Prior to the launch of ELE's new Cyclic Triaxial Soils Testing System, the only equipment available to conduct such tests was complicated to use and very expensive. ELE has now resolved these problems, and the following paragraphs, written by researchers at England's Sheffield University, describe the new system and its relevance to liquefaction of soils in particular.

Summary
One of the most frightening and destructive phenomena of nature is a severe earthquake. An earthquake is a sudden movement of the Earth, caused by the abrupt release of strain that has accumulated over a long time. For hundreds of millions of years, the forces of plate tectonics have shaped the Earth as the huge plates that form the Earth's surface slowly move over, under, and past each other. Sometimes the plates lock together, unable to release the accumulating energy. When the accumulated energy grows strong enough, the plates break free and a major earthquake occurs. If the earthquake occurs in a populated area, it may cause many deaths and injuries and extensive property damage.

Repeated shear stress in saturated sandy soil during earthquakes causes a rise in pore water pressures and a corresponding decrease in effective stress. If the excess pore pressure generated exceeds the in-situ effective overburden pressure then liquefaction occurs.

In recent years cities have expanded on to marginal areas which have often been landfilled and increasing use is made in coastal areas of reclaimed land and artificial islands. As a result the liquefaction of ground has become an engineering problem. Liquefaction causes sand boils and springs of water, ground settlement, ground oscillation, flow slides of slopes, loss of bearing capacity, failure of retaining walls and quay walls and buoyant rise of buried structures. All of these result in a loss of function e.g. subsidence of road embankments, cracking of surface pavements leading to non-serviceability, building subsidence and tilting, lateral movement of bridge abutments and piers resulting in collapse of the deck, breakage of buried gas and water mains, floating of sewage treatment tanks and buried pipes, instability and overtopping of earth dams and distortion of harbour quays making them unusable.

Recent years have seen a number of large earthquakes, such as the 1995 Great Hanshin (Kobe) earthquake in Japan and the 1999 earthquakes in Turkey and Taiwan earthquakes where severe damage occurred due to soil liquefaction. However liquefaction problems can also occur due to wind and wave loading on coastal and offshore structures. The types of soils that are most susceptible to liquefaction are loose, water saturated, fine-grained sands and non-plastic silts that lie close to the ground surface.

The design of remedial treatments such as soil compaction or solidification, gravel drains to dissipate excess pore pressures and even diaphragm walls to block propagating pore water pressures all require an assessment of the soils liquefaction potential. Despite the increasing need for the evaluation of the liquefaction potential, there is still insufficient laboratory testing before design and construction because cyclic triaxial testing procedures can be both complex and costly requiring skilled technicians.

The assessment of the cyclic strength of soils is now relatively straightforward using ELE's new Cyclic Triaxial System which applies cyclic loads to soil specimens using a simple computer controlled menu driven system. Current "state-of-the-art" testing machines use very large scale integrated circuitry within the electronic control and data acquisition system and the operator is confronted with two interfaces: (1) the testing frame/specimen interface and (2) the interface between his own test program and the control electronics.

The first interface is related to specimen configuration and usually takes the form of grip, screw or load platen attachments. The second has, conventionally, been by way of knobs, dials and switches which the operator must set before he can commence a testing sequence. Conventional electronic systems are capable of carrying out very complex tests but this often increases the number of controls required, making it difficult and time consuming for the operator to set up even a simple test. A further complication arises when a complex test, developed on a flexible system using careful, skilled operators, must be introduced into a quality control environment. Often, expensive remodelling of the equipment is needed to adapt it to its new place of work.

With the rapid change in electronic technology over the past decade it is now possible to use high speed digital signal processors to automatically control the hardware. This allows the operator to express his needs by means of a computer program, which in turn makes the calculations and controls the testing machine. The development of ELE's Cyclic Triaxial System, as a general purpose, low cost and computer controlled loading machine, provides researchers and engineers with a tool capable of conducting a range of tests in association with an integrated software package.

The system comprises a compact loading frame, fitted with a servo-controlled pneumatic actuator assembly, a triaxial cell, a triaxial control panel, and a control and data acquisition system to be connected to a PC. Loading forces are transmitted to the specimen using high pressure air acting on a high speed low friction double acting piston, modified to have a displacement transducer mechanically coupled to the actuator shaft, enabling both displacement and load control.

Electric signals applied to the electro-pneumatic converter from the control and data acquisition system enable computer controlled adjustment of the air pressure for the pneumatic actuator. The flow of air pressure is controlled by a servo valve in which small electric currents are used to open and close the control spool of the valve.

The servo valve is controlled by a closed loop system. The feedback signal from the transducer is connected to a summing junction within the control and data acquisition system and compared with the command signal. The difference between the two signals is used to drive the servo valve to regulate the flow of air pressure in the direction to eliminate the error. Thus, the response of the system does not rely on the alertness of an operator as the control loop is closed electronically. This is a very important feature because recent research at Sheffield University in England has shown that open-loop systems are in serious danger of providing inaccurate data, which underestimates the level of risk in foundation design.

The control and data acquisition system is linked to a PC and provides all critical control, timing and data acquisition functions for the testing frame and transducers. The operator uses digital control of the pneumatic servo-valve to apply the requested loading rate or wave form. The system can generate a variety of desired loading wave-shapes and operate with sinusoidal loading at frequencies of up to 70 Hz. All the usual sine, square, haversine, ramp, triangle etc., wave-shapes are provided, plus the facility to input any wave-shape that can be defined by the operator.

The operator can condition these wave-shapes even during the running of a test. Such performance has not previously been achieved by pneumatic systems.

The software is user friendly and menu driven. The loading rate and loading sequence are pre-specified in each test programme by the operator. This allows the operator to select maximum position, force and strain limits for different parts of a test sequence. In operation the system gathers the dynamic data from the transducers attached to the specimen under test then displays plots of, for example, stress vs strain, strain vs time (as appropriate for each test type and function mode) in real time on PC. The software automatically saves test information in binary files which then provides the off-line facilities of reviewing previously run tests through the graphics screens of the system or generation of data files for importing into a spreadsheet package.