On a deep excavation in soft soil, there is one question everyone wants answered at all times: "How far has the wall moved?" An inclinometer answers it in terms of both the magnitude and the shape of the movement with depth. This case study comes from a real job SPN carried out on a construction project at Bang Phli, an area with thick soft clay where the excavation used sheet piles as the earth-retaining system. The article covers the installation steps, how to read the plots and lessons from the site. If you are not yet familiar with how the instrument works, read the basics first in What Is an Inclinometer?

What Was the Brief for This Job?

The brief was to monitor lateral movement of the soil behind the sheet pile line throughout excavation. The project was a deep excavation for an underground structure, using steel sheet piles as the retaining wall together with a strut system. With other construction and live roads around the site, the designer specified an inclinometer with alarm levels that must be reported immediately when readings approach them. For the pros and cons of sheet piles compared with other wall types, see 7 Types of Retaining Walls

The challenge at this site is Bangkok soft clay, which around Bang Phli is often many meters thick before reaching the stiff clay and sand layers. Soft clay has low shear strength: when soil is removed on one side, the earth pressure on the other side pushes the sheet piles toward the pit, and the soil behind the wall moves with them. Read more about this stratum in Bangkok Soft Clay and Foundations, and for an overview of what a deep excavation needs to monitor, see Deep Excavation Monitoring in Bangkok Soft Clay

SPN team installing an inclinometer casing beside a steel sheet pile line on a deep excavation at Bang Phli, Samut Prakan, with a crane and a borehole location sign
Inclinometer installation beside the sheet pile line at Bang Phli. The casing must be installed and baselined before excavation starts (Photo: SPN Soil Engineering)

How Is the Inclinometer Casing Installed, and How Deep?

The inclinometer casing is installed in a borehole in the soil behind the sheet pile line, close enough to the wall to reflect the movement of the retaining system. For depth there is one rule: the casing toe must extend below the sheet pile toe and be embedded in a non-moving stratum. There are 4 main steps.

  1. Drill the borehole with the same type of rig used for soil investigation, deeper than the sheet pile toe and deep enough for the casing toe to be embedded in a non-moving stratum. The key principle is that the casing toe must sit in stiff soil or a sand layer considered stable, because the displacement of the whole column is calculated with the toe as the fixed reference. If the toe moves too, every result will be wrong.
  2. Lower the grooved ABS casing section by section, aligning one pair of grooves perpendicular to the wall line (A axis) so the probe's main axis picks up movement toward the excavation directly.
  3. Backfill the annulus between casing and borehole wall with a material that lets the casing move with the soil.
  4. Build the casing head and protective cover with a sign showing the borehole number, which is essential on a site where machinery runs all the time.

The measurement method follows ASTM D6230 (Monitoring Ground Movement Using Probe-Type Inclinometers), which covers installation, two-direction readings (A+ / A−) to cancel probe error, and data quality checks using the checksum.

The 3D view below places the casing in its site context. Note 3 things. First, the casing is in the soil behind the sheet pile line, not in the sheet pile itself. Second, the casing toe extends below the sheet pile toe and is embedded in stiff soil. Third, the profile lines beside the casing change shape with each excavation level and bulge most below the lower strut level. The number of strut levels and excavation levels in the figure are illustrative only.

Conceptual 3D model of an inclinometer casing in the soil behind the sheet pile line of a strutted deep excavation in soft clay, and readings by excavation stage Conceptual 3D view of a soil block with the front corner cut away to show the inside of the excavation. The upper layer is soft clay and the lower layer is stiff soil considered stable. Steel sheet piles surround the pit, with two levels of steel struts. Behind the pit is a road still in use. Number one is the inclinometer casing installed in a borehole in the soil behind the sheet pile line. Number two is the casing toe, which extends below the sheet pile toe and is embedded in the stiff layer, serving as the fixed reference. Number three is the two strut levels. Number four is where the profile bulges most, below the lower strut near formation level. On the pit wall are three dashed levels: excavation stage one, excavation stage two and formation level. Beside the casing are four profile lines: the straight gray line is the baseline before excavation, the blue line tilts like a cantilever at the first stage, the orange line begins to bulge mid-depth at the second stage, and the red line bulges most when excavation reaches formation level. All lines return to zero at the casing toe. The right panel summarizes the readings at each stage and the installation principles. This is a conceptual image with movements exaggerated for clarity, not actual project measurements. 3D model: inclinometer casing behind sheet pile line Conceptual view · Not to scale 1 Inclinometer casing in soil behind sheet piles 2 4 3 2 strut levels Steel sheet pile line (Sheet Pile) Live road Stage 1 Stage 2 Base Soft clay Stiff soil/sand (stable) Piles before dig Dashed = excavation stages Read each stage vs. baseline Baseline 2–3 reads before digging Dig to strut level 1 Cantilever tilt pre-strut Dig to strut level 2 Profile starts to bulge Dig to base Max bulge below low strut Key points 1 Casing in soil behind piles 2 Toe below sheet pile toe 3 2 strut levels hold top 4 Max bulge near base A-axis groove ⊥ wall line Read every 0.5 m from toe up Conceptual, not measured data Movement exaggerated
Conceptual view of a strutted deep excavation behind a sheet pile line in soft clay. ① Inclinometer casing in a borehole in the soil behind the sheet pile line ② Casing toe below the sheet pile toe, embedded in a stratum considered stable, serving as the reference ③ Struts holding the upper part of the wall ④ Point of maximum profile bulge, below the lower strut near formation level. Dashed lines on the pit wall mark each excavation level, in the same color as the profile line for that reading. (Conceptual image with movements exaggerated for clarity; not the project's actual measurements, depths or number of strut levels) (Image: SPN Soil Engineering)

When Are Readings Taken, and What Are They Compared With?

Every reading is compared with the baseline taken before excavation starts. After installation, the team takes at least 2–3 baseline readings on different days to confirm the casing is stable and the readings are repeatable. Every later reading is reported as the "change from baseline". A reading is taken by lowering the probe to the bottom of the casing and pulling it up 0.5 m at a time, recording the tilt at each interval, then integrating from the casing toe upward to obtain the displacement profile.

The reading schedule is tied to the excavation sequence, not the calendar. The team reads every time excavation reaches a new level, before and after each strut level is installed, and most frequently as excavation reaches formation level. The maximum value of each reading is compared with the alarm levels set by the designer from the analysis, divided into alert, alarm and stop-work levels. Read about the principle of 3-level alarm values in Deep Excavation Monitoring

Illustrative example plot of an inclinometer lateral displacement profile of a retaining wall by excavation stage, and how to read the profile shape In the left plot, the horizontal axis is cumulative displacement toward the excavation, 0 to 60 millimeters, and the vertical axis is depth, 0 to 20 meters. The vertical line at zero is the baseline before excavation. When excavation reaches strut level one, the maximum is about 7 millimeters near the surface. When excavation reaches strut level two, the maximum is 18 millimeters at 6 meters depth. When excavation reaches formation level, the maximum is 30 millimeters at 8 meters depth, which is formation level. The point of maximum moves down as excavation proceeds. The orange dashed line is the alert level of 35 millimeters and the red dashed line is the alarm level of 48 millimeters. The casing toe at 20 meters is zero. On the right are three shapes: a mid-depth bulge near formation level is the normal pattern for a strutted wall; maximum at the casing head decreasing with depth is a wall tilting as a cantilever, usually before the first strut level is installed; and the whole line shifted with the toe not at zero suggests the casing toe is not embedded deep enough. The strip at the bottom gives the reading principles: take two to three baseline readings before excavation, read every time excavation reaches a new level, and compare the maximum with the alarm levels while watching the rate of change. This is an illustrative example plot, not actual project measurements. Excavation inclinometer: profile bulges by stage Conceptual graph · Not measured data 0 10 20 30 40 50 60 0 4 8 12 16 20 Cumulative displacement toward pit (mm) ← Depth (m) Alert 35 Alarm 48 Base −8 m 7 18 30 Baseline To strut level 1 To strut level 2 Dig to base ○ Max point moves down What the shape tells Mid bulge near base Strutted wall · normal Max at top Cantilever tilt (pre-strut) Whole line shifts, toe ≠ 0 Toe too shallow / toe moving 1 2–3 baselines Before digging 2 Read by stage Not by calendar 3 Max + rate vs. 3 alarm levels
Inclinometer result pattern in a strutted deep excavation in soft soil. With each deeper excavation stage, the profile "bulges" more and the point of maximum displacement moves down toward formation level. The engineer compares the maximum of each reading with the predefined alarm levels. The right side summarizes the 3 profile shapes you must be able to tell apart. (Illustrative example plot showing the result pattern found in this type of work, for explanation only; not actual project measurements)

How Do You Interpret the Plot, and What Does the Shape Tell You?

Interpreting an inclinometer plot means looking at 2 things together: the shape of the profile and the rate of change, not just the maximum value. Deep excavations show 3 profile shapes, summarized in the table.

← Scroll to see the full table →

Profile shapeMeaningUsually seen when
Mid-depth bulgeNormal pattern for a strutted wall; maximum displacement near formation levelStrutted excavations in soft soil; the maximum moves deeper with each excavation stage
Maximum at the casing head, decreasing with depthWall tilting as a cantileverFirst excavation stage, before struts are installed
Whole line shifted as a block, toe not at zeroSuspect the casing toe is not embedded deep enough, or the stratum at the toe is moving tooCasing toe has not reached a stable stratum

The second is the rate of change. Readings that step up with excavation and then stabilize are normal, but readings that keep increasing with no further excavation are a signal to increase reading frequency and notify the designer immediately. Where the struts also carry strain gauges, reading strut forces alongside the inclinometer helps confirm the system is carrying load as designed. Read more in Strain Gauges in Geotechnical Engineering

What Lessons Came from the Bang Phli Site?

This job produced 3 lessons that apply to inclinometer installation on deep excavations in general.

  1. Finish installation before excavation. A baseline taken after excavation has started will "swallow" the early movement, so on this job the drilling and installation were scheduled to be complete before machinery entered the pit.
  2. Protect the casing head. Casing heads run over by vehicles or buried by fill are a leading cause of data gaps. A steel cover and a clearly visible sign help a lot.
  3. Read the data as a set, not a single number. Every report must include the profile, the maximum value, and a plot of maximum versus time with the alarm lines, so the site supervisor can decide immediately without having to interpret it themselves.

If you are not sure whether your project needs a monitoring plan, see the criteria in When Is Instrumentation Required? For how retaining systems and monitoring plans are set up for deep excavations in other regions, where the soil profile differs from Bangkok, compare with Case Study: Temporary Earth Retaining System for a 4 m Deep Excavation, Khon Kaen, and see other work in the Bangkok Metropolitan Region at Soil Investigation and Pile Testing, Bangkok Metropolitan Region

Fees for instrument installation and reading depend on the type and number of instruments, installation depth and reading frequency, so they are estimated per project. Contact us for a free quotation within 24 hours. Prices exclude travel and equipment mobilization costs, which are charged extra depending on the distance to site.

⚠️ Note on the case study data

Project details and photographs are from an actual SPN installation. All graphs in this article are conceptual examples showing result patterns typical of similar work, not measured values from this project, because the monitoring data belongs to the project owner. Alarm levels, instrument locations and reading frequencies must be set for each job by the design engineer based on its specific conditions.