Retrievable Extensometers and Pile Load Testing

Conventional bored pile instrumentation for static load testing generally relies on strain gauges and tell-tales cast in the pile to measure axial loads and movements at various levels and at the pile toe. One drawback with this method is that the instrument locations need to be predetermined and the instruments have to be pre-assembled and installed onto the steel cage prior to concreting. Also of concern is that strain gauge measurements can reflect localized strains due to variations in pile cross-section and tell-tales can yield unsatisfactory results due to rod friction, bowing, eccentricity of loading, reference beam movement and the fact that it can be impractical to put tell-tales at multiple levels where complex stratigraphy exists due to congestion in the set-up at the pile head.

To address these concerns a new generation of extensometer, the GEOKON Model 1300 (A-9) Retrievable Extensometer (Patent No. 5,585,555), has been developed to simplify, and allow, highly accurate measurements of relative deformations of multiple segments along the entire pile length. It is designed to be installed in a two-inch PVC or steel pipe cast into the pile (thereby minimizing the risk of damage during the concreting process) and consists of pneumatically actuated anchors with spring-loaded transducers that are connected to one another in series by a single connecting rod, which is held in tension to eliminate errors due to bowing and friction. When installed, the anchors are fixed in place and the transducers measure the deformation between the anchor positions over specific increments, thereby integrating strains over a larger and possibly more representative sample than that which can be obtained using conventional strain gages.

To install the extensometer, the string of sensors is assembled with variable lengths of connecting rods to enable positioning of its anchors at the required depths and inserted into the access pipe, bottom anchor first, lowering in a large arc to not permanently deform the rods. The string is then locked into position by pneumatically actuating the various anchors, starting with the deepest anchor. The transducers are set, deepest segment first, to the required range by pulling on the extension rod protruding from the borehole until the desired reading is obtained, and then locking the transducer in place by turning on the pressure valve. This procedure is repeated for each subsequent segment in the extensometer assembly.


The anchors contain eight pistons, which are pneumatically actuated and forced against the sides of the borehole. The pistons are spring-loaded and automatically retract once the pressure is released.

A pressure manifold, containing on/off valves and check-valves, connects to each of the inflation lines leading to the anchors, enables each of the anchors to be actuated in turn, and maintains the anchor pressure during the monitoring period. Pressure is provided using commercially available, pressurized, nitrogen cylinders. When monitoring has been completed, pressure is released, which allows for removal of the extensometer and future reuse.

Each anchor is attached to a highly accurate and sensitive vibrating wire sensor (or, optionally, to a DCDT or linear potentiometer sensor) by means of Swagelok fittings that grip the interconnecting rods. Rods may be made from stainless steel, invar or carbon graphite. Readout is accomplished via portable readouts and/or data acquisition systems, the latter being preferred for automated testing so that displacements can be recorded along with the applied load at each specific loading increment

The A-9 Retrievable Extensometer has been successfully used on many projects all over the world, including the Bibliotheca Alexandrina (Egypt) and in many pile load tests in Malaysia where comprehensive studies have been carried out by Lee Sieng Kai, managing director, Glostrext Technology Sdn. Bhd., Kuala Lumpur, Malaysia et al.

In their paper “Application of Global Strain Extensometer (GLOSTREXT) Method for Instrumented Bored Piles in Malaysia,” published in the 10th International Conference on Piling and Deep Foundations, May 31st to June 2nd in Amsterdam, Netherlands, Abdul Aziz Hanifah, senior assistant director, public works department, Kuala Lumpur and Lee Sieng Kai report several conclusions comparing the results from the A-9 Retrievable Extensometer with conventional pile instrumentation:

  • The back-calculated concrete modulus values measured by two independent systems (conventional strain gauges and Retrievable Extensometers) agree reasonably well.
  • The axial strains measured by the two independent systems are in good agreement.
  • Using the Retrievable Extensometer measurement of the pile shortening over the whole pile length can be reliably measured in segments. This enables the movement of the pile and strains at various levels down the pile shaft to be determined accurately, thus permitting an improved load transfer distribution of piles in static load tests.

For additional information please contact GEOKON, INCORPORATED at 603-448-1562 or visit