What is Subsidence?

Subsidence is a term that strikes fear into the hearts of property owners and potential buyers alike. It refers to the downward movement of the ground supporting a building, which can lead to structural damage. This phenomenon can be caused by various factors, including soil type, water content, and human activities. Understanding the soilscape of a property is crucial in assessing the risk of subsidence and making informed decisions.

The Importance of Soilscapes in Property Assessment

Soilscapes, or the different types of soil found in a particular area, play a significant role in determining the stability of the ground beneath a property. Each soil type has unique characteristics that can influence its behaviour under different conditions. By understanding these characteristics, property buyers and investors can better assess the risk of subsidence and take appropriate measures to mitigate it.

Types of Soil and Their Impact on Subsidence Risk

Clay Soils

Clay soils are known for their high plasticity and ability to retain water. They can expand and contract significantly with changes in moisture content, making them particularly prone to causing subsidence.

Characteristics of Clay Soils:

• High Plasticity: Clay soils can change shape and volume when wet or dry.
• Water Retention: They can hold a large amount of water, leading to swelling.
• Shrink-Swell Potential: The soil can shrink when dry and swell when wet, causing ground movement.

Impact on Subsidence Risk:

• Seasonal Movement: The shrink-swell behaviour of clay soils can lead to seasonal ground movement, causing structural damage to buildings.
• Tree Influence: Trees and large shrubs can exacerbate the problem by extracting water from the soil, leading to further shrinkage.

Sandy Soils

Sandy soils are composed of larger particles and have good drainage properties. They are less prone to significant volume changes with moisture content but can still pose subsidence risks under certain conditions.

Characteristics of Sandy Soils:

• Good Drainage: Sandy soils allow water to pass through easily, reducing the risk of waterlogging.
• Low Cohesion: The particles do not stick together well, making the soil less stable.
• Erosion Prone: Sandy soils can be easily eroded by wind and water.

Impact on Subsidence Risk:

• Erosion: The loose nature of sandy soils can lead to erosion, undermining the foundations of buildings.
• Compaction: Sandy soils can compact under heavy loads, causing ground settlement and potential subsidence.

Silt Soils

Silt soils have smaller particles than sandy soils and can retain more water. They are more cohesive than sandy soils but less so than clay soils.

Characteristics of Silt Soils:

• Moderate Water Retention: Silt soils can hold water but are not as prone to swelling as clay soils.
• Smooth Texture: The fine particles give silt soils a smooth texture.
• Moderate Cohesion: Silt soils have better cohesion than sandy soils but less than clay soils.

Impact on Subsidence Risk:

• Waterlogging: Silt soils can become waterlogged, leading to ground instability.
• Compaction: Like sandy soils, silt soils can compact under heavy loads, causing settlement.

Loam Soils

Loam soils are a balanced mixture of sand, silt, and clay. They are considered ideal for gardening and agriculture due to their good drainage and nutrient-holding capacity.

Characteristics of Loam Soils:

• Balanced Composition: Loam soils have a mix of sand, silt, and clay, providing good drainage and water retention.
• Nutrient-Rich: They are fertile and support plant growth well.
• Good Structure: Loam soils have a stable structure, reducing the risk of erosion.

Impact on Subsidence Risk:

• Low Risk: The balanced nature of loam soils makes them less prone to subsidence compared to other soil types.
• Tree Influence: While generally stable, the presence of large trees can still affect loam soils by extracting water.

Peat Soils

Peat soils are composed of decomposed organic matter and are found in wetland areas. They have high water content and can be very compressible.

Characteristics of Peat Soils:

• High Organic Content: Peat soils are rich in organic matter.
• High Water Content: They can hold a large amount of water.
• Compressibility: Peat soils are highly compressible and can settle significantly under load.

Impact on Subsidence Risk:

• High Risk: The compressible nature of peat soils makes them highly prone to subsidence.
• Waterlogging: Peat soils can become waterlogged, leading to ground instability.

Assessing Subsidence Risk: A Step-by-Step Guide

Step 1: Conduct a Soil Survey

The first step in assessing subsidence risk is to conduct a thorough soil survey. This involves collecting soil samples from different locations on the property and analysing their composition and characteristics.

How to Conduct a Soil Survey:

• Hire a Professional: Engage a qualified geotechnical engineer or soil scientist to conduct the survey.
• Sample Collection: Collect soil samples from various depths and locations on the property.
• Laboratory Analysis: Analyse the samples in a laboratory to determine their composition, moisture content, and other relevant properties.

Step 2: Analyse Soil Characteristics

Once the soil survey is complete, the next step is to analyse the characteristics of the soil. This includes understanding its plasticity, water retention capacity, and shrink-swell potential.

Key Factors to Analyse:

• Plasticity Index: Measure the soil’s ability to change shape and volume with moisture content.
• Moisture Content: Determine the amount of water the soil can hold.
• Shrink-Swell Potential: Assess the soil’s tendency to shrink when dry and swell when wet.

Step 3: Evaluate Environmental Factors

Environmental factors such as climate, vegetation, and water table levels can also influence subsidence risk. It is essential to consider these factors when assessing the overall risk.

Environmental Factors to Consider:

• Climate: Understand the local climate and its impact on soil moisture levels.
• Vegetation: Assess the presence of trees and large shrubs that can extract water from the soil.
• Water Table: Determine the depth of the water table and its potential impact on soil stability.

Step 4: Assess Building Foundations

The type and condition of a building’s foundations can significantly influence its susceptibility to subsidence. It is crucial to evaluate the foundations and their suitability for the soil type.

Foundation Assessment:

• Foundation Type: Identify the type of foundation used (e.g., shallow, deep, pile).
• Foundation Condition: Inspect the condition of the foundations for any signs of damage or deterioration.
• Foundation Depth: Ensure the foundations are deep enough to reach stable soil layers.

Step 5: Implement Mitigation Measures

If the assessment reveals a high risk of subsidence, it is essential to implement appropriate mitigation measures to protect the property.

Mitigation Measures:

• Soil Stabilisation: Use techniques such as soil compaction, grouting, or chemical stabilisation to improve soil stability.
• Foundation Reinforcement: Strengthen the foundations by underpinning or using piles to reach stable soil layers.
• Water Management: Implement drainage systems to control water levels and prevent waterlogging.

The Role of Professional Surveyors

Engaging a professional surveyor is crucial in accurately assessing subsidence risk and implementing effective mitigation measures. Surveyors have the expertise and tools to conduct thorough soil surveys, analyse soil characteristics, and evaluate environmental factors.

Benefits of Hiring a Professional Surveyor:

• Accurate Assessment: Professional surveyors provide accurate and reliable assessments of subsidence risk.
• Expert Advice: They offer expert advice on suitable mitigation measures and foundation solutions.
• Peace of Mind: Hiring a professional surveyor gives property buyers and investors peace of mind, knowing that potential risks have been thoroughly evaluated.

Conclusion

Understanding the soilscape of a property is essential in assessing the risk of subsidence and making informed decisions. Different soil types have unique characteristics that can influence their behaviour under various conditions, impacting the stability of the ground beneath a building. By conducting a thorough soil survey, analysing soil characteristics, evaluating environmental factors, and assessing building foundations, property buyers and investors can better understand the subsidence risk and take appropriate measures to mitigate it.

At Flettons, we’re committed to safeguarding your investment. When considering a property purchase, trust our seasoned expertise to reveal any hidden threats. For a thorough building survey, get your instant quote through our quote calculator or reach out directly at 0203 691 0451. Your home’s safety is our top priority.

---