7.3 Bearing Capacity and Slope Stability
Key Takeaways
- Bearing capacity protects foundations from shear failure, while settlement checks protect serviceability; WRE structures often need both.
- Allowable bearing pressure is ultimate bearing capacity divided by a factor of safety, with attention to whether the value is gross or net.
- Groundwater can reduce effective unit weight and shear strength, so bearing and slope problems near basins, channels, and excavations should not ignore pore pressure.
- Slope stability depends on the balance between driving forces and resisting shear strength, including the effects of seepage, surcharge, toe erosion, and rapid drawdown.
- For WRE exam questions, identify whether the failure mode is footing shear, sliding, overturning, embankment instability, trench instability, or slope serviceability before calculating.
Soil Strength Problems Behind Water Projects
The NCEES WRE soil mechanics scope includes bearing capacity and slope stability because water projects are built on and in soil. A small pump station, valve vault, tank, clarifier, basin embankment, channel bank, or retaining wall footing may be controlled by the ground beneath it. WRE candidates do not need to become geotechnical specialists, but they do need to recognize the failure mode and use the correct stress basis.
Bearing Capacity Basics
Bearing capacity is the soil support limit under a footing or mat. Ultimate bearing capacity is the pressure at failure. Allowable bearing pressure is usually ultimate bearing capacity divided by a factor of safety, unless a code, report, or problem statement gives a serviceability-based allowable value directly. Many mistakes come from mixing gross and net pressures. Gross pressure includes the full applied pressure at foundation level. Net pressure subtracts the pressure from soil removed by excavation or existing overburden, depending on the stated convention.
A general bearing expression has three sources of resistance: cohesion, surcharge at foundation level, and soil weight below the footing. The exam may provide bearing factors or a simplified ultimate value, so focus on setup and interpretation rather than memorizing every correction factor.
| Bearing item | What it checks | WRE example |
|---|---|---|
| Ultimate capacity | Shear failure below foundation | Pump station mat, tank footing, wall base |
| Allowable pressure | Ultimate divided by safety factor or report limit | Spread footing service check |
| Net pressure | Added pressure beyond removed overburden | Excavated vault or basement-like structure |
| Settlement | Vertical deformation under service load | Clarifier levelness, pipe penetrations |
| Eccentricity | Uneven pressure distribution | Retaining wall or signaled overturning risk |
Groundwater matters. If the water table rises near the foundation, the effective unit weight contribution can decrease and pore pressure can reduce effective stress. For WRE structures near basins, streams, or wet wells, do not assume dry soil unless the problem says so.
Slope Stability Basics
Slope stability compares resisting shear strength to driving forces. A factor of safety greater than 1 means resistance exceeds driving demand under the assumed model, but real design criteria often require higher values. WRE slopes include levee embankments, pond banks, channel side slopes, landfill or sludge lagoon berms, temporary excavations, and cut slopes near facilities.
Slope risk increases when the driving force rises or resistance falls. Added fill, traffic, stockpiles, or water stored on an embankment can increase driving stress. Seepage, high pore pressure, weak clay seams, poor compaction, undercutting at a channel toe, or loss of vegetation can reduce resistance. Rapid drawdown is a classic WRE condition: outside water level drops quickly, removing support from the slope face while internal pore pressures remain high.
Calculation and Judgment Workflow
Use this sequence:
- Identify the structure or slope and the likely failure surface.
- Decide whether the problem is short-term undrained, long-term drained, or a stated simplified model.
- Use effective stress when pore water pressure is relevant to drained strength.
- Convert ultimate to allowable with the specified factor of safety.
- Compare service load to allowable pressure or computed slope factor of safety.
- Check whether groundwater, surcharge, toe erosion, or rapid drawdown changes the conclusion.
WRE Exam Traps
A bearing answer can be arithmetically correct but still wrong if the service pressure is net and the allowable is gross, or if the water table condition was ignored. A slope answer can be wrong if it treats a stable dry cut as stable after seepage begins. When a problem statement describes cracks, seepage exits, soft clay, recent filling, or channel erosion, the safest answer is usually the one that addresses the geotechnical mechanism, not the one that only changes a hydraulic parameter.
A geotechnical report gives a net ultimate bearing capacity of 5,400 psf for a pump station foundation soil. Using a factor of safety of 3, what is the net allowable bearing pressure?
A stormwater embankment slope is stable when the pond is full, but the pond is drawn down rapidly before the embankment drains internally. Why can this condition be critical?