Managing hydrostatic pressure with heavy ballast - LKAB Minerals
Old Elbe Tunnel - St.Pauli Elbe Tunnel

You can solve the need to deal with pressure caused by hydrostatic uplift using heavy ballast. This is a proven concept for tunnels, underground railways and train stations, as well as underwater ramps and other constructions below groundwater level.

According to calculations from LKAB and Sweco*, it does not necessarily make the project more expensive; rather, the opposite.

High density and the Archimedes’ principle

Heavy ballast often refers to our MagnaDense, which we manufacture from iron ore. This product has a particle density that can reach up to 5 tonnes/m3. In principle, you can use heavy ballast in two ways, as:

  • Ballast; either within cast-in-situ concrete or in prefabricated concrete modules,
  • Loose ballast; in cavities (pockets, between walls).

The higher density provides greater weight for a given volume. When combining this with the Archimedes’ principle, heavy ballast material can provide significant advantages due to its higher weight per unit of volume where structures are submerged or where constructions sit below the groundwater level. LKAB Minerals, in collaboration with Sweco, compared the total cost of different solutions which balance hydrostatic uplift on foundations underwater or below groundwater level. We report the results in this article.

From offshore to infrastructure

Using heavy ballast such as concrete and loosely placed ballast has so far been common in connection with various types of offshore structures, such as platforms, top layers on large-diameter pipelines and wind turbine foundations. Additionally, bridges, quays and harbours are other areas where our customers use this in many projects. However, we see a limited transfer of knowledge to the more common types of infrastructure and design projects. This lack of transmission is probably because other technologies have been predominant or because the focus has been on unit costs rather than the total cost.

Dealing with lifting forces

The need to deal with pressure is common, but it is especially apparent in cases where the design has an air-filled volume, which produces major uplifting forces relative to the dead weight.

For example, there may be tunnels where the lifting force is higher than if it had been a solid construction without cavities. Other large open structures such as railway tunnels, train station and ramps below groundwater level also require additional weight where there is a limited load on top of the structure. Another example of using heavy concrete technology was a building with an atrium. In this case, it meant that the load on the base plate was too small and the simple solution was to cast parts of the base plate in heavy concrete to cope with the uplift pressure from the water.

However, it may also be relevant to choose heavy concrete or ballast for solid concrete structures, such as the base plate in a shaft where piling is underwater, for example, bridge foundations.

Although, there are probably more examples of applications which can be developed if usage and knowledge increase in the industry.

Underwater weight

The importance of heavy ballast for the weight of concrete underwater is illustrated in Figure 2. As shown in Figure 1 (more weight per m3), the weight of the concrete underwater will be more than double the weight of standard concrete. This means that for a concrete volume underwater, you can get twice the weight with the same volume, or achieve the same weight with half the size.

Naturally, the above are apparent matters for road and water construction. However, it is not similarly obvious to use a technical solution with high density ballast.

Examples of existing techniques for dealing with uplift pressure are:

  • Pumping during the construction period to reduce the uplift pressure. After construction, a permanent reduction of the groundwater level may cause other problems such as settlement.
  • Increase the concrete volume with either ordinary or heavy concrete.
  • The concrete cross-section, regardless of concrete quality, is given increased weight by applying heavy ballast.
  • Use of different types of rock anchoring that can absorb upward traction forces.

Product cost price vs total cost

When directly compared, heavy ballast is a more expensive option than ordinary ballast. However, experience shows that heavy concrete can be handled cost-effectively and with standard equipment. Moreover, reducing costs is about planning and consulting with those who have experience in the field.

In conclusion, if we consider the total cost for the entire construction, heavy ballast or concrete is not necessarily more expensive than other options. This fact, however, is not widely known.

Following the study conducted by Sweco in collaboration with LKAB Minerals, it was concluded that the total cost could be similar or lower than well-known solutions, supporting the potential for considering the use of heavy ballast as a design option. This conclusion has been further qualified in a recent UK report, conducted in collaboration with Fairhurst,  where it was found that high density concrete can be a cost-effective solution in Radiation Shielding applications.

Cost calculation for underwater structures

The starting point for the calculation is two typical constructions below groundwater level or underwater:

  1. A foundation for an air-filled volume below the surface of the water (figure 2)
  2. A deep shaft inside piling with high water pressure in the bottom of the shaft (figure 3).
Figure 2 – Tunnel cross-section; the uplifting water pressure is greater than the dead weight of the tunnel
Figure 3 – Deep shaft with piling

Based on actual unit costs from various projects in which Sweco was involved, they conducted a cost calculation for both underwater structures. The following options to cope with the uplifting forces from the water were studied:

  1. Using rock anchoring
  2. Increasing the mass by increasing the amount of standard concrete in the cross-section
  3. Replacing the regular concrete with heavy ballast concrete
  4. Increasing the mass by adding loose heavy ballast to a cross-section with normal concrete
  5. Increasing the mass by adding loose heavy ballast to a cross-section with heavy concrete

In the case of a deep shaft within piling, they studied options 2 and 3, as well as an option with only loose ballast and a waterproof membrane. However, we can see that although the last option is possible, it is perhaps a rarely tested option.

The total cost was at an even level for the options studied. In particular, only the option to increase the volume of normal concrete stood out as being about 30% more expensive.

The reasons for this are the savings that you can, first of all, make on the shaft volume; secondly on the amount of moulding and reinforcement; and finally the decrease in the total volume of concrete when you use a heavier ballast.

In other words, the increased expenses for high density MagnaDense were recovered on other costs when using heavy ballast.

From theory to practice

In addition to the two primary examples, two real cases that already had been implemented were also studied. The first case concerns the construction of a bin housing wood chippings and the second example is a mill foundation. Both projects involved foundations below groundwater level. In both situations, it showed that switching to heavy concrete would have reduced the costs relative to the method chosen, which included the use of different types of rock anchorings.

Unlike the use of anchors, using ballast or concrete negates the need to include additional personnel during the construction phase. The reason for this is because you can handle the manufacture or application of the materials by existing staff responsible for concrete casting. This advantage will most likely also result in additional time-saving, not included in the reported study.

Using heavy materials that do not involve additional excavation or rock anchoring minimises the shaft volume and thus the impact on the environment. The exclusion of rock anchoring also means that underlying infrastructure such as various lines and tunnels are not affected.

Calculations form the basis

Calculations and comments in this article are based on the use of heavy ballast materials that LKAB Minerals produces and markets under the trade name MagnaDense. At present, we supply three different grades that are CE certified as ballast in concrete.

Finally, to understand which products are most applicable to your next construction project, we advise you to contact our sales managers for more information and support.

*All calculations used for this article are based on Swedish projects and Swedish price levels in 2018. Consult with your area sales manager and local parties for current prices. LKAB Minerals can help you with calculations for high density concrete mixes.