Deep Injection vs. Piering

Deep Injection vs. Piering

Mechanism of Polymer Injection

1. Injection Process: The polymer is injected through small holes drilled into the ground. The depth and location of the injection points are carefully planned based on the soil's condition and the load requirements.
2. Determining Injection Depth with DCP Testing: Dynamic Cone Penetrometer (DCP) testing can be used to identify the depth at which polymers should be injected. DCP testing involves driving a cone-tipped rod into the soil and measuring the resistance to penetration. This test provides a profile of soil strength with depth, highlighting weak layers that require stabilization. By analyzing DCP results, engineers can pinpoint the exact depths where the soil is weakest and where polymer injection will be most effective.
3. Polymer Expansion: Once injected, the polymer expands and hardens. This expansion process compacts the surrounding soil particles, fills voids, and displaces weak or loose soil. The hardened polymer acts as a solid inclusion within the soil, enhancing its overall strength and stability.
4. Increased Density and Load Capacity: The increase in soil density due to the polymer injection improves the load-bearing capacity of the soil. This process can be particularly effective in soils that are loose, granular, or exhibit poor compaction characteristics.

Relation to Boussinesq Theory

The Boussinesq theory helps us understand how loads are distributed through soil. This theory describes the stress distribution beneath a point load applied to a homogeneous, isotropic, and elastic half-space, which is often approximated for soils. According to Boussinesq, a load applied to the surface of the soil spreads out in a conical fashion, distributing the stress over a larger area as depth increases.

When polymers are injected into weak soils:

• Load Distribution: The injection of polymers creates a composite material (soil and polymer) with improved mechanical properties. This composite material can distribute applied loads more effectively than the weak soil alone. The stress from surface loads spreads more uniformly, reducing the concentration of stress in any one location and preventing excessive settlement or failure.

Piering and Load Distribution

Piering, or underpinning, involves driving or drilling piers (columns) into the ground to support a structure. These piers extend down to more stable soil or bedrock. However, piering has limitations:

• Point Load Support: Piers provide support directly beneath their locations. The load from the structure is transferred through the piers to the stable soil or bedrock below. This method is effective for supporting point loads but does not distribute the load over a wider area.
• Lack of Load Distribution: Unlike the polymer-injected soil, which improves the entire soil mass, piering only supports the load directly above each pier. This means that any load not directly above a pier must still be supported by the weaker soil, potentially leading to differential settlement or other issues.

Comparison

• Polymer Injection: Enhances the entire soil structure, allowing for a more uniform load distribution and improved overall stability. This method addresses both point loads and distributed loads, making it suitable for various applications, including expansive soils and loose, granular soils.
• Piering: Provides localized support and is effective for specific, heavily loaded points such as building columns or other concentrated loads. It does not improve the soil’s properties between the piers.

In summary, deep injection of polymers stabilizes weak soils by increasing their density and load-bearing capacity through expansion and hardening of the injected material. Utilizing DCP testing can help determine the precise depths for injection, ensuring targeted stabilization. This process, when combined with the principles of the Boussinesq theory, enables better distribution of applied loads over a greater area, enhancing overall soil stability. In contrast, piering supports only the loads directly above the piers without distributing the load across the entire area, limiting its effectiveness for broader stabilization.