“Basement level minus 2” is now an increasingly common sight when travelling in an elevator. With pressures to increase living, retail or parking space, and the restrictions on height and available land, constructing additional floors underground is becoming increasingly popular.

Whilst generating a valuable asset, adding basement levels has its own distinct risks and hazards. In particular, when constructing below the water table, inflowing groundwater can create a unique challenge to the main contractor.

Groundwater in isolation rarely causes a ground problem, and can often be managed by a number of sump pumps. The main groundwater hazard to the construction project is the result of excess groundwater pressures on fine grained sands or silts. This is often referred to as “running sand”.

As excavations become ever deeper, groundwater flowing into an open excavation results in the ground losing its strength. As a result, the ground becomes fluid, described by many site agents who have experienced this phenomenon, by the geotechnical expressions of “porridge”, “treacle”, “custard”, or “pea soup”.

Over the past 18 years OGI has developed a unique blend of practical site capabilities and computer modelling design techniques for the purpose of abstracting groundwater from fine grained running sand and silts.

Main contractors are now realising the benefits of advanced dewatering techniques to stabilise the ground as part of the enabling works. These benefits include the production of a dry, stable and safe working environment, resulting in a shorter period of groundwork construction.

As groundwater can often make up 30-40% of the volume of saturated soil slurry, the excavated soil is significantly lighter per cubic metre making disposal cheaper. In addition, the soil can then be defined as a solid, and so making it easier to transport than a liquefied soil slurry.

OGI’s team comprises a blend of experienced engineering technicians, geological, geotechnical and civil engineers, together with designers, mathematical modellers and project administrators.

With a standard ground investigation rarely providing the detail required to predict dewatering requirements, OGI’s operation commences with additional groundwater investigation.The investigation boreholes, used initially to assess the groundwater behaviour, are also designed to be part of the final dewatering system.

As information is gathered in the field, the impact of pumping groundwater from the abstraction boreholes on the drawdown measured in nearby observation boreholes is communicated to the office team of designers and mathematical modellers.

This information is processed within 24 hours, which keeps the model “live” providing a “real time design” to the site team so that the location and depth of the remaining boreholes can be optimised.

A “real time design” approach to dewatering is of particular benefit to the main contractor when it is prudent not to over-dewater the ground. It is a regular occurrence for OGI to be requested to lower the water table to a precise target level within a tolerance of 50mm.

Too little drawdown results in a wet formation, whereas too much drawdown can result in excessive water abstraction and so disposal, together with an unnecessarily wide radius of drawdown influence.

Perhaps the most valuable benefits to the main contractor from combined field management and in-house design and modelling is the predictability of the outcome.

Pre-works meetings between OGI’s designers, modellers and field engineers are held in advance with other parties that are affected by the groundwater abstraction. Meetings with the EA, SEPA or water companies can resolve water disposal issues in advance so avoiding costly delays.

Groundworks sub-contractors can be assured of stable pile cap excavations and crane pits, and the predictive groundwater models assure neighbours that the drawdown beneath surrounding structures will not exceed a pre-determined level.

In conclusion, an engineered approach provides a favourable outcome for all parties. For the client, knowledge that the approach is economic and fulfils safety regulations. For the main contractor, knowledge that the site is stable and other contractors can complete their works in a timely manner. For other stakeholders, the knowledge that the local geotechnical and hydrogeological environment is protected.

For OGI, the reward is sustained involvement in interesting and challenging projects throughout the UK.