Drainage Contractor

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Of soils and plows: soil mechanics fundamentals

Back to the basics of soil mechanics.

May 2, 2019
By Gordon Spoor foreword by Peter Darbishire

Modern plow points and leading edges are wide and have a shallow approach angle. Photo courtesy of Peter Darbishire.

It’s been 40 years, but the theory hasn’t altered. What has changed is the advancement in drainage technology and a machine’s capacity to install drainage pipe. A new generation of business owners and machine operators bring new ideas, but the fundamentals remain.

In the late 1970s, I enlisted the help of an expert to explain soil mechanics theory and practice as they apply to drainage contractors. Gordon Spoor was head of soil mechanics at Silsoe College, near Bedford, U.K. He penned several articles about soils and presented this knowledge to contractors who attended the Drainage Contractor Workshop in Indianapolis in the 1978-80 period.

At the time, one issue was in certain conditions, trenchless drainage plows could cause smearing of trench walls at and above drain depth. As a result, trenchless drainage was banned in parts of the Netherlands, where research was conducted on the problem. This was, incidentally, also used by some trencher manufacturers as a reason that other contractors should not consider trenchless machines.

This article, originally published in the 1977 winter edition of Drainage Contractor, explains how although smearing can be a problem, it can be managed with proper attention to soil conditions and the design of the plow point. Plow designs at the time tended to have a V-shaped frontal profile, with a 45-degree leading edge, rather than one that was flat and of shallower angle.

Critical depth explained

One of the objectives in pipe installation is to bed the pipe well and surround it with permeable material. The backfill should be relatively loose and the walls of the slot or trench should be fissured and loosened rather than compacted. Whether the slot wall is fissured or compacted depends upon how the trenchless plow moves the soil under the prevailing conditions.

The plow can disturb the soil in two ways, depending on its working depth and the soil conditions. A four-inch wide plow working relatively shallow (28 to 30 inches), in dry conditions will disturb the soil, as shown in Figure 1. Soil is broken loose right from the base of the trench, heaving forwards and upwards, and falling back around the pipe as the plow moves on. This type of disturbance creates cracks and fissures without compaction.

The same plow working at a much greater depth (5.5 feet) in the same dry soil will tend to move the soil as shown in Figure 2. Disturbance near the surface will be similar to that in Figure 1, but at depth, in the vicinity of the pipe, the soil is compressed sideways and this results in compaction and result in a reduction of soil permeability.

Between these two extremes of working depth there is a certain depth, termed the critical depth, where the transition between one type of disturbance and the other occurs. This critical depth represents the maximum working depth of the plow if compaction around the pipe is to be avoided.

The position of the critical depth depends on these factors: forward inclination of the plow point and leading edge: a shallower angle is better; width of the plow point: a wider the plow point, the deeper the critical depth; soil moisture content; and soil density.

The soil conditions near the working depth and in the surface layers have the greatest influence on the position of the critical depth for a given plow. How wet, loose and soft a soil is at working depth will increase the chance of causing compaction and smearing. Also, the drier, denser and stronger the surface soil is, and particularly if the soil is moist at working depth, the shallower the critical depth will be. In this case, the surface layers are preventing the lower layers from heaving upwards, so sideways compaction occurs.

(Top) Soil disturbance caused by trenchless plow working relatively shallow (28 in.) versus (Bottom) soil disturbance caused by trenchless plow working relatively deep (5 1/2 ft.).

Practical thinking

It would be useful to predict the position of the critical depth before starting the job. However, the number of variables involved make this impracticable. The only satisfactory way to determine the critical depth is to excavate a trench across the line of pull and observe the soil disturbance.

Here are some practical figures to use: when working in uniform, compact dry seedbed producing conditions, the critical depth will be at a depth approximately 15 times the width of the plow point/leading edge (for a four-inch plow, the critical depth is about five feet). As the surface layers become very dry and strong or the soil at depth becomes moister, the critical depth will reduce to 10 times plow point/leading edge width (about 40 inches) or less. As the soil becomes wetter and more plastic, the critical depth becomes shallower.

There are two changes that can be made to deepen the critical depth: either the plow point and leading edge can be made wider, or the surface layers above the proposed line can be loosened by chiselling the soil or pre-ripping at partial depth before the pipe is installed to grade. This should be done to a depth of between eight and 16 inches to disturb the surface soil in an area, with an approximate width of 1.5 times the pipe depth. Pre-ripping will also reduce the pulling force required for the pipe installation pass, perhaps by as much as half. This might be done on the return pass from one line to the next.

If the top soil is frozen, fracturing the frozen layer would certainly relieve any surcharge on the lower layers allowing them to fail upwards more readily. Any benefit would depend on how and where the frozen layer cracks ahead of the advancing plow. If the leading share of the plow is well under the frozen soil before it cracks, and if the crack is only ahead of the share, cracking a wider surface width beforehand would be of significant benefit in relieving the surcharge.

Increasing plow point width and surface loosening will not lower the critical depth below working depth under all soil conditions. This is particularly the case with firm plastic soil conditions at a depth where compaction and smearing can have a very serious effect on drain performance. Wherever possible, pipe laying should not be carried out under these conditions. Even so, there have been reports of working well below critical depth in very soft, loose saturated clays, with little evidence of structure damage, smear or compaction at depth. The reason for this would be due to the high moisture content, the soil particles would be too widely spaced to form a significant smeared layer. In such a situation, it would be important that the first soil to fall onto the pipe is dry, well-structured surface soil, rather than the wet soil from depth. This situation would certainly be one for opening up a trench across the line of pull to check the resulting soil disturbance before starting work.

It is impossible to predict the position of the critical depth for all soil conditions, but it is important to recognize that its position can have a serious impact on the success or failure of a drainage system installed with trenchless machinery. Casual observation of surface soil heave is not a good indicator of what is happening at depth. It is well worth examining the disturbance produced by excavating a cross section and modifying techniques to minimize deep compaction and smearing.

Editor’s note: The original material for this article is courtesy of retired soil scientist Gordon Spoor. Peter Darbishire, editor of Drainage Contractor from 1976 to 2008, provided the foreword.