a pressure of 300 bar (4,350 psi), with an approximate grout flow of 230 liter/min, an upwards velocity of 25 cm/min (0.8 ft/min), and a rotational velocity of 6 to 8 rpm. The jetting nozzles used were 2 x 3.5 mm (2 x 1/8 in) in diameter, the typical targeted jetting energy was 40 MJ/m, and the aim was to achieve a grout column of 1.2 to 1.5 m (4 to 5 ft) in diameter. Slurry Capping Trench The slurry mix used consisted of bentonite at 25 kg per cu m (1.6 lb per cu ft), cement at 468 kg per cu m (29.2 lb per cu ft) and water at 840 liters (222 gallons). When rockfill fines were added to the slurry mix, the final mixture consisted of bentonite at 8.3 kg per cu m (0.5 lb per cu ft), cement at 156 kg per cu m (9.7 lb per cu ft), water at 280 liters per cu m (214 liters per cu ft), and rockfill fines. Verification Verification of the success and effectiveness of the grouting proved to be difficult, with a variety of methods attempted, including falling/rising head testing, excavation of observation trenches, diamond core drilling and water level monitoring. The most successful methods of verification are described below. Piezometers The strategic use of standpipe piezometers placed along the inside of the grout alignment to the depth of the base of rockfill proved to be a reliable verification technique. The standpipes were installed at the start of the grouting process at approximate intervals of 10 m (33 ft) and offset from the landward side of the grout alignment between 3 and 10 m (10 and 33 ft). Vibrating wire piezometer sensors were installed into the standpipes and attached to data loggers at the surface, and recorded water levels at 20-minute intervals throughout the works. The data loggers were programmed to send data, via telemetry, to a central computer, which could be accessed remotely over the internet. Groundwater levels on the landward side of the seepage barrier were observed to diminish significantly as the grouting progressed and the hole spacing tightened to 1 m (3.3 ft) on center. Sensor readings indicated initial coupling of groundwater and tide levels; however, decoupling (or significant reduction) was observed in the latter stages during grout curtain closure. A typical series of piezometer readings during a three week period of tidal cycle (i.e., spring tide to spring tide) is shown in the graph below. In the graph, the second cycle of spring high tidal movement shows significant changes in groundwater behaviour, including a delayed response to tidal rise and fall, and a progressive reduction in the magnitude of the rise and fall of groundwater within certain standpipes as the grout curtain continued to be sealed. Uphole Circulation The observed behaviour of the fluid returning to the surface during jet grouting operations was a significant early indicator of success. Primary holes were grouted at center-to-center spacing of about 4 m (13 ft). It was noticed that, during grouting of secondary infill holes at 2 m (6.6 ft) spacing and, more noticeably, while grouting tertiary infill holes at 1 m (3.3 ft) spacing, progressively more fluid would return to surface, which was seen as an indication of the progressive success of the grouting. There was a distinct pattern of behaviour that would not only indicate success, but would also highlight porous areas, which likely corresponded to zones where variations in the seawall fill existed. Final Spring Tide Testing More convincing verification was by observation of performance inside the seawall on the first spring tide after the nominal closure of the grout curtain. At this stage, the grouting plant and equipment was maintained on location in standby mode, in the event that additional grouting was required. The performance during the spring tide testing was considered suitable and fit for purpose by the Stage 4 Standpipe Piezometer Monitoring 18 • DEEP FOUNDATIONS • MAR/APR 2017
