<p>The <a href="https://en.wikipedia.org/wiki/Strait_of_Gibraltar" rel="nofollow noreferrer">Strait of Gibraltar</a> is 14.2 km wide and between 300 meters and 900 meters deep. Any tunnel would have to accommodate the maximum depth of the water above the seabed in the strait, the tunnel would need to be at least 900 m deep.</p>
<p>To begin with, this would require lengthy access ramps at each end of the tunnel get to that depth. If each such access ramp had a gradient of 1 in 10, the length of each access ramp would be 9000 meters. Add this to the width of the strait and the overall length of the tunnel would need to be 32 km (14+9+9).</p>
<p>A gradient of 1 in 10 would be sufficient for conventional road transport, but if rail was to be utilized, the gradient would need to be shallower and the access ramps even longer.</p>
<p>Ventilation of the tunnel would need to be considered as well as an emergency escape tunnel. This could be provided by a second, small diameter tunnel, parallel to the main tunnel. This would significantly increase the cost required to establish a tunnel. Both the tunnels would need to be connected by short perpendicular tunnels at regular, but short, intervals to allow people to escape the main tunnel easily and safely.</p>
<p>At a depth of 900 meters, rock temperature would need to be considered as well, and if high enough, the tunnel may need to be artificially cooled, which would add to both the capital and operating costs of the tunnel.</p>
<p>Also, at depths of 900 m, ground stresses could be problematic, requiring the need for stronger ground support and reinforcement measures. This again, would increase the cost of the tunnel.</p>
<p>The seabed of the strait is composed of hard rock which means drilling of the rock would take longer than for softer rock and would increase the cost of the tunnel.</p>
<p>Compared to the <a href="https://en.wikipedia.org/wiki/Channel_Tunnel" rel="nofollow noreferrer">Channel Tunnel</a>, which is 50 km long, but only 75 meters below sea level.  A Gibraltar Strait tunnel would be significantly deeper and subject to higher rock temperatures. The Channel Tunnel was established in chalk which is relatively softer than the seabed of the Gibraltar Strait.</p>
<p>The other thing makes a Gibraltar Strait tunnel difficult is the region is highly seismically active.</p>
<p>The other unknown is how would most people feel about being 1 km underground, allowing for a reasonable amount of ground cover between the seabed and the tunnel.</p>

CC BY-SA 4.0
2024-09-26T15:00:19.247Z
If a Gibraltar tunnel ever gets built, it would probably be a [submerged floating tunnel](https://en.wikipedia.org/wiki/Submerged_floating_tunnel), so it would not have to be 900 metre deep.
gerrit
CC BY-SA 4.0
2024-09-28T03:30:59.197Z
Fred, good approach but: 10% cross gradient cannot be maintained for vehicular access as it is unsafe for trucks and buses, so you need deceleration lines; there aren't vehicular tunnels on that length, 900m is the depth, tunnel shall we lower... gerrit is right and it shall be a submerged floating tunnel. Another challenge is it is an active tectonic area and the Strait it is becoming shorter as Europe and Africa are approaching. Submerged tunnel or bridge: Look at amberengineering.com for tunnel or opacengineers.com for bridge.
David García Bodego
CC BY-SA 4.0
2024-09-30T00:31:12.627Z
It is seismically active but none of the major faults or zones are actually across the strait, so you really only have normal earthquakes to deal with.
John
CC BY-SA 4.0
2024-09-26T04:10:02.867Z
2024-09-28T03:36:26.013Z
2024-09-28T03:36:26.013Z
Fred
<p>There have been some recent news (e.g., <a href="https://www.vozpopuli.com/espana/gobierno-contrata-servicio-geologico-eeuu-para-avanzar-tunel-marruecos.html" rel="nofollow noreferrer">involvement of USGS in the tunnel</a>) about a potential <a href="https://en.wikipedia.org/wiki/Strait_of_Gibraltar_crossing" rel="nofollow noreferrer">tunnel across the Strait of Gibraltar</a>.
Considering the estimated depth of the tunnel (~300 m) and the challenging logistical issues, what are the geological and engineering challenges that they are likely to encounter?</p>
<p><a href="https://i.sstatic.net/9nEC1qqK.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/9nEC1qqK.jpg" alt="Strait of Gibraltar: Wikipedia image" /></a></p>

<geology><oceanography><mediterranean>
What are the Geological and Engineering challenges of a tunnel crossing the Strait of Gibraltar?
arkaia
# What are the Geological and Engineering challenges of a tunnel crossing the Strait of Gibraltar?

(Asked by: username_0 on 2024-09-20)

There have been some recent news (e.g., [involvement of USGS in the tunnel](https://www.vozpopuli.com/espana/gobierno-contrata-servicio-geologico-eeuu-para-avanzar-tunel-marruecos.html)) about a potential [tunnel across the Strait of Gibraltar](https://en.wikipedia.org/wiki/Strait_of_Gibraltar_crossing).
Considering the estimated depth of the tunnel (~300 m) and the challenging logistical issues, what are the geological and engineering challenges that they are likely to encounter?

[![Strait of Gibraltar: Wikipedia image](https://i.sstatic.net/9nEC1qqK.jpg)](https://i.sstatic.net/9nEC1qqK.jpg)

**Tags:** geology, oceanography, mediterranean


---

## 1 answers

### Answer by username_1 on 2024-09-26. Score: 3

The [Strait of Gibraltar](https://en.wikipedia.org/wiki/Strait_of_Gibraltar) is 14.2 km wide and between 300 meters and 900 meters deep. Any tunnel would have to accommodate the maximum depth of the water above the seabed in the strait, the tunnel would need to be at least 900 m deep.

To begin with, this would require lengthy access ramps at each end of the tunnel get to that depth. If each such access ramp had a gradient of 1 in 10, the length of each access ramp would be 9000 meters. Add this to the width of the strait and the overall length of the tunnel would need to be 32 km (14+9+9).

A gradient of 1 in 10 would be sufficient for conventional road transport, but if rail was to be utilized, the gradient would need to be shallower and the access ramps even longer.

Ventilation of the tunnel would need to be considered as well as an emergency escape tunnel. This could be provided by a second, small diameter tunnel, parallel to the main tunnel. This would significantly increase the cost required to establish a tunnel. Both the tunnels would need to be connected by short perpendicular tunnels at regular, but short, intervals to allow people to escape the main tunnel easily and safely.

At a depth of 900 meters, rock temperature would need to be considered as well, and if high enough, the tunnel may need to be artificially cooled, which would add to both the capital and operating costs of the tunnel.

Also, at depths of 900 m, ground stresses could be problematic, requiring the need for stronger ground support and reinforcement measures. This again, would increase the cost of the tunnel.

The seabed of the strait is composed of hard rock which means drilling of the rock would take longer than for softer rock and would increase the cost of the tunnel.

Compared to the [Channel Tunnel](https://en.wikipedia.org/wiki/Channel_Tunnel), which is 50 km long, but only 75 meters below sea level. A Gibraltar Strait tunnel would be significantly deeper and subject to higher rock temperatures. The Channel Tunnel was established in chalk which is relatively softer than the seabed of the Gibraltar Strait.

The other thing makes a Gibraltar Strait tunnel difficult is the region is highly seismically active.

The other unknown is how would most people feel about being 1 km underground, allowing for a reasonable amount of ground cover between the seabed and the tunnel.

> **Comments:**
> **username_2**: If a Gibraltar tunnel ever gets built, it would probably be a [submerged floating tunnel](https://en.wikipedia.org/wiki/Submerged_floating_tunnel), so it would not have to be 900 metre deep.  
> **username_3**: username_1, good approach but: 10% cross gradient cannot be maintained for vehicular access as it is unsafe for trucks and buses, so you need deceleration lines; there aren't vehicular tunnels on that length, 900m is the depth, tunnel shall we lower... username_2 is right and it shall be a submerged floating tunnel. Another challenge is it is an active tectonic area and the Strait it is becoming shorter as Europe and Africa are approaching. Submerged tunnel or bridge: Look at amberengineering.com for tunnel or opacengineers.com for bridge.  
> **username_4**: It is seismically active but none of the major faults or zones are actually across the strait, so you really only have normal earthquakes to deal with.  

---

