The report of the study of the tunnel of the intersection of Africa Street with Mamoun El Behairy Street - Afra tunnel
Urban Development Initiative
Study the tunnel of the intersection of Africa Street With Mamoun Al Behairy Street (Afra Tunnel)
Engineering report for field visit and technical notes on works Ongoing maintenance, required emergency treatments and recommendations
Document number: NS-REP-TEC-19-01
Date: November 6, 2019
The tunnel of Africa Street intersection with Mamoun Al Beheiri Street (known as Afra Tunnel) is the most important project for infrastructure development being the first module for a tunnel made as a solution for traffic in Khartoum as a leading project for the development of roads network in the state, therefore, the importance of this study in the maintenance of this experience is the development and push towards the localization of expertise to expand in other infrastructure projects, taking advantage of the lessons learned and practical experience of the successes and failures made in this project .
This study includes the collection of basic data on the current maintenance of the tunnel, which was done through a field visit to the site, and identify the maintenance work that has been completed and planned work in the next stages and evaluation, to provide technical observations, recommendations and proposed treatments.
2- General engineering description of the tunnel
The Afra Tunnel Project has had many technical problems since its inception, which resulted from several causes, the most important of which is the poor design from the technical point of view for not applying the appropriate design standards, in addition to the weak implementation specifications from the structural point of view. , non smooth deviations in the structural parts, road path, entry and exit slots, turn-back paths, as well as primitive bridge design.
Figure 1: Examples of tunnel design defects according to the current design
These disadvantages can be seen from the comparison of the tunnel shape and dimensions with existing tunnels in developed countries or Gulf countries that follow high design specifications.
Figure 2: Model for a standard design of a tunnel and a crossover traffic signal
This design weakness was accompanied by significant problems in the distribution of traffic with the upper signal, which did not work properly, especially after the work of many untreated processors that worsened the situation, and led in the end to prevent many turns, which is undesirable as a solution, because it weakens One of the basic benefits of tunnel work is to serve as many cars as possible and reduce the delay and distance to cross the intersection, which was not achieved in this project despite the relatively high budget of the project, as this reflects the seriousness of technical corruption in the country and its significant negative effects on the country along with financial corruption that has been rampant over the past decades.
The design problems and poor implementation have resulted in several problems, the last of which has emerged during the past months when a leak occurred in the water lines near the tunnel area, which led to the infiltration of soil under the tunnel, causing the possibility of collapses requiring urgent maintenance work, where the design of the tunnel did not take into account the discharge of Surface water and groundwater, which is observed every year with the rainy season, where the accumulated water is discharged by the primitive methods such as suction tank, rather than the application of ground trap systems with lift pumps to the level of the drainage system or the bottom drainage system Wells
3- Field visit
Engineer Mohammed Abdul Qader Kadam made a field visit to the site of maintenance of the tunnel on Tuesday, 5 November 2019, where he collected technical information from the team at the site, in addition to preview and take notes and pictures, including the following:
- Determine the description of the previous problems and their causes.
- Know the details of the proposed design.
- Identify the work done so far.
- Identify remaining work.
- Identify the ones in charge of designing, executing and supervising the maintenance.
- Assess the work performed and the extent to which it can successfully address current problems.
The following pictures show the location of the work and the current work.
4- Technical notes on the current maintenance work in the tunnel
It was clear from the field visit that the current maintenance work is being carried out by the Ministry of Infrastructure, and the following observations were found:
- The lack of a supervisory authority on the site, which causes the absence of a mechanism to control and receive the completed work properly to ensure its conformity with the approved work plan, although there are technical and administrative differences between engineers on site.
- Work is currently underway in the western side of the tunnel only to allow vehicles to use the opposite direction of movement.
- A review of the timetable for the work accomplished so far revealed a slow progress in the progress of work despite the availability of mechanisms and workers on site.
- The presence of a number of different contractors to carry out tasks in a heterogeneous manner.
- In spite of Test results that has proven that the leak's source is filtered water processed with Chlorine, The Water association has not yet declared the reason of the leakage or the possibility of it happening in the future.
- From the inspection the solution that has been made has indeed stopped the water leakage into the tunnel.
- A 4-inch drainage network has been made over 40 cm of broken stone to collect water that may enter into the tunnel It will be connected in the future with the network that will work in the east, to be able to pass more than 5 liters per second 300 liters per minute knowing that the pipes are placed in the center of the tunnel and perpendicular to it from the west To the east , because the drainage method is the gravity flow at the lowest point.
- The drainage layer is followed by a polyethylene sheets. The purpose of this layer, according to the designer's statement, is to prevent the water inside the drainage layer from reaching the top layers. This error is incorrect. This step is totally unsuccessful for the following reasons:
1.Given the nature of the design, it is illogical to talk about water pressure downward with loads like the road will be exposed by a polyethylene layer of a maximum thickness of 1 or 2 mm, as this layer will prevent any water from a surface source such as rainwater or other access to the drainage layer in case they leak into the back fill before they reach the surface drainage network that we'll talk about later.
2.Given the method of implementation, on the one hand, although the welding of polyethylene sheets was made by adhesive in areas and by flame in other places, the two methods have not proved useful, and there is no effective way to weld the sides with concrete walls, although the sheets are sufficient for the upper range of 30 To 40 cm.
3.Also, although the engineer responsible for the implementation of the landfills showed high experience and skill in the management of mechanisms in a way that does not harm the layer of protection, but the probability of damaging the protection is very large, least sliding plates and winding during the process of dispersing tippers.
- Work is currently underway in the base layer and despite the quality of the materials used (virtual inspection) where mixed with broken stone and sand kangar, but it was noted that the project engineers did not complain about the absence of standard laboratory tests and random mixing of materials and amounts, as this layer will be 20 Cm, followed by reinforced concrete slab.
- Structural joints will be made in the concrete slab, and an asphalt layer will be made over it, but there is a dispute over the thickness of the desired layer, where there are two views: a layer with a thickness of 5 or a thickness of 15 cm.
- The work of the Trench stream in the middle of the tunnel from east to west has been proposed to collect surface water, but the lid of this stream is expected to cause a design problem.
5-Technical recommendations and suggested Emergency treatments
In order to address the negative observations identified in this report, the following points are recommended to ensure the successful maintenance of the tunnel and to avoid failure that may cause significant material and human losses in the future.
- Suspend any further work on the project until the design review and agreement on a well thought-out work plan and design, taking into account all the observations mentioned in this report.
- Organizing the structure of the Ministry's working group and allocating responsibilities.
- Assigning an office or consultant to supervise the execution of works on site.
- Organize the work of contractors, and follow them directly to the site engineer to coordinate the various work to one side.
- Reassess the feasibility of the insulation layer because there are high risks in preventing the discharge of surface water, and only the geotextile layer to prevent the leakage of soft soil to the broken stone layer and avoid clogging drainage holes in the pipes.
- Make a water collecting room connected to an 8-inch pipe if the proposed Trench is not needed in the middle of the surface to collect surface water.
- To seek the help of experts and to take the opinion of more than one consultant to verify the correctness of the engineering treatments followed in the maintenance work.
- Conducting a comprehensive traffic study at the intersection of Africa Street with Mamoun El Beheiry Street above the tunnel to address the design defects at the intersection, adjust the times and phases of the traffic signal, and improve the flow of traffic with this important intersection.
- Establishment of Gantry metal girders before the tunnel entrances enough distance from both sides to prevent heavy trucks from entering the tunnel.
In conclusion, the proposed design of maintenance work in its current form will cause the repetition of previous problems in the future, with the possibility of new problems, and the design of the current problem is not feasible in view of the financial value of Money Value where we found that it has been allocated with a large budget compared to the volume of work, However, this is not reflected in the quality of the design and the effectiveness of the solution, which may be considered a waste of public money in the event of proven failure of these processors in the future as we expected technically.