Trenchless construction operations, such as horizontal directional drilling, pipe ramming and pipe jacking, possess numerous benefits over traditional excavation. However, while these methods have gained favor with public and private sector organizations, they also carry an element of risk. One of the main concerns associated with trenchless construction is soil heaving. This phenomenon has the potential to cause extensive damage to nearby buried utilities and existing surface structures, such as pavements and foundations.
Heaving is defined as the swelling or upward movement of soil. It is typically associated with an increase in moisture content in expansive clay soils. In this case, when water comes into contact with unsaturated clay soils, it gets absorbed quickly. Since clays tend to hold more water than granular soils, its volume increases significantly. Since the soil cannot expand downwards or sideways, the surface of the soil rises upwards as a result.
This upward movement can apply pressure to nearby structures and infrastructure, or even cause them to shift out of place. As a result, cracks can develop, which can ultimately lead to structural failure. While upward displacements are generally small – about a few inches – even this level of movement can cause extensive damage to buildings and foundations.
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Signs of Soil Heaving
Soil heaving is generally viewed as the opposite of subsidence or settlement. Some of the most common signs of soil heaving include:
- Lifting of road pavement and sidewalk structures
- Cracked concrete and masonry in buildings
- Misalignment of doors and windows within their frames
What causes soil heaving during trenchless construction?
While heaving is usually associated with the expansion of clay soils due to moisture ingress, during trenchless construction, the mechanisms for heaving are fundamentally different. During pipe jacking and horizontal directional drilling, heaving can be caused by the pumping of excess drilling fluid into the soil formation. This condition usually occurs when pumping operations resume after the loss of fluid circulation. Sudden pressures applied by the pump to the drilling fluid can cause pressure in the formation to rise above the soil’s overburden pressure and cause heaving at the ground surface.
Another mechanism by which heaving occurs during pipe jacking and horizontal drilling is soil displacement. As the cutting head advances, the soil can move outward and away from the excavated face. This displacement causes the soil to move upward, creating a heave profile at the surface.
During horizontal directional drilling, heaving can also be caused by back-reaming too close to the ground surface. The reaming process, which is meant to enlarge the pilot hole, applies pressure on the walls of the bore. This applied pressure can create visible heaving on the surface if back-reaming is performed at a depth that is considered to be too shallow.
Another trenchless construction procedure that can also contribute to soil heaving is pipe ramming. Unlike pipe jacking, where the soil is excavated using a cutting head, pipe ramming involves using a pneumatically or hydraulically powered piston to hammer the pipe through the earth. With this method, the soil pressures on all sides of the pipe are significantly increased as the soil is displaced radially outward and upward.
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Methods of Preventing Soil Heaving
Several factors can contribute to soil heaving during trenchless construction. These include:
- The rate at which back-reaming is performed
- Borehole pressure
- The type of downhole tooling used
- The depth of the cover between the reamer and the ground surface
- The size of the annular space during pipe ramming
- The geotechnical properties of the surrounding soil
The key to preventing soil heaving is to consider these various factors early in the design and conceptual phase of a trenchless installation project. Some of the practices that contractors can adopt to mitigate heaving include:
Have qualified staff to oversee the trenchless installation process
The presence of qualified supervisors on-site cannot be overstated. Knowledgeable persons can interpret drilling and fluid records and identify irregularities in ongoing operations, which may result in heaving at the ground surface. Qualified supervisors must, therefore, be well informed on the drilling processes and have some proficiency in geotechnics.
Perform continuous monitoring of surface conditions
By constantly monitoring the condition of the surface in the vicinity of trenchless operations, contractors can detect early signs of heaving and evaluate the effect of ongoing activities on nearby structures and utilities. Real-time measurements are compared to existing geotechnical data to facilitate decision-making. Instant readings during construction allow engineers and contractors to make instantaneous adjustments to minimize the risks associated with the installation process.
Measure and maintain appropriate drilling fluid pressures
As mentioned previously, the over-pressurizing of drilling fluid is a contributing factor to soil heaving. Modern trenchless installation projects now employ several types of downhole monitoring tools. These tools manage risk by monitoring pressure buildup, which can give early warnings of possible inadvertent returns. The threshold for the maximum allowable fluid pressure is usually determined by geotechnical engineers and is based on current soil conditions.
Implement foundation strengthening methods for critical structures that may be at risk
In situations where there is a high probability of heaving, foundation strengthening may be necessary. Foundation strengthening usually involves underpinning the foundation of buildings to improve their strength and stability. Underpinning can be achieved in several ways; however, the two most common methods include:
- Enlarging or adding new foundations to the existing foundation
- Improving the soil properties below the existing foundation
Soil heaving during trenchless construction has the potential to cause extensive damage to various types of structures and infrastructure. The key to preventing this phenomenon is understanding the multiple factors that contribute to its occurrence. While trenchless projects can vary widely, qualified supervising personnel and efficient monitoring of surface and subsurface conditions form the basis of effective prevention techniques.