How Do Piling Companies Choose the Right Piling Methods?
How Do Piling Companies Choose the Correct Piling or Ground Improvement Method?
It’s important for Ground Engineering specialists to select an appropriate method for each individual project, taking into consideration details such as, but by no means not limited to –
- Access, headroom and working room constraints
- Proximity to neighbouring structures
- Ground conditions
- Loading requirements
- Required load/settlement performance
- Long term structural durability
The vast range of methods available in the Ground Engineering marketplace provides contractors with a lot of potential options, and on many occasions, there will be more than one method suitable for a given project.
It is important that whatever system is adopted it is safe to construct and will result in a foundation which will remain fit for purpose throughout the design life of the completed structure.
In the highly subjective discipline of Geotechnics, the term ‘correct’ is itself subjective, and it must be recognized that even the most thorough review of ground conditions is limited by the scope and quality of site investigation information made available.
In this blog, we will provide an insight into how we at GeMech select what we consider to be the most appropriate piling or ground improvement method for a project under review.
Access, Headroom and Working Room Constraints
Before making any assessment of the technical aspects of pile structural capacity, load/settlement response or ground conditions etc., firstly it is important to understand which equipment can be safely delivered and used on site efficiently. This can often be confirmed via a desk study, but in some cases a site visit is necessary before a design can be meaningfully progressed.
Soil Type and Ground Conditions
To safely and efficiently design deep piled foundations or ground improvement, it is important to have a detailed understanding of the ground conditions across the entire building footprint area. Some key considerations when preparing a ground investigation scope and specification are –
- Ensure levels are taken at each borehole and trial pit location, allowing the soil profile to be related to proposed building foundation levels.
- If bedrock is anticipated, ensure that the scope of intrusive investigation works is sufficient to record the gradient of the rockhead and the direction of dip.
- Accurately characterise strength and stiffness of all soil layers, using a combination of in-situ and laboratory techniques.
In soft or loose soils displacement methods, or vibro ground improvement might be preferable, provided location and structural intent are suitable.
In stiff clays, or weak rock formations CFA methods might be preferred.
Where competent rock formations are present, particularly at shallow depths, or where rock sockets are required to generate large lateral or tensile capacities, rotary bored or rotary percussive methods are often best.
Load Requirements
Each method of piling or ground improvement is associated with a range of safe working load capacities in the compressive, tensile and lateral directions. Large diameter rotary bored piles are associated with the highest capacities available for traditional piling projects, whilst rigid inclusions offer the largest capacities available for ground improvement projects.
Where access, working room, ground conditions and structural form are best suited to CFA piling for example, the most cost-efficient piling solution will be achieved with the smallest potential pile diameters appropriate to the specified loading conditions.
For ground improvement projects, where the shallow soil conditions permit, vibro stone columns will be the most cost-effective solution for supporting bearing pressures of up to 150kPa and 50kPa for isolated foundations and ground bearing slabs respectively.
Alongside considerations of ultimate load capacity, the likely settlement performance of the chosen solution under working load conditions must also be factored into the decision-making process. As a guide, a hierarchy of expected load/settlement performance, from highest to lowest anticipated settlements, can be listed as –
- Vibro ground improvement
- Rigid inclusion ground improvement
- Bearing piles – all/a proportion of load transferred to pile toe level
- Bearing piles – all load carried in shaft friction alone
Environmental Considerations
Environmental factors can play a role in determining the most appropriate piling or ground improvement method.
For example, factors such as ground water level, proximity of the site to bodies of water and rivers, or the presence of neighbouring structures can influence decision-making.
Vibration damage is a function of both the magnitude of vibration – measured in peak particle velocity – and the frequency of vibration. As a rule, high levels of low frequency vibration should be avoided in close proximity to existing structures, particularly sensitive structures, and first consideration should be given to replacement piling methods.
In close proximity to bodies of water, or where ground water is at shallow depth, pre-formed driven displacement pile types might be favourable, giving the combined benefit of not generating saturated spoil arisings at working platform level, in addition to avoiding the environmental concerns around placing concrete near watercourses.
If the near surface soil is contaminated, displacement piling or ground improvement techniques are typically preferred as they do not produce contaminated arisings for off site disposal, and they limit exposure of the site workforce to contaminants during the construction phase. However, it is important to consider the potential detrimental effects to shallow aquifers if displacement piling methods lead to either preferential pathways for downward migration of contaminated surface water, or to contaminated materials being displaced into the aquifer during the construction process.
A solution such as vibro stone columns would be good option where no concerns with downward migration of contaminants exist. However, if contamination to shallow aquifers is of concern, steel cased bottom driven piling might be preferred, as the pointed tip of steel cased bottom driven displacement piles tends to result in lateral displacement of material during driving, rather than downwards displacement of material into the underlying soil layers.
In a more unusual set of circumstances, GeMech have been involved in projects where the use of displacement driven piling or vibratory ground improvement methods have been specifically precluded due to the potential detrimental effects on migratory fish in a neighbouring river. Under these conditions only replacement or displacement augered methods could be safely used.
As an overarching environmental sustainability consideration, the chosen solution should minimise, as far as possible, the use of carbon intensive materials such as concrete. The carbon reduction associated with a conversion from traditional piling methods to vibro ground improvement must be recognised – particularly on projects where columns can be constructed using recycled aggregates – and every effort made to convert to ground improvement methods wherever the project requirements allow.
Material Availability
Whilst this is seldom an issue for projects in the UK, the local availability of suitable materials must be part of the decision making process. Some examples of potential constraints and concerns are provided below –
Remote sites, located a significant distance from the nearest concrete plant – can the project be serviced by the concrete supplier to enable the use of CFA piles, or will the material be out of specification before it can be placed?
Remote sites with limited access for articulated delivery lorries – can U/C sections, or pre-formed pile sections be safely and efficiently delivered to the working area?
For ground improvement projects, can a sufficient quantity of suitably graded aggregate be procured locally?
GeMech offer a broad range of piling and ground improvement techniques, allowing us to offer the most cost efficient and technically appropriate system, taking all of the various constraints into account.
Want to know more? Contact us today to find out how we can help you with your vibro stone column project.
