Bond Strength Values in Soil Stabilization Design by Nailing Method

Introduction to Bond Strength in Soil Nailing

The pullout resistance of a nail installed using the drilling-grouting method depends on the nail size and the ultimate bond strength, qu. Bond strength is the shear resistance mobilized along the soil-grout interface. There is no standardized method to directly evaluate bond strength, so correlation studies and local experience play a key role.

Due to this local dependency, contract specifications typically require a percentage of installed nails (soil nails or reinforcing elements) to undergo in-situ pullout load tests to verify bond strength design.

Factors Influencing Bond Strength

Based on observations for drilled and grouted soil nails, bond strength depends on:

  • Surrounding soil conditions (type and characteristics)

  • Installation method, including:

    • Drilling process

    • Grouting procedure

    • Nature of grout

    • Grouting pressure (e.g., gravity or pressured)

    • Size of the grouted zone

For drilled and grouted nails in cohesionless soils, overburden pressure and soil grain characteristics affect the soil friction angle, which in turn influences bond strength. For fine-grained soils, bond strength is generally a fraction of the undrained shear strength, Su. The ratio qu/Su is higher in soft soils compared to stiff soils.

Typical Bond Strength Values by Soil Type and Construction Method

Table 1 presents typical ultimate bond strength values (qu) for drilled and grouted nails in various soils and with different installation methods. These values correspond primarily to gravity grouting.

These ranges represent conservative minimum and maximum values for specific soil types and construction methods. For example, for granular soils with wide ranges, the upper and lower bond strengths correspond to dense and loose soil conditions, respectively.

Local project-based testing (e.g., Pressuremeter Test – PMT) can estimate bond strength using the correlation:

qu=0.25×PL1.25qu = 0.25 \times PL^{1.25}

where PL is the pressuremeter limit pressure in MPa, and qu is in kPa.

For rocks, bond strength can be correlated with the unconfined compressive strength of intact rock. The Post-Tensioning Institute (PTI, 1996) suggests bond stress as approximately 10% of unconfined compressive strength, up to a maximum of 4000 kPa, but values should not exceed typical values shown in Table 1.

Effect of Grouting Pressure on Bond Strength

Bond strength can significantly increase with increased grout pressure. For soils under grout pressures less than 350 kPa, bond strength is roughly twice that of gravity grouting. For rocks, the increase can be even more pronounced but plateaus after a certain pressure. Therefore, in-situ load tests provide the most reliable bond strength estimate.

Shear Bond in Driven Soil Nails

In rare cases where driven nails are used in granular soils, bond strength is primarily frictional and influenced by:

  • Overburden pressure along the bond length

  • Relative soil density

  • Installation method

The apparent friction angle at the soil-nail interface can be estimated between 0.75 to 1 times the effective friction angle of the soil.

Table 1: Estimated Ultimate Bond Strength (qu) for Soil Nails and Reinforcements

Material Installation Method Soil/Rock Type Ultimate Bond Strength, qu (kPa)
Rock Rotary drilling Marl-Limestone 300 – 400
Phyllite 100 – 300
Chalk 500 – 600
Soft Dolomite 400 – 600
Fractured Dolomite 600 – 1000
Weathered Limestone 200 – 300
Weathered Shale 100 – 150
Weathered Schist 100 – 175
Basalt 500 – 600
Hard Shale or Slate 300 – 400
Cohesionless Soils Rotary drilling Sand or Gravel 100 – 180
Silty Sand 100 – 150
Silt 60 – 75
Piedmont Deposits 40 – 120
Soft Colluvium 75 – 150
Driven casing Sand or Gravel (low overburden) 190 – 240
Sand or Gravel (high overburden) 280 – 430
Dense Marine 380 – 480
Colluvium 100 – 180
Auger Silty Sand Fill 20 – 40
Soft Silty Sand 55 – 90
Silty Clayey Sand 60 – 140
Pressurized grouting Sand 380
Sand or Gravel 700
Fine-Grained Soils Rotary drilling Silty Clay 35 – 50
Driven casing Silty Clay 90 – 140
Auger Loess 25 – 75
Soft Clay 20 – 30
Stiff Clay 40 – 60
Stiff Silty Clay 40 – 100
Calcareous Clayey Sand 90 – 140