The cracking that appears on the bottom of a trench signals basal failure, a severe form of ground instability often termed basal heave or bottom heaving. This phenomenon represents a dangerous imbalance of forces within the soil surrounding the excavation. Recognizing this distress sign immediately is paramount, as basal heave precedes a catastrophic loss of ground support and potential trench collapse. This failure mechanism demands immediate attention and a structured, engineering-based response to protect personnel and stabilize the construction site.
Identifying Basal Heave and Failure
Basal heave is the upward movement of soil at the base of the excavation. Initial signs appear as small cracks that rapidly develop into a noticeable bulge or upward doming of the trench floor. This upward movement is often accompanied by a corresponding subsidence, or sinking, of the ground surface just outside the trench walls.
As the trench bottom heaves, the soil structure is compromised, causing a loss of lateral support against the trench walls. This loss can cause the lower portions of the protective system, such as shoring or shielding, to shift inward. If unchecked, the failure progresses to a complete shear failure, resulting in the collapse of the trench walls inward and upward into the excavation. This failure is particularly hazardous because it can occur even when side walls are adequately protected.
Understanding the Geotechnical Mechanism
The physics behind basal heave involves a pressure differential created by the excavation process. Before digging, the soil’s weight provides a counter-balancing force, maintaining equilibrium with the lateral pressure from the surrounding ground. Excavating the trench removes the vertical weight of the soil column from the interior, significantly altering the stress distribution.
This removal of mass reduces the bearing capacity of the soil remaining at the base of the cut. The lateral stress and weight of the soil adjacent to the trench walls then exert a net upward pressure on the weakened trench floor. When this upward pressure exceeds the soil’s reduced capacity, the soil mass is forced to deform and displace upward, similar to a bearing capacity failure under a foundation. The resulting shear failure zone typically extends from the base of the excavation, curves outward and upward through the soil mass, and may reach the ground surface outside the trench perimeter.
Primary Causes and Contributing Factors
A combination of specific site conditions and excavation geometry significantly increases the likelihood of basal heave. These factors interact by reducing the soil’s strength and increasing the net upward force acting on the trench floor. Geotechnical site investigation is performed specifically to identify these conditions before excavation begins.
High Water Table and Hydrostatic Pressure
Groundwater saturation is a major factor because water greatly diminishes the shear strength of most soil types. A high water table introduces upward hydraulic forces, known as seepage forces, at the base of the excavation. If the upward flow of water is not controlled, the hydrostatic pressure can exceed the weight of the soil plug at the trench bottom, leading to a condition called hydraulic heave. This upward pressure can essentially liquefy the base soil, rendering it incapable of supporting the lateral stress from the trench walls.
Soft or Cohesive Soil Types
Basal heave frequently occurs in deep excavations within soft, cohesive soils such as silts and clays. These fine-grained soils have low undrained shear strength ($S_u$), meaning they cannot resist lateral stresses effectively in the short term, especially when saturated. The failure surface in these soils is often a deep-seated rotational shear that pushes the base upward. The risk is compounded when a stiff layer of soil or rock lies beneath a softer layer, as the failure plane is forced to travel through the path of least resistance in the weak soil above.
Trench Depth and Width Ratios
The geometry of the excavation plays a role in determining the magnitude of the pressure differential. Deeper trenches require removing a greater mass of soil, which magnifies the imbalance between lateral pressure and reduced vertical stress. A narrow trench is generally more susceptible to basal failure than a wide one of the same depth. This is because a narrow excavation concentrates the lateral load from the side walls over a smaller area of the trench floor, thereby increasing the effective upward stress.
Immediate Safety Protocols and Site Actions
The appearance of cracking or heaving at the trench bottom must be treated as an immediate, life-threatening emergency. Work must cease instantly, and all personnel must be removed from the excavation and the area immediately adjacent to the trench edge. The site’s designated competent person, as defined by the Occupational Safety and Health Administration (OSHA), must be notified to take control of the situation.
The competent person has the authority to stop all work and prevent re-entry until the condition is corrected, aligning with OSHA 1926 Subpart P. No equipment, personnel, or further loading should be permitted near the trench perimeter, as the vibrations or added weight could trigger an immediate and full collapse. The area must be secured, and the incident documented for engineering review before stabilization efforts begin.
Remedial and Preventive Measures
Immediate Remedial Actions
Stabilizing a trench experiencing basal heave requires a multi-faceted approach focused on reducing upward pressure and increasing the base’s resistance. The first remedial action is the partial backfilling of the trench with a suitable, stable material. Placing soil or aggregate back into the cut restores some of the removed vertical weight. This action temporarily stabilizes the base and prevents further upward displacement until a permanent solution is designed.
Long-Term Stabilization
A long-term solution must address hydrostatic pressure and insufficient soil strength. Implementing a robust dewatering system is essential to lower the groundwater table below the trench floor level and reduce uplift forces. This may involve installing deep wells or well points around the perimeter to actively pump water away from the excavation zone. For highly impermeable soils, the eductor well method may be necessary to achieve the required drawdown.
If the existing protective system failed, a complete redesign is required. The revised plan must consider deeper embedment of sheet piling or diaphragm walls, anchoring the walls into a stable stratum below the zone of potential failure. A qualified geotechnical engineer must review the soil data and use analytical methods to calculate the required factor of safety against basal failure.