What Does Box Jacking Mean?
Box jacking is a tunneling method that involves the jacking of rectangular reinforced cement concrete (RCC) sections into the ground. The practice is used in highway construction as well as in trenchless operations such as constructing culverts under road and rail embankments.
Precast reinforced concrete box sections are jacked horizontally through the ground using high capacity hydraulic jacks. Only that portion of the earth that the jacked box will occupy is excavated. This method does not require any intermediate ground support, and since it is built away from the roadway, shoring or traffic control is also not necessary.
Box jacking is also called tunnel jacking or jack box tunneling.
Trenchlesspedia Explains Box Jacking
The box jacking method was adopted in the 1960s based on the success achieved with pipe jacking techniques. As a trenchless construction method, box jacking offers the advantage of minimal disruption while the tunneling is underway. It has been used for railway or subway tunnels, as well as the installation of culverts and other smaller projects.
One of the best examples of box jacking is the huge Central Artery/Tunnel Project in Boston, Massachusetts. Precast concrete sections with walls two-meters thick were used in the construction that started in 1999. Jackhammers and other equipment were used to excavate, and powerful jacks moved the sections along until the three tunnels were complete.
Benefits of Box Jacking
The box jacking method can adapt to shallow overburden and higher structural section utilization compared to circular pipe jacking. Since the method is trenchless, it has a minimum impact on surface traffic and on surrounding structures and does not require intermediate ground supports. The process is simple, safe, economical, and well-established. The method can be used in many applications, including:
- To install storm drains.
- Pedestrian trails.
- Under crossings for livestock and wildlife, in many industrial settings.
- For roadways.
The Box Jacking Method
The precast concrete boxes are either constructed at the site or transported to the site before the operation can begin. First, a shaft is sunk up to the level of jacking, and supports are provided to facilitate the jacking process. The box is lined up and placed exactly in line with its final position, and the soil at the face is excavated using an excavator or a tunneling shield provided at the face. The spoil is removed for safe disposal.
As excavation proceeds, the hydraulic rams push the concrete box into the ground. During jacking, a lubricant is pumped around the box to facilitate easy passage. The process is continued till all the concrete box sections are inserted into the ground safely. Once the installation is completed, grout is injected to displace the lubricant.