Cohesive soil on construction sites – properties, risks and solutions

What You Need to Know

Cohesive soil such as loam or clay changes its load-bearing capacity depending on the moisture content – and quickly becomes a problem for heavy machinery when wet.The result: subsidence, poor trafficability and unstable construction site conditions.

With targeted soil improvement, drainage, geogrids and off-road technology such as crawler platforms or all-terrain forklifts, you can still work safely and efficiently.Careful analysis of the subsoil is crucial – especially in changeable weather or on uneven surfaces.

Whether on construction sites, in agriculture, or during soil investigations – the composition of the soil plays a crucial role . Cohesive soils, in particular, are challenging because their load-bearing capacity changes significantly depending on their moisture content .

For the safe operation of aerial work platforms, forklifts, and construction machinery, a realistic and professional assessment of the ground conditions is therefore essential. This is the only way to reliably avoid stability issues , accident risks , and operational interruptions .

Fundamentals and properties

What is cohesive soil?

Cohesive soil consists of fine-grained material such as clay or silt. It is called cohesive soil because the fine particles retain water and thus hold it together. Unlike non-cohesive soils , which consist of coarse-grained material such as sand or gravel and drain water quickly, cohesive soil can absorb large amounts of moisture. This causes it to become soft and muddy when wet, while during dry periods it shrinks, hardens, and cracks – leading to a loss of load-bearing capacity on construction sites.

Cohesive vs. non-cohesive soil – The most important differences

Geotechnical characteristics of cohesive soils

Certain geotechnical parameters are crucial for the technical assessment of cohesive soils . These include the plasticity index (PI) and the consistency limits , specifically the liquid limit (LL) and plastic limit (PL) . These parameters indicate the degree to which a soil deforms in response to changes in moisture content . A high PI value signifies highly plastic behavior and is typical for clay-rich soils .

To assess bearing capacity, CBR values ​​or Proctor density tests are frequently used. These indicators show how load-bearing and compactable the soil is under real-world loads – a key factor for planning access roads, support areas, and machine locations .

Standards and classification

Cohesive and non-cohesive soils are classified according to DIN 18196. DIN EN ISO 14688 further regulates the description and classification according to grain size and plastic behavior . For construction projects, DIN 1054 is also relevant, as it specifies the requirements for geotechnical reports and soil surveys .

A geotechnical report should always be obtained before the use of heavy machinery . This is the only way to reliably assess the bearing capacity and settlement tendency of the soil and to identify risks such as subsidence or instability at an early stage.

Impact on construction sites

What does this mean for construction sites?

• Cohesive soil is problematic for heavy machinery because it deforms when wet and offers little stability.
• Non-cohesive soil is better suited for the use of work platforms and forklifts because it is more load-bearing and resistant to deformation.

Problems with cohesive soil in construction

On construction sites, cohesive soils often lead to:

• Sinking of machines such as aerial work platforms or telescopic handlers.
• Instability in buildings occurs because the ground expands when wet and shrinks when dry.
• Difficult to drive on , especially after rainfall or freeze-thaw cycles.

Solutions and measures

Soil improvement on the building site

To improve the load-bearing capacity of cohesive soils , base layers of crushed stone or gravel are often installed. These should be at least 20–30 cm thick to distribute loads over a large area . Additionally, geogrids or geotextiles with a surface load capacity of 20–40 kN/m² can be used to further stabilize the subsoil .

For permanently damp soils , drainage with a minimum gradient of 1.5% is recommended to ensure controlled water runoff . This helps regulate the moisture content and maintain load-bearing capacity in the long term .

Choosing the right machine

Machines with low ground pressure should preferably be used on cohesive soils . These include, among others:

• Tracked aerial work platforms with low ground pressure and even load distribution.
• All-terrain telescopic loaders with all-wheel drive and differential lock.
• Work platforms with hydraulic support systems for additional stability.

The selection depends not only on the load-bearing capacity , but also on the soil structure and moisture content . Therefore, a prior soil test according to DIN 18196 is always recommended to ensure safe and predictable preparation for use.

Pay attention to the weather

• Adjust operating times to avoid extreme humidity or dryness.
• Conduct a soil test before starting work to ensure stability.

Economic efficiency and sustainability

The choice of soil improvement measure directly impacts safety and cost-effectiveness . Gravel base courses are usually inexpensive and effective , while cement stabilization incurs higher initial costs but offers less maintenance and long-term stability . The additional effort often pays for itself through fewer machine breakdowns .

Sustainable solutions such as recycled aggregate are increasingly being used. Recycled gravel conserves resources and improves the CO₂ balance . Regardless of the method, groundwater and soil ecosystems must not be adversely affected.

Practice and safety

Practical experience and case studies

Practical projects show that construction sites with soil improvement using geogrids and gravel base courses experience up to 60% less machine sinking. In a construction project in the marshy soil of northern Germany, the failure rate of platforms and forklifts was reduced by a third through targeted subsoil improvement.

Safety and occupational health

Working on cohesive soil requires a risk assessment according to DGUV Regulation 1. The most important measures are:

• Wear non-slip footwear and PPE on wet surfaces.
• Securing work areas with a risk of falling using traffic cones and warning signs.
• Prohibition of standing under raised machinery on unstable ground.

Modern soil analysis

In modern construction site planning, 3D ground scans and ground-penetrating radar (GPR) are increasingly used to digitally visualize load-bearing zones .

Integrating geotechnical data into BIM models allows for realistic simulation of loads from machinery and structures . This enables early risk assessment and helps avoid incorrect decisions during the planning phase .

Conclusion

Cohesive soil presents a particular challenge in construction. Geotechnical analysis, soil improvement, and the use of suitable machinery can significantly increase safety. By adhering to the relevant standards (DIN 18196, DIN EN ISO 14688, DIN 1054) and using digital planning tools, construction projects can be implemented efficiently and sustainably even on difficult soils.

About the author

Martin Biberger

Managing Director

Martin is the founder and managing director of BIBERGER Arbeitsbühnen & Forklifts.

He is responsible for thetechnical areaTogether with his team, he is responsible for thetechnical purchasingthe machines thatFurther development of the machine inventoryand the smooth operation of over 1,500 BIBERGER rental devices.

From many years of experience he knows theStrengths and weaknesses of all device classes, the possibleAreas of applicationand thetechnical possibilities– always with a view to theDevelopment of the entire industryand future innovations.