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The design of structures such as embedded retaining walls, cut-and-cover tunnels, and bored tunnels in over-consolidated clays requires detailed and reliable information on the lateral stresses in the ground.
It is possible to obtain good measurements of insitu stress and its variation around the axis of the instrument by observing the membrane lift-off pressures when using a self-boring expansion pressuremeter.. However the strain arms must be in perfect condition and well adjusted. Even so considerable engineering judgement is necessary in interpreting the readings. There are at least four sources of ambiguity and the measurement cannot be repeated.
Much better readings of the insitu stress can be obtained using a self-boring instrument fitted with load cells on its outside curved surface. The original Hughes and Wroth instrument had two Total Stress load cells on opposite sides. These load cells were subject to gas pressure on their inside surface which pressure was adjusted to keep the output of the Total Stress cells close to zero at all times. Thus to a first order the instrument was a null sensing device involving no movement of the sensitive areas of the load cells.
Later versions of the original instrument continued with two total stress cells and the null sensing control system but added a Total Pressure cell sensing the actual pressure in the instrument and a Pore Water Pressure transducer
The Transport Research Laboratory (TRL)
Design
In the present TRL inspired design the opportunity has been taken to adopt
six load cells equally spaced around the instrument with each cell being
provided with its own individual pressure controller. Each cell also has
its own total pressure transducer and its own pore pressure transducer with
a high air entry ceramic filter.
Complex interpretation of the readings is not necessary. The instrument is direct reading and the nulling characteristic of the cells seriously reduces the effects of cell action factor.
In addition the instrument is made to have the same dimensions as the Cambridge Self-boring Pressuremeter. Thus all the drilling equipment used for one can be used for the other.
A section through one of the ‘null sensing’ load cells is shown in Fig. 3.
Fig.2 Staggered cross-section through the load cells and piezometers
The cells used are similar in design to the contact stress transducer of
Arthur and Roscoe (1961) using strain gauged pillars to support the cell
top plate. The cells are arranged to be sensitive to normal stress and not
to shear stress. Whilst the load cells will measure soil lateral stress directly
with no balancing internal gas pressure, it is intended that the instrument
will normally be used in null sensing mode.
Fig.3 View of the cells
If used in direct mode the instrument would be subject to error owing to the small movements of the top plate developed under load. Although this movement is small, about 16 microns at 2 MPa pressure, it will still cause significantly low readings in the measured soil stress due to the operation of Cell Action Factor. The magnitude of this under-registration would depend on the relative stiffness of the cell and the soil (Carder and Krawczyk, 1975).In null sensing mode the load and total stress cell control systems are zeroed on the ground surface before the pressuremeter enters the borehole. Thereafter the null sensing is continuously operated for 24 hours per day. As the external stress acting on the instrument changes with depth, the internal gas pressure to each individual cell is automatically adjusted by the control system at the surface to maintain the load cell output at a nominal zero. Throughout the drilling and when testing at each depth, the internal gas pressure is in balance with the external soil stress.
In practice, the response time of the control system is such that small differences from nominal zero are measured on each load cell. These are corrected by summing the measurements from the total pressure cell and the load cell to calculate the external soil stress.
The six piezometers have detachable high air entry ceramic elements which need to be deaired before the instrument is used. A strain gauged diaphragm for the measurement of porewater pressure is included behind the ceramic element of each piezometer. The piezometers are direct reading and use the same technology as in the expansion pressuremeter.
Control system and signal conditioning
Signal conditioning of the 18 electrical outputs from the six cell clusters
occurs down the hole, within the pressuremeter. The outputs are amplified,
multiplexed in a 16 bit analogue to digital converter and relayed by the
umbilical cable to the control system on an RS232 link. The control system
provides power to the pressuremeter as well as allowing individual zeroing
of the load cell and internal gas pressure control systems. The data from
the pressuremeter are transmitted by the control system also on an RS232
link and the results are stored on a portable computer with hard copy provided
on a printer.
Fig.4 View of the electronics compartment
A complete set of the 18 voltage outputs is recorded every 10 seconds as an ASCII text string which can then be imported into a spreadsheet for subsequent analysis. In the first field trial, the calculated total lateral stresses and porewater pressures were immediately printed and used to assess the test duration required for stresses to reach a steady value.
The TRL load cell pressuremeter with its control system is powered by an external 12 volt lead acid car battery. During the test the power consumption is about 6 watts. When setting zeros at the start of the borehole or during bench calibration the control system is in ‘reset’ mode and power rises to about 45 watts.
Bench Testing and Calibration
The pressuremeter is calibrated using a cylindrical collar which enables
a known external pressure to be applied simultaneously to all six cell clusters.
Piezometers
The piezometers, being direct reading devices, are calibrated by applying
gas pressure to the external collar in increments up to 800kPa. The output
voltage at each increment is then plotted against the applied pressure and
a best fit calibration determined for each piezometer. During calibration,
the ceramic elements are removed to eliminate any possible effect on the
response time of the piezometers owing to the permeability of the ceramic.
Load Cells
The load cells are calibrated by applying gas pressure to the collar in 10kPa
increments up to a maximum of 50kPa. This covers the operating range in null
sensing mode. The best fit calibration obtained is adopted.. In the event
of a failure of the control system, the load cells are designed to withstand
a maximum stress of 2MPa. This corresponds to about 30 metres depth in a
stiff overconsolidated clay with Ko = 3.5
Total stress cells
Calibration of the total stress cells, which measure the internal balancing
gas pressure, is carried feeding gas pressure directly to the pressuremeter
and by-passing the control system. The same gas pressure is simultaneously
applied to both the instrument and the external calibration collar so preventing
a large differential pressure from overloading the load cells.
Conclusions
References
Arthur, J.R.F. and Roscoe, K.H. (1961) ‘An earth pressure cell for the measurement of normal and shear stresses’. Civ. Engng & Publ. Wks Rev. 56. No. 659, 765-770
Carder, D.R. and Krawczyk, J.V. (1975) ‘Performance of cells designed to measure soil pressure on earth retaining structures.’ TRRL Laboratory Report 689. Transport Research Laboratory, Crowthorne, Berks.
Darley, P., Carder, D.R. and Steele D.P. (1999) ‘Field Evaluation of the TRL load cell pressuremeter in Gault Clay’. TRL Report 426. Transport Research Laboratory, Crowthorne, Berks.
Further details about the Cambridge self-boring pressuremeters are given in other leaflets of ours which we supply on request. There is also a set of detailed specifications available relating to the present instrument.
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Cambridge Insitu
Little Eversden,
Cambridge
CB3 7HE
Telephone: 01223 262 361
Fax: 01223 263 947
E-mail: caminsitu@aol.com