Longyan Bridge is the first elevated bridge in Longyan urban area. It is not only a key bridge on the central axis of Longyan City, but also a landscape bridge. The main bridge has a total length of 340m and is designed as a 190+150m asymmetric single-tower double-cable-plane steel box girder cable-stayed bridge. The main tower is a "gem"-shaped bridge tower with a tower column height of 118m. The bridge crosses the Longjin River, two urban main roads, and two railway trunk lines.

Rendering of Longyan Bridge after completion

The main tower pile foundation of Longyan Bridge has poured 13 piles since the first pile foundation was poured on April 17, 2016. The main tower pile foundation of Longyan Bridge consists of 21 piles, with a pile diameter of 2.5m, a maximum pile length of 86m, and a maximum drilling depth of 97m. The long construction time and great construction difficulty of its pile foundation are rare in China.

As the saying goes, laymen watch the excitement, experts look for the know-how

People who don't know the specific situation will ask

Why does construction take so long?

Why is it slower than other bridge pile foundation construction?

Difficulty 1

Complex and narrow surrounding environment

Plan of main tower pile foundation of Longyan Bridge

The middle of the above figure is the location of the main tower cap, with a total of five rows of 21 piles (pile diameter 2.5m). The north side of the cap has partially intruded into the Longjin River, the east side is adjacent to the Longxia High-speed Railway, the south side encroaches on part of Luolong Road, and the west side is close to shops. The construction space is extremely narrow, and railway safety must be ensured (the closest distance to the railway is only 15m), the traffic on the south road must not be affected, and the environmental protection responsibility of the Longjin River is also shouldered. That is to say, apart from the main structure space, there is very little space around for construction.

Difficulty 2

Complex underground karst cave geology, rare in China

Section of main tower pile foundation

The circles in the figure indicate the locations and sizes of karst caves. As can be seen from the above figure, karst caves of various sizes are almost everywhere in the entire underground space. Pile No. 21 has the most karst caves, with a total of nine. The largest karst cave is the second karst cave of Pile No. 10, nearly 30m high. According to their vertical positions, these karst caves are called first-layer karst caves within 30m from the ground, second-layer karst caves in the range of 30m-60m, and third-layer karst caves in the range of 60m-100m.

The depth of the railway piles on the east side is about 40m, located between the first-layer karst caves and the second-layer karst caves. That is to say, the weight of the railway is all pressed on the roof of the second-layer karst caves. The piles of Longyan Bridge must all penetrate the second-layer karst caves to reach the top or bottom of the third-layer karst caves, and the roof of the second-layer karst caves must also be protected. The construction difficulty is extremely high.

The karst caves at the pile foundation location are extremely developed, not only numerous but also large in cavity size, and they are connected with the karst caves at the bottom of the adjacent Longjin River, forming an underground river. This situation is rare in China.

Difficulty 3

Hard rock strata, rock-socketed depth unique in China

Rock samples taken from underground

Roller cones worn out every day

△ Underground karst caves found during geological survey

The rock samples taken are mainly limestone and granite, either crescent-shaped in karst cave areas (extremely prone to collapse underground) or slightly weathered rock (extremely hard, with hardness up to 115 MPa). Basically, 6 to 10 alloy roller cone drill bits are used per meter. For large bridges in China, the rock-socketed depth is generally 4 to 5 times the design pile foundation. For a 2.5m pile, a rock-socketed depth of about 10m is sufficient. Because it needs to pass through karst caves, the rock-socketed depth of this bridge's pile foundation reaches about 70m, which is 7 to 8 times the normal rock-socketed depth and is unique in China.

Many difficulties, more solutions

Domestic first construction technology overcomes difficulties

Due to the special geological conditions of Longyan Bridge, the technical difficulty of its pile foundation construction is very high. Therefore, the builders of China Construction Bridge under the Sixth Bureau adopted the construction technology of staged drilling and staged lowering of steel casings. The construction technology is complex and is also the first of its kind in China.

Considering the extremely developed underground karst caves, we adopted the full casing follow-up method for construction. The casings were up to 6 layers at most, with diameters of 3.6m, 3.3m, 3.05m, 2.86m, 2.65m, and 2.42m.

Spare casings

First, the project department drilled with a large-diameter drill bit to the top of the first-layer karst cave, and then lowered a 3.6m or 3.3m casing to seal the first-layer karst cave and ensure safety inside the casing.

Then, a medium-sized drill bit was used to drill to the bottom of the second-layer karst cave, and a 2.86m or 3.05m casing was lowered to seal the second-layer karst cave.

Next, a drill bit not smaller than the design pile diameter was used to drill to the bottom of the third-layer karst cave, and a 2.65m casing was lowered.

Finally, a 2.5m drill bit was used to drill to the design elevation to form the hole. Considering that there are still some fissures at the bottom of the third-layer karst cave, the project department also prepared a 2.42m casing, which was套 on the reinforcement cage and lowered to the specified position. At this point, pile pouring could be carried out with confidence.

Workers are lowering the 2.65m casing

According to the site layout and geological environment, the project introduced two CRRC 460 rotary drilling rigs and one German Bauer 460 rotary drilling rig (both are leading in China and can construct rock-socketed pile foundations up to 120m deep).

Left: Bauer 460 rotary drilling rig; Right: CRRC 460 rotary drilling rig

Due to the need to lower casings of different diameters and handle different rock conditions, the project department prepared more than 30 types of drill bits, including drill bits of different diameters and drill bits for different functions (such as core drill bits, full-face drill bits, slag fishing drill bits, guide drill bits, etc.).

Various drill bits

The above technologies and equipment alone could not overcome the problem of super-high karst cavities. Once collapse occurs in the karst cave during drilling, it is very likely to jam the drill pipe and drill bit, making the drilling rig unable to move. Based on relevant domestic and foreign technologies, the project overcame difficulties and improved and designed 15m and 30m long roller cone barrel drills.

Awesome! Super-long barrel drill

With this super-long barrel drill, during drilling, both the upper and lower openings of the drill bit are in a safe geological environment outside the karst cave area, effectively preventing a series of problems caused by collapse.

At this point, the key difficulties of pile foundation construction were overcome.