To determine the performance of piled foundations, it is necessary to load test the pile either to failure in the case of test piles, or to a factor (usually 1.5) above working load for working piles.
Testconsult offer both Static and Dynamic Load testing services, however, dynamic load testing offers considerable advantages in term of speed and cost.
SIMBAT Large Scale Dynamic Pile Testing
The SIMBAT test is a method of predicting the static load/settlement behaviour of cast-in-place piles from dynamic measurements. Pile cap preparation is straightforward, and up to 10 piles per day can be tested. SIMBAT testing costs are a fraction of traditional load testing costs.
Pile performance can be determined on the day of the test and Testconsult can provide the complete service, including provision of hammer and cranage. We also operate a ‘mini-rig’ system that is hand carried and can operate inside buildings on piles up to 300mm diameter and tested up to 600KN. By comparison, the largest SIMBAT dynamic load test to-date was on a 1.5m diameter pile in Taiwan, tested to 30000KN.
The SIMBAT method is based on the propagation of waves in long elastic cylinders. When the pile top is struck with the falling weight the pile section is deformed (enlarged) and this enlargement travels down the pile to the toe where it is reflected back up. The waveforms are analysed to determine In the forces within the pile and calculate dynamic soil reaction and then static reaction. The test procedure can also include a computer simulation to produce a pile/soil model, which enables the distribution of soil resistance down the shaft and at the pile toe to be determined.
Static Load Testing
Static load tests on piles are a way of determining the performance of a pile when subjected to a given load. The test can be used to determine the settlement that can occur at working load, or a multiple of it, and can also be used to verify the ultimate bearing capacity of a pile.
- Maintained Load Test: Each increment of load is held constant for a specified period of time or until the rate of settlement falls below a specified value.
- Constant Rate of Penetration Test (CBP): The load is applied to the pile in order to maintain the rate of penetration at a constant speed. This test is usually carried out to determine the ultimate bearing capacity of a pile.
- Tension Test: Similar to a maintained load test, except that the purpose of the test is to determine how the pile will perform under a tensile load.
There are a number of different published specifications for carrying out static load tests, In addition, a number of geotechnical consulting engineers have developed their own.
Loading can be applied in one of two ways. Either using a jack which reacts against a kentledge heavier than the required test load or using a jack which reacts against beams held down by tension piles acting as anchors. Pile settlement is recorded using dial gauges or electrical displacement transducers (LVDT's). The applied load is monitored using calibrated load cells, and on more up to date systems, can be controlled automatically using a portable computer and a compressed air pump.
Instrumentation of Foundations
In certain instances, it is desirable to obtain information on how foundations are reacting to their loading and environment. Information such as lateral movement, concrete temperature, internal stresses vibrations and even loading at the pile base can be determined by selecting the appropriate transducers. Where readings are required continuously, an automatic data logging system can be installed, which can send the information by GSM link to a central point to store data and trigger alarms. A few examples are given below :
- Inclinometers & Electrolevels: to monitor lateral movement and tilt
- Thermocouples: to monitor concrete temperature
- Strain Gauges: to monitor internal strains and hence concrete stresses under load. Usually vibrating wire gauges attached to reinforcement.
- Geophones: to monitor vibrations occurring at depth
- Load Cells (Osteberg type): Flat type load cells installed at the very base of the pile, to measure the proportion of load that reaches the pile toe. This technique is quite popular in the US and has been used as an alternative to carrying out expensive static load testing.
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