Your upcoming project is in the procurement phase and you have that vital task of sourcing and selecting piling contractors.
The foundations are the bedrock of any build, providing support to the superstructure whilst channelling static and dynamic loads to the layers of soil and rock down below.
Our mission at C-Link is to make a more collaborative and profitable industry for us all and that’s why we’ve created this procurement guide for piling. The guide has tips on how to:
- Analyse tender submissions
- Identify Value Engineering opportunities
- Remove risks on site and deliver on programme
Let’s get started!
The Tender Analysis
Upon receipt of the Subcontract tender documents, it’s natural that different contractors will each have their own way in creating a quotation.
Identifying benchmark comparatives is important to ensure you are comparing apples with apples (other fruit is available). Six checks you can make when reviewing contractor proposals are:
Preliminaries (management & staff, health & safety, design, etc.) may be included within the specialist’s measured work rates or as a separate lump sum. When assessing quotes, it’s prudent to request a breakdown of what has been allowed.
This allows you to not only understand how rates compare for specific items, but also make an assessment on whether a reasonable estimation has been made.
For example, Specialist A has allowed for 100 Design Hours at £40p/hr, versus Specialist B who has allowed for 200Hrs at £37.50p/hr. Whilst both specialists have similar rates, the quantity of hours for Specialist B is double than that of Specialist A, resulting in a £3,500 higher cost. Why has Specialist B allowed for double the hours? Has he identified unforeseen issues, or is it an overcautious allowance? These lines of enquiry will assist during tender interviews and negotiations.
2. Classification and Method
Piles can be classified in two ways:
- By the technique of construction
- By their basic design function
Methods include ‘displacement’, where piles are driven into the ground and subsequently displace the soil, or ‘replacement’, where the soil is excavated or bored.
The method adopted will determine plant selected, productivity and waste. Broadly speaking, replacement tends to be more expensive than displacement due to the additional logistics involved in disposing larger quantities of soil off-site.
Similarly, the design function of a pile will influence material cost, plant selection and productivity. For example, ‘end bearing piles’ develop most of their friction at the toe of the pile and loads are transmitted directly onto firm strata with lateral restraint from the subsoil. This may mean deeper excavation to reach hard rock and in turn a larger quantity of concrete to be poured. Compare this with ‘friction piles’ that develop most of the pile-bearing capacity by shear stress along the sides of the pile, which may be mean less excavation but thicker piles.
Understanding the technique and function is key as different approaches could achieve the same engineering intent but come in at different costs.
3. Integrity Testing
Organisations such as NHBC and Building Control have integrity testing as a requirement. This form of testing verifies the structural integrity of the pile and checks for cracks throughout the pile. Some contractors may not include Integrity Testing, instead including an extra over rate ‘if required’. Check this is included as it is required for all piles and costs approximately £11.50 per pile, including mobilisation.
4. Concrete Cube Testing
Concrete Cube Testing should be included by all contractors. In the Structural Engineers specification this is typically a test required on 1 set of cubes per 50m³ of concrete delivered to site. Again, contractors sometimes include this as an extra over rate but, given it’s a specific requirement, it must be included in the tender – double check that it is.
5. Static Load Testing
This testing is influenced by the factor of safety (or Eurocode equivalent) used by the pile designer. It’s important to check the piling specification and if the factor of safety is reduced then Static Load Testing will be required. Again, always check this is included across all your tenders as necessary.
Pile type will determine the plant required, which is often incorporated into the specialist’s measured work rates.
When using a replacement method such as boring, excavation will be achieved by augering equipment. The auger will be attached to a telescopic arm and rotated into the ground. It can vary in length, gauge, and spacing between helices and width.
In comparison, displacement driven piles will utilise percussion machines, which rely on impact blows to the top of the pile to push it into the ground. Commonly used equipment includes: Hammering (Drop Hammers, Air Hammers, Diesel Hammers), Hydraulic Drivers and/or Vibratory Drivers.
Other considerations will include the rigs to which the pilie driving equipment is attached, along with other accessories such as excavation buckets or requirements for dumper trucks.
Request that the specialist provide a list of plant allowed for along with the rates, against which you can check against the market.
The process of constructing piles can be split into two parts: excavation of the shaft and insertion/forming of the pile. Time taken against each of these elements is known as productivity.
In instances where an all-encompassing rate has been provided, you should request a schedule of productivity times against each activity. This will aid assessment against other quotes and act as a benchmark for any future variation works. With these time schedules, labour and plant costs can be forecasted. An example of which is below:
|Depth of Strata (m)||Type of Rock||Production Rate (m/hr)||Plant Used||Time (Hrs)|
|0 – 20||Sand/minor rubble||2.00||Grab||10.0|
|20 – 35||CDG less than 150||1.50||RCD/Grab||10.0|
|35 – 47||CDG more than 150||1.00||RCD||12.0|
|47 – 57||CDG > 200/corestones||0.50||RCD||20.0|
|57 – 60||Rock socket||0.20||RCD||15.0|
When reviewing quotes, pay attention to the materials selected by the specialist. When assessing which specialist to appoint, you want to choose the contractor who meets the specification at the best value.
Comparing items such concrete type/strength, steel reinforcement thickness, steel casing thickness, sheeting piling profiles with the specification is useful to identify any fluctuations between rates of the various suppliers.
The method of constructing piles has not evolved particularly in recent decades. The benefit of this is that there is a wealth of empirical data that specialists can call upon to create accurate quotes. That said, there is always scope for value engineering. In this section, we highlight a few:
1. Design and Engineering
Design and Engineering is the single biggest factor that can result in value engineering opportunities. By optimising an existing pile design, you could reduce material quantities, construction costs and the construction time. You’ll also improve the performance and quality of the piling works.
The specialist’s designer will re-analyse the design via the use of industry software such as Infocad or Plaxis and identify whether the number of piles could be reduced, if there is a more efficient arrangement or whether different materials be used.
2. Pile Debonding
This is an intelligent method to create a cost reduction across the whole project. The Groundworker will always have to cut the top of the piles and this can be challenging. A way to simplify this is to install debonding foam around the reinforcement at the top of the pile. This will make the Groundworker’s life simpler and reduce cost.
3. Pile Diameters
The Structural Engineer will often over-engineer the initial pile design as, at that point, it is indicative only. In some cases, the more detailed design by the Piling Contractor should be able to reduce the diameter of the pile (e.g. from 600mm to 400mm) and in doing so save a lot in material/concrete costs.
4. Pile Quantities
Another means to reduce material costs is the replacement of fewer larger diameter piles with more smaller piles (e.g. instead of 20 nr. 600mm piles you could have 25nr. 400mm piles). The saving made on the material is significant and often outweighs the cost of labour, as the larger the pile, the longer it takes to drill to the required depth.
Whilst the type of plant will often be determined by the methodology, VE opportunities are still available. Questions to consider:
- Is the plant hired or owned? The latter may result in better value rates for machines, especially in the latter part of their service life.
- Productivity of the machine: If you have enough programme float, you may not need that new expensive machine which excavates twice the rate of an older machine. This is where time schedules are helpful.
- Consider the rigging size and its capability: do you need a rig that can bore down 60m when you only need to reach a depth of 32m?
The specialist will devise their preliminaries in support of the construction activities as they see fit. Whilst you do not want to undermine their expertise, it is reasonable to compare against other specialists to see if there are any excessive or under estimations.
VE considerations may include:
- Rationalise the of site management. If the specialist has concurrent sites in close proximity, could the management be split?
- If the project is simple and low value in nature, is it appropriate to appoint a larger specialist with a higher level of overheads apportioned on their preliminary rates.
- Could the responsibility of Health & Safety Site visits be shared?
Labour will be factored into the measured work rates, with the level required linked to the schemes design. It is prudent to establish a labour histogram from each specialist for ease of comparison.
Such histograms assist in understanding the reliance on manual labour as opposed to plant, giving you the ability to conduct a ‘breakeven analysis’ i.e. a comparison of the volume that could be excavated by manual labour and its associated cost versus plant. For example:
8. Ancillary Services
Look for services within the quotation that are not in strict relation to the process of pile driving, services that you, the developer, may be able to provide.
For example, Section 184 traffic management is a common bolt on service provided by specialists, but you may wish to procure it directly. Other examples include site engineering and logistics. In each case it’s prudent to consider the risks involved.
In line with industry trends of pre-fabrication, it’s beneficial to investigate off-site production, either for the reinforcements (such as steel cages) or the piles themselves (pre-cast concrete).
Programme and scheme dependant, you may find off-site production offers a significant time benefit, which outweighs any additional (if at all) of off-site production.
One of the top priorities and success measures of any given project is completion on programme. In this section we offer tips on how this could be achieved.
1. Planning, Planning and more Planning!
Prior to commencement, devote time in reviewing each element of the build phase. Continually refresh your project knowledge by reviewing information like layouts quantities, depths and logistic plans.
Don’t just focus on the piling package in isolation, as other trades will need to be considered. For example, do the concrete lead times meet programme deadlines? Do you have a contingency if, during the build phase, an issue arises and the concrete supplier is temporarily shut down?
For central government awarded schemes, BIM (Building Information Modelling) is mandatory and is a tool the private sector is eagerly adopting. BIM sparks collaboration between trades early on, creates a visual representation of the scheme, the ability to foresee any build clashes and general transparency.
Software is also increasing in use for real-time programmes, whereby the whole team will be notified of changes in programme and any scheduling conflicts.
3. Water Establishment
Water is needed on site to wash out the concrete drum and pump. This is a necessity for a piling operation as if this wasn’t available, concrete would build up within the equipment and stop the plant working.
4. Design Sign Off
A minimum of 3 weeks before piling commencement on site, the piling specialist will need final construction issue drawings to finalise the pile design and submit it to the engineers for sign off, prior to a start on site.
5. Attendant Excavator
To maximise productivity, it’s important that the Groundworker’s attendant excavator is in full time attendance with the Piling Specialist and not working elsewhere on site. Consider referencing this in the Groundworker’s Scope of Works.