The bridge over Boho river
The project of the bridge over Boho river, in which Eng. M. Yuger served as a soil consultant on behalf of the contractor Sollel-Bona, was intended for the railway line from Ashkelon to Be'er Sheva and in accordance with the constraints of preserving the environment and the complex infrastructure for a railway bridge, and was a unique project in Israel.
The bridge is located above Boho river, west of the city of Netivot, an environmentally sensitive area and for the purpose of building the bridge, it was necessary to carry out a strict and gentle process in regards to the preservation of nature.
The project
The planning began in November 2010 and the execution began in February 2011 and ended in October 2012 - the executing contractor was "Sollel Bona" and the land consultant on behalf of the contractor: Eng. Moti Yuger.
The bridge includes eight equal spans 32.5 meters long, 12 meters wide and 260 meters long. The bridge spans have a hollow circular section with a diameter of 4 meters and reaches a height of about 20 meters. The upper structure of the bridge consists of two massive prefabricated beams 2.2 m high, weighing about 200 tons each, cast on site and hoisted on top of the bridge girders. After placing the beams, a 30 cm thick bearing plate is cast. The bridge is a continuous bridge with seams in the end commissioners and two internal commissioners, where some of the commissioners are harnessed to the lower structure. In the end commissioners and some of the internal commissioners, special EKH-type authorizations were assembled. These braces prevent displacements across the bridge and thereby also share the unharnessed commissioners with the journey in receiving the horizontal forces obtained from loads, such as centrifugal loads, wind loads and earthquakes. In the longitudinal direction of the bridge, the supports allow the movements in order not to receive large forces due to loads such as temperature, shrinkage and creep. The documents were designed to receive horizontal forces of up to 200 tons. A detailed interaction calculation (bridge rail) was made for the bridge using MIDAS CIVIL software in accordance with the European standard EN 1992-2.
Quality control during execution
During the casting of the prefabricated beams, a discrepancy was opened by the contractor's quality control regarding the strength of the concrete that came from one of the mixers for casting the beam. Two separate tests were conducted by two different test institutes. According to the results of Institute No. 1, a compressive strength was obtained at the age of 28 days of 66 MPa compared to 5.30 MPa according to the results of the second Institute. At this point it was decided to wait for the concrete strength results at the age of 50 days to verify the results. Upon receiving results after 50 days, it was decided to conduct an in-depth examination to clarify the issue.
The actions taken:
The instruction to remove cylinders in suspicious places in order to be sure that there is concrete of low strength in a certain area of the beam. For this purpose, a typical beam casting was followed in order to discover the location of the defective mixer.
A meeting with the concrete technologist to clarify the issue.
Schmidt hammer test, which is a test that allows you to get an idea about the differences in the density of the concrete in different areas. The test does not give the strength of the concrete but only indicates changes in its uniformity. As mentioned, in order to give instructions for removing cylinders to test the strength of the hardened concrete in the places where the low-strength concrete was poured, a basic sketch was made, based on the order of casting the beam, for the location where the weak concrete is suspected. The execution of this sketch was possible only thanks to the execution engineer who documented in an orderly manner the order and method of casting that were actually carried out.
The results of the rolls obtained definitely showed a decrease in the strength of the concrete:
Rolls No. 1,2,3 in the area of the weak concrete showed the strength of the concrete lower than planned - Rolls No. 4,5,6 in the normal area showed the strength of the concrete as planned.
At this stage, an in-depth examination was made into the way the problem was handled and after additional tests, consultations with the client of the work and the project manager, it was decided to reinforce the beam in order to qualify it for full function as planned.
Despite many pressures to approve the beam because there was a laboratory test that proved that the strength of the concrete is 60 and the concrete technologist's claim that the concrete that arrived at the site is definitely of the 60 type, it was decided to carry out an in-depth examination of the issue in order to eliminate the smallest possibility that there is concrete with a lower strength than planned. It should be noted that the inspection could not have been done without full cooperation on the part of the contractor and on behalf of the quality control on the part of the contractor. Thanks to the in-depth inspection that was done, a concrete beam with a lower strength than planned was discovered and corrective measures were taken.
Published for the first time in the newspaper of the Union of Construction and Infrastructure Engineers, Issue No. 66