J Washbourne, MSc, BSc (Eng) Hons, CEng, MICE. This paper was first published in GE’s March 1986 edition.
Thixothropic slurries have been used for many decades in the oil industry for drilling oil well holes, and in the last 30 years they have been used increasingly in the piling and diaphragm walling industry.
Fig. 1 shows the unimpeded use of a clamshell grab in a diaphragm wall panel excavation which is supported by ‘bentonite’ slurry. At the end of excavation the steel reinforcement can be placed and finally in its fluid state the slurry can be displaced by tremied concrete which virtually wipes it clean from the steel – see Fig. 2.
Only in the last ten years have serious attempts been made in the tunnelling industry to tap the full benefits of these slurries. Thixotropic slurries can be made to perform such functions as earth support, lubrication or be used as a transportation medium for soil particles. The performance of these functions is, to a substantial extent, dependent upon thixotropy. The nature of this property is that the slurry, in its agitated state and for a short time afterwards, has a high level of fluidity but if it is left undisturbed for a quarter of an hour or so, it gels and loses its fluidity. The two states, that is, fluid and gel are infinitely reversible; disturb the gel and fluidity is achieved; leave the fluid undisturbed and gelling takes place.