Excavation below sea water level

Excavation below sea water level and in locations subject to water ingress into the site

As the population in the Maltese Islands increases, it creates a larger demand for more accommodation and buildings for leisure, offices and commercial spaces. With the increasing price of land and restrictions imposed by the local Planning Authority on height and parking spaces, one way of constructing a profitable building whilst keeping within the remits of the restrictions, is to dig deeper. Factors such as the surroundings, geology and proximity to the sea need to be studied and investigated to understand limits and extent of excavation and construction.

The number of projects in Malta that require deeper excavations has in the past few years skyrocketed with several projects having excavations several meters below mean sea water level. Notwithstanding difficulties façed during the excavation processes, the foundations , ground slab and surrounding walls have to be detailed and constructed in such a way and manner that will impede water ingress into the building as this would render spaces damp, humid and un-functional.

Prior to commencing excavation, the bearing strata have to be studied in detail by a geological study which normally consists of a number of test boreholes which furnish the designer with a sample of the underlying rock. Adjacent third party properties must also be studied if they may be impacted by the excavation and construction.

Several methods of excavation below sea water level exist depending on the site conditions and geological surveys that have been carried out. It is important that every site and condition is specific and needs to be studied separately while being guided and monitored by a professional or expert in the field of study.

Impervious rock strata such as clay (tafal) and blue clay (kahla) do not normally contain any fissures or cracks. This normally works both ways, that is water that is external to the site does not percolate into the site while water that may be trapped in the site from surface waters and rain fall does not find its way out and has to be pumped away. Therefore, excavation to depths below the mean sea water level does not normally require additional efforts in these conditions. Areas in Malta with such geology include Imriehel and Santa Maria Estate in Mellieha.

In Areas that contain permeable rock or fissures in the rock such as globigerina limestone, upper and lower coralline limestone, a mixture of limestone or weak strata such as ‘turbazz’, interventions using grout may be required. The water pressure increases with the depth of excavation so it is very normal that as the excavation progress more water will percolate into the site.

One method is pressure grouting which involves injecting a cementitious, grout material the fissures to improve the bearing strata being soil or rock. The purpose of injecting cement grout in fissures, fractures and cracks in the rock is two fold, namely to strengthen the supporting strata increasing the bearing pressure of the soil while reducing the water flow through interconnected pores or voids linked to the site when excavation occurs below mean sea water level.

In general, the process of pressure grouting injected into rock fissures consists of drilling small diameter grout holes at planned intervals and heights followed by the insertion of small diameter grout perforated steel pipes to the target improvement depth using a drilling and grouting machine. Pressure washing of the fissures and cracks with pressured water to remove mud, drill cuttings, and other materials that may interfere with the grout injection. After cleaning the fissures, cracks and holes, a low-slump grout under pressure is injected into the fissure. This may consist of cement, fine sand, water and admixtures such as plasticisers or superplasticisers and accelerators for rapid hardening of the grout before the water pressure can find its way through the grout. This process is repeated several times depending on the quality of rock and fissures encountered throughout the target zone that requires improvement.

A high water : cement ratio is normally used in circumstances of small fissures and narrow cracks while a grout with a low water : cement ratio is typically used to fill large fractures and fissures in the bearing strata.

This sequence is typically repeated several times as the excavation gets deeper and new fissures, fractures and cracks are exposed, whilst the pressure of the water increases as the depth below mean sea water level increases.

Precautions such as installation of pumps, excavation and construction of temporary sumps for the collection of water that percolates into the site should be taken and revised continuously to avoid site flooding, whilst draining ingress water to specific pre-determined and planned collection points is essential so that the area being excavated is kept free of flowing water. 

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