10.5 Static and Seismic Design of Basement Walls

10.5 Static and Seismic Design of Basement Walls

Basement walls should be designed against the more critical of the following conditions: (1) At-rest soil pressure and (2) active soil pressure plus dynamic increment. In addition, effects of surcharge loads (traffic and adjacent building foundation, if not underpinned) should be considered.

When calculating hydrostatic pressure, the design groundwater table with consideration of sea level rise and seasonal fluctuation of groundwater table should be identified and used. If a drainage system is not installed behind the basement walls above the design groundwater table, the basement walls should be waterproofed beginning at the ground surface. In this case, the basement walls should be designed per code requirements and checked for the groundwater table being at the ground surface, but using a load factor of 1.0 as opposed to 1.6 for this check.

Commentary: According to the load combination in current building code, a factor of 1.6 is applied to hydrostatic pressure. The resulting pressure in most cases accounts for effects of sea level rise, fluctuation in groundwater table, or effects of a temporary buildup of water behind the basement walls due to a possible breakage in a water conveying pipe adjacent to the site. Care should be exercised to avoid undue conservatism in design against hydrostatic pressure.

Resistance to lateral loads could be calculated by considering frictional resistance on basement walls and beneath the foundation (if not pilesupported) and passive pressure against the basement walls, pile caps, grade beams, and foundation edge extending below the basement walls. In calculating frictional resistance, the effects of the presence of a waterproofing membrane (if used) on allowable frictional resistance should be accounted for.

A load-deflection curve for passive resistance should be developed by the GEOR and used by the SEOR to account for displacement compatibility within various components contributing to lateral resistance.

For basement walls in contact with slopping ground conditions, the effects of unbalanced soil pressure on basement walls should be considered.

11. FOUNDATION SUPPORT

Shallow or deep foundation systems may be appropriate for support of tall buildings depending on the ground conditions, structural loads, and performance criteria. Unless it could be demonstrated through comprehensive geotechnical and structural studies that the computed total and differential settlement will not compromise the safety and functionality of the structure and its components, foundation systems should be designed to meet the following criteria using the best estimate soil properties: (1) the total short-term and long-term computed settlement of the foundation under gravity and seismic loads should not exceed 4 inches, and (2) its differential settlement under gravity and seismic loads should not exceed an angular distortion of 1/500. Nonstructural components such as cladding or partition walls may control the acceptable threshold of differential settlement. The amount of dishing of the site under building load should be communicated to the SEOR in the geotechnical report, so that the appropriate building camber could be provided.

Commentary: The inherent variability of natural soil deposits often causes tilting of the foundation (rigid body rotation), which would add to differential settlement (dishing) caused by the applied structural loads. The magnitude of foundation tilting is directly related to the extent of total settlement. Some tilting can be compensated for during construction; however, some tilting may occur after construction is completed. If settlement of more than 4 inches is calculated, GEOR and SEOR should work together and carefully evaluate the impact of settlement larger than 4 inches on the structural system and nonstructural components. Factors to be considered include the amount of settlement occurring after placement of the mat and before the lowest floor is constructed, the timing of placement of cladding and ability to correct foundation tilting before cladding is installed, and of course, the tolerance of cladding to differential settlement caused by tilting and/or by dishing of the mat foundation. Settlement analyses are often made using the approximation that the foundation soil deposits are uniform, homogeneous layers. If this simplification is adopted, it is recommended that the GEOR perform analyses to evaluate the sensitivity of the computed settlement on the input soil parameters.

For shallow foundations, the factor of safety against bearing failure (both global failure mechanism and punching shear failure mechanism) should be evaluated. A minimum factor of safety of 2.0 should be maintained under anticipated gravity loads considering the above bearing failure mechanisms.

If ground improvement is used to mitigate the effects of compressible, weak, liquefiable, or other problematic soil conditions, the GEOR should review design calculations by the design-build (DB) contractor to check that the integrity of ground improvement elements is maintained during both static and seismic loading conditions; that is to say, the replacement ratio and geometry of grid pattern should be such that the ground improvement system maintains its integrity under structural gravity loads, seismic loads (base shear and overturning moment applied by the structure), and seismic loads due to vertical propagation of seismic waves.

Commentary: Recent research indicates that individual columns of deep soil mix (DSM) would bend during design-level ground shaking, thereby limiting the effectiveness of DSM columns for prevention of soil liquefaction. In addition to lateral movement, individual unreinforced DSM columns could crack in bending and with excessive repeated loading and extensive cracking, could have the effect of losing the cohesive strength associated with cementation, with a residual strength related to contact through friction only. Unreinforced individual columns of DSM are brittle and could fail to transfer gravity loads to more competent soils at depth.

If deep foundations are used to bypass compressible, soft, or liquefiable soils, the following construction design issues should be addressed: