When it comes to placing pipes under railroads, embankments, roads, and landscapes, pipe ramming is the preferred trenchless method because it is cost efficient, time saving and non-disruptive. Traditional methods that require open excavation for placing pipes under active vehicular zones require major excavation work and diversion of regular traffic.
While it is time consuming and costly for the contractor, it also has indirect economic implications on the municipality. Pipe ramming is also an excellent method for the installation of pipes where ground loss is expected due to the shallow depth of installation, because it utilizes displacement methods unlike other trenchless techniques, such as pipe jacking and heading, that can lead to considerable ground settlement for shallow installations.
This method works most efficiently above the water table in soft clay, organic deposits, silt, loose or dense sand, and cobbled soil with cobbles less than the pipe diameter.
While this method can also be used in harder soil conditions, it is difficult and time consuming. For instance, pipe ramming can be used in rocky ground but the process will require the use of a pneumatic tool to punch a pilot hole before the pipe can be rammed. Pipe ramming utilizes limited equipment, and hence, can be performed in places that are space constrained and cannot support heavy equipment like those required for directional drilling or auger boring.
As with all trenchless pipe installation methods, pipe ramming also involves excavation of two pits – the insertion pit, and the receiving pit. The equipment required for the ramming process is first inserted into the insertion pit and lined up in the designed direction that the pipe is to be installed in.
The insertion pit is sloped and leveled according to the grade of the borehole. In case of embankments and slopes, the insertion pit may not be required, as the ram can be designed to start at the side of the slope. Thrust plates are not required for pipe ramming, as in the case of pipe jacking, however; guide rails are recommended to maintain the grade.
Factors for Effective Pipe Ramming
Pipe ramming can be carried out at depths varying from 18 inches to 20 feet below the surface. For very shallow excavation, it is very crucial to monitor the ground surface above the line of installation. The ramming process is not steerable, and thus, pipes installed using this method are laid in a straight line.
The design phase is very crucial in pipe ramming to avoid damage to other utilities by direct hit or vibration caused during the ramming process. Mapped data is used to determine the location of utilities passing through that area, however; it may not be exhaustive and precise. Therefore, it is advised to use tolerance boxes, the size of which will depend on the accuracy with which the utility lines are identified.
The route layout can then be planned from the tolerance boxes instead of the utility lines. During the design phase, soil clearance around the pipe should be considered by allowing for a slight overcut of the borehole. This reduces friction between the soil and the pipe and helps to maintain the grade at the time of installation. Tis process is where hole deviation, inclinational deviation and all forms of deviation have contingency plans made for them.
Depending on the depth of installation, soil condition and ramming tool diameter, the nominal designed overcut is between ¼ inch and 1½ inch. To reduce the friction between the soil and the pipe during the ramming operation, especially above the water table in medium to dense sands, lubrication with a lubricant like bentonite is very essential. For porous, sandy soils, gravels, and cobbles, lubrication may not be necessary because the lubricant may just dissipate into the surrounding soil.
The ramming pipes are made from steel conforming to ASTM specifications, with a minimum yield strength of 35000 psi. There are two types of pipes used in the ramming process namely, closed-end, and open-end pipes. Of these methods, open-end pipe is preferred because the ramming force required to drive the pipe is lesser, consequently reducing the chances of heaving and pipe deflection.
Closed-end pipe is used where using open-end pipe could disturb the stability of utility lines in the vicinity or cause surface subsidence due to lack of self-support from the flow of soil into the pipe. Also for smaller pipes with diameters up to 6 inches, closed-end pipe is preferred.
How Pipe Ramming Works
Once the prerequisites are completed, the leading edge of the first pipe segment is fitted with a lubrication pipe, and a soil cutting shoe or a welded special band on the leading edge that protects the pipe and also provides a slight overcut to reduce friction between soil and pipe. Once the pipe is well supported it is firmly attached to the ramming tool.
The ramming tool rams the pipe into the soil using repetitive percussive blows until the pipe exits at the receiving pit. Ramming can be done for an entire length of pipe or for shorter lengths depending on the availability of space and the prevailing ground conditions. For shorter pipe segments, the first segment is rammed in and the ramming tool is returned to its original position.
The next segment is then welded or fitted onto the first segment already in the ground and the process is repeated. Compressed air powers the pneumatic hammer which rams the open ended casing into the soil until it reaches the exit.
To lengthen the drive, additional pipe lengths can be welded onto the casing. For shorter rams up to 80 feet long, spoil removal can be done at the end of the ramming process. For longer lengths, periodic cleaning is required to lessen the drag due to the weight of the soil accumulated in the pipe. Spoil is removed by using compressed air or water jets for smaller diameter pipes and by augers for larger diameter pipes.
After the casing exits at the receiving pit, a plug or pig is inserted through the insertion pit into the pipe and the air compressor is allowed to build pressure behind it enabling the spoil to be completely extracted through the other end.
Pipe ramming is a straightforward process that involves ramming the casing pipe into the ground along the designed route after careful consideration of other utility pipelines running through the area. Pipes laid using this process can only be installed in a straight line as the process is non-steerable.
A lubricant such as bentonite is used to lubricate the pipe to reduce friction between the soil and the pipe. Spoil is removed from the pipe either after the ramming process is complete or is periodically cleaned depending on the pipe length.