Why Do Deep Excavations in Bangkok Soft Clay Need Monitoring?
Deep excavations in Bangkok need monitoring because the Bangkok soft clay layer is about 12-18 m thick, has low shear strength and deforms easily. When soil is removed from one side of a retaining wall, the earth pressures on the two sides become unbalanced: the wall moves toward the excavation, the soil behind it settles, and the effect spreads to roads and adjacent buildings. The real behavior of soft clay has never matched the design model one hundred percent. Read about the characteristics of this layer in Bangkok soft clay and foundations
Monitoring is therefore the tool that closes the gap between "design values" and "actual behavior on site". If the measured values stay within limits, work can continue; if they start to drift outside, the team knows before damage occurs. The same data also serve as numerical evidence in disputes with neighbors, so monitoring should go hand in hand with a pre-construction condition survey of adjacent buildings (dilapidation survey)
Legal safety-factor requirements for earth retaining systems are covered in Retaining walls under the 2023 Ministerial Regulation on foundations. If you are not sure whether your project requires a monitoring plan, see the criteria in When is instrumentation required?
Which Instruments Does a Deep Excavation Need?
Deep excavations use five main instrument sets, grouped by the four questions to be answered: how much the wall moves, how much the surrounding ground settles, how the groundwater changes, and how much load the struts carry. This table summarizes what each instrument set measures and where it is installed.
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| Equipment | Measures | Where installed |
|---|---|---|
| Inclinometer | Lateral movement of the wall and soil with depth | Embedded in the retaining wall (D-wall / sheet pile) or in the soil behind the wall |
| Settlement Point + Tiltmeter | Ground surface settlement and tilt of adjacent buildings | Along roads around the excavation and on adjacent building structures |
| Piezometer + Observation Well | Pore water pressure in each layer and overall groundwater level | Behind the retaining wall and beneath the excavation base |
| Strain Gauge | Strut forces compared with design values | On each level of steel struts |
| Deep Settlement Point | Settlement of deep soil layers at each level | Borehole beside the excavation |
The system has two key instruments. The first is the inclinometer, which measures the lateral movement profile with depth, showing both the magnitude and the shape of wall deflection. The second is the piezometer, which measures pore water pressure, monitoring both the effects of dewatering and the water pressure beneath the excavation base that could cause base failure. Settlement and tilt of structures around the excavation are covered in detail in Settlement points and tiltmeters, and strut force measurement in Strain gauges in geotechnical engineering
Where Is Each Instrument on an Actual Site?
The 3D view below cuts away a corner of the excavation to show the whole system at once. On the left is the adjacent building and behind is the road. Note three things. First, the red dashed line on the wall, which deflects toward the excavation most near the base level. Second, the line of settlement points laid out perpendicular to the wall, to show the shape of the whole settlement trough rather than settlement at a single point. Third, the orange arrows at the excavation base, which show basal heave.
Basal heave occurs when the soft clay below excavation level is pushed by the weight of the soil behind the wall to flow toward the excavation and rise. The signs this instrument set picks up are the wall toe moving toward the excavation on the inclinometer plot and faster settlement of the ground behind the wall at the settlement points. Where adjacent buildings already have cracks, designers usually specify crack gauges across them to see whether they widen; read how cracks are assessed in Crack assessment and monitoring. The type and number of instruments depend on each project's specification.
How Do Trigger / Alarm Levels Work?
Trigger levels are values set in advance by the design engineer for each instrument, based on the design analysis. They are usually divided into three levels, each with its own response plan. A monitoring system is only meaningful if "what value requires what action" has been agreed beforehand.
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| Level | Measured value | Action |
|---|---|---|
| Alert level | Starting to rise above the normal trend, but still well below the design limit | Increase reading frequency, verify data accuracy and tell the team to watch closely |
| Alarm level (Alarm / Action) | Approaching the design limit | Hold a meeting to review the excavation procedure, slow down or re-sequence the work, and prepare additional measures |
| Stop Work level | Reaches the unacceptable limit | Stop excavating immediately, add bracing or partially backfill; resume only when the engineer confirms it is safe |
The values for each level come from project-specific analysis and depend on excavation depth, wall type, distance to adjacent buildings and the actual soil profile from soil investigation results, so there are no generic values that apply to every job. Besides the maximum value, engineers also look at the rate of change: a value that keeps increasing even though no further excavation has taken place is a signal that must be reported to the designer.
See examples of earth retaining systems and monitoring from real projects in Case study: temporary earth retaining system for a deep excavation. Reading inclinometer data through the excavation stages is covered in Case study: inclinometers in a deep excavation, Bang Phli. If neighboring work involves pile driving or other activities that cause vibration, also read Vibration monitoring for pile driving
How Often Are Readings Taken at Each Stage?
Reading frequency is divided into four work stages and adjusted to the risk level of each stage. The principle is "the more the ground is disturbed, the more often you read". The actual reading schedule for each project is set in its monitoring specification.
- Before excavation: take several rounds of baseline readings after installation to obtain stable reference values and filter out instrument scatter. Every value after this is reported as "change from baseline".
- During excavation and strut installation: the most frequent readings, because every excavation stage changes the force balance of the system. The wall is most heavily loaded when excavation passes each strut level, before that strut level is installed.
- During substructure construction and strut removal: removing each strut level transfers load back into the system, so readings must continue until the structure has fully taken over the load from the struts.
- After the underground work is complete: reduce frequency to long-term monitoring until readings stabilize.
Whatever the stage, if values reach the Alert level, frequency must be increased immediately according to the response plan.
Which Standards Does SPN's Deep Excavation Monitoring Follow, and What Does It Cost?
There is no specific Thai DPT standard for geotechnical instrumentation. Measuring movement with inclinometers follows ASTM D6230, and acceptance testing of vibrating wire piezometers before installation follows ASTM D7764. Installation locations, trigger levels and reading schedules follow the project-specific specification.
SPN provides the full service, from laying out monitoring points, drilling and installation, baseline readings and scheduled readings through to data processing with plots against trigger levels. Reports are certified by a civil engineer licensed by the Council of Engineers Thailand (COE), with signature and registration number, and can be submitted to the relevant authorities. See the full scope of services on the Geotechnical instrumentation and monitoring page
Fees depend on the type and number of instruments, installation depth and reading schedule, so they are estimated per project. Contact us for a free quotation within 24 hours. Prices do not include travel and equipment mobilization, which are charged extra according to the distance to site.
This content explains the principles of deep excavation monitoring systems for basic understanding. The type and number of instruments, installation locations, trigger values for each level and reading frequency for each project must be set by the design engineer and the responsible engineer according to the specific conditions of that job.