Análisis de resultados de las encuestas aplicadas a los docentes

7.1.1. Necessity of controlling trajectory deviations

It is essential for the success of a microtunneling project to follow the trajectory and minimize the deviations.

Firstly, when creating a gravity system, it is essential to fix a well-defined angle.

In addition, trajectory deviations brought about by the misalignment of the pipes and by the fact that the drive thrust is not centered on the axis of the pipes and the distribution of stresses is no more uniform on the section. For the same jacking thrust, the maximum stress applied on the section of the pipes will therefore increase. When calculating the acceptable thrust on the pipes, an angulation of 0.5°

between the pipes is generally taken into account by the manufacturers (see section 6.3). Thus it is necessary to minimize the angulations between pipes by ensuring continuous trajectory corrections.

Finally, sharp increases in thrust at the head can occur due to trajectory corrections; the contact stresses on the body of the machine during changes in direction become more significant (see paragraph 3.1.3.5).

Furthermore, it is noted that during stable excavations, the horizontal trajectory deviations were caused by an average increase of 50% in frictional forces (see paragraph 6.2.4.2.2). According to the studies carried out by Milligan and Norris

(1995), the amplitude of these increases in friction is linked to the curvature of horizontal deviations.

7.1.2. Guidelines for the measurement of deviations

The position of the machine is marked according to the planned course. The parameters measured are: vertical deviation (EV), horizontal deviation (EH), the angle (IV) and the azimuth (IH) of the axis of the machine with respect to the theoretical axis. The operator modifies the trajectory based on this data; it is thus essential that these measurements are precise and reliable.

Vertical and horizontal deviations are obtained using a laser beam. The laser is fixed in the starting shaft. It emits a beam in the direction set beforehand according to the desired trajectory. This beam is reflected onto a target located at the back of the machine body.

The laser must be fixed to a part of the shaft, which is not attached to the dead man. In fact, the latter supports the thrust frame, which can be subjected to movements during the implementation of the thrust cylinders. Even a slight modification of the inclination of the laser stand can cause significant differences at larger distances in the deviation readings. These deviations naturally increase with the length of the shaft; furthermore, the increase in drive thrusts at the end of the section can lead to significant movements of the thrust massif.

Thus, experimental data has shown that from 80 m onwards guidance problems inherent to these measurement inaccuracies appear: the amplitude of most of the deviations at the end of a section are between 10 and 30 mm, whereas half of them are less than 10 mm before 80 m.

Thus it is advisable to fix the laser stand at the base slab, which is less likely to move than the walls of the shaft on which the dead man is attached. Another alternative would be to fix it on a support attached to the surface of the ground and independent from the walls of the shafts.

Some lasers have an automatic vertical leveling based on the concept of a plumb line, thus compensating for the vertical movement of the laser stand. As regards its positioning in the horizontal plane, a continuous adjustment is required.

7.1.3. Guidelines for the monitoring of deviations

Experimental data helped provide some explanations on the problems of trajectory deviations, which could be noted and thus draw up recommendations for

entry ring is correctly aligned on the trajectory and does not lead to an initial misalignment of the pipes.

The difficulties caused by reworking of the ground near the walls of the shaft to retain a correct slope and direction of the boring machine can also be emphasized.

As a result, the entry of the boring machine into the ground is a tricky phase that requires frequent monitoring and adjustments. During this phase it is advised that the boring machine be guided using its body rather than its head rams.

7.1.3.2. Corrections during jacking

The National Microtunnels Project has shown that a sudden correction in the trajectory often led to an increase in deviations. The following recommendations can therefore be suggested:

– to minimize the amplitude of deviations, the operator should anticipate his action on the steering cylinders by taking into account the response time of the machine. In fact, as shown in paragraph 3.1.2.1, the body of the machine follows the angle imposed on the head by the steering cylinders only after several meters of jacking (an average of 4 m). While covering this distance, the boring machine continues to follow the previous direction, thereby creating a new trajectory deviation in the opposite direction (see Figure 7.1);

– successive small corrections should be preferred even though they are slow in restoring the theoretical trajectory rather than sudden corrections, which can lead to small radii of curvature and thus cause significant angulations between pipes (see Figure 7.1). More than the amplitude of the deviations, it is the angulations between pipes, which cause most problems (see paragraph 7.1.1). It is advised that an

angulation of 0.5° not be exceeded (see section 6.4). Furthermore, with small corrections even the amplitude of over-corrections mentioned earlier is reduced.

Figure 7.1. Trajectory followed by the boring machine according to the anticipation of corrections

It is to be noted that the PJA (Pipe Jacking Association) provides assistance in decision-making for manual control. The objective is to determine the possible corrections on the head rams by fixing a limit for the angulation between pipes (for example α = 0.1°). There is also software that helps automate the corrections made to the boring machine (Phelipot, 2000). Some are based on the concept of fuzzy logic, taking into account at each step the response of the machine during previous stages. Other software (Iseki and Decon-Soltau) relies on a database collected from different soils. However, according to Anheuser, automatic guidance is not necessarily the best solution (Anheuser, 1994).

7.1.3.3. Adjustment of the overcut

The value of the overcut also comes into play in facilitating the steering of the boring machine and thus reducing the deviations. A minimum overcut is in fact required to guide the boring machine, but it should not be too large, as it would make the corrections on the head cylinders less effective, the machine having no sufficient support on the walls of the excavation. In France, for boring machines having diameters between 600 and 1,200 mm, an overcut (difference in radius between the cutterhead and the pipes) of 20 to 30 mm is generally used.