During the construction of infrastructure, for instance the Schiphol line, the A4 Motorway and the Circular Canal Aqueduct, the groundwater caused problems with subterranean wells. The HSL runs partly through polders, such as the Haarlemmermeer Polder. The groundwater may also cause problems for the construction of the HSL. For instance, it would be difficult to dig a tunnel or building excavation in places where there is high groundwater. The excavation would fill with grondwater, making it impossible to work. A number of years ago, the Dutch builders took drastic measures to lower the local groundwater. With powerful pumps the water was drained, as long as needed to make the building site dry. But draining the groundwater proved to have great disadvantages: for miles around the land dried out. Ditches ran dry, farmers started to complain. So a better solution had to be found. And this turned out to be underwater concrete. The HSL Project Organization applies this construction method to a great extent, but also takes a number of other measures to prevent well water, and uses other (innovative) techniques to build below groundwater level.

In polders (reclaimed land) groundwater may reach the surface, the so-called seepage water. Often this water, coming from some depth, is somewhat salty. When a lot of seepage water wells up in one particular place, it is called a well. Wells make terrain more difficult to drive on and they also increase the salinity of the surface water. Possible consequences of a well are: decline in agricultural produce and damage to the natural surroundings.

The formation of wells is stimulated by cracks or perforations in the top layer of soil. When this covering layer erodes or is removed, the water-containing layer beneath is not sealed insufficiently and the amount of seepage water increases. This situation may arise as a result:

• digging, for instance digging ditches or tunnels;
• deformation of the subsoil when ground is raised or deeper layers are drained;
• soil investigation; 
• introduction of vertical drainage below raised ground;
• removal of drainage; 
• installing sheet pile walls.      

Besides, wells also occur as a result of natural processes.

During the construction of the HSL, measures are taken to counter well formation as much as possible. These are:

• Raising ground as gradually as possible;
• No raising of ground in sensitive areas, such as the reclaimed lands in the southern part of the Haarlemmermeer Polder. For this reason, the greatest part of the HSL is built on pile supported no-recess slabs. The space below the concrete slab is filled with earth, which makes raising of ground unnecessary;
• Railway ditches are dug parallel to the track. During digging it is examined if an increase in seepage water may be prevented by making the soil impermeable;
• No large (pressurised) drainages for building excavations; for sunken constructions underwater concrete is used.        

If despite all precautions wells do occur, they are sealed with filling material (bentonite). However, this method does not guarantee a complete sealing of the soil.

In 1998, Grondmechanica Delft and physical research institute TNO researched the wells and salinization problem under the auspices of the HSL Project Organization and with assistance of the relevant provinces, polder boards and district water boards. The research comprised:

• studying measures to prevent or counter well formation;
• an inventory of the situation of that time (as a preliminary soil analysis). As a result, an estimate of the future development was made.