Introduction
Self-drilling anchors (SDAs) are increasingly used in slope stabilization and ground support applications due to their ability to be installed rapidly in difficult ground conditions. Unlike conventional anchors, SDAs combine drilling, grouting, and anchoring into a single operation. Understanding the load transfer mechanisms of self-drilling anchors in soil and rock slopes is essential for ensuring effective design, stability, and long-term performance.
Overview of Self-Drilling Anchors
A self-drilling anchor consists of a hollow threaded steel bar, a sacrificial drill bit, grout injected through the hollow core, and a bearing plate or anchorage head. Once installed, the anchor transfers tensile forces from the unstable slope mass into the surrounding soil or rock through the grout–ground interface.
Fundamental Load Transfer Concept
The primary load transfer mechanism in self-drilling anchors is mobilization of bond resistance along the grouted length. Tensile load applied at the anchor head is gradually transferred to the surrounding ground through shear stresses developed at the grout–soil or grout–rock interface.
Load Transfer in Soil Slopes
Bond and Frictional Resistance
In soil slopes, load transfer occurs mainly through friction and adhesion between the grout and soil. The effectiveness depends on soil type, density, moisture content, and grout penetration. Cohesive soils mobilize adhesion-based resistance, while granular soils rely primarily on frictional resistance.
Stress Distribution Along Anchor Length
Load transfer in soils is non-uniform, with higher shear stresses concentrated near the anchor head. As loading increases, the bond resistance progressively mobilizes along the anchor length until equilibrium is achieved or pull-out failure occurs.
Load Transfer in Rock Slopes
Mechanical Interlocking and Bond Strength
In rock slopes, load transfer is dominated by mechanical interlocking between grout and rock asperities, joints, and fractures. Rough and fractured rock surfaces enhance bond strength and load transfer efficiency.
Influence of Rock Mass Quality
The quality of rock mass, degree of weathering, joint spacing, and joint infilling significantly influence anchor performance. Anchors embedded in competent rock provide more uniform load distribution and higher load-carrying capacity.
Role of Grout Properties
Grout strength, stiffness, and permeability play a critical role in load transfer. High-quality grout ensures effective stress transfer and minimizes debonding. Proper grout pressure improves penetration into surrounding ground, enhancing bond resistance.
Effect of Anchor Geometry and Installation Parameters
Anchor diameter, length, inclination, and grouted zone length influence load transfer mechanisms. Longer bonded lengths increase load capacity, while appropriate inclination improves alignment with dominant slope forces. Installation quality directly affects bond performance.
Time-Dependent Load Transfer Behavior
Over time, load redistribution may occur due to creep in soils, stress relaxation, or grout hardening. In cohesive soils, long-term performance is influenced by consolidation and creep behavior, while in rock slopes, load transfer remains relatively stable.
Interaction with Slope Deformation
Slope movement affects load transfer by altering stress distribution along the anchor. Progressive slope deformation may increase anchor load demand, highlighting the importance of sufficient bond length beyond potential failure surfaces.
Failure Modes Related to Load Transfer
Common failures include pull-out due to insufficient bond strength, grout cracking, bar yielding, or progressive ground failure. Understanding load transfer mechanisms helps prevent such failures through optimized design.
Monitoring and Verification
Load tests and field monitoring using load cells or strain gauges provide valuable data on actual load transfer behavior. Monitoring helps validate design assumptions and detect early signs of anchor distress.
Conclusion
Load transfer mechanisms of self-drilling anchors in soil and rock slopes are governed primarily by bond resistance along the grout–ground interface. Soil type, rock mass quality, grout properties, and installation parameters significantly influence performance. A clear understanding of these mechanisms enables efficient design, improved safety, and reliable long-term performance of self-drilling anchor systems in slope stabilization projects.
