Testing of soil nail systems
C.1.2 The designer should specify the bond length of the test nail (the length of the nail in contact with the surrounding ground).
When designing the bond lengths for the nail test programme, the following should be taken into account:
- a) geological stratigraphy and potential variations;
- b) the location of the potential slip surfaces, defining active and passive zones in the design model;
- c) the typical (or average) nail length within the passive and active zones used in the design model (i.e. the test nail bond length shall be representative of the lengths, providing the stabilising force within the structure).
C.1.3 It is essential that the test nail be axially loaded; this can be achieved by cutting the slope perpendicular to the nail or with a stressing chair.
C.1.4 The proof load, Pp, should be defined in the design.
C.2.1 Proof loading set-up
C.2.1.1 The proof loading system comprises the jack or stressing device, displacement and load monitoring devices, a reaction system, associated locking nuts, extension pieces, etc. A schematic nail proof loading system is shown in Figure C.1.
C.2.1.2 The proof loading system should be capable of functioning safely without excessive deformation at the maximum proof load.
C.2.1.3 Where the facing system, or the load test reaction system, could influence the test result, then the test nail should be de-bonded over the zone of influence.
C.2.1.4 Prior to final assembly of the displacement monitoring system, it may be necessary to apply a datum load to minimise the movement of the nail test arrangement on initial loading. The datum load P0 should not exceed 10 % of Pp.
NOTE The datum load is the initial force P0 applied to a test nail before commencing the load test to take up slack in the test system (sometimes referred to as a seating load).
- 1 facing
- 2 rigid reaction frame
- 3 extension of soil nail for testing
- 4 locking nut and plate at top of jack
- 5 displacement gauge on independent support frame
- 6 hydraulic jack for stressing
- 7 calibrated pressure gauge to control jack force (load cell can be used as alternative, see 7* in the figure)
- 8 pump
- 9 soil nail
- 10 bonded length
- 11 de-bonded length
C.2.2 Reaction system
C.2.2.1 The reaction system shall be designed to have a structural resistance at least equal to the maximum proof load (Pp), according to the relevant European Standards.
C.2.2.2 The reaction system should be designed so as not to affect the measured pullout resistance of the test nail or to impose bearing pressures in excess of the safe bearing capacity of the facing.
NOTE This could be accomplished by using a special load distribution device at the front of the facing. This is especially important for soft facing.
C.2.3 Loading service
The loading device, normally a hydraulically operated jack, should have a nominal capacity at least equal to the maximum proof load, Pp. The extension of the jack should preferably be sufficient to avoid reseating during the test. The design of the loading device should permit the proof load to be applied smoothly and axially to the test nail, in both increasing and decreasing load increments.
C.2.4 Load measurement
The load in the test nail may be measured either indirectly by means of a calibrated pressure gauge, monitoring the hydraulic pressure in the loading device, or directly by the use of a load cell. Pressure gauges and load cells should be calibrated to an accuracy of ± 2 % of the maximum proof load, Pp. If load cells are used, it is recommended that a secondary method of measurement is used due to the difficulties of their use in the field.
C.2.5 Displacement measurement
The displacement monitoring system should be calibrated to have a reading accuracy of ± 0,1 mm. The support for the displacement gauges should be remote from the loading device and reaction system. The independent support should be sufficiently rigid, so as not to be influenced by climatic effects or background vibrations. Displacement gauges should be capable of monitoring the extension of the test nail throughout the test, without the need for reseating.
C.2.6 Time and temperature measurement
C.2.6.1 The accuracy of the equipment for measuring time shall be ≤ 1 s.
C.2.6.2 The accuracy of the equipment for measuring temperature shall be ≤ 1 °C.