Question

3. You are a construction engineer working for a large utility contractor. The project you are working on is in New York City (Manhattan). You are installing approximately 1,000 L.F. of 48” dia. RCP for storm drains. Invert elevation ranges from 9’ to 14’ below grade.
   • Pay lines for the trench are indicated in the attached specification.
   • Soil characteristics range from SM to SC, based on the Unified Soil Classification.
   • What considerations would you make with regards to a trenching & shoring specifications for this pipe run? Your considerations should include:
   • Working on active & congested city streets
   • Encountering of groundwater
   • Anticipate encountering other underground utilities crossing over the trench +/- every 10 L.F.
   • Soil removal, storage, & backfilling. Soil can be re-used as suitable fill, providing concrete encasement & pavement specifications are met (make an assumption for pavement details, based on previous class notes).
   • Shoring specifications (type). Shoring must conform to OSHA requirements

• NOTE: It is not necessary to design a shoring system or calculate soil volumes. The answer should be a procedural or methodology consideration.

Answer 1

The following are the considerations for trenching and shoring works in a congested city like New York.

Trenches should be provided with egress. They should have a proper protection system, like benching, shoring, shielding etc, implemented by a competant person.

It has to be made sure that the traffic is least affected because of the trenching work. Proper barricading and sign boards are to be installed giving instructions to the public regarding diversions of traffic, if any, or regarding the inconvenience caused.

Heavy equipments should be kept away from the edges of the trenches to prevent falling off of the soil as well as the equipments

Workers should be wearing warning vests

Proper lighting should be available in the workplace

Barricades should be used effectively

The toxicity of the gases in the surroundings where the workers are to work have to be properly studied. Proper oxygen supply should be ensured.

The soil removed during the trenching work should not be carelessly left over at the pavements. It should be used for backfilling and until the required point of time, it should be held in a proper location, not disturbing the flow of traffic.

The workers should be well trained and competant

The surface water that flows to the concerned area should be removed on a regular and frequent basis. Water removal equipments should be kept ready for use whenever required

Possibility of trees falling should be checked when the ground gets disturbed

Ramp or ladders should be placed at appropriate locations for the easy movement of the workers

The possible hazards that may occur due to poisonous gases may be looked into and all the precautionary measures should be taken.

No worker should be allowed to stay in the work place during rains

If presence of poisonous materials like asbestos is found, clear instruction has to be given to the workers to immediately stop the work and to come out of the trench

The water that flows to the trench should be completely removed before the start of a day's work

Proper and timely inspection of the work place is important. Every progress should be properly monitored.

Cases of inappropriate seepage, bulging of soils, tension cracks etc should be carefully monitored and necessary actions should be taken with immediate effect

Any changes or movement to the structures in the locality should be observed with atmost care.

SHORING-

Shoring can be done using the following types of methods:

• Boards
• Screw Jack (Pneumatic )
• aluminium shoring
• hydraulic methods
• Other types of supports to prevent failure of soils
• Underpinning may be done at required points

Care has to be taken while encountering other utility lines while trenching.

Any possibility of leakage should be studied. There are chances of electrocution also. To avoid this, the city's plan that contains the location of utility lines and pipes should be taken and studied before commencement of the trenching activity.

Proper first aid should be made available in case of any mishaps. No worker should be left unattended. Proper care has to be given immediately after an accident, in case if any.