Annual settlement rates of up to 100mm are not unheard of in Holland's soft compressible soils. These movements regularly cause extensive damage to pavement structures. With rising labour costs making repair and maintenance of pavement structures increasingly expensive, there has been a resurgence in the use of pre-consolidation to improve ground conditions before construction.
Pre-consolidation has been used in the Netherlands since the 1940s, with sand drains used to accelerate primary consolidation and surcharge loading (using sand fill) to accelerate secondary consolidation.
More recently, Dutch consultant and instrumentation developer IFCO has developed a vacuum drainage system, which it claims is a faster, more effective and economic alternative to conventional pre-consolidation methods. IFCO's system, intensive forced consolidation, uses groundwater lowering and under-pressure (equivalent to setting up a vacuum in the ground) to pre-consolidate low permeability cohesive soils. IFCO claims to have achieved rapid results, obtaining over 2m of settlement in just 30 days for one site.
Intensive forced consolidation relies upon a temporary but substantial reduction in pore pressure within the ground, causing an increase in effective stress of the soil. As treatment progresses, shear strength is increased, further increasing stability. The method can also be combined with sand fill surcharge loading to produce an increase in total stress.
The technique uses a series of parallel sand-filled trenches installed a few metres apart across the site. Trenches are typically 250mm wide, 7m deep and a few hundred metres long and are excavated by a deep trenching machine, which is capable of forming up to 1km of trench every day. These sand walls act as drains, groundwater being extracted by a pump placed at one end of the drain running along the base of the trench.
IFCO says the system is more effective than a vertical drainage solution. Groundwater flow to the sand walls is horizontal and as horizontal permeability can be five to ten times higher than vertical in clay and as the sand walls are only a few metres apart, maximum drainage length is limited. Additionally, the contact surface between the sand walls and the soil is substantially bigger than in vertical drain systems.
With time, less water and more air is expelled from the ground and an under-pressure is set up in the sand wall. The under-pressure replaces surcharge - 0.5bar of under-pressure is equivalent to the load applied by 3m of sand fill at ground level. This forces substantial pre-consolidation, reducing the amount of secondary consolidation that will occur after treatment. The more under-pressure, the greater the secondary consolidation achieved during treatment.
Under-pressure will only be achieved if the soil is sealed off from the atmosphere. To this end, the trenches are sealed at ground level and cannot be in contact with any naturally occurring sand layers, which of course restricts the suitability of the method, which is most effective in cohesive soils with low permeability at and below the water table.
The amount of under-pressure is dependent on the capacity of the pumps and the amount of ground- water and air leakage, which must be kept to a minimum - a good seal is essential - to achieve the best results. However, as each drain acts separately, IFCO says that leakage should not affect the entire system. And because the under-pressure is continuously monitored and recorded, leaks can be detected. IFCO claims that under-pressure obtained is much greater than conventional vertical drainage systems, with up to 95% vacuum achievable.
Additionally, extracted ground- water can be used as further surcharge to accelerate settle- ment, by laying down plastic sheeting (acting as the seal to the system) and building levees around the area, allowing the groundwater to pool.
After the required settlement is achieved, the pumps are shut down, removing under-pressure and therefore removing the surcharge, allowing the groundwater to regain its natural pressure distribution. Fill to compensate for settlement is often placed only after under-pressure is achieved, a more rapid solution to conventional methods, where fill must be laid in distinct layers of limited thickness, with rest periods between.
The shear strength of the soil increases instantaneously due to the under-pressure and a further increase is gained under pre-consolidation. As primary consolidation and a large amount of secondary consolidation is achieved during treatment, there should be little or no settlement after the pavement structure is laid, reducing maintenance and remedial work. IFCO says cost savings will be most notable in structures built on young river and marine deposits.
Direction and length of the sand walls can be designed in accordance with project requirements. According to IFCO, narrow drains are best placed parallel to the road axis, while for a runway, perpendicular placement of trenches is usually preferable.
One main advantage is that the entire system remains accessible throughout treatment, allowing regular inspections and mainten- ance of pumps, which can become fouled up through biological action in the water.