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FWD SURVEYS
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29/11/2005 |
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Falling Weight Deflectometer (FWD) surveys
Grontmij | Carl Bro Pavement Consultants participates in projects all over the world. Projects funded by local highway authorities and internationally funded contracts to measure the bearing capacity of roads or taxiways and runways of airports. The final reports are used by design engineers as a basis for the preparation of feasibility or rehabilitation design studies.
FWD-measurements with PRI2100 FWD/HWD International consultants and donors i.e. World Bank, TACIS, EBRD, ICAO and many others have used our services. Today’s standard for determination of the structural condition of road and airfield pavements uses the FWD for this purpose. The FWD simulates the actual wheel loads in both magnitude and duration. For all tests the pulse time duration must be between 20-30 ms. The dynamic load for road is normally 50 kN and for airfields up to 250 kN and in some cases up to 300 kN. Linear bearing capacity calculation is often impossible if a runway pavement is measured with a load lower than kN 250 - especially when an airfield should be capable of withstanding the load from a Boing 747 aircraft.
Quality assurance with PRIMA 100 PRIMA 100 is primarily used for compaction control in connection with laying of asphalt, cobbles and flagstones, but is also ideal for measuring bearing capacity where geogrids are to be used or where pipeline trenches are to be reestablished. PRIMA 100 is also indispensable for compaction control prior to industrial flooring.
Grontmij | Carl Bro Pavement Consultants yields this service to customers but customers may also rent equipment to make measurements.
Analysis
Prior to a design analysis several external and internal parameters need to be input to the computer program. External parameters, which are all site-specific, consist of information on material, traffic prognoses, layer thicknesses, pavement temperature and data collected by the FWD. Internal parameters consist of information on distress relationship, design parameters etc. Layers of different stiffnesses are transformed into a single homogeneous semi-infinite layer. Equations are used for back-calculation of the modulus of each layer (up to 4 layers) by comparison of the theoretically calculated deflections and the deflection bowl actually measured in the field. Then the critical stresses and strains in each layer are calculated for the design wheel load. Furthermore, a pavement overlay is calculated for each test point, if the residual life is shorter than the required design life
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