Evaluation of Construction Adequacy in Nailing (Nail) Stabilization of Excavations

Introduction

To verify that the design loads and adequate safety factors are achieved without excessive movement or displacement, nails (or nailing reinforcements) undergo field load testing. These tests evaluate the adequacy of excavation, installation, construction, and grout injection operations. If soil conditions or construction methods change, additional tests may be required to assess nail performance under the new conditions.

It is common practice to complete load testing of each nail row before excavating and installing the subsequent lower row. For very tall walls, construction testing requirements for the upper rows may be somewhat relaxed. If test results indicate improper construction or insufficient nail capacity, the contractor must modify the installation and construction methods. Testing procedures and acceptance criteria should be clearly specified in project specifications.

Types of Load Tests

Evaluation (Preliminary) Tests

Evaluation tests are conducted on sacrificial or non-operational nails. They may also be performed during construction to assess capacity under different construction methods and site conditions encountered. While reaching pullout failure during evaluation tests is ideal and optimal, it is not always feasible. These tests provide the following information:

  • Determination of ultimate bond strength (if pulled to pullout failure)

  • Assessment of design safety factors

  • Identification of load levels causing additional creep

The evaluation test load should be at least the product of the pullout safety factor and the allowable pullout capacity specified in the design. For example, if the safety factor is 2.0, the test load should be 200% of the allowable pullout capacity. These are minimum values, and testing up to failure is preferred as it yields valuable data and may lead to more economical drilling and installation methods. The number of evaluation tests depends on project size and major soil variability. At least two tests per distinct soil layer should be performed. Evaluation tests are done on sacrificial nails not intended for permanent use.

Witness (Verification) Tests

Witness tests are conducted during construction on a specified percentage of operational nails, typically around 5%. These are single-cycle load tests where load is gradually applied up to the maximum test load, generally 150% of the design load capacity. Witness tests verify the contractor’s construction methods and ensure soil conditions remain unchanged. They confirm that operational nails will safely resist design loads without excessive displacement or long-term creep. Both bonded and unbonded operational nails are tested.

Creep Tests

Creep tests may be part of evaluation or standalone tests. Displacement versus logarithmic time data is plotted on semi-log graphs and compared to acceptance criteria. Typical criteria require creep displacement between 1 and 10 minutes under maximum load to be less than 1 mm, or between 6 and 60 minutes to be less than 2 mm. These criteria are primarily based on empirical experience from ground nailing practices. Safe long-term load tolerance of nails is ensured through proper operational methods.