A Case Study of waterproofing Strategies for a Typical Mat and Deep Foundation System

Transcription

1 A Case Study of waterproofing Strategies for a Typical Mat Deep Foundation System Andrea B., PE, LEED AP BD+C; Stephen T., SE Simpson Gumpertz & Heger Inc. The One Market, Suite 600, San Francisco, CA Phone: Fax: stbono@sgh.com 2 8 t h R C I I n t e R n a t I o n a l C o n v e n t I o n a n d t R a d e S h o w M a R C h , B o n o a n d B o n o 1 2 7

2 Abstract Current construction methods such as piles, micropiles, tiebacks permit building of foundations at sites that were previously impossible or impractical. Coordination of a fully integrated below-grade waterproofing design is required to ensure successful performance of buildings employing these structural foundation systems. As the percentage of new construction at sites with less-than-ideal soil increases, waterproofing detailing should be considered during the schematic phase of a project continue throughout construction. This presentation will include waterproofing considerations for typical structural foundation systems through several case studies where the waterproofing was considered independent of the foundation. Speakers Stephen T., SE Simpson Gumpertz & Heger Inc. - San Francisco, CA STepHeN T. BoNo, Se, is a Senior Staff I at national engineering firm Simpson Gumpertz & Heger Inc. His experience includes both performance-based code-based design, evaluation, repair of steel, concrete, masonry low- to high-rise structures incorporating both linear nonlinear techniques. He specializes in evaluation rehabilitation of commercial institutional facilities. He is a member of the existing Buildings Committee, the Building ratings Subcommittee, the Sustainable Design Committee of the Structural engineers Association of Northern California (SeAoNC). Andrea B., PE, LEED AP BD+C Simpson Gumpertz & Heger Inc. - San Francisco, CA ANDreA B. BoNo, pe, leed Ap BD+C, is a Staff II at national engineering firm Simpson Gumpertz & Heger Inc. She has experience in the design, investigation, rehabilitation of commercial, healthcare, civic, residential buildings related to below-grade spaces, podium decks, exterior components cladding, roofs. Andrea works closely with architects consultants who specialize in the specification design of roofing, waterproofing, exterior wall systems to design, analyze, repair aspects of the building envelope. She is the branch secretary for the U.S. Green Building Council s Northern California Chapter, San Francisco Bay Bridge Branch B o n o a n d B o n o 2 8 t h R C I I n t e R n a t I o n a l C o n v e n t I o n a n d t R a d e S h o w M a R C h ,

3 A Case Study of waterproofing Strategies for a Typical Mat Deep Foundation System INTRODUCTION A case study of a low-rise building is presented to illustrate that coordinating a fully integrated below-grade waterproofing design among all disciplines of a project team can protect the building its occupants from potentially adverse site conditions. In particular, the coordinated effort between the structural engineer waterproofing consultant is discussed. As the percentage of new construction at sites with less-than-ideal soil increases, structural waterproofing integration should occur in the schematic phase of a project continue throughout construction to ensure successful building performance. service life of a building by designing the structure against poor soil conditions such as the presence of high variable water tables, liquefiable soils, corrosive deteriorating agents, hazardous substances, pollutants, /or contaminants. Common structural protection methods include: Sacrificial (extra) steel Increased concrete cover epoxy coatings Uncertainties in Design prior to any new development, a site assessment is typically performed to evaluate a site s above-ground subsurface characteristics, including its structure, geology, hydrology. However, due to size, quantity, cost constraints of sample collection, not all site conditions can be discovered from a site assessment. Designers should be cognizant of these uncertainties underst that potential findings during construction could impact their design the design of other disciplines. If variations of the conditions do occur design changes are made, collaborative coordination among all disciplines, as well as input from the building owner, are necessary to ensure the project objectives are met. To mitigate encountered design uncertainties, a collaborative effort among all disciplines generates the best solution to protect the building its occupants. Figure 1 demonstrates how collaboration is essential as modification to building systems becomes significantly more challenging costly as construction begins access becomes restricted. Common Means of Protecting Structure Structural engineers can prolong the Figure 1 Below-grade waterproofing membrane under mat slab reinforcement (shown horizontal) concrete column reinforcement (shown vertical) with a rock anchor centered in the column. 129

4 Figure 2 Elevation view of a foundation wall with negative-side waterproofing a foundation wall with positive-side waterproofing. passive or active cathodic protection These means of protection are generally intended to preserve only the structural elements of a building for a finite amount of time. However, waterproofing is a nonstructural means of protecting extending the life of structural elements, with the added benefit of protecting occupants interior contents from uncontrolled water infiltration consequential damage. Waterproofing design strategies are implemented at either the positive or negative side of the structure as shown in Figure 2. positive-side waterproofing protects a structure at the source of hydrostatic pressure by placing a barrier at the exterior face of the structure, whereas negative-side waterproofing is applied at the opposite side of the source of hydrostatic pressure permits water to pass through the structure before meeting a barrier at the interior space. Both strategies protect the interior contents from damage; however, negative-side waterproofing does not protect the structure in front of the membrane. Common methods of positive- negative-side waterproofing include loose-laid or adhered sheet membranes, liquid-applied or slurry-applied coatings, concrete admixtures. These waterproofing strategies require coordination with the structural engineer. A well-integrated well-designed waterproofing system can protect the structure, its occupants, interior contents, thus making the integration of structural components waterproofing systems critical for both building durability performance. Integrating for Performance In order to design a fully integrated waterproofing system, it is important to identify the key persons involved in the design of a building. These persons include an architect, a contractor, a geotechnical engineer, an owner or tenant, astructural engineer, if hired, a waterproofing consultant. Without a waterproofing consultant on the design team, waterproofing systems may be selected without the full consideration of risk of water or vapor intrusion, the impact of water or vapor on occupants the intended use of interior spaces or structural service life. Designers can develop appropriate design criteria based upon the site conditions, their previous experience, an owner s interpretation of acceptable or unacceptable performance; however, all design considerations by key personnel should be developed concurrently in order to satisfy manage owner or tenant expectations of the final design. Case Study The following case study demonstrates the importance of coordination communication among the design team, with particular emphasis on the interaction between the structural engineer waterproofing consultant. The subject building is a six-story, steel-framed structure over two levels of below-grade parking supported by a concrete foundation. The two stories below grade are within the water table. The site is in an industrial area, the former site user contaminated the soil with single-phase hydrocarbons (SpH) heavy Figure 3 Isometric view of foundation mat, piles, rock anchors that support the structure B o n o a n d B o n o 2 8 t h R C I I n t e R n a t I o n a l C o n v e n t I o n a n d t R a d e S h o w M a R C h ,

5 Figure 4 Steel HP members being driven into rock. Figure 5 Shotcrete leveling layer with wire mesh cast against secant auger-cast pile walls to address out-of-plane variation surface irregularities. metals. The site was excavated to remediate petroleum-contaminated soils backfilled with granular material. Due to the site s close proximity to a bay, the soil has a high concentration of salts. The southern edge of the site was previously used as a shipping channel, creating additional project challenges. originally, the shipping channel was unsystematically filled with granular material that included fill as large as cobbles. This filled shipping channel acts as a direct source for water into the site. The bay s tidal influences high water table were also considerations in the below-grade structural waterproofing design. Foundation Design As the structural engineer of record, we designed a mat foundation to bear mostly on soil, with the remaining area supported by piles rock anchors socketed into bedrock structure. Figure 3 below the basement shows an isometric view of the foundation mat, piles, rock anchors indicates three general regions of the foundation. one small region of the mat bears directly on native rock. The largest region of the mat bears on 2 ft. of engineered fill (competent soil). The remaining region of the foundation is supported on groups of piles at the former shipping channel, which are socketed into native rock. piles are used to transfer loads through the poor soil to the native rock below. The piles used on this project are 150-ton, nominal-capacity, Hp steel members (also known as bearing piles). (Figure 4 shows the equipment used to drive the Hp members through the soil into the rock.) Groups of piles are placed below basement shear walls columns. A thickened mat provides additional shear flexural resistance at the pile groups. The remainder of the mat is nominally 2 ft. thick. Supplemental rock anchors, used to resist hydrostatic forces at regions of the mat that are not supported on piles, prevent the structure from uplifting. During excavation of the site, secant pile walls, with alternate piles reinforced with steel W-section (wide flange) soldier members, provided shoring to allow for excavation of the site (shown in Figure 5). These secant pile walls also acted as a dewatering cutoff to reduce the amount of dewatering within the site. An interior reinforced-concrete structural shear wall was constructed inboard of the secant pile wall serves the following purposes: transfer of superstructure loads to the foundation, protection of the vertical belowgrade waterproofing membrane, lateral restraint of hydrostatic pressure due to the elevation of the water table. 131

6 the different combinations of structural waterproofing systems. The owner then determined their acceptable risk/cost basis for the waterproofing structural systems, we designed our waterproofing system to the owner-accepted performance level. At this site, the owner would accept a level of risk for water intrusion resulting in isolated staining. MAT FOUNDATION SYSTEM INTEGRATION Mat foundations are a shallow foundation system generally encompass a building s entire footprint consist of heavily reinforced concrete. Mat foundations are appropriate for the following site conditions: Soil types that are susceptible to significant differential settlements or expansion that could cause differential heaves Unpredictable structural loads lateral loads that are not equally distributed lowest elevation of the foundation is within the water table Figure 6 Mat foundation supported on soil. DESIGN PARAMETERS Geotechnical Considerations Resulting Waterproofing Design The elevation of the water table provided by the geotechnical engineer indicated that the subterranean concrete mat slab resides in the water table. During construction, site dewatering was required to lower the elevation of the water table until we determined that the building s self-weight could resist the hydrostatic uplift pressure. Due to the soil characteristics on-site the elevation of the water table, a secant auger-cast pile wall was selected to shore the excavation. Site dewatering was required to capture the inflow of groundwater into the excavation. Structural Foundation Wall Construction Resulting Waterproofing Design In general, every shoring system requires treatment or modification to provide a sound substrate for installation of waterproofing. Unmodified shoring systems lead to high probability of water intrusion into building structures. In our case, secant auger-cast pile walls acted as vertical shoring would require a leveling layer for placement of the waterproofing. We recommended the installation of a shotcrete leveling layer to provide a smooth substrate to compensate for the out-of-plane surface irregularities, prior to application of the waterproofing membrane (Figure 5). prior to determination of the basement shear wall construction, we discussed both structural waterproofing concerns with the building contractor regarding a preferred construction method. Structurally, the strength of either cast-in-place or shotcrete is comparable as long as the concrete is well consolidated. From the contractor s perspective, shotcrete is typically faster less expensive to install. From a water- 132 B o n o proofing perspective, use of shotcrete with waterproofing is currently less reliable than cast-in-place concrete with waterproofing. 1 When utilizing cast-in-place concrete, the placement vibration of plastic concrete allows it to fill the majority of voids but does require the use of one-sided forms in shored excavations. In contrast, when shotcreting foundation walls, waterproofing problems can develop because of voids shadowing during the placement process from broken penetration seals resulting from rebar cage anchor vibration. Figure 6 shows a mat foundation bearas the owner s waterproofing consuling directly on competent soil. In a mat tants, we defined the levels of risk for water intrusion based on common waterproofing product service lives. Waterproofing performance levels were defined as expected system behavior a variable amount of observable moisture inside the building. For example, a performance objective at the highest performance level would be negligible staining on structural surfaces, whereas a performance objective at a lower performance level would be water flow at isolated areas: a leak. Understing that wellthought-out design can limit building damage but cannot eliminate it, we recommended a waterproofing system that considered the design water table, acceptable risk of incident water intrusion, assumed construction sequence for the foundation below-grade exterior walls. The contractor priced Figure 7 Mat foundation rebar.

7 Figure 8 Blind-side, below-grade waterproofing: loose-laid sheet membrane. Figure 9 Blind-side, below-grade waterproofing: fully adhered sheet waterproofing. foundation, the loads from the superstructure are assumed to be evenly distributed through the mat to the soil below. In the condition where the mat foundation is in contact with the water table, such as at this site, waterproofing becomes an important consideration when designing the mat foundation. Mat Foundation Waterproofing Recommendations In below-grade construction, because the waterproofing is placed below several feet of concrete layers of steel reinforcing, the ability to make repairs to the waterproofing, requiring soil excavation or concrete removal for mitigation of leakage, is often infeasible (as indicated by Figures 1 7). To minimize leakage potential, we recommended the following primary waterproofing systems for blind-side, belowgrade waterproofing in hydrostatic conditions: (1) loose-laid sheet membrane, (2) fully adhered sheet waterproofing, or (3) bentonitebased systems. (See Figures 8-10 for examples of each type of waterproofing system.) regardless of the below-grade membrane type, we recommended installation of an unreinforced mud slab to provide a suitable substrate for installation of the horizontal below-grade membrane, similar to the shotcrete applied to the augercast piles. on top of the installed waterproofing, we recommended a 4-in. reinforced protection slab to Figure 10 Blind- side, below-grade waterproofing: bentonite-based system. 133

8 prevent damage to the membrane during construction from rebar placement, stored materials, welding, equipment. The three below-grade waterproofing types as presented to the owner, as well as their advantages disadvantages, are described below. Ultimately, we designed the waterproofing system based on the cost/risk determination of the owner. PRIMARY waterproofing SYSTEMS Loose-Laid Sheet Membrane Systems Traditional polyvinyl chloride (pvc) systems could not be used at this site due to potential SpH contamination from adjacent sites over the life cycle of the building. While alternate materials exist that combine the benefits of heat-welded seams, isolation from substrate cracking, large sheet installation with hydrocarbon resistance, it would have been cost-prohibitive when compared to the other membranes. Also, because of its loose-laid nature, water that bypasses the membrane through minor defects could migrate laterally behind the membrane, making it difficult to locate the source of water entry. Fully Adhered Sheet Waterproofing Fully adhered membranes are installed on cast-in-place concrete. The membrane can either consist of rubberized asphalt with a polyethylene carrier sheet for positive-side applications or adhere to the concrete as it is placed in blind-side applications. This type of membrane consists of an inert highdensity polyethylene (HDpe) sheet with a proprietary adhesive acrylic binder that bonds to plastic (wet) concrete. leaks in the fully adhered membrane are localized so that repair locations can be easily identified. However, this system has the risk that failures could occur at membrane seams due to incorrect installation. not exp in the presence of saltwater. However, products with additives are available that perform up to a given concentration of salt in the water. Based on the above considerations for each of the systems the owner s cost/ risk preference, we designed a bentonitebased system for the mat foundation s waterproofing. As a condition of specifying this product, we recommended that the groundwater be tested for chloride content to determine if bentonite-based systems were appropriate. SECONDARY (BACKUP) waterproofing SYSTEMS Due to the cost-prohibitive nature of repairs system failures, we recommended including redundancy into the below-grade waterproofing system design. The two recommended methods of backup waterproofing included interior drainage concrete admixtures. Interior Drainage In lieu of a drainage field below the basement slab requiring an additional two feet of excavation, the owner chose an interior drainage system to collect water. This bypasses the primary system then transports it to a sump location for removal from the building. At basement slab-toexterior wall locations, we recommended a trough (shown in Figure 11) to intercept collect any wall-water leakage. Crystalline Waterproofing Admixtures We also recommended crystalline waterproofing admixtures for the mat foundation. Crystalline waterproofing admixtures can help minimize water migration by reacting with water to grow crystals that block passage through the concrete pore structure. The admixture helps the concrete resist fluid flow of water through its own pores reduces the risk of interior leakage /or staining. For hydrostatic conditions, we recommend against concrete admixtures as the primary waterproofing since concrete admixtures are limited by the quality of the concrete placement. Additionally, concrete tends to crack due to shrinkage /or building movement, if cracks widths Bentonite-Based Systems Sodium bentonite is a naturally occurring mineral that, in the presence of uncontaminated water confining pressure, swells to form a waterproofing gel layer over walls under slabs, preventing water infiltration. The membrane consists of a layer of bentonite surrounded by geotextile fabric on either side to form a membrane. There must be adequate coverage confining pressure for the water-gel reaction to be effective in waterproofing. Bentonite does Figure 11 Perimeter waterproofing section B o n o a n d B o n o 2 8 t h R C I I n t e R n a t I o n a l C o n v e n t I o n a n d t R a d e S h o w M a R C h ,

9 Photo 12 Rock anchors with lower mat reinforcing in place. Figure 13 Rock anchors in a foundation resisting hydrostatic pressure. exceed the waterproofing capabilities of the concrete admixtures in the presence of water, leaks can occur. Due to this possibility, we considered the admixture as secondary (backup) waterproofing. See Figure 11 for a detail indicating the structural waterproofing elements of the mat foundation at a basement shear wall. ROCK ANCHOR SYSTEM INTEGRATION Anchors are a deep foundation system are used to restrain foundations resisting tensile forces or uplift due to hydrostatic pressures, soil heave, or superstructure loads. A rock anchor, shown in Figure 12, is one type of anchor with a small-diameter, high-capacity rod. rock anchors are constructed by drilling through the soil into rock; placing the rock anchor into the hole; grouting the anchor; tensioning the rod; regrouting, if necessary; locking the rod into position. (Figure 13 shows rock anchors resisting hydrostatic pressure in a mat foundation.) The mat slab spans between rock anchors resists hydrostatic water pressure from the underside of the mat. The rock anchors transfer the hydrostatic water pressure deeper into the soil, where the tension loads can be resisted keep the mat from lifting up. Rock Anchor Waterproofing Recommendations In terms of waterproofing, rock anchors create a bypass in the mat foundation s horizontal below-grade waterproofing sys- tem (refer to Figure 12). rock anchors act as straws reaching to below-grade depths, into the water table, bringing perched water to the surface from subsurface depths via capillary force. With their irregular profile, anchors can be difficult to detail require coordination between the structural engineer the waterproofing consultant to ensure both adequate bearing-plate embedment sufficient height to install waterproofing to the anchor. To seal any voids between the rock 135

10 Figure 14 3-D rendering of waterproofing at a rock anchor including bentonite waterproofing membrane, bentonite waterproofing membrane patch, bentonite mastic, block waterstop, injection tube waterstop. anchors the mat foundation pour, we recommended installation of injection tube waterstops. These injection tube waterstops are in addition to the block waterstops normally detailed as part of a bentonite waterproofing system. Injection tube waterstops consist of wire tubing covered by a reinforced membrane that is injected with polyurethane grout. See Figures for a detail indicating the strucfigure 15 Typical detailing around a rock anchor. tural water- proofing elements of the rock anchor. Figure 16 shows the the manufacturer s requirements but prior detailing of bentonite around the irregular to decommissioning the site dewatering rock anchor penetration. In general, injec- wells. on this project, injection tube watertion tube waterstops are injected after the stops were installed as a backup means in concrete mat slab has sufficiently cured per the event of water intrusion. Currently, the injection tube waterstops are not grouted are left in place as a supplemental waterproofing measure. PILE SYSTEM INTEGRATION Figure 16 Installed detailing around a rock anchor prior to bentonite mastic, block waterstop, injection tube waterstop installation. 136 Another deep foundation system used on the project was piles with pile caps. piles are prefabricated structural members made of wood, concrete, or steel (as in this case) drilled or driven into the ground typically extended to depths on the order of 50 ft. below the ground surface, but that can also extend to depths of 150 ft. or greater. The piles transfer loads from the superstructure, through weak soil layers that cannot support the applied loads, to competent layers of soil. Deep foundations are used when competent soil layers are located well below the surface, it is not practical to excavate down to their elevation. (Figure 17 shows a pile-supported mat foundation with piles bearing directly on rock.) A pile cap is the reinforced concrete element that connects a column from the superstructure to a group of piles,

11 perimeter. See Figures (the last of these is a detail indicating the structural waterproofing elements of the typical pile pile cap). SUMMARY OF waterproofing RECOMMENDATIONS In summary, we designed the waterproofing system based on the following site conditions structural foundation system: Subterranean levels in the water table potential SpH contamination potential saltwater contact Auger-cast pile shoring wall Mat foundation rock anchors piles pile caps Based on these design parameters, we recommended the following installation: opposite flanges to form a metal cap. This Shotcrete skim coat over auger-cast, pile-shoring wall to act as a watersteel cap provided a suitable substrate for proofing substrate installation of waterproofing at each pile s Figure 17 Pile-supported mat foundation with piles bearing in rock. tying them all together. Summarized from Foundation Design: Principles Practices, when designing pile foundations, designers must consider the following parameters:2 Applied loads required diameter required length Availability of pile type Durability Anticipated driving conditions In the field, the impossibility of determining assumed design strengths Due to the above considerations, additional piles may need to be installed. This can influence construction time cost, along with altering building performance. Consideration must also be given to the waterproofing detailing of piles within the water table. Pile Waterproofing Recommendations Similar to rock anchors mat foundations, piles are best detailed with collaborative efforts. Deep foundations, because they transmit some or the entire applied load to soils well below the ground surface, are likely to penetrate the groundwater table bring water to the surface through capillary force. on this site, we recommended waterproofing each individual pile along with the pile caps. To prepare the piles for installation of waterproofing, three steel plates were welded to the steel Hp members one at the top end of the pile the two welded to Figure 18 3-D rendering of a pile pile cap. M a R C h ,

12 An unreinforced concrete mud slab to provide a suitable substrate for the sheet waterproofing Welded steel plates at Hp members to provide a suitable substrate to terminate waterproofing Waterproofing the subterranean level with bentonite membrane with lapped fastened seams detailed penetrations reinforced protection slab over the horizontal below-grade waterproofing to protect the waterproofing from construction damage (not implemented) Crystalline waterproofing admixture at mat slab cast-in-place walls Drainage channels Supplemental injection tube waterstops Figure 19 3-D rendering of waterproofing at a pile in a pile cap showing bentonite waterproofing membrane, bentonite mastic, block waterstop, injection tube waterstop. These recommendations were based on the waterproofing performance desired by the owner, cost considerations from the contractor, available construction techniques at the time of construction, past experience on similar below-grade local projects. CONCLUSION Design team coordination between the structural engineer the waterproofing consultant led to a well-integrated structural waterproofing system. Without this team effort, opportunities to select the best waterproofing system for a building may not have been possible once construction had begun. However, even with coordination integration, conflicts errors may not be eliminated but their severity is minimized, providing the owner an economical suitable building that meets performance expectations. REFERENCES 1. D.G. Gibbons J.l. Towle, Waterproofing Below-Grade Shotcrete Walls, The Construction Specifier, V. 62, No. 3, March 2009, pp D.p. Coduto, Foundation Design: Principles Practices, second edition, prentice Hall, Upper Saddle river, New Jersey, Figure 20 Waterproofing at a typical pile cap. 138 B o n o