Pilot pipe jacking is accepted worldwide as a method for trenchlessconstruction of sewers. This extremely economic machine and process technologybecame more and more established during the last years, especially for theconstruction of small sewers.
In the beginning of 2005, Herrenknecht and Bohrtec sold a pilot jackingmachine of the type BM 400 in Johannesburg, South Africa, where it wassuccessfully used for an extremely difficult project. A little later, but stillin the first quarter of that year, additional machines were sold in Tanzania.The following report discusses the advantages of this simple method, even whenused for a non-common application in the capital of Tanzania, Dar Es Salaam.
The Dar Es Salaam Water Supply And Sanitation Project has the aim toguarantee a reliable, economically bearable and effective water supply to thepopulation of Dar Es Salaam and to improve the sewage system of the catchmentarea of the Dar Es Salaam Water and Sewerage Authority (DAWASA), covering theCity of Dar Es Salaam, as well as a part of the coastal area. In the long run,this shall improve the hygienic conditions and thereby the population’s healthin an area that is still known for cholera epidemics and other diseases causedby bad water quality.
The project, totalling $161 million and subdivided into five differentindividual projects, was partly financed by the World Bank and the AfricanDevelopment Bank. One of these individual projects is the reconstruction ofsewers, with a total length of 28,180 m and nominal widths of DN 200, DN 250, DN315, DN 400 and DN 550 mm.
In November 2004, the first conversations were held between DAWASA, theChinese contractor, China Electric Power Technology Import & Export (CETIC)and Herrenknecht and Bohrtec about execution possibilities with regard tomachine and method.
Due to the ground conditions onsite (sand and low-cohesive grounds with lowcompactness) and the reach lengths of a maximum of 80 m, the pilot pipe jackingmethod was preferred to the conventional microtunnelling method from thebeginning. Alternatives were developed by the machine manufacturer in closecoordination with CETIC with regard to the nominal widths. Due to the crampedspace conditions, the diameter ranges could be covered by the pilot jackingmachines BM 400 and BM 500. With the BM 400 — minimum required launch shaftdiameter 2 m — pipe up to an OD of 620 mm and with BM 500 — required launchshaft diameter 3.2 m — pipe up to an OD of 1,020 mm could be jacked. Howeverthere was a substantial need for discussion due to DAWASA’s request to installexclusively low-cost plastic pipe.
The basic procedure of the classic pilot pipe method provides for a guidedpilot jacking in the first step, the reamer drilling in the second step, withthe pushing in of the product pipes in the third step. In larger nominal widths(approximately from DN 400 and larger), this technology is changed to the effectthat after the first reamer drilling with the steel casings, a second reamerdrilling with a counter-clockwise conveyance of the ground into the target shaftand simultaneous jacking of the product pipe is performed. Both procedureoptions are characterized by the jacking of the product pipe in the last step.Due to the insufficient allowable jacking forces, the use of plastic pipe cannotbe considered. The jacking forces resulting from the jacket friction in reachlengths of up to 80 m would be too strong.
For this project, Bohrtec first recommended to DAWASA that it use“conventional” jacking pipe (pipe materials: polymer concrete pipe, clay pipe,glass-fiber reinforced pipe (GRP pipe)) for all nominal widths. Due to thecustomer’s request for the use of plastic pipes, however, an alternative methodwas developed for the smaller nominal widths.
In the first step, the guided pilot jacking is performed. In the second step,a drilling with steel casings is performed, in which the casings are geared tothe pilot rods. Afterward, the conveyor augers are pulled and the steel casingsare cleaned. In the last step, the product pipes are inserted into the steelcasings and finally the steel casings are pulled out while holding the plasticpipe at the same time.
This procedure option ensures a minimum stress of the plastic pipe. Due toBohrtec’s experience with the scope of dead-end drillings — this method iswidely used in the field of house connections and also provides for pushingproduct pipe into the steel casings and pulling out the steel casings in thelast procedure step.
A conclusive concept was quickly presented to DAWASA. Among other things, thesteel pipe diameter was optimized with regard to the outside diameter of theproduct pipe. Practical and simple detail solutions were developed to secure theproduct pipe during pullback of the steel casings.
The above-described method could not be used for all diameters. On one hand,the steel casings for the large diameters (up to 660 mm OD) would have been tooexpensive and on the other hand, the retraction forces of the machines providedfor the use in Dar Es Salaam for steel casings with an outside diameter of 660mm and reach lengths of up to 80 m would not have been sufficient withoutspecial additional measures. On this account, it was agreed to use DN 400 and DN550 GRP jacking pipes as jacking pipe. The corresponding outside diametersamounted to 470 and 662 mm, respectively. For these jacking pipes, theabove-described method is used, in which the product pipe is jacked in the laststep, while the ground loosened at the face is transported through the steelcasings of the first reamer drilling in direction of the target shaft.
A procedural concept was quickly developed by the company Firma Bohrtec andthe syndicate Bohrtec/Herrenknecht, which received in February 2005 the orderfor delivery of two BM 400 machines and one BM 500. In early May 2005, the threemachines left the factory in Alsdorf, Germany.
In late July 2005, training began at the Dar Es Salaam jobsite. Two Bohrtecmachine drivers remained onsite for three months in order to make use of allthree machines in a minimum of time and within the scope of a training programdetermined in cooperation with CETIC and DAWASA.
As a result of CETIC’s good jobsite preparation, the machine could beimmediately installed in the first launch shaft after acceptance of the fullcomplement of equipment. The first jacking length of 35 m was completed in onlythree days, including pulling of the steel casings. A brilliant achievement, ifyou take into consideration that several machine drivers and workers wereintensely trained during this drilling.
Intensive preparation for the complete project, including the simulation ofpossible problem scenarios, led to a smooth performance of the first drilling.Afterward, the machine was repositioned and the second jacking length of 35 mwas also completed in only three days.
At the beginning of the onsite training, there was only one BM 400 in use.After the first two drillings, the CETIC personnel were able to perform jackingswith both BM 400 machines on their own. The Bohrtec trainers were only on handto survey and assist if needed. After approximately four weeks, the BM 500machine came into operation. Since that time, the work has been performed withall three machines.
More than 70 jacking lengths of the nominal widths wereachieved. DN 200 to DN 550 mm were completed, despite extremely difficultmarginal conditions like unknown positions of crossing service pipes, hightraffic volume, etc.
Even under extremely difficult and marginal conditions in a central Africancountry, the use of this progressive underground technology was quitesuccessful. This was a tribute to the robust machines and technology, as well asof the professional approach to the planning of the contract by (DAWASA), theexecution by (CETIC) and the training by Bohrtec.
Dr.-Ing. Gregor Nieder is managing director at Bohrtec GmbH, Aldorf,Germany.