What Is Groundwater and Why Is Groundwater Control Needed?

Water doesn’t just flow across the surface of the earth. A vast, largely invisible reserve of it exists beneath us, locked inside rock, soil, and sediment feeding rivers, filling wells, sustaining ecosystems, and quietly shaping the ground on which we build. This is groundwater. And for anyone involved in construction, civil engineering, or excavation, understanding what it is and why it needs to be controlled is not optional. It’s foundational.

This guide explains what groundwater is, how it forms and moves, where it comes from, and why managing it properly is one of the most critical challenges in below-ground construction.

What is Groundwater?

At its simplest, groundwater is water that exists below the surface of the earth, filling the spaces pores, cracks, and fractures within soil, sediment, and rock.

When rain falls or snow melts, some of that water runs off across the surface into rivers and streams. But a significant portion seeps downward through the soil, drawn by gravity. It passes through an upper zone where air and water share the available pore space called the unsaturated zone or vadose zone until it reaches a depth where every pore and crack is completely filled with water. This is the saturated zone, and the water it contains is groundwater.

The boundary between the unsaturated zone above and the saturated zone below is called the water table. Think of it as the “surface” of the underground water body the level to which water would rise in an open borehole drilled into the ground.

The water table is not fixed. It rises and falls with the seasons, responding to rainfall, evaporation, snowmelt, and human activities like pumping. In wet seasons, the water table rises as recharge outpaces discharge. In dry seasons, it falls. In areas with very shallow water tables, the land can become waterlogged after heavy rain. In others, the water table sits hundreds of metres below the surface.

How Does Groundwater Form?

Groundwater begins its journey as precipitatio rain, snow, sleet. Once it reaches the ground, some evaporates immediately, some is absorbed by plants and transpired back into the atmosphere, and some runs off across the surface. What remains filters downward through the soil.

This downward movement is called infiltration or percolation. As water moves deeper, it passes through layers of soil and rock with varying characteristics. Loose sand and gravel are highly permeable water moves through them quickly. Dense clay and solid bedrock are much less permeable water moves through them slowly, if at all.

Eventually, the percolating water reaches a level where all available pore spaces are saturated and joins the groundwater body. From there, it moves slowly through the ground, generally from areas of higher elevation to lower, eventually discharging at springs, seeping into rivers and lakes, or being intercepted by wells.

Aquifers: Where Groundwater Lives

Not all underground formations hold or transmit water equally well. A geological formation that stores and transmits significant quantities of groundwater is called an aquifer.

Aquifers come in two main types:

Unconfined (water table) aquifers sit directly below the surface, with the water table forming their upper boundary. They are recharged directly by rainfall percolating down from above. Most shallow wells draw from unconfined aquifers.

Confined aquifers sit deeper, sandwiched between layers of impermeable rock or clay called confining layers (aquitards). The water in a confined aquifer is often under pressure when a borehole is drilled into one, the water can rise above the top of the aquifer and sometimes even flow freely to the surface. These are called artesian aquifers, and the wells that tap them are artesian wells.

Aquifer materials range from loose sands and gravels which are highly productive to fractured limestone, basalt, and sandstone. Groundwater flow velocity varies enormously depending on permeability: in coarse gravel, water might move several metres per day; in clay, it might move less than a millimetre in the same period.

How Does Groundwater Affect the Ground Above It?

Groundwater isn’t just a below-ground phenomenon. It has a profound influence on the physical behaviour of the ground at and near the surface which is why it matters so much to engineers and construction professionals.

Soil Strength and Bearing Capacity

When soil becomes saturated with water, its mechanical behaviour changes dramatically. Water fills the spaces between soil particles and exerts pressure called pore water pressure that acts to push the particles apart. This reduces the effective stress between grains, which is what gives soil its strength and stiffness.

The practical result is that saturated soils bear loads less effectively than dry or unsaturated soils. Foundations built on saturated ground can settle unevenly, crack, or shift over time. In the worst cases, this leads to structural failure.

Slope Stability

Elevated groundwater levels increase pore water pressure within slopes, reducing their shear strength. A slope that is stable under normal conditions may become unstable when the water table rises, either through heavy rain, seasonal change, or construction activities nearby that alter drainage patterns. The result can be slope creep, shallow slipping, or, in severe cases, full-scale landslides.

Heave and Uplift

In deep excavations, if groundwater pressure beneath the excavation base is not properly controlled, the base can “heave” pushing upward under hydrostatic pressure. This is particularly dangerous in confined aquifer conditions where the upward water pressure can be very high. For deep excavations with high groundwater tables, contractors often rely on deep well dewatering systems to reduce hydrostatic pressure.

Subsidence

The relationship between groundwater and ground movement goes both ways. When large volumes of groundwater are pumped from an aquifer either for water supply or construction dewatering the reduction in pore water pressure can cause the ground to compress and settle. This ground subsidence can damage roads, utilities, and the foundations of nearby buildings if not carefully managed.

Why Is Groundwater Control Needed in Construction?

Construction rarely stays above the water table. Basements, tunnels, bridge foundations, underground car parks, pipeline trenches, and utility ducts all require excavating into or through water-bearing ground. When that happens, groundwater control becomes a fundamental requirement not a luxury.

Worker Safety

The most immediate reason to control groundwater is the safety of the people working below ground. Uncontrolled groundwater creates a cascade of hazards:

Excavation walls in saturated ground can collapse with little warning. Saturated soils, especially sands and silts, lose their cohesion rapidly when exposed and can flow into an open trench a process called piping or running sand. Flooded excavations create drowning risks and can trap or crush workers when they collapse.

Dry, stable ground is the foundation of safe below-ground work. Groundwater control is the means of achieving it.

Structural Integrity

Water and concrete structures are adversarial by nature. Groundwater exerts continuous hydrostatic pressure against basement walls, slabs, retaining walls, and tunnels. If the structure is not designed to resist this pressure or if the groundwater is not lowered before and during construction water will find every crack, joint, and imperfection.

Beyond pressure, groundwater can carry dissolved salts and aggressive chemicals that attack concrete and corrode steel reinforcement over time, causing long-term structural degradation that can take decades to become apparent.

Project Schedule and Cost

Water is one of the most reliable schedule killers on a construction site. An excavation that floods overnight requires pumping out, drying, re-inspection, and often remediation before work can continue. Repeated flooding events or a single catastrophic inflow can set a programme back by weeks or months and inflate costs enormously.

Proper groundwater control, planned from the outset and designed for the specific site conditions, eliminates these unpredictable delays. The cost of a well-designed dewatering system is almost always a fraction of the cost of managing the consequences of inadequate groundwater control.

Protecting Adjacent Structures and Services

Groundwater doesn’t respect site boundaries. Uncontrolled seepage and poorly managed dewatering can affect properties well beyond the construction footprint. Lowering the water table too aggressively can cause subsidence beneath nearby buildings. Discharging turbid or contaminated water can damage watercourses. Altering drainage patterns can affect the water supply of local wells.

In urban environments especially, the impact of groundwater mismanagement can extend far beyond the site boundary creating legal liability, regulatory penalties, and community relations problems that can be far more costly than the construction work itself.

Environmental and Regulatory Compliance

Groundwater is a protected resource in most jurisdictions. Construction activities that disturb, contaminate, or uncontrolled discharge groundwater are subject to regulatory oversight and the penalties for non-compliance are substantial. Environmental regulators can halt a project entirely until compliance is demonstrated.

Groundwater control systems must therefore not only keep the excavation dry but must also manage how and where the pumped water is discharged, what treatment it receives before discharge, and how the system’s impact on the broader aquifer is monitored and reported.

The Two Approaches to Groundwater Control

Engineers and geotechnical specialists use two broad strategies to manage groundwater in construction:

1. Pumping (Dewatering)

Pumping methods lower the water table by extracting groundwater from the ground either from sumps within the excavation or from wells installed around it. The main techniques include sump pumping, wellpoint systems, deep-well systems, and eductor systems. Each suits different soil types, excavation depths, and water volumes. Pumping is the most widely used approach for temporary works during construction. Learn more about how deep well dewatering systems work for large-scale excavation projects.

For shallow excavations with manageable groundwater inflow, our sump pump dewatering services provide an efficient solution for temporary water removal.

2. Exclusion

Exclusion methods don’t lower the water table they keep water out by installing low-permeability barriers around the excavation. Sheet pile walls, concrete diaphragm walls, jet-grouted cut-offs, and ground freezing are all exclusion techniques. These are often used in combination with pumping the exclusion barrier reduces the volume of water that needs to be pumped, making the overall system more manageable.

Which approach or which combination is right for a given project depends on the site’s geology, the depth and extent of the excavation, the proximity of adjacent structures, and the duration of the works. This is specialist territory, and getting it right requires proper ground investigation and expert input.

Projects involving trenches or shallow foundations often use wellpoint dewatering systems to lower groundwater levels efficiently.

What Happens When Groundwater Control Goes Wrong?

The consequences of inadequate groundwater management in construction are well-documented and consistently severe:

Trench collapses and fatalities are among the most tragic and preventable outcomes of groundwater mismanagement. Saturated, unstable ground gives way without warning.

Foundation failures occur when structures are built on saturated ground that settles unevenly under load, or when the groundwater exerts uplift forces that were not accounted for in the structural design.

Adjacent property damage through settlement-induced cracking is a common cause of insurance claims and litigation on urban construction projects.

Environmental damage contaminated discharge reaching rivers, aquifer contamination from construction materials, and disruption of groundwater-dependent ecosystems can have consequences that outlast the construction project by decades.

Project insolvency is not unheard of on projects where a groundwater problem that was not anticipated or properly managed drives costs and delays to a point where the project can no longer be funded.

Conclusions

Groundwater is one of the most abundant and important resources on earth a fundamental part of the global water cycle, vital for drinking water supply, agriculture, and ecosystems worldwide. But in the context of construction and excavation, it is also one of the most significant hazards a project can face.

Understanding what groundwater is how it forms, how it moves, where it sits, and how it behaves when disturbed is the essential starting point for managing it effectively. From that understanding flows everything else: the selection of the right investigation methods, the correct dewatering approach, the appropriate monitoring systems, and the regulatory compliance framework that protects the project, the public, and the environment.

Groundwater doesn’t disappear just because you start digging. But with proper planning and the right controls in place, it doesn’t have to stop you either.