Tunnels pass through seven different rock types in 6km. Engineers mapped every fault before boring to prevent collapses in the unstable riverbed.
A riverbed of geologic hazards
Boring a tunnel under any major river is a challenge, but the Yangtze River at Nanjing presents a particularly hazardous sub-surface environment. The geology is a complex mix of hard rock and soft soils, a condition engineers call "soft upper and hard lower" strata. The upper layers consist of Quaternary-period loose sediments like silty clay, fine sand, and highly permeable gravel. Beneath these soft deposits lie much older, fractured bedrock formations, including sandstone and mudstone. This creates unpredictable conditions for a tunnel boring machine (TBM), which can struggle when the top of its cutting head is in soft material while the bottom is grinding against hard rock.
The main geologic dangers are the numerous fault zones that cut through the bedrock directly beneath the river. The Heyan Road Cross-River Tunnel, for example, had to be engineered to pass through five distinct fault zones. These zones are areas of crushed and weakened rock, which increase the risk of collapse and water inrush during excavation. The maximum water pressure faced by boring machines can reach 0.79 Megapascals (MPa), equivalent to the pressure at a depth of nearly 80 meters of water. Some tunnels must pass through these formations with as little as 10.25 meters of overburden—the thickness of the riverbed sediment—separating the top of the tunnel from the river itself.
Engineering on a fault line
To navigate this underground maze, engineers rely on enormous, custom-built slurry shield TBMs. For the Nanjing Yangtze River Tunnel, two machines with diameters of 14.93 meters were used. The slightly larger "Zhenxing" TBM, used for the Heyan Road tunnel, has an excavation diameter of 15.03 meters. These machines maintain pressure at the tunnel face using a bentonite slurry, which prevents the surrounding soft ground and water from flooding the excavation. The machine's cutting head is equipped with tools capable of handling the mixed-face conditions, grinding through sandstone one moment and loose gravel the next.
Before any machine begins boring, an exhaustive geological survey is required. This involves extensive seismic profiling to map the various layers and pinpoint the location of faults. Hundreds of boreholes are drilled into the riverbed to retrieve core samples, giving engineers a direct look at the materials the TBM will encounter. For the Nanjing Metro Line 4 tunnel, the route crosses silty clay, fine sand, coarse sand, gravel, and about 700 meters of moderately weathered mudstone and sandy rock. This detailed mapping allows the tunnel's path to be optimized, avoiding the worst of the fractured zones where possible and preparing for the specific conditions of each geologic layer. The data informs every aspect of the project, from the design of the TBM cutter head to the precise pressure needed to keep the tunnel face stable through every meter of its journey.
💡Fun Facts
The age of the Yangtze River itself is debated, with estimates ranging from 40 million years to as recent as 3 million years, based on analysis of ancient river gravels found near Nanjing.
One of the major risks during construction is liquefaction, where the vibration from the TBM can cause water-saturated sandy soil layers to lose their strength and behave like a liquid.
Some of the TBMs used, like the German-made Herrenknecht machines, have a torque of almost 35,000 kilonewton-meters (kNm) to grind through the mixed rock and soil layers.
The Heyan Road Crossing River Tunnel is China's first super-large-diameter shield tunnel engineered to traverse both fault zones and ancient lava formations under the river.
The tunnels are public infrastructure (road and metro) and are generally open 24/7 for transit.
Admission
Varies by tunnel. Road tunnels may have tolls, while metro tunnels require a fare.
Accessibility
The geological formations are not directly accessible. The point of interest can be experienced by driving or taking a metro train through one of the several cross-river tunnels.
Nanjing Yangtze River Crossing GeologyJiangsu, China