Although trenchless construction and rehabilitation methods remove the need for open trenching along the duration of the underground piping installation, they often still require the establishment of an entry and exit pit at each end of the pipe. Sometimes, intermediate pits are also needed in between the entrance and exit pits for longer pipelines. Entry pits are always the largest excavations, as they are required to house the trenchless equipment and piping or casing segments, as well as the thrust block for pipe jacking projects.
The size and depth of these pits is dependent on which trenchless technique is being used, the diameter of the pipe being installed and the ground conditions. Types of soil one may encounter in excavations are gravel, sand, silt and clay. Soils are classified in these groups according to their particle size and moisture content. (Learn more in "The Right Drill Bit for Soft, Medium & Hard Ground Conditions.")
OSHA regulations stipulate that a pit of more than 5 ft. deep must be supported to prevent wall collapse. If the excavation is to be deeper than 20 ft., then a structural engineer is required to sign off on the construction plan. Elevators are required for pits that are more than 50 ft. deep.
Temporary pits for trenchless construction are only used for the duration of the project. Once the pipe installation is complete, the shaft is refilled or converted into an access point. Shoring material is therefore saved for reuse. Concrete and steel are two of the predominant materials in use for shoring of these temporary trenchless pits. This article examines the pros and cons of each and answers the question of which is better than the other.
Steel For Temporary Trenchless Shoring
Vertical sheeting uses steel beams and sheets to absorb the load of the ground wall. Sheets are progressively installed by slipping them into place behind the beams as the excavation deepens. This system is not watertight and cannot be used in conditions where groundwater is present.
Sheet piling uses interlocking steel sheets, which are braced with ring beams along the shaft depth. They are either driven into the ground or installed using vibration. Sheet piling is watertight and distributes the ground loading through the metal sheets. It cannot be used for small diameter shafts and an added disadvantage is the noise and vibration during construction. Sheet piling is the cheapest form of temporary trenchless shoring, but problems may be encountered with rocky ground. In this case, pre drilling may be required in order to prepare for the sheet piling installation.
Trench boxes are prefabricated steel structures that are lowered into the shaft as the excavation is progressing. The lower edge of the trench box is sharpened so that the weight of the box keeps pushing it further into the excavation. Transportation is a limitation for this technique, as prefabricated units must be transported to site, and larger size loads require special transport permits and vehicles.
Concrete For Temporary Trenchless Shoring
One method of concrete trenchless shoring is the secant piling method. Alternating reinforced and unreinforced concrete columns are placed on top of the other to form a watertight shoring support. This method is suitable for excavation of pits up to 25-meters deep. Advantages of this method is the rapid installation time and small working area requirement. However, the technique is expensive due to the drilling rig required for installation.
Concrete sinking shafts use prefabricated concrete segments, which are installed and bolted or pinned together. A watertight solution means that concrete sinking shafts can be used in any groundwater conditions and depths can reach a up to 131-feet. In a similar method to trench boxes, the lower edge of the concrete segments is sharpened so that they sink under their own weight as the soil is excavated from inside the concrete walls. If the pit extends below the natural height of groundwater, a concrete plug is installed once the excavation is complete. Water is then pumped out of the pit. The watertight seal keeps the shaft free of water while the trenchless construction activity continues.
A significant advantage of concrete shoring is its suitability for pipe jacking methods.
Is Steel Better Than Concrete for Temporary Trenchless Shoring?
The most influential factor in choosing steel or concrete for the temporary trenchless shoring is the analysis of the soil conditions and the level of groundwater. Project schedule and budget also play a role as some shoring techniques are quicker to implement than others, and costs vary between applications too.
Steel temporary trenchless shoring solutions like sheet piling are quick and cheap to install. However, if hard rock is encountered in the ground, drilling may be required, which will rapidly escalate the cost and duration.
Concrete temporary trenchless shoring solutions like concrete sinking shafts, are suitable for very deep pits and can absorb the load required for pipe jacking thrust.