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Soil Mechanics and the Data Logger

Soil Mechanics and the data logger (by Steve Ackerley, Imperial College)

The Soil Mechanics laboratory at Imperial College is one of the most sophisticated in the world. As the laboratory title describes we are concerned with the mechanical behaviour of the soil. We carry out high quality laboratory tests to see the effects of movements and settlements caused by construction work.

In our laboratory we take intact or reconstituted samples and test them by various means. One such test is the triaxial test. We place a cylindrical sample enclosed within a waterproof jacket vertically inside a cell. Pressure is then applied to the diameter and a load to the top of the sample. This as you can imagine would put the sample into the same conditions it would be if it were in the ground. (The sample would have the soil around it and a load from the soil or construction above it.)

  Soil Mechanics Image

Now the sample is in the conditions we require we can monitor the effects of the sample with changing stresses. (i.e. radial stress and load). We also take a number of other criteria into account. Water pressure and volume from inside the sample and small strain measurements directly on the sample need to be monitored. These are just some parameters that we can measure.

 

When stress changes are made to the sample they are normally carried out very slowly to allow the water pressure within the sample to drain. Samples with a high clay content dissipate very slowly and can take one to three months to complete.

 

Some samples are nearly as strong as concretes therefore we have to apply large loads to fail them. We also have the problem of measuring very small strains down to 0.00001mm and also need to shear the sample up to around 25mm due to the highly non linear characteristics of the materials.

 

Datascan 7220 units suit our application perfectly as they integrate over one mains cycle. Even under our laboratory conditions with lots of other equipment around we usually get two to three bits stability.  With this level of stability we do not need amplification or filtering on our strain gauged transducers. These loggers are very stable and give a high resolution.

 

We have used Datascan units in the Soil Mechanics laboratories for over 20 years and have found them to be extremely reliable. There are around thirty 7220 units that are in constant use. Our Structures and Concrete laboratories have also been using them for about 10 years. We have also installed many Datascan loggers in other universities in the UK and overseas.

 

Any problems that may occur are easily overcome. The logger is addressed by simple commands that may be sent stage by stage via a computer and the response can be monitored. If all else fails a few words with the friendly experienced staff at Measurement Systems usually sorts things out.