UFC 3-220-01N
15 AUGUST 2005
Methods described in UFC 3-220-10N may be used to evaluate the stability of open
excavations in soils where behavior of such soils can be reasonably determined by field
investigation, laboratory testing, and analysis. In certain geologic formations (stiff clays,
shales, sensitive clays, clay tills, etc.) stability is controlled by construction procedures,
side effects during and after excavation and inherent geologic planes of weaknesses -
Table 8-1.1 (modified from Effects of Construction on Geotechnical Engineering, by
Clough and Davidson) presents a summary of the primary factors controlling excavation
slopes in some problem soils. Table 8-1.2 (modified from UFC 3-220-10N) summarizes
measures that can be used for excavation protection for both conventional and problem
soils.
8-1.2.2
Vertical Cuts. Many cuts in clays will stand with vertical slopes for a
period of time before failure occurs. However, changes in the shear strength of the clay
with time and stress release resulting from the excavation can lead to progressive
deterioration in stability. This process can be rapid in stiff, highly fissured clays, but
relatively slow in softer clays. (See UFC 3-220-10N for critical heights for vertical cuts in
cohesive soils.) For cuts in hard unweathered rock, stability is mostly controlled by
strength along bedding planes, groundwater condition, and other factors (see UFC 3-
220-10N, and Stability of Steep Slopes on Hard Unweathered Rock, by Terzaghi for
detailed discussion on the effects of rock discontinuities). Cuts in rock can stand
vertical without bolting or anchoring depending on rock quality and joint pattern.
Trenching
8-1.3
8-1.3.1
Site Exploration. Individual trenching projects frequently extend over
long distances. An exploration program should be performed to define the soil and
groundwater conditions over the full extent of the project, so that the design of the
shoring system can be adjusted to satisfy the varying site conditions.
8-1.3.2
lateral earth pressures on the wall support system, bottom heave, and the pressure and
erosive effects of infiltrating groundwater (see Chapter 6 and UFC 3-220-10N). External
factors that influence trench stability include:
Surface Surcharge. The application of any additional load between the
edge of the excavation and the intersection of the ground surface with
the possible failure plane must be considered in the stability analyses
for the excavation.
The effects of vibrating machinery, blasting or other dynamic loads in
the vicinity of the excavation must be considered. The effects of
vibrations are cumulative over periods of time and can be particularly
dangerous in brittle materials such as clayey sand or gravel.
Ground Water Seepage. Improperly dewatered trenches in granular
soils can result in quick conditions and a complete loss of soil strength
or bottom heave. (See UFC 3-220-10N.)
Surface Water Flow. This can result in increased loads on the wall
support system and reduction of the shear strength of the soil. Site
8-2
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