Failure Modes of Facing Connection in Nailing (Nailing) Stabilized Walls

Common Failure Modes in Facing-to-Nail Head Connection

The most common failure modes in the facing-to-nail head connection are illustrated with details in Figure 1:

• Flexural Failure: This failure mode occurs due to bending stresses exceeding the flexural capacity of the facing. It must be considered for both temporary and permanent facings.

• Punching Shear Failure: This failure mode occurs around the nail (or nailing reinforcement) and should be evaluated for both temporary and permanent facings.

• Tensile Failure of Nail Heads: Tensile failure of the nail heads is relevant only for permanent facings.

To resist these failure modes, the capacity of the nail head (or nailing reinforcement) and facing must exceed the maximum tensile force at the nail head, T0T_0T0​, on the surface of the stabilized wall by nailing. To achieve design capacities with sufficient safety factors for all potential failure modes, appropriate dimensions, adequate strength, and reinforcement of the facing and suitable stiffeners for the nail head (such as bearing plates and nail studs) should be used. The following sections describe the calculation of the maximum tensile force at the nail head, the failure modes described above, and the equations used to calculate the capacities of these failure modes.

Tensile Forces in the Facing of Nailing-Stabilized Walls

The tensile force in the nail (or nailing reinforcement) at the face of the stabilized wall, T0T_0T0​, is less than or equal to the maximum tensile force of the nail, TmaxT_{\text{max}}Tmax​ (Byrne et al., 1998). Figure 2 shows the dimensionless measured tensile forces of nails (or reinforcements) in the facing of a nailing-stabilized wall. These values correspond to long-term forces and do not include frost or other loads.

Dimensionless nail forces in the facing have distributions comparable to the maximum dimensionless tensile nail forces. The ratio of the dimensionless nail head force to the maximum nail force varies between 0.6 and 0.1. In the upper half of the stabilized wall, the average dimensionless nail head force varies between 0.4 and 0.5, and in the lower half, it gradually decreases to zero at the bottom of the wall.

Considering the dimensionless parameters and influence zone, the tensile force at the nail head, T0T_0T0​, generally varies between:

T0=0.60KAγHtoT0=0.70KAγHT_0 = 0.60 K_A \gamma H \quad \text{to} \quad T_0 = 0.70 K_A \gamma HT0​=0.60KA​γHtoT0​=0.70KA​γH

These results align well with experimental data from trial walls in Germany and France. In Germany, actual earth pressure measurements via total stress cells at the soil-shotcrete interface indicated that under most conditions, the equivalent earth pressure on the facing ranges from 60% to 70% of the Coulomb active earth pressure (Gässler and Gudehus, 1981).

Failure Modes and Experimental Observations

In French experiments, the ratio of nail head force to maximum nail force in the upper sections of stabilized walls typically ranged from 0.4 to 0.5 (FHWA, 1993a). These experiments also showed that due to the soil arching effect between nails, reducing the nail spacing lowers forces in the facing compared to expected values.

Based on these results, recommended operating tensile force ratios for nail heads (or reinforcements) according to Clouteree (1991) are:

• 60% of maximum service load for vertical spacings of 1 meter or less.

• 100% of maximum service load for vertical spacings of 3 meters or more.

• Linear interpolation for spacings between these values.

Flexural Failure of Facing

The facing of a stabilized wall can be modeled as a continuous reinforced concrete slab, where lateral earth pressure acts as load and nail tensile forces act as supports.

Lateral earth pressures and nail reactions produce bending moments in the facing: positive moments (tension at the outer section) between nails and negative moments (tension at the inner section) around nails. If moments exceed capacity, flexural failure of the shotcrete facing may occur. Due to the similarity between the facing and continuous concrete slabs supported by columns, design methods for concrete slabs can be applied to facing design.

Flexural Failure Capacity

Flexural failure progresses after the first cracking in the facing section. As lateral pressure increases, cracks propagate on both sides of the facing, with increasing deflections and tensile nail forces. Dispersed cracks and failures indicate reaching flexural capacity.

Ultimately, cracks connect forming a hinge mechanism, known as the critical failure line. This failure pattern depends on factors like earth pressure, nail spacing, bearing plate size, facing thickness, reinforcement layout, concrete strength (Seible, 1996), and maximum earth pressure.

The earth pressure causing failure can be theoretically applied in the nail influence zone to derive the nail tensile reaction force, defined as the flexural capacity RFFR_{FF}RFF​, proportional to the flexural capacity per unit length of facing.

Reinforcement and Design Considerations

The reinforcement ratio $\rho$ in the facing (vertical or horizontal, at nail head or mid-span) should lie between minimum and maximum limits to ensure adequate capacity and ductility.

For punching shear failure, localized failure around nail heads must be checked, especially at plate-support connections (temporary facing) or nail stud connections (permanent facing).