Mobile Crane Safety Distance Near Excavations | TCVN 4244

Amidst the hustle and bustle of a construction site, the sight of a lifting machine weighing tens of tons reaching out near the edge of a narrow excavation pit always brings extreme tension. In that moment, the "safe distance" is absolutely not dry technical specifications on paper. It is the fragile boundary determining the life safety of the people working hard below, as well as the success or failure of an entire project.

Just one misaligned decision to place an outrigger on weak sandy and gravelly soil can lead to catastrophic landslides and crane overturning. So, how is this red line quantified and protected by law through technical factors?

Legal framework and national standards for lifting equipment

The operation of lifting equipment at a construction site must strictly comply with current technical regulations and standards to ensure the safety of workers, equipment, and structures.

QCVN 18:2021/BXD Regulation – Proactive prevention principle

QCVN 18:2021/BXD stipulates mandatory requirements for contractors and construction units regarding mechanical safety. According to this regulation, when mobile lifting equipment operates near pits, trenches, or ditches, the operating unit must establish a safe distance to ensure the stability of the equipment's foundation.

This requirement stems from the fact that the ground must simultaneously bear the static load from the equipment's weight and the dynamic load generated during the load lifting process. Compliance with QCVN 18:2021/BXD is a mandatory technical condition for the safety department to assess risks and issue a Permit to Work.

TCVN 4244:2005 – The quantitative figure of the red line

Table 19 of Vietnam Standard TCVN 4244:2005 provides detailed technical parameters on safe distances based on the physical and mechanical properties of each soil type and the excavation depth.

For an excavation pit with a depth of 1.0 meter constructed on sandy and gravelly soil, the mandatory minimum safe distance is 1.5 meters.

Method for determining distance on site: This distance is measured straight from the outermost edge of the hydraulic outrigger pad (the location where the equipment transmits the greatest pressure to the ground) to the upper edge of the excavation pit wall.

In practical construction situations where this minimum distance of 1.5 meters cannot be met, the construction unit must mandatorily design and apply measures to reinforce the excavation wall structure (such as sheet piling, installing a bracing system). A shortfall in safe distance without compensatory reinforcement measures is a violation of technical standards, leading to the risk of soil wall subsidence and crane overturning.

=> Read more: Lifting Equipment Safety Regulations: Latest Standard Update 2026

Why 1.5 meters? The mechanical nature of sandy and gravelly soil

The minimum distance requirement of 1.5 meters for sandy and gravelly soil stems from the distinct physical and mechanical parameters and stress states of this soil type under the action of external loads.

Cohesionless nature and soil wedge sliding risks

Sandy and gravelly soil is classified into the granular soil group, with the basic characteristic being that the cohesive force between particles is zero (cohesion c = 0). The shear strength and bearing capacity of the soil mass depend entirely on the internal friction angle and the mechanical interlocking between the graded particles.

Due to the lack of cohesive force, the projected sliding surface (sliding wedge zone) of an excavation pit on sandy and gravelly soil has a flatter slope angle, extending deeper into the construction site compared to clay soil. If the crane's outrigger pad is placed too close to the pit edge, the transmitted stress will superimpose on the shear stress of the soil wall, exceeding the apparent shear resistance. The distance of 1.5 meters is established to ensure the concentrated load from the outrigger lies entirely outside the critical sliding prism, preventing the phenomenon of pushing and sliding the soil mass into the pit.

Mechanical impact of vibration loads

The operation process of a mobile crane (lifting, lowering, slewing, braking) continuously generates dynamic loads and vibration cycles transmitted down to the foundation. Granular soil environments like sand and gravel are particularly sensitive to these impulses.

Under the effect of vibration forces, the static interlocking between gravel and sand particles is broken, causing a sudden decrease in internal friction and leading to uneven subsidence right at the contact position of the outrigger pad. Moving the load transmission center back to 1.5 meters helps increase the intermediate soil mass, attenuating the amplitude and intensity of vibration waves before they impact the open excavation pit wall.

Piping phenomenon under the impact of seepage flow

Sandy and gravelly soil has a high permeability coefficient. In construction conditions where groundwater emerges or surface water seeps down, the water head difference between the natural ground elevation and the pit bottom will create a seepage flow toward the pit.

This seepage flow generates hydrodynamic pressure, which can wash away loose fine sand particles within the grading band. This phenomenon (piping) alters the soil's porosity, creating localized voids beneath the surface. The massive pressure from the crane's outrigger, if placed on a piped soil area, will cause sudden subsidence and lead to equipment overturning. The 1.5-meter distance acts as a safe median, limiting the placement of equipment on the soil area with the highest unstable hydraulic gradient.

=> Read more: Crane Safety Factor: TCVN Regulations & Detailed Calculation Formula

"Pressure Bulb" - The mechanism of outriggers transmitting loads to the ground

To clearly understand the necessity of a safe distance, it is necessary to analyze the mechanism of mechanical load distribution from the lifting equipment to the natural ground. The "pressure bulb" concept is the key geotechnical basis for explaining this phenomenon.

Contact pressure distribution at the outrigger pad

In the working state, the mobile crane relieves its weight from the wheel axles and concentrates the entire load on the 4 hydraulic outrigger pads. This load includes static load (the dead weight of the base carrier, crane, counterweight) and dynamic load (the weight of the lifted load, inertia force generated when slewing or braking).

Because the surface area of the outrigger pads is relatively small compared to the total equipment mass, the contact pressure transmitted to the ground is extremely large. This pressure parameter usually fluctuates at a very high threshold, from 300 kPa to 800 kPa, posing a risk of local foundation failure if not distributed properly.

Stress propagation model in 3D space

According to the principles of soil mechanics, the pressure from the outrigger pad does not transmit straight down vertically in a prismatic shape, but disperses widely outwards in a 3D space. Points with the same stress value in the soil connect to form isobars (stress contours).

The collection of these isobars forms a hemispherical influence zone, known as a "pressure bulb". The stress intensity reaches its maximum right near the contact surface under the outrigger pad and gradually decreases as it expands outward or goes deeper. The most dangerous core zone of the pressure bulb (where 10% to 20% of the maximum stress is concentrated) usually develops to a depth approximately 1.5 to 2 times the width of the outrigger pad.

The dangerous intersection and the 1.5-meter boundary

When an excavation pit is opened, the pit wall forms an unconfined face with absolutely no lateral resistance pressure. If the outrigger pad is placed too close to the pit edge, the boundary of the "pressure bulb" will directly intersect with this unconfined face.

At this time, instead of being neutralized by the resistance force of the surrounding soil mass, the lateral stress generated from the crane's load will push directly into the excavation wall. Because the loose sandy and gravelly soil has no cohesion, this lateral stress will quickly overcome the soil's shear resistance, thrusting the soil mass into the pit and causing the equipment to lose its footing.

The regulation of maintaining a minimum distance of 1.5 meters is precisely the technical solution to shift the entire "pressure bulb" deep into the dense, solid ground. This distance ensures the force transmission trajectory of the outrigger system lies completely outside the natural sliding prism of the excavation pit, eliminating the risk of unconfined wall landslides due to mechanical overload.

Conclusion

Establishing and complying with the minimum safe distance of 1.5 meters when operating a mobile crane near an excavation pit on sandy and gravelly soil is a mandatory technical requirement according to TCVN 4244:2005. This parameter must be directly integrated into the construction organization measure design documents to prevent the risk of localized geotechnical instability, neutralizing the intersection of stress transmitted from the outriggers into the pit wall.

To ensure comprehensive safety, maintaining a mere geometric distance is not enough. Construction units need to closely combine this task with continuous monitoring of the ground's physical and mechanical changes at the site. Specifically, the standard TCVN 4447:2012 (Earthworks - Codes for construction and acceptance) should be applied in parallel to implement an effective surface and groundwater collection and drainage system. Properly controlling seepage flow and eliminating hydrodynamic pressure will help permanently prevent piping, thereby maintaining the original bearing capacity and shear resistance of the granular soil mass under the action of mechanical loads.

In summary, lifting equipment safety work requires synchronized coordination between theoretical construction measure calculations and practical geotechnical surveying and monitoring. Construction enterprises, installation units, and Health, Safety, and Environment (HSE) departments must take strict compliance with the national standards and regulations system as the core foundation. This is the solidest technical basis for building safe operational procedures, completely eliminating risks and equipment overturning incidents, and maximizing the protection of workers' lives on the construction site.