What Is an Observation Well and How Does It Work?

An Observation Well is an open-standpipe well for measuring groundwater level, and the simplest groundwater instrument in principle. Groundwater around the well seeps into the pipe through the slotted section (screen) until the water level in the pipe equalizes with the surrounding groundwater level. The reader lowers a tape from the pipe top down to the water surface and reads the depth from the pipe top, whose reference level has already been surveyed.

It has 3 key advantages: easy to install, easy to read and low maintenance, so it is found on almost every project that needs groundwater level monitoring, and it is 1 of the 10 main geotechnical instruments actually used on Thai projects. It has 2 limitations. First, the reading is the combined water level of every layer the pipe is open to, so individual layers cannot be distinguished. Second, the well responds slowly in low-permeability soils.

SPN crew installing the filter-wrapped PVC pipe of an Observation Well into a borehole on site
Lowering the filter-wrapped PVC pipe into the borehole. Joints must be tight and the pipe lowered plumb so it does not catch on the borehole wall (Photo: SPN Soil Engineering)

The 3D model below cuts away a corner of the ground to show both the above-ground and below-ground parts at once. Numbers 1–4 are the above-ground parts: locking cap with protective steel casing, bollards with well ID sign, concrete pad, and the water level tape. Numbers 5–8 are the below-ground parts: cement-bentonite grout, bentonite seal, filter sand with slotted pipe, and the groundwater level, which equals the water level in the pipe.

3D model of an Observation Well with a corner cut away to show the subsurface construction, plus a close-up of reading with a Dip Meter A 3D view of a block of ground with one front quarter cut away through the axis of the observation well, showing two faces of the well section. The upper layer is clay and the lower layer is sand. At the wellhead are a concrete pad, a protective steel casing with a locking cap, red-and-white bollards linked by a chain, and a well ID sign as in the site photo. Below ground, a PVC pipe sits in the center of the borehole; the upper part is backfilled with cement mixed with bentonite, followed by a bentonite seal, then filter sand around the slotted section of pipe in the sand layer. The blue dashed line is the groundwater level, which equals the water level in the pipe; soil below the water level is shaded light blue. A technician holds a water level tape reel and lowers the tape into the pipe until the probe touches the water surface. The right panel is a close-up of the reading: the tape length is read at the reference point on the pipe top, the tape is graduated, and when the probe touches water the circuit closes and the reel sounds a buzzer and lights a lamp. 3D Model · cutaway view showing the subsurface Not to scale 1 2 3 4 5 6 7 8 Clay Low perm. Sand Flows in freely Key to numbers 1 Locking cap + steel casing 2 Bollards + well ID sign 3 Concrete pad 4 Dip Meter (water level) 5 Cement-bentonite grout 6 Bentonite seal 7 Filter sand + slotted pipe 8 GWL = level in pipe Reading with a Dip Meter d Ref. point Same spot Scaled tape Read length at ref. point Probe contact Circuit closes Light + beep on water contact
3D model of an Observation Well — the wellhead as SPN actually installs it (concrete pad, bollards, well ID sign) and reading with a Dip Meter at the same reference point on the pipe top every time

Why Measure the Groundwater Level?

Measuring groundwater level is essential because the water level is one of the variables that most changes how soil behaves. When the water level rises, pore water pressure increases and the soil’s bearing capacity decreases. When the water level is pumped down during excavation (dewatering), the surrounding soil carries more load and settles accordingly. Many buildings next to deep excavations are damaged by groundwater lowering, not by the excavation itself. Read more in How Does the Groundwater Level Affect Foundations?

The figure below shows this mechanism. When water is pumped from the excavation, the water level around it drops in a cone shape — a large drop near the excavation, decreasing with distance. Well OW-1 near the excavation and well OW-2 farther away therefore show how far the drawdown cone extends toward adjacent buildings.

A deep excavation with dewatering, and the locations of Observation Wells monitoring the groundwater level of the adjacent area A conceptual cross-section of a deep excavation with a retaining wall and struts. Inside the excavation is a dewatering well pumping water up for discharge. The blue dashed line is the original groundwater level before pumping; the solid line is the water level during pumping, which drops sharply near the excavation and gradually returns to the original level farther away, forming a drawdown cone. Observation Well OW-1, near the excavation, reads a large drop in water level; OW-2, farther away, reads a small drop. Adjacent buildings within the drawdown zone are at risk of settlement, because the soil carries more load when the water pressure falls. The strip at the bottom summarizes 3 uses: take a baseline reading before pumping starts, read frequently during pumping, and notify the designer when readings exceed the threshold in the monitoring plan. Deep excavation + dewatering: what Observation Wells monitor Conceptual view · Not to scale Wall + struts Excavation Pump well (Dewatering) Pumped water OW-1 OW-2 Near excavation Large drawdown Farther away Small drawdown Adjacent building Soil carries more load Settlement risk Original GWL Pumped GWL Saturated soil Sand layer (aquifer) Clay 1 Baseline before pumping Know the site’s normal GWL 2 Read often during pumping Per project monitoring plan 3 Drawdown over limit Notify design engineer now
Conceptual figure — Observation Wells near and far from the excavation track the extent of drawdown. Actual well locations and alert thresholds are set by the design engineer in the project monitoring plan.

Groundwater levels also change with the seasons; in some areas the rainy-season and dry-season levels can differ by several meters. A design based on the water level from a single soil investigation may therefore not cover actual conditions. Projects sensitive to the water level, such as work on Bangkok soft clay, should have continuous water level data to support decisions.

What Are the Parts of an Observation Well?

An Observation Well has 6 main components, from the wellhead down:

  1. Locking cap with concrete pad protects the wellhead and is opened only for readings
  2. Backfill (grout) of cement mixed with bentonite fills the annulus around the upper part of the pipe
  3. Bentonite seal prevents surface water from flowing down the well
  4. Solid PVC pipe (riser) runs from the wellhead down to the slotted section
  5. Filter sand (filter pack) surrounds the slotted section and extends slightly above it
  6. Slotted pipe (screen) wrapped in filter fabric is where water enters the pipe; the bottom is closed with an end cap

The cross-section below uses figures from SPN’s example deep-well design: a Ø 50 mm PVC pipe in a Ø 100 mm borehole, with the slotted section at 6.00–11.00 m depth and an example groundwater level of 8.00 m.

Cross-section of Observation Well construction, based on SPN’s example deep-well design Cross-section of an Observation Well, example deep-well design. A 50 mm PVC pipe sits in a 100 mm borehole. At the wellhead is a 4-inch protective steel casing standing 0.80 m above ground with a locking cap, a 1.0×1.0×0.1 m concrete pad and red-and-white bollards. The annulus around the upper pipe is backfilled with cement mixed with bentonite, followed by a bentonite seal against surface water, then filter sand around the slotted section from 6.00 to 11.00 m depth. The pipe bottom has an end cap. The example groundwater level is at 8.00 m, and the water level in the pipe equals the groundwater level around the well. The water level tape is lowered from the pipe top until the probe touches the water surface, and the depth d is read from the pipe top. Groundwater level equals pipe-top level minus d. In the example, d equals 8.80 m and the pipe top is 0.80 m above ground, so the water is 8.00 m below the ground surface. Observation Well cross-section · SPN deep-well example Not to scale Depth (m) 0.00 6.00 8.00 11.00 Clay Low perm. Sand Screened zone d Depth from pipe top Dip Meter (water level) Probe touches water → beep Locking cap Opened only for readings Pipe top (Top of Pipe) Reference point; level must be surveyed Protective steel casing Ø4 in. 0.80 m above ground Bollards + concrete pad Pad 1.0×1.0×0.1 m, sloped away from well Backfill (grout) Cement mixed with bentonite Solid PVC pipe (riser) Ø50 mm in a Ø100 mm borehole Bentonite seal Keeps surface water out of the well Filter sand (filter pack) Around screen, slightly above it Groundwater level 8.00 m In pipe = around well (example) Slotted pipe (screen) 6.00–11.00 m Filter-wrapped; water enters here End cap Borehole bottom at 11.00 m GWL = pipe-top level − d Survey the pipe-top reference level first then re-read until stable = baseline This deep-well example Tape reads d = 8.80 m from pipe top Pipe top 0.80 m above ground → water 8.00 m below ground
Cross-section of Observation Well construction, based on SPN’s example deep-well design — the upper bentonite seal is critical: if surface water can leak into the well, the reading will not be the true groundwater level.

5 Installation Steps

1. Drill the borehole: Drill with a soil boring rig to the depth specified in the monitoring plan, then check that the hole is clean with no collapse at the bottom.

2. Install the pipe: Lower the PVC pipe, with the filter-wrapped slotted section at its tip, to the bottom of the hole and center it in the borehole.

3. Place the filter sand and seal: Pour filter sand into the annulus around the pipe to cover the slotted section and extend slightly above it, then cap it with a bentonite seal so surface water cannot seep down and mix with the groundwater. Then backfill the remaining annulus up to the wellhead with a backfill material such as cement mixed with bentonite, so the filter sand does not connect separate water-bearing layers.

4. Build the wellhead: Cast a concrete pad around the pipe top, install a cap with a lock, and mark the well number clearly.

5. Develop the well and take baseline readings: Pump the well to flush out drilling sediment (well development) until water flows freely into the well, then take repeated readings until the water level stabilizes and use it as the reference baseline.

Example Installation Designs: Deep Well and Shallow Well

Well depth and wellhead type are set according to the depth of the water level to be monitored and how the area is used. The two designs below are installation designs SPN actually uses. They differ in 2 respects: the depth range of the slotted pipe (6.00–11.00 m vs 1.00–5.00 m) and how the wellhead is protected (a steel casing raised 0.80 m vs a cover set flush with the ground).

Deep-well Observation Well installation design: 50 mm PVC pipe, solid section 0.80 to 6.00 m, slotted pipe 6.00 to 11.00 m, in a 100 mm borehole; wellhead protected by a 4-inch steel casing 0.80 m high with a lock, concrete pad and bollards
Deep-well design — ⌀50 mm PVC pipe in a ⌀100 mm borehole, solid section 0.80–6.00 m connected to slotted pipe at 6.00–11.00 m. Filter sand surrounds the slotted section and extends above it, capped with a bentonite seal and then backfilled with cement mixed with bentonite up to the wellhead. The water level read in this design is 8.00 m. The wellhead is raised with a 4-inch steel casing 0.80 m high with a lock, a 1.0×1.0×0.1 m concrete pad and bollards (Drawing: SPN Soil Engineering)
Shallow-well Observation Well installation design: 50 mm PVC pipe, solid section 0.00 to 1.00 m, slotted pipe 1.00 to 5.00 m; wellhead is a 4-inch threaded stainless-steel cover flush with the ground
Shallow-well design — ⌀50 mm PVC pipe with a solid section of only 0.00–1.00 m connected to slotted pipe at 1.00–5.00 m, used where the water level is shallow, around 1.00 m. The wellhead is a 4-inch threaded stainless-steel cover set flush with the ground (flush mount), suitable for areas with vehicle or machinery traffic (Drawing: SPN Soil Engineering)
Completed Observation Well with concrete pad, protective posts and well ID sign Monitoring Well No.1
A completed well with concrete pad, bollards and well ID sign, to prevent damage from machinery and to reference the location correctly (Photo: SPN Soil Engineering)

How Are Readings Taken, and How Often?

Observation Well readings use a water level tape (Dip Meter or Water Level Meter): a graduated tape with an electric probe at the tip. When the probe touches the water surface, the circuit closes and a buzzer sounds. The reader records the tape length (d) at the same reference point on the pipe top every time, then calculates from the surveyed pipe-top level: groundwater level = pipe-top level − d. In the example from SPN’s deep-well design, the tape reads d = 8.80 m and the pipe top is 0.80 m above ground, so the water level is 8.00 m below the ground surface.

Every reading must always be recorded together with the date and time, recent rainfall, and any nearby construction activity or pumping, because this information is used to explain changes in the readings.

SPN staff reading the groundwater level in an Observation Well with a Dip Meter water level tape
Taking a reading with a Dip Meter on site. Each recorded value must note the date, time and activities around the well during that period (Photo: SPN Soil Engineering)

Reading frequency follows the monitoring plan set by the design engineer. The general approach is to read more often during activities that affect groundwater, such as dewatering or filling, then reduce to weekly or monthly rounds once the readings stabilize. Seasonal monitoring usually continues for at least a full year to capture the actual highest and lowest levels.

How Does an Observation Well Differ from a Piezometer?

An Observation Well measures the combined groundwater level, while a Piezometer measures pore water pressure only in the sealed layer. Both are groundwater instruments, but they answer different questions: an Observation Well answers “what is the groundwater level?”, while a Piezometer answers “what is the pore water pressure in that layer?” Choosing the wrong one is one of the most common mistakes in monitoring work. For the working principle and how to choose a sensor type, see What Is a Piezometer? Measuring Pore Water Pressure in Soil. For a real project with a Monitoring Well installed alongside Piezometers, read Case Study: Piezometer and Monitoring Well in Samut Prakan

The cross-section below places both instruments in the same soil profile. The Observation Well is open over a long section, so it reads the combined groundwater level, while the Piezometer has its filter tip sealed within a single layer. When the clay layer beneath the fill has excess pore water pressure, the water level in the Piezometer pipe rises above the groundwater level. That difference is information an Observation Well cannot measure.

Comparison of an Observation Well and a Standpipe Piezometer in the same soil profile A conceptual cross-section with a fill layer over soft clay and a sand layer. On the left is an Observation Well whose long slotted pipe and filter sand are open to several layers, so the water level in the pipe equals the combined groundwater level. On the right is a Standpipe Piezometer with a short filter tip in the soft clay, sealed with bentonite above and below, so the water level in its pipe reflects the water pressure in that layer only. In this example the weight of the fill generates excess pore water pressure in the clay, so the water level in the Piezometer pipe is higher than the groundwater level; the difference Δh is the excess pore water pressure. The strip at the bottom summarizes that each instrument answers a different question and they are commonly used together. Observation Well vs Standpipe Piezometer: they measure different things Conceptual view · Not to scale Δh Observation Well Measures overall GWL Standpipe Piezometer Measures pore pressure in one layer Fill surcharge Water level in pipe = overall GWL Long slotted screen Open to several layers Fill Soft clay Low permeability Sand High permeability GW level Pipe level higher = excess pore pressure Top bentonite seal Blocks other layers Filter tip Reads this layer only Lower bentonite seal Isolates the layer 1 Well: where is the water? General, seasonal, environmental 2 Piezo: layer pressure? Fill, PVD, deep excavation, dams 3 Difference = information so often installed as a pair
Conceptual figure — in the same soil profile, the water levels in the two instruments’ pipes can differ. The difference Δh reflects excess pore water pressure in the clay layer.

← Scroll to see the full table →

ComparisonObservation WellStandpipe Piezometer
What it measuresCombined groundwater level of the layers the pipe is open toPore water pressure only in the sealed layer
SealingOpen over a long sectionBentonite seals above and below, so it takes water only from the zone of interest
Layer separationNot possiblePossible, by sealed zone
Response in claySlow; may take several days to equalizeFaster; the vibrating wire type responds almost instantly
Suitable applicationsGeneral water level monitoring, seasonal and environmental workMonitoring pore pressure dissipation in fills, deep excavations and dams
CostLowerHigher, especially the vibrating wire type

Another name often confused with it is the Monitoring Well used in environmental work. Its construction is similar to an Observation Well, but its purpose is to collect groundwater samples for quality and contamination analysis, so its material and well-development requirements are stricter. Sampling is usually done by the low-flow method per ASTM D6771. SPN provides both types; see the scope of environmental assessment and groundwater quality monitoring wells on the service page.

What Kind of Projects Is an Observation Well Suited For?

An Observation Well suits work that needs to know the combined groundwater level of an area and how it changes over time. It is most often used in 4 groups of work:

  • Deep excavations with dewatering — installed around the excavation to monitor how far pumping draws down the groundwater level in the adjacent area. See the instruments used alongside it in Deep Excavation Monitoring in Bangkok Soft Clay
  • Earth filling and ground improvement — used with Piezometers and Settlement Plates in PVD and Preloading work, with the Observation Well providing the site’s reference water level
  • Long-term seasonal monitoring — records the highest and lowest water levels to support the design of foundations and basements
  • Environmental work — monitors groundwater level and quality around landfills, factories or fuel depots

There is no specific DPT standard for Observation Well installation. The international standard referenced for well design and installation is ASTM D5092/D5092M (Design and Installation of Groundwater Monitoring Wells), used together with each project’s specifications, the equipment manufacturer’s manuals and soil investigation practice.

SPN’s monitoring reports are certified by a licensed civil engineer (COE license), with signature and registration number, and can be used for submission to the relevant authorities. See the scope of instrumentation installation and performance monitoring on the service page. Fees depend on depth, number of points and reading rounds, so they are estimated per project. Contact us for a free quotation within 24 hours. Prices exclude travel and equipment mobilization, which are charged extra according to site distance.

⚠️ This article provides general information

This content explains the working principle and use of Observation Wells for basic understanding. The location, depth, number of wells, reading frequency and the choice between an Observation Well and a Piezometer for each project must be determined by the responsible engineer based on the specific conditions of that project.