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Lankelma undertook a programme of cone penetration testing (CPT) combined with seismic cone penetration testing (SCPT) at the Cinque Port of Rye in East Sussex.
Harbour History
The harbour of Rye comprises all the tidal channels seaward of the four tidal sluices on the river Rother and its tributaries. Rye became a Cinque port in the 13th century, however, it lost its importance as a port when the coastline changed, leaving the town as it is now, some 2 miles (3.2Km) from the sea.
The sheet piled wall along the entrance of the river Rother was originally installed in 1914 and extended and refurbished in the 1920s and 1930s. In the 1970s the eastern side of the harbour wall was replaced. There is now a severe reduction in the strength and safety of the western wall.
Objective
The CPTs and SCPTs were carried out to determine the depth of the sheet pile toe along the length of the western river wall and to provide geotechnical data on the underlying soils for the design and installation of a new proposed sheet pile retaining wall.
Scope of works
The scope of works included 15 cone penetration tests with 9 of these including seismic shear wave tests at 1m centres.
Intertidal conditions
The ground surface consisted of very soft marine muds, inaccessible on foot. This restricted access was further complicated by the adjacent river and immersion at incoming and high tides.
Underlying the very soft muds were units of medium dense to dense granular soils. To cope with this, Lankelma mobilised its new rubber tracked light weight crawler rig (UK8). With very low ground pressure and sufficient reaction to penetrate dense granular soils, UK8 exceeded expectations.
Combined tests, specific locations
The seismic tests were carried out at specific locations along the length of the sheet piled wall. A combined seismic cone penetration test (SCPT), as well as taking seismic readings, takes continual tip and sleeve readings to establish the soil type with the change in depth. Previous estimations of the client expected the toe depth to be approximately 7m – 8m below ground level (see results in table below).
Parallel seismic techniques
Parallel Seismic techniques for pile geometry measure the time taken for a seismic wave to propagate (or travel) from a pile head to a sensor in the ground. The interpretation of the travel times to sensors at different depths can be used to estimate the lengths of a pile (Butcher, 2006).
During an SCPT the sensor is pushed into the ground to a measured depth close to the pile. Once at the required depth, the source is activated and the data recorded. The sensor is then pushed to the next measurement depth and the procedure repeated.
Fieldworks
These seismic shearwave tests were performed every metre, from between 3-4m below ground level down to depths of 21m. At each test depth, the top of the pile was struck vertically with a sledgehammer and the resulting seismic waves travelling down the sheet pile were detected by two sets of triaxially mounted geophones.
The shear wave velocity was recorded by a Geometrics Geode signal enhancement seismograph. An inertia switch on the hammer was used to trigger the seismograph's time base. Typically, 2 to 4 hammer blows were stacked and the resulting six channel seismic record was stored in SEG Y format on the hard disc of the laptop computer controlling the seismograph.
The two sets of triaxial geophones were mounted 0.5m apart with the lower set located 0.75m above the cone tip. The probe was hydraulically pushed into the ground using the CPT rig. The rig’s positioning meant that the geophones were between 3m and 4m from the wall (the wall being the point of contact with the hammer).
The travel time of the shear waves were then corrected for the horizontal distance offset between shear wave source and the cone string to give corrected (vertical) travel times.
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