Beams & Lagging
The beam & lagging method is the most common type of temporary excavation support and may be less costly than other excavation support systems in the right ground conditions. Beams may be drilled or driven or, in very difficult ground, installed as micropiles. Lagging is installed top-down as excavation proceeds. This is a free-draining system; where the excavation extends below the groundwater table, water inflow during the construction project can be controlled by installation of a dewatering system around the excavation perimeter.
Driven or drilled soldier beam and timber lagging walls are the most widely used form of temporary earth support for large open excavations. Micropiles may be used in lieu of conventional soldier beams in areas of low headroom, restricted access or when difficult drilling conditions are encountered, such as dense soil or when there is the presence of cobbles and boulders.
For above ground permanent retaining walls, precast concrete panels can be used in lieu of timber beams for the permanent facing. Moretrench has served as the retaining wall contractor on a number of above ground projects, including installation of rock anchors for a 50-ft high exposed wall in Emlenton, PA, and installation of a retention system on a steeply sloping site to replace a failed retaining 175- year old retaining wall protecting the historic Old Croton Aqueduct.
Beams are driven or drilled in place, typically at 6 to 10-ft spacings. Lagging boards are installed as excavation progresses in approximately 5-ft lifts until the final subgrade is reached. For deeper cuts, tieback anchors are typically installed to provide lateral restraint to the excavation support.
When a permanent final face is required, Precast concrete lagging panels can be installed between the beams from the bottom up. The use of a one-sided pour-in-place concrete facing can also we used as the permanent face.
Advantages & Limitations
- Rapid construction
- Can be economical compared to other excavation support systems in the right ground conditions.
- Multiple level of tieback anchors can be installed thus making it ideal to handle most excavation depths.
- The system is less economical is difficult ground conditions, i.e., the presence of cobbles and boulders.
- A dewatering system is required for excavation below the water table
- Beam and lagging systems are less rigid than other earth support systems and have more deflections.
- The system can be very costly when rock is present due to high drilling costs.
Our experience as earth retention contractors covers such projects as excavation support for the auxiliary spillway and in-service sluiceway at Bear Creek Dam in AL; and installation of a tied back soldier pile and lagging wall in conjunction with perimeter dewatering at Colgate University in Hamilton, NY.