Monitoring of Nailing-Stabilized Retaining Walls

Introduction

Although thousands of nailed (nailing) retaining structures have been constructed worldwide, only a limited number have been instrumented for monitoring. Monitoring the performance of nailed retaining walls during construction and operation is considered essential, especially when such walls are built for the first time in a city or for permanent applications. To assess operational displacements, stress development in nails (reinforcing elements), and facing behavior as a function of load, time, and environmental changes (such as winter freeze-thaw cycles), a minimum monitoring period of 2 years after construction is recommended.

Parameters Measured During Monitoring

Measurements focus on the overall performance of the stabilized wall, including deformation of the wall or soil slope during and after construction. Inclinometers installed along the wall at various intervals provide detailed data on ground deformations.

Key elements for developing a systematic monitoring plan include:

  • Local displacements or failure of facing elements

  • Groundwater drainage behavior

  • Performance of soil-retained structures such as roads, bridge footings, and slopes above the wall

  • Nail loads with special attention to the magnitude and position of maximum loads

  • Load distribution in nails caused by surcharge loads

  • Temporal variations of nail loads

  • Temperature effects (which can cause significant changes in other parameters and influence instrument readings)

  • Rainfall effects (often affecting other parameters)

Performance Monitoring Program for Nailing-Stabilized Walls

To acquire comprehensive performance data, monitoring should begin during the wall construction. A minimum monitoring program typically includes the following assessments:

  • Horizontal displacement of the wall facing measured by surveying methods and inclinometers, along with surface markers on the facing

  • Horizontal and vertical displacements at the top of the facing and ground surface behind the shotcrete facing

  • Local displacements or failure of facing detected by visual inspections and instruments such as crack gauges

  • Drainage behavior, especially if groundwater is observed during construction. Drainage can be monitored visually by observing exit points or using piezometers installed behind the facing

Performance monitoring can be extended over a longer period for the following purposes:

  • Verification of design stress levels and safety monitoring during construction

  • Process improvements for safety or economic goals

  • Construction rate control

  • Enhancing knowledge of structural behavior to improve future designs and reduce costs

Conceptual Monitoring Plan

The plan should include:

  • Strain gauge monitoring along the nail length to determine the magnitude and location of maximum nail loads. Ideally, paired strain gauges are installed on the reinforcement bars at 1.5 m intervals from top to bottom. The nail ends should be marked to enable final strain angle assessment.

  • Load cells to measure loads at the nail heads; these provide high-quality load data near the head and can substitute for strain gauges on the nail body.

  • Inclinometers to measure horizontal displacements of the structure. Care should be taken to install inclinometers at the wall height and behind the facing.

A systematic plan must be developed for every monitoring program. The first step is to define the measurement objectives. Without clear questions or goals, instrumentation has little meaning.

Instruments Used for Monitoring Nailing-Stabilized Walls

Instruments should be selected based on the parameters to be measured, required accuracy, and ease of use. They should also be compatible with data reading systems. Below is a brief overview of commonly used monitoring tools:

Survey Points

Wall facing deformation can be measured indirectly by visual or electronic distance measurement (EDM) techniques. Reflector prisms mounted on nails enable electronic displacement monitoring from points on the wall surface. Typical surveying systems achieve horizontal and vertical displacement accuracy of about 3 mm or better.

Strain Gauges

Nails equipped with strain gauges allow estimation of load distribution along the nail length. Strain gauges measure loads carried by both grout and the nail. This approach can address interpretation challenges related to grout-nail interaction since grout has some tensile strength and carries part of the load. Although strain measurement on grout and nails is feasible, accurately converting these measurements into nail loads can be difficult.

Load Cells at Nail Heads

Load cells installed at nail heads provide direct, high-quality measurements of nail loads near the head and are often preferred over strain gauges for this purpose.