Moles are non-steerable devices. This means that it is not possible to change the direction of operation LEFT / RIGHT and UP / DOWN of the machine during thrusting. It is only possible to change the direction of REAR / FORWARD, through the valve in the oiler. However, the adjustment of the machine's course itself is only possible at the beginning of the piercing being carried out, before the whole machine disappears into the ground. How you set it at the beginning will determine the accuracy of the piercing being performed.
One of the most important components of a mole, which has a direct impact on making boreholes accuracy, is the head, or more precisely, its shape. Moles that have a stepped head perform best:
This head shape minimizes the possibility of changing the machine's track upon encountering hard obstacles. The obstacle gets broken up by consecutively increasing head diameters until the machine has paved its way in the hard ground throughout the entire diameter.
A mole equipped with another type of head - a conical head - shows a slightly higher speed in making boreholes. However, it loses significantly in accuracy. And it is accuracy that is the key feature of moles. An inaccurate piercing, especially in the vertical axis, can result in unplanned high costs due to damage to the pavement under which the piercing is carried out. A mole with a conical head, having encountered an obstacle, usually slides over it, changing the direction initially set for it. Changing the angle of the mole's track by just 1o, causes a deviation of as much as 20 cm from the planned exit point of the machine, over a distance of 10 meters of piercing. The cone head is good only in places where we are dealing with uniform soil, with the same degree of compaction regardless of depth, without inclusions of hard structures. However, such soil conditions are difficult to find, so it is worthwhile, for safety reasons, to always use machines equipped with a stepped head. All our machines have such heads. On special request we can make conical heads, according to individual expectations, but we definitely do not recommend this solution.
In the following pictures we show how the shape of the head affects the accuracy of making boreholes. In the first picture, we see how the mole copes when encountering a hard obstacle. The impact energy of the machine is high enough that it is able to crush such an obstacle. For example, the MAX K130S has a single impact energy of more than 400 J. Such a value corresponds to the situation if we dropped a 20 kg weight from a height of 2 meters.
In the next illustration, we show how a mole with a stepped head deals with encountering a hard obstacle at its extreme point. Segment by segment, diameter by diameter such a head will push this obstacle until it is completely removed from the intended piercing corridor.
And in the last illustration we show what happens in such a case, when our mole is equipped with a conical head, or when the stepped head is so worn/gripped that it can be compared to a conical head. Through its conical shape, the machine will succumb to the obstacle, change its initial direction and continue already following an unexpected trajectory.
In the best situation, when a mole runs away downward or in some horizontal direction, you will be forced to find it somewhere near the intended destination. Worse, if a mole runs away upward, as a result of encountering a hard obstacle. As you reach a depth above the safe depth for making boreholes (which depends on the diameter of the mole and the degree of soil compaction), the surface under which you are working will lift and a hump will form on the road or sidewalk. The cost of repairing such damage to the surface can far exceed your income from the installation being carried out.
The situation is similar when making boreholes with a mole, the head of which shows significant wear. Such a head is shown below.
In that case, it is really worth investing in a new head.
Other important parameters are the correct ratio between the machine's diameter, length and weight, and the resulting position of the center of gravity. These parameters are optimized as early as the design phase, and then verified during testing of the prototype machine. In our moles, during design, we adopted the principle that the mole's center of gravity (CG) should be located around half the mole's length, but at the same time closer to the mole's head than the mole's center of pressure (CP). The location of the center of gravity of the mole (CG) around half of its length, positively influences on maintaining the set trajectory during piercing, eliminates the influence of the weight of the machine on the potential possibility of deviation from the set course. Such mutual positioning of these centers (CG in relation to CP) is aimed at maintaining a constant preset trajectory during making boreholes, and the automatic return of the mole to the preset, proper trajectory, especially in the case of variable and difficult ground conditions, where we are dealing with different density, moisture content and different types of soil. In addition, the distance itself between the indicated centers is important.
This principle, unfortunately, cannot be applied to the so-called short versions of moles. In these devices, unfortunately, due to the need to provide them with sufficient energy and frequency of operation, the mole's center of gravity (CG) is located at a greater distance from the mole's head than the mole's center of pressure (CP). This unfortunately results in the mole's tendency, after breaking out of its preset trajectory, to plunge further in the direction of that deviation instead of returning back to the trajectory set at the beginning. Therefore, wherever field conditions permit, we suggest using standard LONG moles. Of course, this is not to say that short moles absolutely do not do well when it comes to accuracy. Their accuracy will be satisfactory at short distances, from 10 to 20 meters, depending on ground conditions. Let's remember what was mentioned earlier: a change in the angle of the mole's track by just 1o, results in a deviation of as much as 20 cm from the planned exit point of the machine, over a distance of 10 meters of piercing. The greater the distance of making boreholes, if the mole deviates (due to ground conditions), the deviation will increase in proportion to the distance of the piercing.
The accuracy of making boreholes is also strongly influenced by the operator himself, through the correct positioning of the mole in the beginning of the piercing, and any further corrections before it sinks completely into the ground. Details can be found in the document - Manual for optical set.
We must remember that when aiming, the Optical Set should be in a perfectly vertical position on the mole. For this purpose, we use the vial (level) (1) located in the front part of the Optical Set housing (5):
After setting up the Optical Set on the mole, first ensure that the Vial (1) is pointing vertically. Only after this condition is met should the mole be set so that the Target Cross (2) is at the correct height of the Optical Set Pole (3).
The accuracy of the moles also allows you to perform sewer installations, so-called gravity.
Finally, here is what our experience looks like when working with a mole in soils of different types:
|
SOIL TYPE |
BEHAVIOR OF THE MOLE |
|
Peat |
It can flee downward due to its mass. The low friction against the ground causes slow completion of the piercing. The ground collapses directly behind the mole. |
|
Silt / sludge |
It can flee downward due to its mass. The low friction against the ground causes slow completion of the piercing. The ground collapses directly behind the mole. |
|
Soft clay |
Be careful when aiming so that the mole does not run away in a downward direction. |
|
Hard clay |
Mole will tend to run upward. |
|
Dry sand |
The mole should maintain the straight direction of the route. It is recommended to pull the pipe directly at once, due to the backfill behind the mole. |
|
Wet sand |
It can flee downward due to its mass. The low friction against the ground causes the piercing to be slow. The amount of air supply should be reduced. |
|
Wet / dry gravel |
Large fractions can affect the mole's deviation from the set course, especially with a conical head. Also, the presence of hollow areas or water pockets can affect the direction of the mole. |
It is worth mentioning that, with our MAX K130S, we won 1st place in the First Official International Jacking Competition, the so-called MOLE’S RODEO. It was accuracy that determined our success.
And our numerous customers consider our machines to be the most accurate among those available on the market. Our customers report to us from time to time about accurately executed thrusting, at distances exceeding even 40 meters. Most recently, NALTEL gave us information about an accurately executed piercing at a distance of 43 meters, using a MAX K180S mole, the heaviest machine in our mole range.
Krzysztof Dydel