What Is Groundwater, and How Deep Is It?

Many projects focus only on whether the soil is "stiff or soft", yet many site problems — collapsed excavations, voids in bored piles, leaking basement floors — have groundwater as the culprit.

Groundwater is the water held in the voids between soil grains and in rock fractures. The top of the saturated zone is called the groundwater level (water table), and it is not constant — it rises and falls with the seasons, rainfall, water levels in nearby water bodies and groundwater pumping in the area. In many parts of Thailand's Central Plain, the groundwater level is only a few meters below the ground surface.

The figure below summarizes all four effects in a single section: a spread footing placed near the water level, an excavation deeper than the water level, a basement pushed up by water pressure, and piles in groundwater containing corrosive substances.

2D model of the four ways the groundwater level affects foundations Conceptual 2D section. The upper layer is clay and the lower layer is sand, with dry-season and rainy-season groundwater level lines. Zone one: a spread footing placed near the water level; upward water pressure makes effective stress equal to total stress minus water pressure, so bearing capacity decreases. Zone two: an excavation below the water level; water seeps through the pit walls, continuous pumping is needed, and the pit bottom in the sand layer may boil or suffer running sand. Zone three: a basement submerged below the water level, subject to lateral water pressure and uplift under the floor; water pressure is about one tonne per square meter per meter of depth below the water level, and the structure is most at risk of floating when empty or lightly loaded. Zone four: groundwater containing chlorides and sulfates corrodes the concrete and reinforcing steel of the piles. The bottom row summarizes the data needed before design: the water level in the boring log, an observation well or piezometer, and chemical testing of the water or soil. 2D model: 4 ways groundwater affects foundations Not to scale 1 Lower bearing capacity 2 Pit inflow/running sand 3 Uplift (buoyancy) 4 Material corrosion Basement Empty/light load most at risk of uplift Water pressure ≈ 1 t/m² per 1 m depth below GWL Water seeps into pit Constant pumping Base boils / running sand Rainy season Dry season GW level σ′ = σ − u Pore pressure u rises → effective stress drops → less bearing capacity Cl⁻ SO₄²⁻ Cl⁻ SO₄²⁻ Coastal areas Saline soil, old estates Attacks concrete Rebar corrodes A Water level in boring log At drilling + when stable, dated B Observation well/piezometer To track rainy-season changes C Water/soil chemistry tests If sulfate/chloride suspected
2D model (section) — ① footing near the water level: water pressure u reduces effective stress σ′ = σ − u ② excavation below the water level: water seeps through the walls and the pit bottom in the sand layer may boil ③ basement subject to lateral water pressure and uplift under the floor, about 1 t/m² per 1 m of depth below the water ④ chlorides and sulfates corrode concrete and reinforcing steel. The two dashed lines are the rainy-season and dry-season water levels (Image: SPN Soil Engineering)

← Scroll to see the full table →

EffectMechanismWorks at riskData needed
Lower bearing capacityWater pressure reduces effective stressShallow spread footingsWater level in boring log
Excavation problemsWater seepage, base boiling, running sandFoundation excavations, basements, bored pilesWater level + soil type (sand) + rainy season
UpliftWater pushes up ≈ 1 t/m² per 1 m of water depthBasements, water tanks, treatment tanksHighest expected water level (observation well)
CorrosionSulfates/chlorides in water and soilCoastal areas, saline soils, old industrial estatesChemical tests: pH, sulfate, chloride

Effect 1: Why Does Groundwater Reduce Soil Bearing Capacity?

Soil carries load through contact between soil grains. When water is present, the pore water pressure "carries" part of the load instead of the soil skeleton, reducing the effective stress actually carried by the grains. By the principle σ′ = σ − u, effective stress equals total stress minus pore water pressure. As a result:

This is why engineers need to know the groundwater level before concluding the allowable bearing capacity of the soil.

Effect 2: What Problems Arise When Excavating Below the Groundwater Level?

  • Water flowing into the excavation — requires continuous pumping, slowing work and increasing costs
  • Running sand (boiling) — when excavating below the groundwater level in a sand layer, water flowing into the pit can carry sand grains with it until the pit bottom fails or adjacent structures settle
  • Bored pile works — high groundwater requires the borehole to be stabilized with a steel casing or a support fluid; otherwise the hole collapses before concrete is placed
  • Dewatering — makes excavation possible, but if done without analysis it can cause the surrounding ground to subside and affect neighboring buildings
🚨 A common case

A foundation excavation that is "easy to dig" in the dry season can turn into a pond in the rainy season because the groundwater level rises — planning excavation works using groundwater data measured only once in the dry season is therefore a risk that must always be allowed for.

Effect 3: How Much Does Uplift Push on Underground Structures?

Structures submerged below the groundwater level, such as basements, underground water tanks or treatment tanks, are pushed up by water pressure (uplift / buoyancy). If the design does not allow for it, the structure may float, the floor may crack or joints may leak — especially while the structure is still empty and lightly loaded. An empty underground water tank is in its own most dangerous condition.

The magnitude of the water pressure is easy to estimate: water pressure increases by about 1 t/m² (approx. 9.81 kPa) per 1 m of depth below the water level. For example, a basement floor 3 m below the groundwater level will be pushed up by about 3 t/m² over the whole floor. The design water level should therefore be the highest level expected in the rainy season, not a value measured once in the dry season.

Effect 4: How Can Groundwater Corrode Foundations?

In some areas the groundwater is high in sulfates or chlorides, such as coastal areas, saline-soil areas or old industrial areas. These attack concrete and accelerate corrosion of reinforcing steel. Where this is suspected, the engineer may specify collecting water or soil samples for chemical testing, such as pH, sulfate and chloride (see water/soil sampling and testing services), to select a suitable cement type and concrete cover (factory projects should check this from the land selection stage — see the soil checklist before buying factory land).

How Do You Find the Groundwater Level? The Role of Soil Investigation

A soil investigation can collect groundwater data at several levels of detail:

  1. Recording the water level in the borehole — standard data in the boring log, measuring the water level encountered during drilling and after the hole is left for the water level to stabilize
  2. Groundwater observation well (observation well / standpipe) — a screened pipe surrounded by filter sand and sealed with bentonite is installed in the borehole, following ASTM D5092/D5092M-16(2024), to measure the groundwater level continuously over a long period; suitable for projects that need to know seasonal fluctuations (details in What is a groundwater observation well?)
  3. Piezometer — an instrument that measures pore water pressure at a specific depth, used in deep excavations, dams or monitoring soil behavior during construction (read What is a piezometer? · real project example Piezometer + Monitoring Well, Samut Prakan · SPN's instrumentation installation services)

The 3D model below compares all three data collection methods on the same soil section, showing that an observation well measures the water level over the screened section, while a piezometer measures water pressure only at the depth sealed off from other layers.

3D model of groundwater level data collection methods: borehole, observation well and piezometer Conceptual 3D view of a soil block cut away to show the section: clay on top and sand below. A blue dashed line marks the groundwater level, and the soil below it is shaded blue. The section shows three items. One: a soil borehole recording the water level in the boring log with the date. Two: an observation well — a plastic pipe with a protective cover at the ground surface; the lower part is a well screen surrounded by filter sand, sealed above with bentonite and cement; the water level in the pipe equals the groundwater level. Three: a piezometer — a sensor embedded in a sand pocket at a specific depth, sealed with bentonite above and below, with a signal cable running up to a readout box at the surface. A box on the right compares the observation well and piezometer: what each measures, which works each suits, and the ASTM D5092 standard reference. 3D model: 3 levels of groundwater data Not to scale Bentonite Screen + filter sand Tip Clay Sand ▽ Ground- water level 1 Boring Log Water level + date 2 Observation Well Obs. well, continuous 3 Piezometer Pressure at one depth GW instruments compared Observation Well Measures "water level" over screen Read with a water level meter Tracks seasonal fluctuations Per ASTM D5092 Piezometer Measures "pressure" at one depth Sealed above/below to isolate Deep excavations, embankments Monitoring during construction Drilling-day value = snapshot GWL varies with the seasons Projects with basement/deep dig should collect data covering the rainy season Pressure rises ≈ 1 t/m² per 1 m depth below GWL Conceptual view, not to scale
3D model — ① the borehole records the water level in the boring log with the date ② observation well: well screen surrounded by filter sand, sealed on top with bentonite, with a cover at the ground surface; the water level in the pipe equals the groundwater level ③ piezometer: sensor in a sand pocket, sealed above and below, with a cable up to a readout box. The box on the right compares their uses. Conceptual figure, not to scale (Image: SPN Soil Engineering)
Example boring log recording the groundwater level together with soil layer data by depth
The groundwater level is standard data recorded in the boring log of every borehole, together with the date and time measured
⚠️ Important note

The water level measured on the drilling day is a snapshot of that moment. If a project is sensitive to groundwater, for example with a deep basement, engineers usually recommend installing an observation well to collect data over a longer period, in particular covering the rainy season.

Summary: Don't Overlook the “Groundwater Level” Line

Groundwater is a silent variable that affects soil bearing capacity, construction methods, excavation safety and the long-term durability of the structure. The good news is that this data can be collected along with the soil investigation with almost no extra steps (see the overview of why a soil investigation is needed). Just don't overlook the "groundwater level" line in the boring log, and ask your engineer how detailed the groundwater data for your project needs to be.

📌 Note

This article is provided as engineering knowledge. Analyzing the effects of groundwater and designing protection systems is the responsibility of a licensed design engineer, based on the actual project data.