United Rentals

Planning for Success in Deep Excavation Work

We’re all familiar with the adage, ‘proper planning prevents poor performance.’ But the concept of proper planning is particularly important in deep excavation construction where the stakes for non-compliance can be extremely high.

Deep excavation construction work is complex, carrying with it a number of challenges that can affect overall project performance, and more importantly, the safety of your worksite. With early planning efforts and involving an expert shoring specialist, potential issues may be successfully avoided, allowing for increased safe working conditions and jobsite productivity gains.

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There are three key steps in how to plan a successful deep excavation project. They are:

  • Consider and understand project challenges, such as excavation size, soil and ground water, structures and accessibility.
  • Select and engage with an expert partner who can assist through the shoring process.
  • Follow best practices in deep excavation project planning.

Challenges in Deep Excavation

are many challenges associated with deep excavations, and all potential issues must be considered. It is important to not get locked in on what is thought to be the single greatest risk. Deep excavations tend to be open for long periods. Those things thought to be not so important early can develop into big problems as the work progresses.

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Excavation Size

The deeper the dig, the greater the lateral earth pressures become. The shoring system must be able to withstand those forces without failure. The width of excavations can also be an issue where cross braces are needed. Traditional cross bracing may be inadequate, depending on the span. Unsupported cross bracing will have the potential to deflect over a long span. A deflected cross brace under a load can be extremely dangerous. Where lateral earth forces grow over time, a deflected cross brace may become a great risk.

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Soil and Groundwater

Many factors go into the selection of an appropriate protective system. One of the most important factors is the type of soil found where the work will take place. After all, the purpose of the retaining system is to hold back the soil to allow the work to happen. Some soils will be more stable than others – typically the more stable the soil, the lower the lateral earth pressures; less stable soils will have greater lateral earth pressure.

It would be a mistake to overlook the presence of, or the sudden appearance of, groundwater. Temporary retaining structures are usually designed to hold back the soil load, not the combination of soil and water. On this point, it is important to note that a number of manufactured systems, and many site-specific excavation plans, call for the groundwater to be kept at an elevation below the bottom of the excavation. The expectation from many designers is that the groundwater removal take place on the exterior of the shoring system – that is the only way to remove any unexpected hydraulic load from the shoring system.

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If it is not possible to remove the hydraulic load via some sort of well point system, then the load must be factored in, which may require a more robust shoring system.

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Any structure adjacent to an excavation must be protected so as to not allow soil to subside from under that structure, which could lead to failure of that structure. Adjacent, in this context, will mean anything within the area away from the excavation equal to the depth of the excavation. As well, any surcharge created by the existence of that structure would need to be calculated into the protective system design. These structures may include roadways, railroads, and structures on floating slabs.

Where the adjacent structures would include traffic, whether motor vehicle or rail, special consideration must be given to both the weight and vibration from those sources.

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Exposed utilities are frequently found during the deep excavation process. Proper support measures need to be factored into the plan not only for those utilities crossing the excavation, but also many times for those running parallel to the site.

United Rentals is a specialist in trench shoring for deep excavations.

Four-sided slide rail system with tie-back beams

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Finally, accessibility to the excavation site is also an important consideration. Tight spaces impose limitations on the size of equipment that can be used for excavating, as well as on how shoring equipment can be handled and maneuvered within the confines of a narrow or otherwise difficult to access jobsite.

A Key Consideration in Deep Excavation

With the wide range of potential challenges on a deep excavation project, it’s often important to consider bringing in a qualified and experienced partner to assist through all aspects of the shoring design process. A trusted source of excavation-specific expertise can make the difference between a well-run and safe project and costly downtime and budget overages.

Engagement with a shoring partner early in the planning stages is critical. Through experience, engineering support and access to the proper shoring equipment and systems, a qualified trench shoring partner can speed the process and raise potential issues before they become significant problems. They can work with you during project execution to manage changes that inevitably arise during construction progress.

Flexibility during execution can be a significant factor in overall project success. Expertise and experience often translates into creativity and flexibility in shoring system design and adjustment as construction moves forward.
Jobsites may require a range of shoring systems such as slide rail, hydraulic bracing and steel sheeting. Ensuring your shoring partner has the capabilities and quantity of equipment on hand to safely and effectively put these solutions in place is an important fallback when conditions and timelines inevitably change mid-project.

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Planning A Deep Excavation Project

As mentioned, planning comes first. Here are some deep excavation project planning best practices.

First, proper planning means having your shoring specialist review plans and schedule a site visit.

Groundwater control and management is frequently one of the initial considerations well in advance of breaking ground. Dewatering to provide a dry site must often be initiated well in advance of beginning the excavation.

As with trench shoring solutions, there are many types of dewatering and your specific system should be configured to the jobsite according to a range of environmental factors.

This is where an integrated provider of shoring and water management solutions can come in handy. They can streamline your planning and engineering process. The provider can also design and supply a shoring and dewatering system that works seamlessly to account for specific jobsite factors. Make sure to ask your shoring partner if they provide dewatering services as well.

Prior to initiating the excavation process, utility locates must be done to discover the exact location of any underground obstacles and utilities.

The planning process also includes identifying the best possible shoring solution for your project. Your system should be jobsite-specific, designed to accommodate the type of construction being conducted on the project. The plan should detail how the shoring system will be designed and constructed. This will typically involve manufacturer’s tabulated data or perhaps a site specific engineered plan.

During the excavation process, any utilities that may cross the excavation or might simply be exposed must be supported and protected.

Once the excavation and protective system installation is completed, then the actual construction can commence.

It is absolutely critical to make certain that no employee is allowed to enter the excavation until the protective system is completely installed. Quality training can educate personnel so they are aware of the potential hazards of entering an unsafe excavation.

Planning Drives Productivity and Efficiency

With proper planning and careful execution, deep excavation projects can be completed safely and without costly damage to utilities, equipment or adjacent structures.

While some may consider extensive planning a costly process which slows down project progress, a thorough plan properly executed with the help of experienced shoring experts can be a productivity driver resulting in cost savings due to substantial efficiency gains.

Jon Preston is regional product development manager at United Rentals. Jerry Wagner is a senior trench shoring specialist at United Rentals.

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