Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
2000-02-16
2001-11-06
Noori, Max (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
C073S803000
Reexamination Certificate
active
06311567
ABSTRACT:
The present invention relates to a method and apparatus for testing the static load-bearing capacity of engineering piles.
Piles, usually made out of concrete, are generally used to form the foundations of buildings or other large structures. Before using the piles as a Foundation for further building work, it is important to test the static load-bearing capacity of each pile. This is generally done by applying a test load to the top of a pile by way of a hydraulic jack braced against a reaction system comprising a cross-beam which is anchored in place at its ends. The test load is generally measured by monitoring the hydraulic pressure supplied to the jack, and the associated displacement of the pile is measured by using a displacement sensor. Frequently, the displacement of the pile is measured for a number of increasing test loads, each applied for a predetermined time. Because the applied test loads tend to be high, there is a significant danger to operating personnel should the cross-beam or its anchorages fail, particularly if the operating personnel are required to read test values from a gauges located on equipment located close to the top of the pile.
Furthermore, because the applied test load has to be maintained and adjusted by operating the jack manually, it is necessary for operating personnel to be in attendance at all times. It is not safe for a single operator to work alone, particularly overnight (the typical time taken to perform a comprehensive static load test can often be as much as 18 hours), and accordingly the present method of static load testing is expensive, as well as being slow.
Another disadvantage of the known static load-testing equipment is that the quality of the data obtained is not always consistently good. Typical data required from a static load test are the record of displacement of the pile head and the load applied. Although manual reading and recording of the dial gauges employed in a static load test should not present an insurmountable difficulty in terms of accuracy and regularity, it is the application of the load that generally is the source of poor quality data. This is principally due to the need to attend continuously to a manual hydraulic pump in order to maintain the load with any degree of constancy. A further source of error arises through the use of a pressure gauge to derive the applied test load by way of calibration charts. The accuracy with which the load can be maintained is governed by the resolution with which the gauge can be read. Assuming the operator performing the load control is entirely dedicated and doing his utmost to maintain the load, he may at best be able to read a pressure or load column gauge to 1%. This implies that the load variation is not likely to be better than around ±2%. This in turn means that the pile head displacement recording of a pile whose elastic shortening alone is, say, 5 mm, will fluctuate by ±0.1 mm according to this load variation.
According to a first aspect of the present invention, there is provided a method of testing the static load-bearing capacity of a pile, wherein:
i) a test load is applied to the top of the pile by way of a jack braced against a reaction member;
ii) the magnitude of the test load is determined by measuring means and communicated to an electronic computer;
iii) the resulting displacement of the pile is measured by at least one displacement sensor and communicated to said electronic computer; characterised in that:
iv) the electronic computer issues control signals to the jack in response to the measured magnitude of the test load so as to keep the test load substantially constant;
v) the electronic computer determines when a definite settlement rate for the pile has been attained and then issues control signals to the jack no as to apply a new test load of different magnitude to the top of the pile in accordance with a predetermined regime of test loads; and
vi) steps ii) to v) are repeated until the test regime is completed.
According to a second aspect of the present invention, there is provided an apparatus for testing the static load-bearing capacity of a pile, the apparatus comprising:
i) an electronic computer with a power supply;
ii) a jack, which in use is braced between the pile and a reaction member so as to apply a test load to the top of the pile;
iii) means for measuring the magnitude of the test load and communicating this to the computer;
iv) at least one displacement sensor for measuring the resulting displacement of the pile and communicating this to the computer; characterised in that:
v) the electronic computer is adapted to issue control signals to the jack in response to the measured magnitude of the test load so as to keep the test load substantially constant;
vi) the electronic computer is adapted to determine when a definite settlement rate for the pile has been attained and then to issue control signals to the jack so as to apply a new test load of different magnitude to the top of the pile in accordance with a predetermined regime of test loads; and
vii) the electronic computer is adapted to repeat steps v) and vi) until the test regime is completed.
By providing computer control of the load testing procedure, together with automatic data logging, the present invention allows a much more detailed analysis of the structural integrity of the pile to be obtained. This analysis can be presented in real-time, advantageously in tabulated and/or graphic form, and reduces the risk of errors being introduced through manual processing of the data.
Furthermore, because the electronic computer receives data regarding the actual test load applied to the top of the pile, operating signals may be sent to the jack in order, for example, to maintain a given test load even when the pile is being displaced. This means that a given test load can be applied for a long period of time without the need for operating personnel to be present in order manually to adjust the applied load.
The computer can be arranged so as to control the jack to apply a number of different test loads to the pile, each for a predetermined minimum period of time or until a definite settlement rate has been achieved. In order to do this, the required load steps and intervals may be defined, together a specific settlement rate. The computer can then control the test load and make the required load changes as required. Load changes may be performed by successively increasing the applied load in small increments until the next desired substantially constant load level is achieved. If the settlement rate during the load change exceeds a predetermined maximum value, then the increase of the applied load may be paused until the settlement rate stabilises.
In preferred embodiments, the jack is a hydraulic jack controlled by the computer by way of a hydraulic control system. The applied test load may be calculated by monitoring the fluid pressure in the hydraulic control system driving the jack. This method, however, has the disadvantage that it is temperature sensitive (due to thermal expansion of the hydraulic fluid), and does not take into account friction between the jack and the point of contact on the top of the pile in the event that the test load is being applied eccentrically.
Accordingly, it is preferred to use one or more electronic load cells, which typically employ balanced strain gauges around a coaxial element. These may be placed above the jack on a spherical seating arrangement so as to reduce the risk of eccentric loading. Because the load cells measure the actual load applied to the pile, it is possible to operate the jack by way of the hydraulic control system so as to apply a substantially constant load, even when the pile is undergoing displacement. This feedback mechanism allows the applied load to be held constant to a degree hitherto not possible with manually-operated systems. The time interval between successive measurements of applied load and pile displacement can be of the order of a few seconds, say from 1 to 5 seconds. With the hydraulic
Browdy and Neimark
Kvaerner Cementation Foundation Ltd.
Noori Max
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