Vertical anchors, often made of steel, are commonly used in geotechnical engineering to resist the lateral movement of soil and to provide horizontal support for structures such as retaining walls and bridges. The effectiveness of these anchors depends heavily on their pullout capacity, which is influenced by several factors, including the type of soil, the anchor's geometry, and the depth of embedment. Steel plate anchors are particularly common in such applications, and their behavior in layered cohesionless soils (soils with no significant cohesion, such as sand) is crucial for optimizing anchor designs.
In layered soils, the characteristics of the different layers, such as their density and internal friction—affect how the anchor interacts with the surrounding material. Understanding these interactions is vital for designing anchors that perform efficiently under load. This article delves into an experimental study that measures the pullout capacity of steel plate anchors embedded in multi-layered cohesionless soils.
Materials and Methods
In this study, a physical model was constructed to simulate the behavior of steel vertical plate anchors in multi-layered cohesionless soils. A wooden box of dimensions 1 × 1 × 1 m was used to contain the soil and anchor systems. Two types of sand were used to create a layered soil structure: dense and loose sands. The dense layer, which has a relative density of 95%, was compacted using an electric vibrating compactor. The loose layer, with a relative density of less than 35%, was placed on top of the dense layer. The total height of the soil column was 80 cm, with each layer being 40 cm thick.
Steel plate anchors with dimensions 150 × 50 × 8 mm were used for the tests. These anchors had L/d ratios (length to diameter) and H/d ratios (height to diameter) that ranged from 1 to 4. The embedment depths of the anchors were varied, with tests conducted at depths of 150, 300, 450, and 600 mm. Three different anchor heights were tested: 200, 400, and 600 mm from the soil surface.
The performance of the anchors was evaluated by measuring the pullout force using a winch motor connected to the anchor via a steel cable. The displacement and force were recorded using a load cell and a linear variable displacement transducer (LVDT), respectively. Additionally, strain gauges were used to measure soil deformation at the anchor's surface.
Results
Pullout Behavior of Anchors
The results from the study show that the pullout force of the anchors increased with the distance of the anchor from the soil wall. This effect was observed at all three embedment depths (200, 400, and 600 mm). As the anchor was placed further away from the wall, the available soil mass in front of the anchor increased, which enhanced the anchor's resistance to pullout. For example, increasing the distance from the box wall by four times resulted in a 2 to 3 times increase in pullout force across all depths.
The depth of embedment also played a critical role in the pullout capacity. Anchors embedded at greater depths showed significantly higher pullout forces. For example, at a depth of 600 mm, the pullout force was much greater compared to anchors embedded at shallower depths like 200 mm. This increase in force can be attributed to the increased friction between the soil and the anchor, especially in denser layers.
Soil Layer Interaction
The study further revealed that the pullout force was highest when the anchor was placed in the dense layer of soil. The dense sand provided greater frictional resistance due to its higher density and internal friction angle, which impeded the anchor's movement. On the other hand, anchors placed in the loose layer exhibited lower pullout forces because the soil’s low density and larger pore spaces allowed for easier anchor movement.
Moreover, the pullout force was lowest when the anchor was placed at the boundary between the dense and loose layers, as the transition between the layers affected the interaction between the soil and the anchor. The change in soil density from dense to loose resulted in less resistance at the boundary, leading to lower pullout forces.
Effect of Embedment Depth
The embedment depth had a significant effect on the anchor’s pullout capacity. As the depth of embedment increased, the pullout force also increased. This trend was consistent across all three anchor height levels. The reason behind this is the larger failure surface area created by deeper anchors, which interacts more effectively with the surrounding soil, especially at deeper levels where the soil is denser.
The results confirmed that anchor depth plays a crucial role in the failure mechanism of plate anchors. Shallow anchors typically experience general shear failure, which results in a larger failure surface reaching the soil surface. Deeper anchors, however, tend to experience localized failure, which results in higher pullout resistance.
Pullout Force Coefficient
The pullout force coefficient, which is a critical parameter in determining the effectiveness of anchors, increased with both the distance from the wall and the embedment depth. This trend was particularly evident for the anchors embedded at deeper depths. The increased depth contributed to greater soil pressure on the anchor, thus enhancing the interaction between the soil and the anchor, resulting in a higher pullout force.
Discussion
The findings of this study contribute significantly to the understanding of vertical steel plate anchors in multi-layered cohesionless soils. The results show that several factors, including soil density, anchor embedment depth, and the distance from the soil wall, can affect the pullout performance of anchors. This has important implications for the design of anchor systems in real-world applications, where soil conditions are often heterogeneous.
The study also highlights the importance of considering layered soil structures in the design of anchors. In real-world applications, soils are rarely homogeneous, and understanding how anchors behave in layered soils is essential for predicting their performance accurately.
Key Takeaways
• The performance of vertical steel plate anchors is significantly influenced by the anchor’s embedment depth, soil density, and the distance from the soil wall.
• The pullout force increases with the distance of the anchor from the soil wall and with greater embedment depths.
• Anchors placed in denser soil layers provide higher resistance to pullout compared to those placed in loose layers.
• The interaction between different soil layers affects the pullout behavior, with anchors at the boundary between dense and loose soils exhibiting lower pullout forces.
• Increasing the anchor’s embedment depth leads to a larger failure surface and higher pullout resistance.
• The pullout force coefficient is directly related to both embedment depth and the distance from the wall.
This research provides valuable insights into the design of vertical anchors in layered cohesionless soils, helping engineers optimize their performance in various geotechnical applications.
