Tunnel in sandy soil

Hello everyone, in this post I am going to answer two other questions I got during the my tunnel lecture.

• How does the method of creating a tunnel change when tunneling through sand?

• Sand is a completely different material compared to hard rock. Building a tunnel through sandy ground could mean several things:
• Ground water and surface water could have a strong influence
• Collapse of tunnel face could impact the surface by building a deflection
• More direct load from above, especially when building shallow
• Positive attributes might be:
• Minimum gap between the outer tunnel lining and the soil surrounding the tunnel - positive interface interaction between tunnel and soil
• Grouting is easier and more successful in sandy ground - but, on the other hand, the grout might go anywhere or gets lost when the pores between the sand grains are large enough.
• The construction of a tunnel through a sandy ground can be done with a conventional excavation in several steps, like heading and bench method or NATM, with immediate support of the face and the tunnel walls. 
• Other options are possible. It all depends on the building site conditions and the contractor how they build tunnels in similar ground before and with what they are comfortable and professional at the same time.

• Cost effectiveness of crating a base tunnel compared to a raised tunnel, material removal, tunnel type (for trains, cars, utilities), aesthetic, ...?
• The effectiveness of a tunnel is strongly coupled with economics like an increase of train speed which results in less time traveled, which will give the people more time for meetings, work, etc.
• Further, the effectiveness is coupled to ecological considerations, like the environmental impact - a base tunnel might have less of a "surface"- impact than a train route partially above the mountain.
• Security during the service is a highly discussed subject as well - for instance, a base tunnel might have a higher risk of fire due to its length than a tunnel which lies higher in the mountain and is shorter for that matter. On the other hand, the road to get to the higher point at the mountain might be riskier, the train/car/truck has to slow down, possibly windy road...
• The older train routes above the mountains might be in very good condition and worth keeping, while others might be outdated and have to be replaced.
• The construction itself has its influences, e.g. the material transportation, concrete fabrication, dealing with excavation material, pollution during construction, and so on.. and a generalization cannot be made, it all depends on the specific project parameters.
• Since the tunneling industry advanced so rapidly in the past decades, I believe that more base tunnels are possible. This would reduce the noise emissions and environmental impact on the "surface" and creates a faster and maybe more secure option for the economy.
• To rap it up: it is hard to decide which one is more cost effective. Other factors might be as or even more important to the owner, the immediate population, and to politics. Large traffic projects, like tunnels are, have almost always political influences which could alter the cost effectiveness dramatically.

So, I hope I could answer - or not answer - the questions.

Please, correct, add, or otherwise comment on this. I am always eager to learn new things and to correct "old" views.

Thank you for reading.

Stephie