How do screw piles compare with conventional concrete foundations?

Answer: Screw piles are quick to install, use no concrete or wet trades, no muck away, no disturbance or contamination, use lightweight plant and are installed to a measured capacity.

What are the advantages of screw piles?

Screw Piles – in addition to the above, work in freezing conditions, are ideal round trees or in expansive soils can take load immediately. If necessary they can be removed and reused; ideal for temporary structures.

Where can you use screw piles?

Screw Piles will work in confined spaces and penetrate most soils – if the ground is fractured to solid rock, then they won’t be economical, but special piles work really well in many soft soils.

Can screw piles be removed easily?

Answer: They can be easily unscrewed, but may need 20-25% more torque for removal than insertion, tent erection contractors use them all the time for reliability and reusability.

Can screw piles be used for tension loads?

Answer: The original piles were for compression loads but AB Chance reinvented them for tension application once installation equipment became available.

Do screw piles offer lateral resistance?

Answer: Intuitively a relatively slim pile shaft has poor lateral resistance. We produce transition extensions (Conical extensions) which give significant improvement and can install piles up to 610mm diameter with ease.

How are screw piles installed?

Answer: Basically just like a woodscrew, using hydraulic motors. For the same holding power, you need to change the length and size of the screw. Measure the torque constantly and you can predict the holding power.

Do screw piles corrode?

Answer: All steels corrode but you need an electrolyte (water) and air so the area at ground level is of greatest concern. Our piles are thicker than our competitors and they can be galvanised. Read our document for more information. Identifying issues leads to a satisfactory result.

How accurately can you install screw piles?

Answer: Hand installed piles will usually be within 50mm of the correct position, use of our new “hole” pile generally improves this to 25mm. Machine installation can usually be more accurate and use of the correct termination will ensure a high level of accuracy.

What is the life expectancy of screw piles?

Answer: We aim for a minimum life of 60 years.

What happens if you hit rock or obstructions?

Answer: We produce a “rock” pile which can penetrate fractured rock, provided there is room for the point and shaft to displace the material. Sometimes we use a rock auger to “pilot drill” a pole through hard layers. The worst situation is where hard material is encountered below 2 metres and the pile “spins out”. It may be possible to reposition the pile or install a pair of piles spanning the obstruction.

How deep can you install screw piles?

Answer: Concrete piles, because they depend on skin friction and end bearing need to be quite deep. 10-12 metres is typical, whereas the helix plates on a screw pile, being in practice like a multiple set of end bearing piles all on a common shaft, typically “torque out” (i.e. reach maximum capacity) at 6-8 metres. We have installed them down to 14 metres before we met good ground.

What happens if you can’t screw the piles in to finish height?

Answer: If the pile “torques out” before hitting level, it can be cut off. Some users who consistently install to maximum capacity have to trim piles to level.

How do you set the finished height of screw piles?

Answer: Use of a laser level will ensure piles are within a few mm of level.

Can piles be installed without using an excavator?

Answer: We have our 400H, 650H and 1000H hand held installers. The 400H igves working loads of 60kN per 60mm pile and is ideal for extensions, underpinning and lighter weight buildings. All parts fit through household doorways.

Do screw piles generate any spoil?

Answer: Screw Piles produce no spoil – no muck away required!

Can screw piles be installed at any angle?

Answer: Screw Piles can be used at any angle. We have a special design to achieve the best results when using them as a spoil nail.

What soils can/cannot screw piles be installed into?

Answer: The only limit on installing a screw pile is if the ground is so rocky that it will not penetrate. Soft rocks are no problem and the light weight of the piles and installation equipment can be a positive benefit.

What distance should screw piles be installed from each other?

Answer: Ideally, piles should be placed 5 of the largest helix diameters apart. A reduction in capacity of 20% will allow this to be reduced to 3 diameters. It is possible to angle the piles so that the tops are touching, yet ? full capacity is achieved.

Can screw piles be terminated above ground level?

Answer: Screw Piles can be used for piers (Brighton pier is an example) with suitable bracing.

What size excavator will I need to install screw piles?

Answer: Having determined the pile size, a suitable torque head can be selected. This governs the minimum excavator size

 

Pile Size Torque Load Torque Head Excavator (tonne)
60RL 2.5kN 28 500X 1.5
60R 4.5kN 56 500X 1.5
76RL 8kN 96 1800X 3.5
76R 16kN 192 1800X 3.5
89R 18kN 180 1800X 5.0
89RHD 25kN 225 2500X 6.0
What load will screw piles take?

Answer: The maximum loads at a FOS of 2.5 are shown above. Where a lot of piles are to be used, it is advantageous to carry out a load test which usually will establish a higher ? torque factor ? – thus allowing higher loads.

What is a FOS?

Answer: The factor of safety (FOS) represents the ratio between the calculated or measured ultimate capacity of the pile and the actual load imposed on it. When the ultimate load has been established with a series of tests, it may be possible for the engineer to allow an FOS of 1.6 but normal practice is to use between 2 and 3; we are using 2.5 in our tables. Because we measure the installation torque of each pile (and this is a well-researched and proven technique), a lower FOS for screw piles is often used,

 

Do I need to inform planning/building control?

Answer: You should always consult an appropriate building control engineer before using screw piles. Most now accept the system; many specify it, especially in difficult situations.

 

Can I use screw piles next to trees?

Answer: Screw Piles work very well in the ? of trees. The general practice is to site the highest helix at least 2m below ground level where the roots have very little effect. Research by Kew Gardens indicates little effect below 1 metre.

 

Will screw piles move if soil heave is an issue?

Answer: Expansive (and contracting) soils present no problem for screw piles. In some places, soils can expand and contract by up to 10cm and screw piles are specified by the local authority. The highest helix may be specified at 4m deep with a total pile length of 6 or 8m. As the load is taken on the helixes, skin friction on the upper shaft is cancelled by similar friction in the lower portion.