Soil excavations are often the highest liability activity taking place on a construction site. In the US, dozens of workers are killed and more seriously injured each year from improperly planned and executed work. In addition, many sites are hit by delays, change orders, and lawsuits resulting from designers improperly addressing site soil conditions and the necessary staging of work related to excavation safety. Safe excavations are not a problem to be sorted out during construction; they should be planned early on by the developer in conjunction with their design consultants. Below are a few preventable scenarios and strategies for excavation safety.
1. Involve qualified engineers for below-grade excavations
On a raw land site, you often have room to slope or step deeper excavations for utility lines, basements, underground tanks, etc. in the middle of the property (see Figure 1). For digs less than 20 feet in depth and above the groundwater table, the slope of the excavation can be determined based on the OSHA soil type determined by the geotechnical engineer. For an infill site, or perhaps even for a raw land site near a property line, shoring will be required to support the excavation, as shown in Figure 2. The shoring should be designed by a qualified structural engineer using data provided in the geotechnical report. For utilities, this is often done using trench boxes which can be placed and moved by excavation equipment. For larger excavations, more sophisticated shoring systems including drilled or driven piles, lagging, soil anchors, rakers, etc. may be needed.
2. Protect yourself and your workers during temporary excavations and required backfill
It is important to remember that these are temporary excavations only, which are to be promptly backfilled and carefully monitored. Material and equipment should not be stockpiled or operated on the edge of the excavations (unless specially planned for), and they should be protected from rain and stormwater outflows. The excavations should also be carefully inspected by the geotechnical engineer to verify that soil and groundwater conditions match what was anticipated. Changed conditions must be noted and safety precautions adjusted if needed. A robust geotechnical exploration should be completed early in the design process to avoid changed conditions for major excavations. In areas where there is possible groundwater, groundwater monitoring should be performed well in advance of the work. If design proceeds based on only limited soil and groundwater data, costly design changes and significant project delays could result. For excavations close to the property line, it is a good practice to photo-document the conditions of the surrounding buildings, roadways, and other infrastructure in the event that neighbors claim damage to their property from your activities. Surveying the support piles and vibration monitoring can also be employed to ensure site safety and to reduce the possibility of damage to neighboring properties.
3. Plan ahead for temporary cuts for new retaining walls
In hilly terrain, retaining walls are often needed for raw land development or the redevelopment of existing sites. This is done to flatten the site and make more room for new buildings. See Figures 3 to 5. While engineers regularly provide the needed information for the design of site retaining walls, analysis of the stability of the temporary cut slopes is often neglected. This is the condition depicted in Figure 4.
This temporary cut slope may create a dangerous working condition for retaining wall builders and also may expose the site to the risk of undermining and damaging uphill structures. Depending on the OSHA soil type, temporary sloped excavation may not be practical as it could extend beyond the property line. The designer should strategize early in the design to select the right type of wall system to be used for sufficient clearance. In some cases, the slope-facing building wall may need to become the retaining wall, creating a walkout or daylighting basement. In other cases, “top-down” wall construction, such as a pile wall, may be needed. In pile walls, piles are installed and the wall is built with lagging from the top down, like a shored excavation. Whatever is chosen, it is important to analyze this situation early in the design process to avoid designing a project that cannot be safely constructed.
While working on a design for new construction, considering the temporary conditions during excavation is of critical importance. If not properly planned, excavations may endanger worker and public life safety and could also cause damage to neighboring properties, resulting in losses and lawsuits. Even if the unsafe conditions are corrected in the field before anybody gets hurt, the failure to properly plan will result in delays and change orders as contractors and engineers scramble to address the unsafe situation. With thoughtful planning and coordination of the consultants and design team, projects can be constructed safely and smoothly.