There are many design and analytic techniques at the disposal of engineers when it comes to determining the most effective installation methods of sheets and piles. In simple terms, the four basic elements to consider in order to achieve a successful outcome are dependent on:
- Topographical features
- Geological conditions
- Use of the most appropriate type of sheet section, plus
- The correct choice of driving hammer
Essentially, the choice of driving hammer will be determined by the first three points in our above list. For example, topographical features such as residential dwellings, nearby roads or buried services will often generate restrictions on noise and vibration. High Frequency Vibrators and Resonance Free Vibrators are often used where there is a need to minimise and localise potentially hazardous vibrations. As an added precaution, pile driving activities can be effectively measured by use of a Vibration Monitor supported with Audible and Visual Alarms to minimise the risk of unwanted damage claims.
Having considered topographical limitations, geological aspects such as the prevailing soil type and condition will determine the best type of pile driver for the job. A site investigation, which may include visual or bore hole samples, should be undertaken to ascertain whether an impact hammer or a vibratory hammer should be used. Broadly speaking, difficult driving conditions such as heavy clays, compacted dry soil and rock strata are examples of when an impact hammer would produce the most effective results. In cases where the ground has varying sub-layers, the use of a Pre Auger can be used to increase the likelihood of successful pile driving when using a vibratory driver.
Vibratory Hammers are more suited to the granular and cohesive conditions found in sand, gravel and other particulate soils. The pile is driven using a counter-balance system which generates a high speed vibration, allowing the pile to travel into the ground as the soil is made fluid or ‘liquefied’ by the vibrations. Unlike Impact Hammers, the pile is usually driven under its own weight plus that of the vibration hammer. Depending on the model of vibrator, additional weight can be applied if necessary to further improve the driving process.
Using a traditional impact driver can be compared to hammering a nail into a piece of wood, whereas using a Vibratory Hammer has been described as the sheet passing through the ground like a hot knife through butter. A vibratory hammer ‘used in the right soil conditions’ can significantly decrease driving time and mitigate groundwork costs.
Selecting the most appropriate sheet pile type and section will depend on a number of fundamentals, including: soil type, required depth of penetration and any specification for water or fine particle exclusion. The principal objective is to ensure that the pile can be driven to the required depth while adhering to any limitations and constraints. Progressive technological development of impact and vibratory driving equipment has meant that wider and thicker section piles can be more effectively installed. However, factors such as sheet thickness still remain a major consideration when using impact hammers as damage to the sheet head is possible if the steel section is too thin in relation to the soil density.
Engineers instinctively strive to create working scenarios where the least number of piles are used for a given design and there will inevitably be a trade-off which aims to strike a balance between cost effectiveness and successful pile driving.
This article comes from vpgroundforce edit released