL | T PROJECT NO. DN45,673-125

The recommended drain system should prevent hydrostatic pressures from developing on below-grade walls. However, high moisture conditions in the natural soils or localized zones of free water could result in moisture transmission through the below-grade walls. If this cannot be tolerated, then some form of moisture retarder or waterproof barrier should be considered. The level of protection desired will depend on the level of performance desired. Building code considerations may control the required level of protection.

FOUNDATIONS

Based on information provided by Martin/Martin, estimated maximum column loads for the current design include a dead load of 150 kips and a live load of 30 kips. We have assumed the maximum tolerable settlement to be a total post-construction movement of 1 inch with maximum differential movement of 1/2-inch between columns. The soils at anticipated foundation levels are variable and may consist of fill and/or native clay, sand, and gravel.

Considering the risk of differential movement of a shallow foundation system, the variable characteristics of the subsoils, and our experience, we believe drilled pier foundations bottomed in comparatively unweathered bedrock are the most reliable foundation alternative. To reduce distress between the existing structure and the proposed additions, connections between the structures should be designed to accommodate differential movements. We recommend allowing for at least 1/2 inch of differential movement.

Information provided by Martin/Martin indicates some load will be added to existing belled piers. Our investigation revealed the weathered bedrock is variable under the site and may limit the capacity of belled piers. The existing foundation system may not have been designed to accommodate additional loading. The owner, design team, and contractor should verify the depth and size of the existing belled piers to evaluate load-bearing capacity. If original as-built construction documents are not available, Sonic Echo/Impulse Response (SE/IR) testing may be performed to determine the length of the existing piers.

CTL | T PROJECT NO. DN45,673-125

If documentation of the existing piers is available or SE/IR tests are performed to determine the pier bottom elevation, the structural engineer should evaluate the existing load applied to the foundations and the total increase in soil pressure due to new loading. If the pier capacity is less than the proposed loading, we should be contacted to provide underpinning alternatives. Underpinning the existing structure may also be necessary if sufficient spacing between new and existing piers, as discussed below, cannot be provided.

Some cosmetic damage to the existing building should be anticipated where new load is applied to existing piers.

We recommend monitoring the existing structure during construction. If excessive movement is observed, the job should be halted and conditions evaluated further. You may wish to ask the structural engineer to identify movement tolerance. It may also be prudent to monitor the structure for movement after the completion of construction, especially in areas where new load will be added to existing foundations.

Design and construction criteria are presented below. These criteria were developed from analysis of field and laboratory data and our experience.

1. Piers should penetrate a minimum of 6 feet into the comparatively unweathered bedrock and have a minimum length of 26 feet. Estimated elevations of the comparatively unweathered bedrock surface are shown on Fig. 2.

2. Piers penetrating 8 feet or less into the comparatively unweathered bedrock should be designed for a maximum allowable end pressure of 30,000 psf and an allowable skin friction value of 3,000 psf. The maximum allowable end pressure can be increased to 50,000 psf for piers that bottom more than 8 feet into the comparatively unweathered bedrock. An allowable skin friction value of 5,000 psf can be used for the portion of the pier penetrating more than 8 feet into the comparatively unweathered bedrock.

3. A minimum pier diameter of 18 inches is recommended. Quantity and size of column reinforcement, or the size of base plates, may dictate the most convenient size of drilled piers. Economy can be achieved by varying the depth of penetration and limiting the number of pier sizes. We recommend the piers be designed with a maximum length-to-diameter ratio (L/D) in the range of 20 to 25, but not to exceed 30.

4. Pier drilling should produce shafts with relatively undisturbed bedrock exposed. Excessive remolding and caking of bedrock on pier walls must be removed. The bedrock surface should be rough or roughened.

IRON HORSE ARCHITECTS

NIST BUILDING ONE RENOVATION PROJECT

CTL | T PROJECT NO. DN45,673-125

5. Shear rings should be installed in the lower 6 feet of bedrock penetration in all piers. Shear rings should extend 3 to 4 inches beyond the pier shaft to increase the load transfer through skin friction. These shear rings should be spaced about 2 feet on-center.

6. Piers should be reinforced their full length and the reinforcement should extend an adequate distance into grade beams, foundation walls, and pier caps. We recommend a minimum steel to pier ratio of 0.005, based on cross-sectional area. Grade 60 (or better) steel is recommended. More reinforcement may be required by structural considerations.

7. A minimum 4-inch thick continuous void should be placed beneath grade beams, pier caps, and foundation walls, between the piers, to concentrate the deadload of the structure on the piers.

8. Should wind, seismic, or lateral load analysis result in tension loads or uplift on the piers, the tension loads should be resisted by skin friction in bedrock.

We recommend using the skin friction values provided above for uplift resistance.

9. Piers should have a center-to-center spacing of at least three pier diameters when designing for vertical loading conditions, or they should be designed as a group. Piers aligned in the direction of lateral forces should have a center-to-center spacing of at least six pier diameters. Reduction factors for closely-spaced piers are provided in a following section.

10. It is critical that the depth, location, and size of existing belled piers be verified early in the design process. If new piers are drilled near the base of existing belled piers, there is risk that existing foundations may be undermined.

Sufficient spacing between new and existing foundations should be provided, as discussed above. The diameter of the belled portion of the existing piers should be used when determining minimum spacing. If this is not feasible, we should be contacted to discuss alternatives to protect existing foundations during construction.

11. Ground water is present in the fill and native soils. We anticipate the piers will require slurry installation techniques and/or temporary casing down to the bedrock for proper cleaning and dewatering of pier holes and placement of concrete. Ground water seepage may also occur above or within the bedrock after casing is set. If excessive water develops (more than about 3 inches at time of placement), pumping or underwater concrete placement are recommended. Concrete should be placed by pumping methods and the pipe should be at the bottom of the pier. If less than 3 inches of water is in the bottom of the pier and concrete placement by free-fall is desired, the structural engineer should specify the maximum free-fall distance.

12. If drilling slurry is used to maintain hole stability, sufficient head should be maintained to reduce the risk of caving or raveling of the pier walls. Concrete should be maintained at least 10 feet above the tremie pipe and at least 5 feet above ground water as casing is removed.

CTL | T PROJECT NO. DN45,673-125

13. Where casing extends into bedrock, or where bedrock is within 3 feet of foundations walls or grade beams, the skin friction should be neglected.

Cased penetration length should be ignored when determining installed bedrock penetration.

14. Concrete should be on site and placed in the pier holes immediately after the holes are drilled, cleaned, and observed and reinforcing steel is set. Concrete placed in pier holes should have a slump of 6 inches (± 1 inch) to promote proper consolidation and reduce arching of concrete on the reinforcement, casing, and sides of the piers.

15. Some pier-drilling contractors use casing with an O.D. equal to the specified pier diameter. This practice results in a pier diameter less than specified. The design and specifications of piers should consider the alternatives. If full-size casing is desired (I.D. of casing equal to specified pier diameter), it should be clearly specified. If the design considers the potential reduction in diameter, then the specifications should include a tolerance for a smaller diameter for cased piers.

16. Formation of mushrooms or enlargements at the top of piers should be avoided during drilling and subsequent construction operations.

17. Some movement of drilled pier foundations is anticipated to mobilize the strength of the bedrock. We estimate the movement will be on the order of 1/4 to 1/2 inch. Differential movement between adjacent piers may equal total movement. Designs should consider these potential movements and accommodate them as much as practical.

18. Installation of drilled piers should be observed by a representative of our firm to identify the bearing strata, confirm the subsurface conditions are as anticipated from our borings, and observe contractor’s installation procedures.

Laterally Loaded Piers

Foundation piers will be designed to resist lateral loads applied to the structure through wind, seismic, and lateral earth pressure. Lateral load analysis of piers can be performed with the software analysis package LPILE by Ensoft, Inc. We believe this method of analysis is appropriate for piers with a pier length to diameter ratio of seven or greater. If you desire, we can assist with these analyses after the structural engineer has developed representative loading criteria. Suggested soil and bedrock input values for use with LPILE software are presented in the table below.

CTL | T PROJECT NO. DN45,673-125

TABLE A

SOIL INPUT DATA FOR “LPILE”

Soil Type Fill, Clay, or

Weathered Bedrock (Above Water)

Sand (Above Water)

Fill, Clay, or Weathered Bedrock

(Below Water)

Sand (Below Water) Bedrock

Density (pci) 0.07 0.07 0.033 0.033 0.07

Cohesion (psi) 8 0 5 0 60 Friction Angle 0 30 0 30 0 ε50 (in/in) 0.007 - 0.007 - 0.004 ks (pci) 500 60 500 40 2,000 kc (pci) 200 60 200 40 2,000

The ε50 represents the strain corresponding to 50 percent of the maximum principal stress difference. The modulus of subgrade reaction for static (ks) and cyclic (kc) conditions are used by the program to generate the slope of the initial portion of the “p-y curves”.

Closely-Spaced Pier Reduction Factors

For axial loading, no reduction is necessary for a minimum spacing of three diameters (center to center). At one diameter (piers touching), the skin friction reduction factor for both piers would be 0.5. End pressure values would not be reduced provided the bases of the piers are at similar elevations. Interpolation can be used between one and three diameters.

Piers in-line with the direction of lateral loads can be designed with no reduction if they have a minimum spacing of six diameters (center-to-center) based upon the larger pier.

If a closer spacing is required, the modulus of subgrade reaction for initial and trailing piers should be reduced. At a spacing of three diameters, the effective modulus of subgrade reaction of the leading pier can be estimated by multiplying the given modulus by 0.6; for trailing piers in line at three-diameter spacing, the factor is 0.4. Linear interpolation can be used for spacing between three and six diameters.

Reductions to the modulus of subgrade reaction can be accomplished in LPILE by inputting the appropriate modification factors for p-y curves. Reducing the modulus of subgrade reaction in trailing piers will result in greater computed deflections on these piers.

IRON HORSE ARCHITECTS

NIST BUILDING ONE RENOVATION PROJECT

CTL | T PROJECT NO. DN45,673-125

In practice, a grade beam can force deflections of all piers to be equal. Load-deflection graphs can be generated for each pier by using the appropriate p-multiplier values. The sum of the pier lateral load resistance at selected deflections can be used to develop a total lateral load versus deflection graph for the system of piers.

For lateral loads perpendicular to the line of piers, a minimum spacing of three diameters can be used with no capacity reduction. At one diameter (piers touching) the piers should be analyzed as one unit.