failure is defined as the collapse of part or all of an excavation wall. The most common soil failure is typically described as an unexpected settlement, or cave-in, of an excavation. Soil sliding is the most common factor leading to soil failure.
Tension cracks. Tension cracks usually form at a horizontal distance of one-half to three-quarters times the depth of the trench, measured from the top of the vertical face of the trench. See Figure 2 for additional details.
Sliding or sluffing may occur as a result of tension cracks, as illustrated in Figure 3.
Toppling. In addition to sliding, tension cracks can cause toppling. Toppling occurs when the trench’s vertical face shears along the tension crack line and topples into the excavation. See Figure 4.
Subsidence and Bulging. An unsupported excavation can create____________ in the soil, which, in turn, causes subsidence at the surface and bulging of the vertical face of the trench. If uncorrected, this condition can cause face failure and entrapment of workers in the trench. See Figure 5.
An Unbalanced stress
Heaving or Squeezing. Bottom heaving or squeezing is caused by the downward pressure created by the weight of adjoining soil. This pressure causes a bulge in the bottom of the cut, as illustrated in Figure 6. Heaving and squeezing can occur even when shoring or shielding has been properly installed.
Boiling is evidenced by_____________ into the bottom of the cut. A high water table is one of the causes of boiling. Boiling produces a "quick" condition in the bottom of the cut and can occur even when shoring or trench boxes are used. See Figure 7.
An upward flow
Cracks in the soil
Unit Weight of Soils refers to the weight of one unit of a particular soil. The weight of soil varies with type and moisture content. One cubic foot of soil can weigh from 110 pounds to 140 pounds or more, and one cubic meter (35.3 cubic feet) of soil can weigh more than
9. A safe slope can be defined as the maximum angle of the edge wall or bank of an excavation at which sliding will not occur. The unique mixtures of the different types of soil (sand, clay, silt and rock) will not necessitate different safe slopes from one excavation site to the next.
10. Excavation workers are exposed to many hazards, but the chief hazard is danger of ______. Contractors may also use a trench box or shield that is either designed or approved by a registered professional engineer or is based on tabulated data prepared or approved by a registered professional engineer. Timber, aluminum or other suitable materials may also be used. OSHA standards permit the use of a trench shield (also known as a welder’s hut) as long as the protection it provides is equal to or greater than the protection that would be provided by the appropriate shoring system. (See Figure 9.)
11. Any large, heavy movement near an excavation results in vibration of the surrounding soils. This movement can result in soil failure. Moving machinery, nearby traffic, pile driving and blasting all cause vibration in surrounding soils.
Strengthening the soil
12. Figure 10 shows typical situations where vibrations can result in soil failure.
13. Clay is a very, very fine-grained soil. In general, water moves very slowly through clay. Large amounts of rainfall may pond on the surface and evaporate before being absorbed.
14. Silt has properties intermediate between fine sand and clay. Silt is the most sensitive to changes in soil moisture content. Silt tends to crumble with drying.
15. To help ensure safety in trenching and excavations, which one of the followings should be taken into account:
Soil types and layers
Nearness of structures and their condition
Surface and ground water conditions
The water table elevation
Overhead and underground utilities
All of the above