Underground Singapore 2011 The final MFC grout mix had a water cement ratio of 1.2, compared with the typical 1:1 ratio used in Sweden. The increased water:cement ratio increased the bleeding for the grout, however the latest grouting research has shown that an increased water cement ratio can be used in a MFC cement grout, as long as the bleeding can be kept at an acceptable limit and the strength requirements are not critical. In this case, the measured bleed with the 1.2:1 mix was 5 – 7 %, which was considered acceptable. The mixing water was cooled to 10°C. The cement power, which was stored in the open, on site, had an initial temperature of more than 30°C. Combining the chilled water and the cement power resulted in an initial temperature of about 20°C. The resulting MFC mix had a pot life of approximately 30 – 40. The final microfine cement mix consisted of a 1.2:1 water to Microcem 650, with 1% Muraplast120S additive. When mixed in the high speed colloidal site mixer and tested with a filter pump, b crit was 95 µm and bmin 41 µm. An OPC grout mix was developed for toe grouting and for the injection of large fissures. For the grouting trial the OPC grout mix was 1:1 water : cement, with 1% bentonite used to control bleeding. As discussed below, a thicker OPC mix was used after the grouting trial.
Figure 8. Filter pump testing on the Bukit Panjang site
Figure 9. The filter used in the filter pump
5.2.1 Outline of grouting trial The grouting trial was carried out to assess the effectiveness of the mixes, injection sequences and limiting pressures proposed. The trial was a ‘working’ trial: the grouting for the trial formed part of the finished grout curtain. Grouting was carried out under 10 secondary secant piles. Each pile included two 110mm preformed ducts (Figure 5), so that drilling commenced just above the toes of the piles. The drill holes were split into A1, A2, B3, B4, C5 and C6 holes, to define the grouting sequence, as shown in Figure 10.
Figure 10. Plan view of the grout trial
9