What is Shoring in Excavation?
OSHA explains it best, saying that shoring is a “support system” installed to eliminate the risk of movement of soil, roadways, foundations, or underground utilities. Often called shielding, as well, it is put in place when the depth or the location of the excavation prevents sloping back to the maximum allowable slope.
Installed in a trench facing, shoring in excavation becomes necessary when the unsupported vertical excavation exceeds 4 feet and if the horizontal distance required for sloping the excavation per OSHA regulations exceeds the distance to property boundaries, existing structures or if the sloped excavation would present access constraints during construction.
At what depth does OSHA require shoring? As stated above, should trenches or excavations exceed four feet, support is required. However, OSHA requires proper soil type determination and evaluation, as well as the use of test equipment for conducting evaluations. Based on those findings, OSHA institutes slope and shield configurations.
Described as sloping and benching, they address the maximum allowable slopes for excavations under 20 feet and take soil type and angle to the horizontal into the equation. The use of benching, which steps the slope, is another factor where shoring in excavation is concerned.
There can be simple and multiple benches, and again the soil type determines the horizontal to vertical ratio of the benched side. Generally, the bottom vertical height must not exceed four feet for the first bench, but subsequent benches may be up to five feet in what is known as Type A soil, but in Type B the total depth cannot exceed 20 feet, with any subsequent benches below the maximum allowable slope for the type of soil in question.
Types of Shoring
There are several kinds of shoring in use. The following are commonly used in trench shoring (for below ground utility construction):
- Hydraulic shoring / Pneumatic shoring – This is one of the most common types and uses a prefabricated strut or wale system made with steel or aluminum. It has many advantages over classic timber shoring, but most important is that workers remain outside of trenches to install and remove it.
- Screw jacks – Requiring the workers to be in the trench to make adjustments, they are unable to preload and can be difficult to handle.
- Single-cylinder hydraulic shores – Utilized in water systems and to assist classic timber shoring systems, this type of shoring requires face stability.
In addition to the TYPES of shoring, there are also many EXCAVATION SHORING TECHNIQUES and primarily include the following:
- Soil Nails and Shotcrete
- Anchored/Tieback Drilled Shaft/Beam and Lagging
- Cantilever Drilled Shaft/Beam and Lagging
- Tangent/Secant Pile Walls
- Reticulated Micropile Walls
Permanent or Temporary Excavation Shoring?
If planned for in advance, excavation shoring systems can serve not only as temporary excavation support, but also as the permanent foundation wall. Often design changes in the building foundation system are required to utilize the temporary excavation shoring system as the permanent foundation wall.
If not used as the permanent foundation wall, the shoring system can be designed as a permanent earth retention system to reduce the lateral load on the below grade structural wall which allows the below grade wall to reduce in thickness. Anchor elements of a permanent shoring system must account for corrosion loss and be protected from corrosion.
Selection of Appropriate Excavation Shoring System
The most appropriate excavation shoring technique and corresponding cost depends upon many variables including:
- Project size and location and available access to mobilize equipment.
- Proximity of shoring system alignment to property boundaries.
- If shoring elements extend beyond property line and easement from affected property owner is required.
- Proximity of shoring system to existing structures.
- Existing structure foundation type and depth relative to elevation of bottom of excavation. Significant surcharge loads may apply, and an underpinning system may be required in addition to an excavation shoring system.
- Clearance between planned construction and existing structures.
- Existence of horizontal projections from existing structures or other overhead obstructions that limit equipment clearance required for shoring system installation.
- Performance criteria
- What are minimum and/or maximum criteria for performance of the system considered, i.e. max. or min. movement, deflection, settlement.
- What is minimum required design life of the system?
- Are neighboring property owners cooperative or litigious?
- Presence of overhead utilities
- equipment needed to install the excavation shoring system must haveclearance from overhead utilities
- Presence of underground utilities
- Density of underground utilities and available ‘windows’ for shoring system components.
- Alignment of utilities and utility material type and presence & extent of bedding material.
- Existing utility trench parallel near shoring alignment presents risk of instability of temporary cut face during construction.
- Ground conditions
- Soil cohesive to allow temporary stable cut face for shotcrete application.
- Groundwater – is groundwater above, or below final depth of excavation.
- Depth to rock and variability of depth to rock
- Local building authority allowable practices
- Temporary construction surcharges
- Proximity of concrete pump trucks
- Location of mobile crane
- Any unusual heavy crane picks and location
- Building components deeper than planned building foundation
- Grease traps
- Stormwater detention vaults
- Elevator shafts
- Required over-excavation below bottom of planned footing due to soil conditions.
- Planned structure waterproofing
- Waterproofing material can be applied atop a shoring system?
- Excavation Shoring System is temporary or permanent?
- Shoring System serves dual purpose as temporary excavation shoring system and permanent foundation wall?
Excavation Shoring System Descriptions
With those basics, it is a good idea to take time to review the more specific information about the types of excavation shoring. Below are full definitions and explanations of the types of excavation shoring, the relative economy, and the conditions to which they are best suited:
Soil nails & shotcrete are generally the most economical excavation shoring system. Soil nails are steel bars installed and grouted in a drill hole placed at regular intervals 5 ft to 6 ft horizontally and the same vertically. The system can conform to complex geometries and is appropriate in a variety of ground conditions provided the soil has adequate cohesion to maintain a 5 ft vertical unsupported face long enough for shotcrete to be applied. Soil nails are first drilled and shotcrete applied in 5 ft vertical lifts or less along the entire perimeter of the excavation. Another 5 ft lift of soil is removed, soil nails installed, then shotcrete is applied and the process repeats until the planned depth of excavation is reached.
Drilled Shaft/Beam and Lagging Walls utilize either concrete drilled shafts or steel soldier piles or ‘H’ piles drilled and installed on 8 to 10 ft centers around the perimeter of the planned excavation. The excavation proceeds in 5 ft lifts as wood or shotcrete lagging is installed atop the drilled shafts or soldier beams. This excavation shoring system can be cantilevered, meaning that no anchors or tiebacks are needed. However, if the excavation height is much higher than 12-14 ft Anchors or tiebacks are installed through the soldier beam into the soil behind and then post-tensioned to provide lateral load to counter the lateral earth loads thereby reducing the net horizontal deflection.
TangentPile Walls are constructed as drilled shafts spaced such that there is little to no clear space between drilled shafts. Secant Pile Walls are constructed as drilled shafts with overlapped spacing, are expensive and are generally only considered if the excavation shoring system will also serve as the exterior wall or shell of the planned structure.
Reticulated Mini Pile Walls are expensive and are only considered when access constraints only allow smaller equipment and anchors are not feasible or allowed due to geometry or because an easement for the excavation shoring system onto the neighboring property is not attainable.
Contact GeoCraft Builders Today
What is the most suitable excavation shoring system for your project? Contact Geocraft Builders, we can evaluate a site, review the best approaches given the project context and soil type, and offer the most economic shoring system. GeoCraft Builders has provided excavation shoring for projects throughout the Front Range and in remote, steep mountain terrain. Let us put our experience and expertise to work in order to help determine the safest and most economical excavation shoring system for your project.