Despite the well-known advantages of the trenchless technologies for installation of pipes, the companies implementing this method commonly face many challenges because of the on site geological conditions. It was exactly such an interesting case that Raicommerce Construction JSC engineering team faced.
Description of the task
The company has been consulted in relation with an unsuccessful attempt for trenchless installation of Ø 1020 / 12 mm guide steel tube for Ø 800 mm sewer of fibre glass reinforced plastic, 36 m long and crossing below two asphalt roads. The construction and assembly works on site were interrupted and these works had been started by a company of similar scope of activity. This company used technology of pushing steel tube using screw machine in order to perform the task. The reason for interruption of the construction works was the considerable collapse found of the front end of the pushed guide steel tube which at 20 m from the start of the inlet shaft was approximately 38 cm – inadmissible lower level from the design elevations as a result of which the installed tube became useless. The review of the available design documentation and preliminary visual inspection on site showed that the layout of the newly designed tube fell partially in a sand stratum in high underground water conditions. The origin of the underground water was result of the immediate vicinity of the site to Tundzha river. The most probable reason for the collapse of the tube installed according to the above technology was the water saturated sand stratum through which the installed tube run.
Selection of suitable technology
On the base of projects given by the Employer, taking into consideration of the unfavourable geological conditions and unsuccessfully implemented screw technology, the engineering staff of Raicommerce Construction JSC performed thorough analysis of the possible solutions for the execution of the assigned task. The results showed that none of the available trenchless methods applied separately was suitable for installation of tube of such large diameters in such unfavourable geological conditions. The specific conditions required to find other non-standard solution. Facing this challenge, the working team selected the option of experimental (for the first time in this country) implementation of two trenchless technologies in parallel:
- Controlled horizontal drilling (HDD);
- Horizontal pushing open end steep tube using hydraulic press.
The designed method included two steps:
1) Independent implementation of the first technology in order to perform guide pilot drilling.
2) Implementation of both technologies in parallel during the installation of the steel guide tube itself in order to provide the required pushing force and accuracy.In order to ensure the execution of the task according to the suggested method, the working process was arranged in two stages
Performed pilot drilling according to the design elevations from inspection shaft 1 to inspection shaft 2 using HDD. Then, the pilot drill hole would be enlarged (reamed) in stages up to Ø 485 mm diameter taking into account that the drill lower part must remain below the underground water level and sand stratum. Figure 1 shows the execution scheme of stage I.
Installation of Ø 1020 / 12 mm guide steel tube using open end segments, 4 m long. The preparation of the segments (lowering and welding) has been performed in the specially adapted inlet hole in inspection shaft 2. The technology for installation of the tube included both pushing using hydraulic press located in inspection shaft 2 and drawing using HDD from inspection shaft 2 to inspection shaft 1. Another steel pipe of Ø 385 mm diameter had been welded on the front face of the first segment of the guide tube according to the details shown in Figure 2. It acted as a guide directing the drawing process with the HDD machine along the design enlarged horizontal drill. This accessory does not allow moving from the pilot drill and from the design layout inclination, respectively.
Due to the tube open front end, during the tube forward movement, the tube cut the remaining earth between reamed pilot drill Ø 485 and the Ø 1020 guide tube contour. The remaining earth had been collected inside the installed tube and had been removed regularly. Figure 2 shows the execution scheme for the second stage of the working process.
The designed method of combined use of two trenchless technologies by Raicommerce Construction JSC has been implemented successfully in order to overcome the unfavourable geological conditions on the site assigned to the company. Because of the site conditions, the implementation of this method gave remarkable good results. The guide steel pipe Ø 1020 x 12 mm, 34.5 m long has been installed with lower than 0.53 % deviations from the designed inclinations and the construction and assembly works have been completed for less than 10 working days.