In carrying out pull-out tests for soil nails, it generally requires the unloading and loading of soil nails of several cycles up to 80% of ultimate tensile strength of soil nails. Principal function of soil nail tests is to verify the design assumptions on the bond strength between grout and soil that is likely to exceed the design values based on past experience. Additionally the ultimate bond strength between grout and soil can be determined and this information is helpful as a reference for future design.

Then someone may query the purpose of conducting load/unloading cycles of soil nails as it doesn't provide information on the above two major purposes of soil nails. Actually loading and unloading soil nails can provide other significant information on their elastic and plastic deformation behaviour. Though as stress levels in soil nails are generally low, knowledge on elastic and plastic performance may not be of significant value. Alternatively the creep and slippage performance of soils nails can also be obtained that may be useful for some soils.

Fig: Typical pull-out test result