625kN Pile Test – Restricted Access London Site

When configuring suitable screw piles for a project, our Engineers rely heavily on soil properties determined from geotechnical investigation. Often undertaking soil testing using the ABC Anchors soil test probe, or commissioning a full soil investigation (SI) report from a geotechnical engineering firm provides us with sufficient design information. On small projects, use of our soil test probe instead can significantly reduce the cost compared to an SI report and provide similar results for specification purposes.

There are occasions however when pile testing is necessary to confirm our pile specifications.

ABC Anchors was recently contacted by a Client looking to install screw piles for a residential project in close proximity to the Thames in London. From an initial review of the SI it was understood that the site was underlain by a thin layer of dense gravels, however the soil properties below this were not described in the report. Being London based, an area known for weak Clays, and with a site in such close proximity to a river; it was decided that the soil report could not be relied upon alone. As the project required over 50 piles to be installed with design safe working loads of 20 tonnes per pile, it was critical to test the pile under load to confirm that any settlement was within acceptable limits.

ABC Anchors installed an 89R test pile to an installation torque of 25kNm using our Autoguide 2500X torque head mounted on a 3 tonne excavator. Typically excavators between 5-10 tonnes are used to install piles of this size, however as the site access was restricted to a small alleyway only 1.5m wide, a 3 tonne excavator was the only option. It was a testament to our Engineering team’s ingenuity and the skill of our installers that installation torques of this magnitude were achieved using such a low powered machine.

The pile was tested using our recently redesigned ABC Anchors testing equipment and experienced only 2mm of settlement when loaded to the pile’s ultimate load capacity of 62.5 tonnes. The Federation of Piling Specialists suggest that allowable settlement should be no more than 10mm under working loads.

The testing was a complete success and confirmed the suitability of our 89R rock tip pile for the project.

New Build – 89R Screw Piles Henley on Thames

89R piles were installed for foundations on this site in Henley on Thames. The site was particularly sensitive as it contained roman artefacts which if exposed would have halted the build. The installation of the piles also included a load test on a single pile which is sometimes asked for by the structural engineer to verify installation torque against pile capacity.

Screw Piles

Screw Pile Capacities and Installation Equipment

  60Rrangeb 76RRangeb 89R-39336b
Screw Pile Range 60R 76R 89R
Torque Head Model 400H 500X 650H 1600X 2500X
  400H 500X_XG 650H 1800X 2500XGb
Max Torque 4kNm 5kNm 6.5kNm 16kNm 25kNm
Machine Type Hand-held Excavator Hand-held Excavator Excavator
Ultimate Pile capacity
(un-factored) torque
120kN 150kN 180kN 450kN 625kN
Safe Working Load
(2.5 FOS)
48kN 60kN 73kN 180kN 250kN
Specification Sheet
(PDF)

Un-factored Load - A Maximum load that the Pile will take before deflection exceeds standard limits.

FOS - Factor of Safety - is the ratio between the un-factored load and the working load

Safe Working Load -The actual load the pile is taking when the building is in place

All machines offer torque read outs, allowing the pile capacity to be calculated instantly.

Screw Pile Specifications

SPECIFICATION DEFINITION 60R 76R 89R
Shaft Diameter Diameter of the screw pile shaft excluding helices 60mm 76mm 89mm
Wall Thickness Thickness of the screw pile tube wall 6.35mm 9.5mm 9.5mm
Torque Limit The ability of the pile to transmit torque is always the limiting factor. This is therefore the ultimate practical pile install torque capacity. 4kNm 16kNm 25kNm
Ultimate Strength Single Helix The load which can be supported by a single helix- most piles use multiple helixes, but if a single one has to support more load (although non standard) this can be accommodated. 90kN 160kN 220kN
Empirical torque factor KtKt m-1 The “Empirical Torque Factor” expressed in metric units when torque is measured in kNm and force in kN. Its value decreases as pile diameter and helix plate thickness increases. This is due to a combination of skin friction and the energy needed to displace the soil. 30m-1 28m-1 25m-1
Tube Specification S355 HFS DIN1629 ST52 or Equal S355 HFS
Helix Specification BS4360 43A BS4360 43A HRMS BS4360 43A
Fasteners Specification Metric M16 GR8.8 Metric M20 GR8.8 Metric M22 GR8.8

Pile Life/Corrosion

In most conditions, provided the top 2 metres are galvanised the pile life will be in excess of 100years. Where the soil resistivity is less than 10 Ohm– further steps must be taken. This means salt water, wet peat and where soils are subject to saturation. For more information please download the soil corrosion chart.

The estimated life is based on the assumption that a loss of 40% in thickness is acceptable without adverse effect on structural integrity. The atmospheric exposed areas of the piles are coated with high build zinc rich coating system or galvanised. There is a safety factor of x2 included for the anode design to take account of variances in corrosion conditions and ground resistivity.

Ground Resistivity(Ohm-m) Corrosion rate based on Uhlig data mm/yr Estimates life with corrosion protection (years) Corrosion protection methods, required for top 2m section of Pile. (Min 9.35mm wall thickness) Estimated life with additional corrosion protection* (years)
Less than 5 0.1-0.2 18-37 Zinc rich coating
4 x 20kg galvanic zinc bracelet
100
5-10 0.033 80+ Zinc rich coating
4 x 20kg galvanic zinc bracelet
100+
10-50 0.025 100 Zinc rich coating 100+
50-100 0.018 100+ n/a 100+
Greater than 100 0.014 100+ n/a 100+