Capstone: planning a utility trench excavation

This capstone brings the course together in one realistic job. You will plan a utility trench from the first look at the drawings to the final backfill, making the same decisions a competent supervisor would make. Work through each step before reading the suggested answer. Your plan may differ in detail; what matters is that every hazard from lessons 1 to 11 has a control, every control has an owner, and nothing depends on luck.

The job

A city has hired your company to replace 150 ft of water main and two service connections along a residential street. The new main will be laid at 8 ft depth to the trench floor in a trench about 4 ft wide. The work will be done in one traffic lane, leaving the other lane open. The geotechnical report from the design stage shows 5 ft of stiff clay fill over silty sand, with groundwater measured at 6.5 ft depth in spring. Records show a 6 in gas main running parallel 5 ft from the trench centerline, a buried telecommunications duct crossing the trench near one end, and electrical services to houses on one side. One house's foundation is 13 ft from the trench. The job is scheduled for five working days starting Monday 16 November.

Step 1: gather information before digging

Start with what can be learned in the office. Read the drawings and the geotechnical report. Identify every buried facility on the record drawings, and note the ones the records might miss: private services to houses, abandoned mains, street lighting cables. Check the municipality's permit conditions, the traffic control requirements and any noise or working hour limits. Find out who provides trench rescue in this municipality and how long they take to arrive.

Step 2: deal with buried facilities

When the locates arrive, compare them with the records and check that every owner has responded. On site, confirm the marks are where the sheets say. Then plan how the trench will deal with each facility:

• The gas main runs parallel 5 ft from the trench centerline. The trench edge will be about 3 ft from the centerline of the gas main. With a 6 in main and an 18 in tolerance zone, the zone extends to 21 in from the main's centerline, so the trench edge sits just outside it. Even so, expose the main by vacuum excavation at intervals before mechanical digging, confirm its depth and position, and ask the gas utility whether it requires its representative on site.
• The telecommunications duct crosses the trench. Expose it by vacuum excavation before the trench reaches it, support it across the trench as the owner specifies, and protect it from the bucket.
• The house services cross the trench. Locate them privately, expose them by hand or vacuum, and support or temporarily relocate them as planned with the owners.
• Mark every exposed facility with stakes or paint so the operator can see it, and brief the crew on the strike response for gas, electrical and communications lines.

Step 3: classify the soil and choose a protective system

The upper 5 ft is stiff clay fill. Fill is previously disturbed soil, so it cannot be Type A. The silty sand below is granular and lies below the water table from 6.5 ft, so water will seep freely: Type C. Plan for Type C and confirm on site with visual and manual tests.

Because the trench passes 13 ft from a house foundation and a gas main, consider whether a trench shield, which allows some wall movement, is enough, or whether shoring that prevents movement is better near the house. If in doubt, ask an engineer. Below the water table, the shield alone will not stop sand from flowing under its edges, so water control is part of the protective system.

Step 4: water and air

Groundwater at 6.5 ft means the bottom 1.5 ft of the trench is below the water table. Plan for dewatering: a sump pump in the shield at minimum, sized with a standby, or well points installed ahead of the excavation if the silty sand runs. Watch the discharge for fines. Discharge to the storm sewer needs the city's approval and sediment control under the site's stormwater requirements.

The gas main nearby and the use of a gasoline cut-off saw for cutting pipe mean the atmosphere could become hazardous. Keep a calibrated four-gas detector on site, test before entry each day and whenever the saw or other engines have run near the trench, and run the saw and pumps on the surface downwind of the trench.

Step 5: the site around the trench

• Access: ladders inside the shield, extending 3 ft above grade, moving with the shield. With the shield in one position at a time, one ladder inside it gives every worker an exit within a few feet.
• Spoil: the street has no room for a large spoil pile. Load spoil directly into trucks and haul it away, keeping spoil at least 2 ft from the edge and placing trucks so they do not load the trench walls.
• Equipment: the excavator works from the open trench line, tracks at right angles where possible, and keeps away from the gas main side. A signaler guides trucks backing up.
• Traffic and public: traffic control to the provincial or state manual, fencing along the work zone, steel plates and barricades for the trench overnight, and safe pedestrian access to the houses.
• Adjacent structure: photograph the house foundation before work, and watch for cracks during the job.

Step 6: people, inspection and emergencies

• Name the competent person for each shift and confirm their training and authority in writing.
• Plan inspections: before work each day, after rain, when the trench reaches the sand and the water table, whenever the box is moved and after any loads change at the edge. Record each inspection.
• Write the emergency plan: the site address, the nearest cross street, who calls 911, the trench rescue response time, first aid supplies and first aiders, utility emergency numbers, who meets responders, and how equipment and traffic will be stopped.
• Brief the crew at the start of the job and each morning: the locates and exposed facilities, the soil and water conditions, the protective system and its limits, staying inside the box, the strike response, the emergency plan and every worker's right to stop work.

Step 7: close-out

Backfill in layers as specified, removing the box or shoring only as backfill proceeds and never leaving walls unsupported over significant height. Have the owners inspect any exposed facility before it is covered, and replace warning tape. Remove the dewatering system only once backfill is above the water table. Restore the surface, remove traffic control, and file the locate records, inspection records, atmospheric readings and any incident reports. Hold a short review with the crew: what went well, what nearly went wrong, and what should change on the next job.

Reflection

Compare your plan with the suggested one. Did you identify the fill as previously disturbed soil? Did you check that the box would extend above grade and was rated for the weakest soil, not the strongest? Did you plan where the spoil would go before the first bucket came out? Did you find out how long a trench rescue would take? Each of these is a decision that, missed, has killed workers on real jobs.