During the cast-in-suit project, the segment casings play a crucial role in allowing the segments to be assembled into the complete tunnel ring. However, there are several factors that have to be taken into account when using the segment casings.
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Inspection of precast segments after demoulding
During the precast segment production process, various quality control measures are in place to ensure that segment casing are produced according to design specifications. These measures include regular inspection of concrete moulds and raw materials, testing of specimens and periodic visual checks of the surface of the segment.
The inspection of the surface of the precast segment may be carried out after demoulding. A qualified inspector will observe the appearance of the surface and look for any irregularities. Immediately, if a defect is found, it should be repaired. Otherwise, it should be rejected.
The inspector will also check the conformance of the segment to its post-concrete dimension and finish. The surfaces will be checked for any irregularities such as stains and holes.
The surface is then sprayed with an approved curing compound. It must harden within five hours. The surface is then covered with a polythene sheet to prevent drying. It is then transferred to the finishing area.
In the past, a large number of non-conformance cases were due to a poor surface of the precast segment. Hence, it is very important to conduct inspection on the segments after demoulding.
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Trial assembly of precast segments into a complete tunnel ring
During construction of a tunnel lining, the precast segments are assembled to form a ring. This process can save a lot of money in terms of lifecycle cost.
The quality of precast segments is controlled through a strict quality control system. Strictly following the manufacturing and assembly procedure can lead to a high quality segment. It is possible through better production tools and material selection.
The precast casing joint segments are produced in high-precision laser-cut moulds. These are then joined together using specialized connecting devices.
The segments are then transported to the site by transport cars. They are then wrapped with polythene sheets to prevent drying. A lifting socket is provided at the center of the segment to stabilize the soil around the tunnel.
The number of segments varies from four to ten, depending on the geometry of the tunnel. The trial assembly of precast segments into a complete tunnel ring is performed during the initial production stage. This is to ascertain the level of integration of the precast segments.
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