What is Subsurface Moisture?

Introduction

When it comes to building safely and sustainably, what lies beneath the surface is just as important as what’s built above it. One of the most critical factors geotechnical engineers analyze is subsurface moisture—the water present in soil below the ground surface. Unlike surface water, which is easy to see and manage, subsurface moisture is hidden, yet it has a profound impact on soil behavior, slope stability, and the performance of structures.

At G3SoilWorks, our team of geologic engineers has decades of experience helping clients throughout Southern California and the western United States understand how moisture affects soils and foundations. With over 100 years of combined expertise in geotechnical, environmental, and engineering geology consulting, we know firsthand that correctly evaluating and managing subsurface moisture can make the difference between a stable foundation and costly structural issues down the road.

What is Subsurface Moisture?

Subsurface moisture refers to the water found beneath the surface of the soil. This water can occupy soil pores, cling tightly to soil particles, or move freely through larger voids depending on soil type, composition, and local hydrologic conditions. It is different from groundwater, which typically exists in aquifers below the water table. Subsurface moisture usually exists above this level, but it plays a major role in soil strength and stability.

Engineers and geologists study subsurface moisture because it directly influences:

• Soil compaction and density (which affects how much soil can support a load). 
• Settlement (how much the ground sinks when supporting a structure). 
• Slope stability (how likely a hillside or embankment is to fail). 
• Expansive or collapsible soils (common in parts of Southern California). 

Understanding moisture beneath the surface is essential before constructing buildings, roads, retaining walls, or other infrastructure.

Why Subsurface Moisture Matters in Geotechnical Engineering

Subsurface moisture plays a pivotal role in geotechnical design. Here are some of the ways it impacts engineering decisions:

1. Foundation Stability
   If soils hold too much water, they lose strength, reducing their ability to support a structure’s load. This can lead to uneven settlement or cracks in foundations. 
2. Soil Expansion and Shrinkage
   Expansive clay soils absorb water and swell, then shrink as they dry. This cycle can damage pavements, foundations, and underground utilities if not accounted for. 
3. Slope Failure and Landslides
   On hillsides, water infiltrates and increases pore pressure, which can destabilize slopes. Engineers evaluate subsurface moisture to assess landslide risks and design protective measures. 
4. Infrastructure Longevity
   Roads, retaining walls, and other civil works depend on stable ground. Excess moisture beneath these structures can reduce lifespan and increase maintenance costs. 

For these reasons, geotechnical engineers invest significant effort in studying moisture before, during, and after construction.

How Moisture is Calculated and Evaluated

Accurately measuring subsurface moisture is vital for understanding site conditions. Geotechnical engineers use a combination of field and laboratory techniques to calculate and evaluate moisture levels:

Field Testing Methods

• Gravimetric Method: Collecting soil samples, weighing them, drying them in the field, then reweighing to determine moisture content. 
• Time-Domain Reflectometry (TDR): Using sensors that measure how quickly an electrical signal moves through soil, which correlates with water content. 
• Neutron Probe/Nuclear Methods: Specialized instruments that measure hydrogen atoms in the soil, giving a direct indication of water content. 
• Electrical Resistivity: Since water conducts electricity differently than dry soil, resistivity tests can map variations in moisture underground. 

Laboratory Testing Methods

• Oven-Drying: A standard method where soil samples are weighed, oven-dried at a controlled temperature, and reweighed to calculate water content. 
• Chemical Methods: In some cases, chemicals are used to detect moisture levels, though less common for geotechnical use. 

The data collected is then interpreted to help determine soil strength, settlement potential, and the need for mitigation measures such as drainage or soil stabilization.

Types of Moisture in Soil

Not all subsurface moisture behaves the same way. Engineers categorize moisture into several types:

1. Gravitational Water

1. • Moves freely through large soil pores. 
   • Drains quickly after rainfall or irrigation. 
   • Usually not available to plants but can reduce soil strength. 
2. Capillary Water 
   • Held in smaller pores between soil particles. 
   • Readily available to plants. 
   • Important for cohesion in soils but can contribute to expansion or shrinkage. 
3. Hygroscopic Water 
   • Thin film of water tightly bound to soil particles. 
   • Not available to plants. 
   • Doesn’t drain or evaporate easily. 
4. Groundwater Influence 
   • While subsurface moisture lies above the water table, fluctuations in groundwater can affect how much moisture is present in upper soil layers. 

Each type has different implications for soil behavior, slope stability, and engineering design.

Geotechnical Implications of Subsurface Moisture

Understanding subsurface moisture isn’t just about knowing how wet the soil is—it’s about predicting how the soil will behave under load, over time, and through environmental changes. Some key implications include:

• Soil Expansion/Shrinkage: Clay-rich soils expand when wet and shrink when dry, leading to cracks and structural movement. 
• Bearing Capacity: Wet soils often lose their ability to carry loads, which can compromise foundations. 
• Slope Stability: Increased moisture raises pore water pressure, lowering friction between soil particles, and increasing landslide risks. 
• Erosion: Subsurface water movement can weaken soil and wash away fine particles, undermining stability. 

In regions like Southern California, where diverse soil types and hillside construction are common, these factors play a huge role in engineering assessments.

Managing Subsurface Moisture in Engineering Projects

Once subsurface moisture is identified and evaluated, engineers can recommend management strategies:

1. Drainage Solutions 
   • Subdrains beneath structures. 
   • Retaining wall drains. 
   • Slope surface protection to control infiltration. 
2. Soil Treatment 
   • Compaction control during construction. 
   • Chemical stabilization (lime or cement treatment for expansive soils). 
   • Grading adjustments to divert surface water away from sensitive areas. 
3. Monitoring Systems 
   • Installing piezometers and moisture sensors for ongoing measurement. 
   • Regular inspections, especially after heavy rainfall or drought cycles. 
4. Long-Term Maintenance 
   • Landscaping choices that minimize water infiltration. 
   • Routine drainage system upkeep. 

At G3SoilWorks, we tailor these solutions to each project’s unique geology and conditions, ensuring long-term safety and performance.

Real-World Example: Southern California Challenges

Southern California presents unique challenges when it comes to subsurface moisture:

• Expansive Clay Soils: Found in many valleys and flatlands, they swell with moisture and shrink during dry periods. 
• Collapsible Soils: Lightly compacted soils that collapse suddenly when wetted, causing rapid settlement. 
• Steep Slopes: Hillsides in areas like Los Angeles and Orange County are vulnerable to landslides when saturated. 

By thoroughly analyzing moisture, G3SoilWorks helps clients mitigate these risks through practical, science-based engineering solutions.

Conclusion

Subsurface moisture may be hidden from sight, but its influence on soil stability, foundation strength, and slope safety cannot be underestimated. For geotechnical engineers, accurately evaluating and managing this underground water is essential to building safe, durable, and cost-effective structures.

With over a century of combined expertise, G3SoilWorks provides the insight and strategies needed to address moisture-related challenges across Southern California and the western United States. From field investigations to long-term solutions, our team ensures that every project is built on a foundation of solid geotechnical understanding.

If you’re planning a project and need expert guidance on how subsurface moisture could affect your site, reach out to G3SoilWorks today. Our experienced team is here to help you build with confidence.

FAQs

Q1. How does subsurface moisture vary between sandy and clay soils?
Subsurface moisture behaves differently depending on soil type. Sandy soils drain quickly, often holding less water, while clay soils retain moisture for much longer, leading to higher risks of swelling and shrinkage cycles.

Q2. Can landscaping choices affect subsurface moisture levels?
Yes. Plants with deep root systems or excessive irrigation can increase water infiltration into soils, while proper landscaping with drought-tolerant plants can help minimize subsurface moisture fluctuations around structures.

Q3. How does seasonal weather impact subsurface moisture?
Seasonal changes—such as heavy winter rains followed by hot, dry summers—cause significant moisture swings in soils. These shifts can stress foundations, slopes, and pavements, making seasonal monitoring an important part of geotechnical planning.