JACKING REINFORCED CONCRETE PIPELINE

This article is reproduced by the kind permission of the American Concrete Pipe Association – The Certificated Engineer April 1965.

CHAPTER 1 - Introduction

The practice of constructing pipelines under highways or railways without Interference to traffic was first initiated by the Northern Pacific Railroad between 1896 and 1900. Since that time this method of construction has become a standard accepted practice; unique construction methods have been devised and the work may be accomplished at relatively low cost. The following is a compilation of the latest and most generally accepted procedures based on the study of a large number of important installations over a period of years.

When conditions are suitable, the installation of concrete pipelines by pushing them into position with jacks is safe, fast and will save from 30 per cent to 50 per cent of the cost f placing the pipe in an open trench. The advantages of the meth dare:

(a) Usually lower cost.

(b) No interruption of street, highway or railway traffic.

(c) A minimum disturbance of roadbed and consequently little or no subsequent settlement.

The Pipe

Reinforced concrete culvert pipe with tongue a groove joint is usually used for jacking, and the minimum inside diameter of pipe that should be used is generally accepted as being 30-inch. It often will be found to be - more economical to use a 30-inch culvert pipe having a capacity greater than called for by the hydraulic design than to install a smaller conduit having the correct hydraulic design.

The Soil

The pipe may be jacked through almost any type of soil, but costs will be higher, although not necessarily prohibitive, in unstable soils and in embankments containing boulders, stumps, waste from rock cuts or similar obstructions. Groundwater in gravel causes no trouble as it can flow back through the pipe and be pumped from the pit. Groundwater in the sand or a sand-clay mixture is much more troublesome as it is difficult to keep the lead pipe properly aligned in the very soft material. It will usually be found necessary to dewater this type of soil with good points before jacking is started.

The Joints

Some form of cushioning between the joints will reduce the likelihood of breakage due to concentrated pressure. A piece of ? -inch manila rope stuck to the pipe with asphaltic cement, or strips of asphalt roofing paper, or metal strips, or cement grout inserted around the circumference in the joints as each piece of pipe is placed ahead of the jacking frame is satisfactory for this purpose. After the pipe is in the final position, the joints may be pointed from the inside with mortar or joint compound.

Unloading the Pipe

Care should be taken in handling the concrete pipe to prevent chipping the edges or cracking the pipe.

In transporting concrete pipe from the truck or railway car to its final location, the pipe sections should not be dropped off the truck on to the ground.

The concrete pipe should not be rolled down embankments in such a way that the pipe gets out of hand and rolls without control. If the sections of the pipe are moved along the ground by pushing with bulldozer r tractors, great care should be taken not to damage the pipe.

Preparing the Approach Pit

It is usually more economical to start the pushing operation in an open trench and therefore the site should be investigated carefully to determine where the jacking operation should start. Generally, the starting place will be the point where further excavation of the fill would be uneconomical or where further excavation would interfere with the highway, railway or building above. If possible, the first excavation should present a vertical face on the side of the bank facing the entering pipe and a vertical face opposite for the support of the reaction blocks. The pit should be only of sufficient length to provide room for the jacking head, the jacking frame, the reaction blocks, the jacks and usually two sections of pipe and should be dug to such a depth that the invert of the pipe, when placed on the guide frame, will be at the elevation desired for the completed line at the point of entering the fill. The pit should be dug sufficiently wide to allow ample working space.

Where the nature of the site is such that it is not possible to dig a pit, the backstays or reaction blocks are constructed so as to carry the reaction of the jacks to the ground. (See Fig. 1.) The backstays may be constructed either of concrete or of timbers with the bottoms set in the ground and the tops supported by diagonals reacting against embedded anchorages. To avoid excessive soil deflection, the bearing area is usually provided equally to 200 per cent of the estimated maximum active jacking pressure. In wet areas, it may be necessary to drain the jacking site sometimes by the use of good points.

On some projects, it may be easier to dig a pit at an intermediate point and jack the pipe in both directions from the pit. This method simplifies the problem of the reaction blocks and decreases the total length of pipe to be jacked in any one direction. 

Erecting the Jacking Frame

The type of jacking frame and the type of backstop will depend necessarily on the nature of the site, and the character of the material to be penetrated. For average conditions backstops such as shown in Figs. 1 and 2 have proved satisfactory.

The backstays, backstops or reaction blocks must be placed with great care. They absorb just as much push as goes into the concrete pipe and if not installed correctly, the work of the jacks will be expended in deflecting the backstops instead of pushing the concrete pipe forward into place. The backstops must be absolutely perpendicular to the jacks so that forces will not be set up which will tend to collapse the structure. The timbers used to carry the push from the jack to the pipe should be placed so that the forces go parallel to the grain of the timber in order to minimize deflection of the timber.

In some instances, the usual type of backstop cannot be used and a special design must be made. Such a condition arose for sewer crossings at the Marine Barracks, Quantico, Virginia, when four reinforced concrete pipelines 30-inch, 36-inch, 60-inch, and 66-inch, every 100 feet to 110 feet in length, were jacked under the Richmond, Fredericksburg, and Potomac R.R. for the Bureau of Yards and Docks, U.S. Navy Department, in 1931. In the tidal marshes through which the railroad passes, the ordinary timber backstops could not be employed. Two wrought-iron pipes, 2 inches in diameter, were pushed through the fill through which cables were threaded and lapped around a bulkhead on the opposite side of the fill, then threaded back through two additional 2-inch pipelines and attached to the backstops. Thus, four cables were used, two on each side of the pipeline, affording ample support for constructing these large pipelines.

A jacking head can be used to transfer the pressure from the jacks or jacking frame to the concrete pipe. Many types of jacking heads have been used successfully but ahead constructed as shown in Figure 3 will protect the end of the pipe and will aid materially in keeping the pipe on line by maintaining equal pressure on the circumference of the pipe. The jacking head must be constructed leaving an opening for entry and exit of men and material.

Guide timbers to hold and guide the pipe are carefully installed to line and grade in the approach trench. They may be constructed of 10-inch x 10-inch timbers with steel angles placed on the inner edges. Excavation for the guide timbers should be made exactly to grade as there may be some settlement if the soil is removed and then replaced. If a large amount of groundwater or rain is expected, construction of a six-inch thick concrete working platform draining to a sump in one corner will be found helpful. If the ground around the jacks and frames is continually water-soaked, it may be difficult to maintain line and grade in the guide timbers. The working area is often protected from rain and sun by an improvised tent.

The Jacks

The number of jacks required and their capacity depend upon the size and length of pipe to be jacked and the nature of the embankment material. For concrete pipe 30-inch to 42-inch diameter in the ordinary clay or sand soils, two 100-ton hydraulic jacks are recommended. For concrete pipe over 42 inches in diameter, three or four 100-ton hydraulic jacks will be needed except for extremely short runs. Sufficient data has not been gathered to date from which formulas or tables can be derived to calculate the pressure that may be required. A typical pressure chart, prepared from data presented by Charles F. Sterling, Superintendent of Building Construction of Elmira, New York, and D. R. Young, Division Engineer, D. L. & W. RR., Buffalo, New York, covering the pressures required to jack 69 feet of 96-inch reinforced concrete pipe, with nine-inch shell thickness, under four mainline tracks of the Delaware, Lackawanna & Western Railroad at East Fifth Street in Elmira, New York, is shown in Fig. 5. The lead pipe was fitted with a steel cutting edge 5/8-inch in thickness and 18-inch in width and bolted to the groove end of the pipe so that 6 inches extended ahead as a shield. The excavation was carried on within but not ahead of the shield, the pipe is actually pushed through the fill and not skidded through an excavated tunnel as is sometimes the case. Slow orders were not issued by the railroad during this operation.

A pressure chart, prepared by Lionel Pedley, Assistant Construction Engineer, Los Angeles City Housing Authority, showing the pressure required to jack 144 feet of 72-inch reinforced concrete pipe through a 20-foot high railroad fill, composed of moist sand, gravel and boulders are shown in Fig. 6. The first section of pipe was equipped with a steel cutting edge of ? -inch greater diameter than the section for its entire circumference; and an additional steel extension projecting 20 inches ahead of this section for the upper one-third of its circumference. The excavation was maintained at all times approximately one foot in advance of the first section of pipe to a diameter of 1 inch more than that of the pipe except at the bottom, where excavation was made to exact grade.

Jacks should be oversized by as much as possible. The resultant of their push should pass below the spring line of the pipe and jacks should be placed on both sides of the line to permit 'rocking' the pipe to overcome 'freezing.'

Frictional resistance can be decreased by the application of bentonite or other suitable lubricants to the exterior surface of the section being jacked. It can be applied to the surface directly or through ? -inch nipples through holes drilled in the cutting shield around its periphery with the aid of a grease gun.

A comparatively small jack, in the 10-ton range, is sometimes u ed at the heading to realign the lead pipe when it deviates from the desired line and grade. When the allowable deviation from final grade is exceeded, a new slope is computed to the station being jacked to and construction is then governed by the new grade. If the location of a change in line is of a considerable distance or if the location of the next jacking pit is not determined, the new slope can be computed to a reasonable distance, say 200 feet from the grade change.

Jacking Procedures

After the preliminary work of excavating the approach trench, installing the guide timbers and constructing the jacking frame, the jacking head and the backstops, the first two pieces of pipe are rolled or hoisted on t the guide timbers and jacked against the bank. Digging is then begun from inside the pipe, the bore being excavated to one inch above grade at the bottom and about one inch larger than the pipe at the top and sides. The man digging casts the spoil back into the pipe where it is loaded into a wheelbarrow or cart by his assistant. Another man wheels the spoil out through the pipe to the waste bank. In constructing smaller size lines, 30-inch to 42-inch, the use of the assistant is not feasible. The mucker loads the spoil on to a small cart or sledge which is pulled out of the pipe by a rope from the outside, emptied and then pulled back into position by the man in the pipe. If the excavated material is dry, it may be deposited on the pipe invert and then pulled to the open end by means of a steel plate shaped to fit the lower portion of the pipe and drawn by a power or hand winch through a pulley arrangement.

The jacking goes forward continuously until the effective limit of the jacks is reached. Blocking is then placed and jacking is continued until additional blocking is necessary. The. the process is continued until there is room for, another piece of pipe. Blocking may be either of steel or timber, burnt or cut to the correct length in the field. The same blocking may be used over and over as each length of pipe is installed. After the jacking is started, it should be kept going insofar as possible. It is better to work three shifts or at least two eight-hour shifts out of twenty-four and to continue over the weekend. When a long stop is made, the pipe may freeze into place and it may be difficult to get started again. The direction of jacking is a matter of personal preference and experience. If the pipe is jacked downhill, it may be more easily kept on grade due to the weight of the pipe tending to work the front end down. However, if the pipe is jacked uphill, it may be more easily drained in case water is encountered in the fill.

The tools used for digging will depend on the soils but short-handled picks and shovels and adzes are most commonly found to be needed. In hard clays and rocks, pneumatic clay spades and rock drills may be necessary. In long pipes, the exhaust from air tools aids in ventilating the pipe. The distance to which excavation can be carried ahead of the pipe will depend on the type of soil, the distance below the surface, and the superimposed surface loading. In some very hard clays it is possible to excavate four feet ahead of the pipe, but, under ordinary conditions, excavations of one to two feet ahead will be all that safety will permit. In extremely soft soils, installation of a cutting edge either on the top and partly down the sides or all around the pipe will minimize the danger of settlement ahead of the pipe. In this case, excavation is carried on within but not ahead of the cutting edge.

If the backstop, jacks, and guide timbers are carefully aligned, little trouble should be encountered in maintaining the line and grade of the pipe. However, the alignment should be checked often using an engineer's transit or surface borings. Alignment of the pipe may be corrected by cutting more material away on the side toward which movement is desired, or by inserting wooden wedges or a piece of scrap iron on the side where it is going out of alignment so that, when the pipe is again pushed forward, it will press against the wedge or scrap iron and be brought back into line. The weight of concrete pipe causes it to work the front end down but in dry soils, this tendency may be easily controlled. However, in very wet soil’s it may be necessary to align the pipelines by the use of jacks at the heading. In some installations, the settlement has been prevented by attaching a steel bar to the crown of the lead pipe to extend into the soil mass ahead of the excavation.

In jacking pipelines through wet soil, sand may get into the joints at the invert causing the line to rise in an upward curve. This can be avoided by the use of one-half inch manila rope stuck to the joint with asphaltic cement, cement grout, or two sheets of asphalt roofing paper or a metal sheet inserted between each pipe at the joint.

A concrete base in the jacking pit will usually pay for itself as it allows for better working conditions, constant grade and the best drainage situation. Guide rails embedded in the base should extend two pipe lengths into the fill under the line at proper grade.

Preventing Erosion of Soil Around the Jacked Pipe

After the pipe has been jacked into place, the back-fill should be tightly compacted around both ends of the culvert to prevent erosion. Concrete collars or cut-off walls, concrete pavement or rip-rap, intake structures and in some cases outlet work for dissipating the energy of the issuing water should be installed wherever necessary.

Conclusion

Jacking of pipe 30-inch to 108-inch in diameter under railroads and congested streets and highways is now a common practice in the United States and Canada, thus eliminating interference with traffic and the costly maintenance which often results from trench settlement. Concrete pipe is particularly suitable to this type of work because of its strength and because the smooth external surface of the pipe offers less resistance to friction and is not easily deflected from established line and grade. Pipelines while being jacked and after they are installed are not subject to unbalanced stresses. When the line is pushed through the soil, it becomes an integral part of the oil mass as it occupies practically the same space as the excavated material.

Four points which should be carefully observed before and while jacking pipe are:

1.      A thorough investigation of the soil through which the pipe is to be jacked. It is not recommended that the jacking method be used to place the pipe in unstable soils. When there is doubt as to the nature of the material, sufficient borings should be made along the proposed pipeline.

2.      Have jacks of adequate size available. It is far better to have available jacking capacity over that required than under that required.

3.      Correct alignment of the pipe guide frame, jacks and backstops. If any part of the jacking set-up is the slightest bit offline, forces are set up which tend to bind the pipe or cause it to wander off line.

4.      Once the jacking is started keep it going. At least two shifts of eight hours each should be worked. In some cases, it is well to station a man on the job during the third shift to pull on the jacks at prescribed intervals, and thus, keep the pipe loosened.

Jacking concrete pipe is a fast procedure. There is a minimum disturbance of the soil above the pipe with little or no subsequent settlement. It is a time tested procedure. It entails no interruption of traffic and it is usually more economical than any other method.

CHAPTER II

RANDOM NOTES ON JACKING

by Howard F. Peckworth

In fifteen years of heavy construction experience I have had occasion to install numerous concrete pipelines by the jacking process. On one project alone, the Santee-Cooper Hydroelectric and Navigation Project in coastal South Carolina we jacked a whole series of concrete pipe culverts under the Southern Railway Tracks as they skirted the project. Some of the lines were large in diameter and others were small. In the course of fifteen years, I ran into almost every kind of soil, including fill. In some cases we had difficulties, others gave us no trouble whatsoever. The following notes are taken somewhat at random from my experiences.

Sizes

I am often asked, 'What is the largest size concrete pipe that can be installed by the jacking process and what is the smallest size?' I think the largest concrete pipe that I know of that was jacked into place was a line of 108-inch ASTM Specification C-76 pipe jacked under the Alexandria Yards of the Richmond, Fredricksburg and Potomac Railroad.

The smallest size concrete pipeline to be jacked into place is 4-inches in diameter and there have been a number of them. 'How did they excavate the earth in such a small pipeline?' That was accomplished by using a hose and washing the earth material from the front of the line back through the line as it was jacked forward.

The type of soil encountered has much to do with the maximum and minimum size pipe capable of being jacked in any given location. For instance, if you are going to run into boulders, naturally you cannot jack a small size pipe using a hose to flush out the excavation. And the limit on the large size is determined somewhat by the length of line to be jacked because the skin friction and the weight in the larger sizes naturally increase the force necessary to push the line forward. If the soil encountered is so tough that it will have to be dug by hand, then the pipe should be larger than 36 inches. A normal size man can go through a 24-inch pipeline by lying on his back on a surfboard running on casters and pushing himself forward with his heels, but this position is so awkward he cannot do much digging at the heading. A small man can work with difficulty in a 36-inch pipe, but for the best efficiency in working men in the heading, the diameter should be larger than 6 inches.

Types of Soil Encountered

In my estimation, the most difficult soil to jack a pipeline through is filled ground, especially the kind f filled ground met up with near large cities. You are liable to run into the trash of all kinds-old auto-mobiles bed springs, masses of rusted tin cans mixed with ashes can make digging at the heading very difficult indeed.

The next most difficult soil for jacking is one containing large random boulders. Parts of the boulders which come within the way of the pipeline have to be removed by chipping off with a paving breaker, by blasting using very small charges, or by chipping off by hand using the old fashioned 'plug' and 'feathers.' In the cramped quarters at the heading, all three of these processes are tedious. The old fashioned plug and feather method of breaking a boulder is where a hole is drilled first, then two wrought iron 'feathers' are placed in the hole and then the wedge-shaped plug is driven in between the two 'feathers,' thus splitting the boulder.

Wet sand can be very easy to jack through or it can be very difficult depending on whether or not it stands of its own accord or runs. Dry sand that runs can also be very difficult. In either case. of running, it may be necessary to sheet the heading with timbers caulked with straw, removing the running sand little at a time while the shield is moved forward, and jacked accordingly.

In spite of these difficult soil conditions which I have mentioned, there are surprisingly few soils in actual practice which will present great difficulties.

Ground Water

Groundwater can give considerable trouble. If you know you are going to run into the water, it can be drained off .by aiming the. grade of the line up a little as you go m, thus allowing the water to drain off. Others like to aim the grade downward thus confining the water to the heading where it can be pumped off. I know of no place where a concrete pipeline has been jacked forward into water pressure balancing the water pressure by means of air pressure: but there is no reason why it couldn't be done similar to the methods used in tunnelling under air pressure.

When you make the preliminary borings prior to jacking, examine particularly for the presence or absence of groundwater.

Jointing Materials

It is necessary for the joints of the line to spread the jacking load from one piece of pipe to the next as the pipeline is pushed forward. This load may be made uniform from pipe to the pipe by many different methods. An easy method is to put a piece of hemp rope around the pipe so that the hemp rope is squeezed between the tongue and the groove of the pipeline. Other materials used are several thicknesses of roofing paper, quick-setting cement, rubber hose, or especially formed wooden shims. The purpose is to have a uniform bearing from one piece of pipe to the next.

Jacking Frames and Backstops

There are probably as many different designs of jacking frames and backstops as there are jacking projects. Typical designs are given in the Concrete Pipe Handbook published by the American Concrete Pipe Association. I suggest that they be sturdily constructed, even to the point of using 12 x 12 timbers if available. Don't forget that the backstop takes just as much push and a little more than the pipeline. So many times I have seen makeshift backstops constructed with the result that the backstop was pushed into the earth while the pipe remained stationary.

The Jacks

I believe in using jacks which are stronger than you need. Some people like mechanical jacks and I know that some mechanical jacks function very well. Personally I prefer hydraulic jacks, probably because I have used them more than I have used mechanical jacks. Also, another prejudice of mine is in favour of the big old 100-ton hydraulic jacks. All the U.S. Navy yards have them and most of the heavy construction contractors who do subway or tunnel work have them and use them off and on continuously for years.

Never try to jack a concrete pipeline with jacks that are not strong enough. Also, it is very important to line the jacks up accurately using lines and levels. Thus, the jacking process will be absolutely straight and in the direction in which you want It to go. If the jacks are not lined up properly the for be will be on a skew, and the line will jam.

General Considerations

One of the main considerations in jacking a pipeline to keep going after you have started. A line will 'freeze' if left too long without working. This is especially true in the case of wet weather or where the line goes under a railway track subject to continual vibration and pounding. After a line 'freezes' it is many times extremely difficult or actually impossible to start it going again. In jacking operations, I generally work two shifts and sometimes three shifts and plan never to shut down operations for rain or holidays until the project is completed.

Prepare for rain before you start by laying a concrete foundation, building a sump and installing a sump pump in the jacking pit. If the jacking pit gets fouled up with mud and water you may be forced temporarily to discontinue operations with the result that your line 'freezes' and sometimes it freezes so tight you can never get it going again. A tarpaulin over the jacking pit to keep the rain out and to keep the workmen dry is a good investment.

In many soils, it is possible to excavate ahead of the heading. Sometimes you can excavate ahead only a few inches. At other times you can go ahead 6 or 8 feet prior to jacking. If the soil is soft and wet it may be more desirable to push the pipeline through the soil and then remove the soil after it has entered. The type of soil will determine many procedures on a jacking project.

It is also important to prevent erosion around the entrance where the line enters the soil. For instance, in jacking under a railroad track you might first dig down to form a vertical heading but always protect the area above the heading in one way or another so that you prevent rain from bringing material in on top of you.

Concluding Statement

The main considerations in a jacking project are first, take good borings and examine them carefully so you know what you are liable to run into. Next, use pipe large enough to get a man inside so that he can work as freely as possible. Set the backstop up properly so it will take the necessary thrust without failure. Make a jacking frame to carry the push of the jacks into the pipeline and have the jacks in line so the push is not wasted and so that the pipeline does not bind. Count on getting rain and be prepared for it by concreting the pit and providing sumps, pumps and tarpaulin protection. Use heavier jacks than you think necessary and heavier timbers than deemed necessary. Make complete preparations at the start so that when jacking commences you can keep on going until completion.

Using these few and common-sense precautions, concrete pipelines can be jacked surprisingly long distances under highways, railways, city streets and buildings at remarkably low cost.