Loads on Foundations and Retaining Walls

The methods used to estimate loads for the design of foundations and walls using LRFD are fundamentally no different than procedures used in the past for ASD. Thus, procedures commonly used by geotechnical engineers to estimate earth pressures, negative loads on piles, seepage pressures due to hydraulic gradients, or surcharge load effects on earth pressure, for example, can be used for LRFD. What has changed however is the manner in which loads are considered for evaluations of foundation/structure stability (e.g., bearing and sliding resistance of spread footing foundations) and foundation movement (e.g., axial and lateral displacement of a pile-supported pier). This section highlights some of the areas in which considerations of force effects for the design of foundations and walls differ between LRFD and ASD.

The design of foundations supporting bridge piers or abutments should consider all limit states loading conditions applicable to the structure being designed. From a review of the Limit States loading descriptions in Section 4.3 and the load factors in Tables 4-10 and 4-11, it can be seen that a variety of Strength Limit States may control the design of a bridge pier or abutment foundation.

• The Strength I Limit State will control for very high live to dead load ratios • The Strength III or V Limit States may control for structures subjected to

high wind loads

• The Strength IV Limit State will control for high dead to live load ratios

The Extreme Limit States may control the design of foundations in a seismically active area (i.e., Extreme Limit I) or for foundations for piers which may be exposed to vehicle or vessel impact (i.e., Extreme Limit II). With respect to deformation, (i.e., lateral deflection or settlement), the Service I Limit State will control. The Service II and Service III Limit States are used to evaluate specific critical structural components and are not generally applicable to foundation design.

For a typical retaining wall, dead weight, earth pressure and live load surcharge will be the predominant loads influencing wall stability. From a review of the Limit States loading

descriptions in Section 4.3 and Tables 4-10 and 4-11, it can be seen that the Strength I and IV Limit States, for which the largest dead, earth and live load factors apply, will control the design of a retaining wall with respect to stability. With respect to deformation, the Service II and Service III Limit States only apply to special structures such that only the Service I Limit State applies to retaining wall design.

Applying the criteria in Section 4.3 to evaluate the resistance of walls and foundations at the Strength Limit State, some general observations can be made. The stability of conventional (i.e., gravity, semi-gravity and cantilever) retaining walls must be evaluated for sliding, bearing and overturning. When applying the AASHTO LRFD Specification, these stability cases are discussed below for a typical analysis of a cantilever wall at the Strength I Limit State.

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Figure 4-11 shows that the vertical earth load, EV, on the rear of a wall is factored by γpmin (1.00)

and the structure weight, DC, is factored by γpmin (0.90) because these forces result in an increase in

the contact stress (and shear strength) at the base of the foundation. The horizontal earth load, EH, is factored by γpmax (1.50) for an active earth pressure distribution because the force results in a more

critical sliding force at the base of the wall. If EH has both horizontal and vertical components (e.g., a wall supporting a sloping backfill) both components are factored by 1.50.

Figure 4-11

Load Combination for Evaluation of Sliding Resistance of a Cantilever Retaining Wall Supported on a Spread Footing

Figure 4-12a shows that the critical load combination for an analysis of bearing resistance of the wall foundation often, but not always, occurs when the structure weight is factored by γpmax (1.25),

the vertical earth load is factored by γpmax (1.35), and the horizontal active earth pressure is factored

by γpmax (1.50). For the case of a wall supporting sloping backfill, both the horizontal and vertical

components of EH are factored by 1.50.

Figure 4-12b shows that the critical load combination for an analysis of eccentricity (or overturning) of the wall foundation occurs when the structure weight is factored by γpmin (0.90), the vertical earth

load is factored by γpmin (1.00), and the horizontal active earth pressure is factored by γpmax (1.50).

For a wall supporting a sloping backfill, both the horizontal and vertical components of EH are factored by 1.50. (This case may also control for analysis of bearing resistance due to the decrease in effective foundation bearing width associated with increasing eccentricity.)

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(a) Bearing Resistance (b) Eccentricity (Overturning)

Figure 4-12

Load Combination for Evaluation of Bearing Resistance and Eccentricity of a Retaining Wall Supported on a Spread Footing

Using the retaining wall examples in Figures 4-11 and 4-12, the vertical and horizontal components of differential water pressure forces, WA, on the wall stem is factored by 1.00.

Because Figures 4-11 and 4-12 only show some of the typical loads applied to a retaining wall without consideration of magnitude, the comments above regarding factored load combinations which could control design represent conditions which may control a majority of cases. For an actual problem, the designer must investigate all viable load combinations and limit states to complete a design using the AASHTO LRFD Specification.

Application of the AASHTO LRFD Specification for design of a driven pile foundation subjected to downdrag loading is presented in Figure 4-13. The neutral plane is located at the elevation where the settlement of the pile equals the settlement of the soil. Above the neutral plane, load in the pile continues to increase with depth due to downdrag and the factored downdrag load adds to the initial factored load in the pile, as indicated by the path A-B in Figure 4-13. Below the neutral plane, skin friction begins to support the pile, and initially offsets the accumulated downdrag load. Along the path B-C in Figure 4-13 skin friction is considered to offset downdrag and is, therefore, regarded as a negative factored load. In the example idealized in Figure 4-13, the skin friction is sufficient to offset all of the downdrag when the load path reaches Point C. Along the path C-D, the resistance of the pile accumulates for a total equal to the factored resistance from skin friction along the path C-D plus the factored tip resistance.

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Figure 4-13 (AC 10.7.1.4-2)

Schematic Representation of Factored Loads on Driven Piles Subjected to Downdrag (AASHTO, 1997a)