During a trench excavation, the competent person determines the soil is Type B. The trench is 12 feet deep. What is the maximum allowable slope ratio for the sides of this excavation?
Correct Answer
A) 1:1 (45 degrees)
For Type B soil, OSHA requires a maximum slope of 1:1 (45 degrees) for excavations. Type B soil has moderate cohesion and requires this specific slope ratio for safe excavation practices.
Why This Is the Correct Answer
According to OSHA 29 CFR 1926.652, Type B soil requires a maximum allowable slope of 1:1 (45 degrees) for excavations. Type B soil has moderate cohesive strength and unconfined compressive strength between 0.5 and 1.5 tons per square foot. The 1:1 slope ratio means for every foot of depth, the excavation must be sloped back one foot horizontally from the vertical, creating a 45-degree angle that provides adequate stability for this soil classification.
Why the Other Options Are Wrong
Option B: 1 1/2:1 (34 degrees)
The 1½:1 (34 degrees) slope ratio is the requirement for Type A soil, which has the highest cohesive strength. Type A soil is more stable and can support steeper slopes than Type B soil. Using this steeper slope for Type B soil would create unsafe conditions as the soil lacks sufficient cohesive strength to maintain stability at this angle.
Option C: 3/4:1 (53 degrees)
The ¾:1 (53 degrees) slope ratio is too steep for any OSHA-classified soil type in excavations. This slope would be unsafe and does not correspond to any standard OSHA excavation requirements. Such a steep angle would likely result in cave-ins, especially in Type B soil which has only moderate cohesive properties.
Option D: 1/2:1 (63 degrees)
The ½:1 (63 degrees) slope ratio is extremely steep and unsafe for excavation work. This slope exceeds all OSHA safety standards and would create immediate hazardous conditions. No soil classification permits such steep slopes in excavation work, as the risk of cave-in would be unacceptably high regardless of soil type.
Memory Technique
Remember 'B for Basic 1:1' - Type B soil gets the basic 45-degree slope (1:1 ratio). Type A is 'A-1' quality soil so it gets 1½:1 (steeper/safer). Type C is 'Can't trust it' so it needs 1½:1 with extra protection.
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