September 2016, Vol. 71 No. 9
Features
Engineering Trench Safety Critical Aspect Of Project Success, Costs
Any project that includes excavation is going to require shielding and shoring to prevent excavation walls from collapsing. For large projects, the design and installation of shielding and shoring is itself an extensive project within the project.
Manufacturers and suppliers agree that shielding and shoring requirements should be identified early in the planning stage, and that types of shielding and shoring design should be part of the engineering process.
There are various types of shielding and shoring systems available today that allow knowledgeable engineers to design systems to meet specific projects. Because each project has individual characteristics, renting shielding and shoring usually is the most cost-efficient option.
United Rentals Trench Safety
“The purpose of the planning process is to stimulate thought and arrive at solutions prior to the start of work,” said Josh Chandler, United Rentals Trench Safety engineering manager for North America. “Excavation due diligence should be taken seriously in order to prevent delays, damage, injury and death.”
Excavation is an area of construction where uniqueness is the norm, Chandler continued.
“It’s well known,” Chandler said, “that within the OSHA standard an excavation requires engineering at depths greater than 20 feet or when lateral loads due to surrounding surcharges are in excess of 72 pounds per square foot (psf). A registered professional engineer (RPE) is required by two of the four options under OSHA 29 CFR 1926 Subpart P – namely, designs using tabulated data from an RPE and site-specific project designs which can also offer benefits due to uniqueness or complexity.
“It is well worth involving a professional engineer at this early stage on projects of any meaningful size.
“Large projects can change quickly as to scope, size or timeline for equipment needs and regulatory review. The excavations are usually deep, with a significant soil load to support. A typical uniform building load is 100 psf or less, whereas loads for deep projects can be in excess of 2,000 psf. Also, when a system extends further into the earth, there tends to be embedment limitations that restrict shoring options, such as underlying rock, underground tunnels or utilities. Pre-existing penetrations create clearance issues that usually require phasing,” Chandler explained.
In addition, large projects are often located in heavily congested areas where structures such as buildings, concrete vaults, storage tanks, bridges and towers increase the lateral load on the shoring system. A congested site can require asymmetrical geometries that make structural bracing layouts more complicated. Above-ground structures can present site access problems for machinery and equipment. Engineering offers safe, workable solutions to these issues.
“Also, a shoring system on a large job typically stays in the ground longer, which can require more stringent deflection constraints.”
Failure to plan an excavation safety system properly can result in loss of control of the outcome.
“For example,” Chandler said, “delays can occur if space restrictions or phasing requirements aren’t thought through in advance. One circumstance triggers another.
If shoring components can’t clear span from the base to the top of the excavation, internal bracing is commonly required to support the applied loads. When this occurs, the infrastructure installation must be completed in phases in order to work around the necessary structural supports.”
Planning also prevents damage to surrounding structures. Stockpiled material, machinery, structures, roadways and other influences resting on the soil surface all have associated loads. This can conceivably increase the lateral pressures on shoring walls. Surcharge loads have to be combined with lateral soil loads in order to determine the total load that will be applied to each shoring system. If this isn’t properly calculated during planning, there is a risk of shoring system failure.
“In our experience at United Rentals Trench Safety, most public and industrial projects account for shoring needs within the bid documents,” Chandler said. “By contrast, many private projects will specify the scope, but leave it up to the contractor to build the project safely. Either way, if the shoring design is site-specific, it should ensure that geotechnical requirements and engineering design standards are being met, the plan is constructible and can be followed, site safety issues are addressed, and workers are protected.”
United Rentals is the world’s largest equipment rental company offering approximately 3,000 classes of tools and equipment. In addition to trench shielding and shoring, the company’s Trench Safety Division locations carry confined space ventilators, air monitor detectors and other shoring equipment, and offer 24-hour emergency response and professional safety training.
National Trench Safety
Tom Hartman, senior vice president of strategic alliances, National Trench Safety LLC, said a shoring system’s complexity is largely a function of the job site including surrounding structures.
“The uniqueness of each project means there are no ‘will fit’ solutions,” said Hartman.
“Shoring system selection has evolved significantly over the last decade. We’re seeing more and more site-specific plans due to the increasing complexity and depths of excavations.
“Municipalities continue to put utilities deeper and deeper below ground and as cities continue to grow, contractors are dealing with job sites that have crossing utilities and many adjacent structure considerations.”
Factors that add to the complexity of a trench system include soil conditions, the water table, adjacent structures and pre-existing utilities, size of excavation and free span requirements, depth of the excavation and project timeline.
Oftentimes a contractor ends up trying to fit a system into the budget and doesn’t get the overall best system, continued Hartman. When assessing a shoring system at the project planning phase, it’s possible to calculate an estimated cost and compare it to production tradeoffs to see what the best total cost
option is.
Hartman said from time to time his company does encounter projects where shielding and shoring were not included, but for the most part contractors are pretty well prepared. Larger contractors recognize the need for trench safety equipment for productivity as well as worker safety.
Not including trench safety in early planning can cause multiple problems.
“The immediate result is a budget not prepared to withstand the cost of a shoring system,” said Hartman. “The contractor must determine how to fit the system into the cost structure under which the job was bid and this may have trade-offs in terms of equipment costs and productivity. It’s always best to have a proactive plan to address potential ways to shore an excavation. Most rental companies, including NTS, will provide support to contractors during the bid phase to review plans and present potential options for consideration by the estimating and project management teams.”
One of the most recent developments in the industry is a push in multiple state jurisdictions to hold companies and key management directly responsible for the failure to provide shoring equipment.
“In one prominent case in California,” he said, “the contractor was sentenced to jail for failing to provide adequate protection on a project.”
Based in Houston, TX, and with locations nationwide, NTS provides trench boxes, trench shields, slide rail systems, barrier walls, message boards and other related safety products, along with trench and traffic safety training.
Efficiency Production
Mike Ross, shoring specialist in the Special Operations Shoring Division of Efficiency Production, said in most regions of the country, design engineers do not specify means and methods of construction, but rely on the contractor to employ the most efficient method of shielding and shoring. Project specifications and contracts will require work to be performed within all local, state and federal laws to include prevailing OSHA standards.
“Contractors,” Ross said, “rely on well-trained local manufacturers representatives to provide solutions that are efficient and compliant.”
The size of the project may or may not reflect the complexity of the shoring system, but will drive the cost of the proposed solution. Larger projects almost always require more costly solutions due to the quantity of equipment and duration that is needed.
“Manufactured shielding and shoring systems are designed to be flexible and cover a large spectrum of situations when used within the tabulated data and OSHA Subpart P,” Ross said. “However, scope of work to be performed and site conditions make owning a ‘fits all’ system difficult, requiring contractors and municipalities to rent equipment for projects falling outside the parameters of existing equipment.”
According to Ross, factors influencing the decision of what type of safety system is needed include:
- Scope of work to be completed
- Total depth, including benching or sloping
- Soil and water conditions
- Possible crossing utilities or overhead
restrictions - Size or model of excavating equipment
“Very few last-minute plans or reactions are profitable to a contractor, and this includes the shielding and shoring,” said Ross. “Drawbacks to waiting can cause delays in progress due to delivery times and reduction of pipe production in wet and unstable soil conditions. Also, without anticipating problems and designing a solution in advance, you could suffer the loss of all progress gained up to that point.
“Failure to plan or poor planning results in lost profits or worse – overall loss on the project as costs go beyond quoted amounts. Although shielding and shoring is a relatively small percentage of most common underground projects, certain projects can turn into losers very quickly if a poor site or soil condition is encountered with no prior planning. In competitive markets the time spent idle by a crew waiting for a solution could be the difference between profit and loss.”
Efficiency Production, Mason, MI, is a leading manufacturer of trench shields, trench shoring, trench boxes and hydraulic shores and the Premier slide rail system. It has branch locations in the U.S. and internationally.
FOR MORE INFORMATION:
United Rentals, (203) 622-3131, unitedrentals.com
National Trench Safety, (832) 200-0988, ntsafety.com
Efficiency Productions Inc., 800-552-8800, efficiencyproduction.com
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