Imagine digging a hole 100 feet deep in wet beach sand, right between skyscrapers on one of a city’s busiest streets — and then making sure the concrete box you build inside doesn’t float back up. That was the challenge for engineers building the underground stations of the Tel Aviv Light Rail’s Red Line. In this Grand Structures documentary, we explain how they fought sand, water and buoyancy to build Israel’s first light-rail subway.
Watch the full documentary
Tel Aviv Red Line at a glance
- Opened: Aug. 18, 2023
- Length: about 24 km (15 miles), with roughly 11 km in tunnel
- Stations: 34, including 10 underground
- Route: Petah Tikva, Bnei Brak, Ramat Gan, Tel Aviv and Bat Yam
- Cost: about NIS 11 billion (roughly $3 billion)
- Station depth: about 30 m (100 ft) — a 10-story building upside down
Tel Aviv vs. Jerusalem: sand instead of rock
In Jerusalem, engineers carved the Yitzhak Navon station out of hard limestone. Tel Aviv is the opposite. The city sits on the Mediterranean coast on layers of wet sand and porous kurkar — a crumbly local sandstone — with groundwater close to the surface. Sand has no strength of its own when it is saturated: dig a hole and the sides slump inward, dragging nearby streets and building foundations with them.
Problem 1: digging without sinkholes
To keep the sand from collapsing into the excavation, engineers first built the station walls before digging the station. This is the diaphragm wall, or slurry wall, method.
- A hydromill trench cutter digs a narrow, deep trench in panels.
- As it digs, the trench is kept full of bentonite slurry — a dense clay mud. The pressure of the heavy liquid pushes outward against the sand and acts as an invisible brace.
- Steel reinforcement cages are lowered into the slurry, and concrete is pumped in from the bottom up through tremie pipes. Heavier concrete displaces the slurry, which is recovered and reused.
According to the documentary, these walls reached about 50 meters (164 feet) deep — far below the station floor — to cut off groundwater and anchor the walls in firmer ground. Only once the walls are in place is the soil inside excavated, with steel struts or concrete slabs bracing the walls as the pit gets deeper.
Problem 2: the floating paradox
Once the station box is sealed and emptied, a new problem appears: Archimedes’ principle. A hollow, watertight box surrounded by groundwater experiences an upward buoyant force equal to the weight of the water it displaces. The video calculates up to about 126,000 metric tons (139,000 U.S. tons) of uplift for a station box — more than twice the displacement of the Titanic. Without countermeasures, the station could literally be pushed upward like a plastic bottle held underwater.
How engineers hold the station down
- Thick base slabs and concrete plugs add weight at the bottom of the box.
- Tension piles extend deep below the floor and work like anchors, gripping the ground through friction so the slab cannot rise.
- Bottom-up counterweight: the weight of the completed structure — slabs, walls and the street above — adds to the downward force.
The design has to balance these forces for every stage of construction, because the box is lightest — and most vulnerable to uplift — before it is fully built.
When the tunnel meets the station
Tunnel boring machines (TBMs) dug the Red Line’s running tunnels. The documentary explains how earth-pressure-balance TBMs passed straight through the station boxes using a tunnel-first sequence, with the surrounding sand strengthened by high-pressure jet grouting — injecting cement into the ground to create blocks of artificial rock where the machines break in and out of station walls. Earth-pressure-balance machines keep the soil at the cutting face under constant pressure so it cannot flow into the tunnel.
3 millimeters of precision
All of this happened under busy streets lined with towers. The documentary notes that engineers kept surface settlement of nearby buildings under about 3 millimeters — a remarkable result in saturated sand, achieved through constant monitoring and careful control of the TBMs and grouting.
Frequently asked questions
How deep are the Tel Aviv Light Rail stations?
The deepest Red Line underground stations reach about 30 meters (100 feet), roughly a 10-story building turned upside down.
What is a diaphragm (slurry) wall?
A reinforced concrete wall built in a deep trench kept full of bentonite slurry, which supports the trench walls until concrete replaces it. It lets engineers build walls before excavating in soft or wet ground.
Why would an underground station float?
A hollow, watertight structure below the water table is pushed upward by buoyancy. If its weight and anchors are less than that upward force, it can lift — so engineers use heavy slabs and tension piles to hold it down.
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