For a 9-foot deep trench in Type C soil, specific safety measures are required to ensure the protection of workers from cave-ins. Type C soil is the least stable soil type, and therefore, requires the most stringent protective measures. Here are the key requirements:

1. Protective Systems: You must use a protective system such as shoring, shielding, or sloping to prevent cave-ins. For Type C soil, the maximum allowable slope is 1.5:1, meaning for every foot of depth, the trench must be sloped back 1.5 feet horizontally [1].

2. Shoring and Shielding: If using shoring, it must be designed to withstand the forces exerted by the soil. For a 9-foot deep trench, horizontal wale with a section modulus of 7.0 and 2-inch diameter cylinders spaced at 6.5 feet horizontally can be used. Alternatively, a horizontal wale with a 14.0 section modulus and 3-inch diameter cylinders spaced at 10 feet horizontally is also acceptable. Both wales should be spaced 4 feet vertically, and 3x12 timber sheeting is required at close spacing vertically [2].

3. Professional Design: Any protective structure for trenches deeper than 6 meters (approximately 19.7 feet) must be designed and certified by a professional engineer. Although your trench is 9 feet deep, it is good practice to have a professional engineer review the design to ensure safety [3].

4. Access and Egress: Safe access and egress must be provided for workers. This includes ladders, stairways, or ramps located within 25 feet of all workers [4].

5. Inspection: A competent person must inspect the trench and protective systems daily and as conditions change to ensure safety [5].

6. Spoil Piles: Keep spoil piles at least 2 feet away from the edge of the trench to prevent material from falling back into the trench [6].

These measures are crucial for maintaining safety in trenching operations, especially in unstable Type C soil conditions.