Deadman Anchor Design Example: The Only Guide You Need!

Deadman Anchor Design Example: A Comprehensive Guide

This guide presents a detailed explanation of deadman anchor design, focusing on practical examples to illustrate the core principles. We aim to provide a clear and accessible resource for understanding the process.

Understanding Deadman Anchors

Deadman anchors are buried structures used to resist tensile loads. They rely on the passive earth resistance and frictional resistance between the anchor and the surrounding soil to provide holding capacity. The design process involves determining the required holding capacity, selecting an appropriate anchor type and size, and verifying its stability under the expected load conditions. These anchors are frequently used in retaining walls, tiebacks, and other applications requiring lateral support.

Types of Deadman Anchors

Numerous types of deadman anchors exist, each suited to different soil conditions and load requirements. Some common types include:

• Concrete Blocks: Simple and widely used, especially for temporary applications.
• Timber Logs: Economical in areas with readily available timber, but susceptible to decay.
• Steel Plates: Offer high strength and can be easily customized.
• Geosynthetic Anchors: Lightweight and effective in sandy soils.
• Helical Anchors: Screwed into the ground, providing significant holding capacity with relatively small footprints.

The choice of anchor type will significantly influence the subsequent design calculations.

Key Design Considerations

Several factors must be considered during the design process to ensure the anchor’s stability and long-term performance. These include:

1. Soil Properties: Accurate soil data is critical. This includes soil density (γ), friction angle (φ), and cohesion (c). Site investigation and laboratory testing are essential to obtain reliable values.
2. Applied Load: The magnitude and direction of the load applied to the anchor. This should include a safety factor to account for uncertainties.
3. Anchor Depth: The depth of the anchor significantly affects its capacity. Deeper anchors generally provide higher resistance.
4. Anchor Spacing: The distance between anchors must be sufficient to prevent interference between their zones of influence in the soil.
5. Anchor Inclination: While often installed horizontally, some designs might require inclined anchors. This affects the calculation of earth pressures.
6. Groundwater Table: The presence of groundwater reduces the effective stress in the soil, thereby affecting the holding capacity.

Deadman Anchor Design Example: Concrete Block Anchor

This example illustrates the design process for a concrete block deadman anchor used to support a retaining wall.

Problem Statement

Design a concrete block deadman anchor to resist a tensile load of 50 kN. The anchor will be used in a silty sand soil with the following properties:

• Unit Weight (γ) = 18 kN/m³
• Friction Angle (φ) = 30°
• Cohesion (c) = 0 kPa

The anchor should be buried at a depth of 1.5 meters. Apply a factor of safety of 2.0.

Design Steps

1. Determine the Required Holding Capacity:
   The required holding capacity is the applied load multiplied by the factor of safety:
   Required Capacity = 50 kN * 2.0 = 100 kN

2. Calculate the Passive Earth Resistance:
   The passive earth pressure coefficient (Kp) is calculated using the following formula:
   Kp = (1 + sin(φ)) / (1 – sin(φ)) = (1 + sin(30°)) / (1 – sin(30°)) = 3.0

   The passive earth pressure (Pp) at the anchor depth is calculated as:
   Pp = Kp γ z = 3.0 18 kN/m³ 1.5 m = 81 kN/m²

3. Estimate the Anchor Dimensions:
   Assume a rectangular concrete block with a width (B) and height (H). Initially, assume B = 1.0 m and H = 1.0 m. This will need to be verified iteratively.

4. Calculate the Passive Earth Force (Front Face):
   Passive Force (Pp_front) = Pp H = 81 kN/m² 1.0 m = 81 kN/m

5. Calculate the Frictional Resistance (Top and Bottom Surfaces):
   Effective Vertical Stress (σv) = γ z = 18 kN/m³ 1.5 m = 27 kN/m²

   Frictional Force per side (Ff_side) = σv tan(φ) B = 27 kN/m² tan(30°) 1.0 m = 15.59 kN/m

   Total Frictional Force (Ff_total) = 2 Ff_side = 2 15.59 kN/m = 31.18 kN/m

6. Calculate the Total Holding Capacity:
   Total Holding Capacity = Pp_front B + Ff_total H

   Total Holding Capacity = 81 kN/m 1.0 m + 31.18 kN/m 1.0m = 112.18 kN

7. Check the Safety Factor:
   Safety Factor = Total Holding Capacity / Required Capacity = 112.18 kN / 100 kN = 1.12

   This Safety Factor is lower than desired.

8. Iterate the Dimensions:
   Increase the height of the concrete block to H = 1.2 m.

   Pp_front = 81 kN/m² 1.2 m = 97.2 kN/m
   Total Holding Capacity = 97.2 kN/m 1.0 m + 31.18 kN/m * 1.2 m = 134.62 kN

   Safety Factor = 134.62 kN / 100 kN = 1.35

   This Safety Factor is still too low; continue increasing height until it’s >2.0.

   Increase height further to H = 2.0 m.

   Pp_front = 81 kN/m² 2.0 m = 162 kN/m
   Total Holding Capacity = 162 kN/m 1.0 m + 31.18 kN/m * 2.0 m = 224.36 kN

   Safety Factor = 224.36 kN / 100 kN = 2.24

   This Safety Factor is satisfactory.

9. Final Design:
   Concrete block dimensions: Width (B) = 1.0 m, Height (H) = 2.0 m. The anchor should be buried at a depth of 1.5 meters. This design provides a factor of safety of 2.24.

Important Considerations

• Soil Disturbance: Minimize soil disturbance during installation to avoid reducing the soil’s shear strength.
• Drainage: Ensure adequate drainage to prevent water from accumulating behind the anchor, which could reduce its holding capacity.
• Long-Term Performance: Consider potential creep and settlement of the soil over time.

Alternative Design: Steel Plate Anchor

This section briefly introduces the design considerations for a steel plate anchor. Steel plate anchors typically involve a similar process of calculating earth pressure and frictional resistance. The primary difference lies in the material properties and the shape of the anchor, which often leads to higher capacity for a smaller overall size. The design requires careful consideration of the steel’s yield strength and buckling resistance. A more detailed design example for a steel plate anchor would follow similar steps, adjusting calculations based on the specific geometry and material properties.

So, there you have it! Hopefully, this deep dive into deadman anchor design examples has given you a solid foundation. Now, go out there and build something amazing – or at least, anchor something really well!