1. Introduction

Inclinometer equipment is used to measure horizontal displacement by means of a probe connected to the data acquisition device, the probe will be pulled along the inclined measuring tube from the bottom to the top of the tube. The inclinometer is installed in a pre-drilled hole in the diaphragm wall or in the ground. Some of its typical applications are as follows:

- Identify areas with landslides.

- Monitoring horizontal displacement of earth dam, embankment on soft soil, deep excavation pit or tunnel.

- Monitoring the displacement of diaphragm piles, diaphragm walls, or earth retaining walls.

Figure 1. Application of Inclinometer equipment in monitoring construction works.

2. Applicable standards

ASTM D6230 “Standard Test Method for Monitoring Ground Movement Using Probe-Type Inclinometers”.

AASHTO T 254: "Installing, Monitoring, & Processing Data of the Traveling Type Slope Inclinometer".

3. Measuring device

Inclinometer device consists of 4 main parts as follows:

Casing casing can be made of plastic, aluminum alloy, fiberglass or steel. It has grooves to orient the transducer, and is installed almost vertically.

- The head has a gravity sensor.

- Data collector.

- Electronic cable connects the probe to the data collector.

- Handheld display unit with Bluetooth connectivity.

Figure 2. Geokon Inclinometer components.

3.1 Wall pipe

                                                                        D = 85 mm D = 70 mm D = 48 mm

The sidewall of the inclinometer is a pre-grooved tube that is used during installation. It performs the guiding function for the transducer of the inclinometer. The diameter of the wall pipe includes large, medium and small types, made from many different materials, depending on the nature of the job, we choose the most suitable type.

Figure 4. Wall pipe

3.2 Tilt probe

Tilt probes have wheels that run in longitudinal grooves in the casing. It includes two force-balanced accelerometers. One tachometer measures the A-axis, the other measures the incline in a perpendicular plane called the B-axis.

Figure 4. Probe system and wall pipe cross-section.

3.3 Data collector

The inclinometer displays data received from the transducer. Data acquisition devices store readings in memory.

Figure 6. Data collector

 3.4 Electronic cables

4. Equipment installation

4.1 Some points to pay attention to

a. Wall pipes and materials

- The diameter of the wall pipe affects the service life and quality of work. Movement of the ground, diaphragm wall causes the casing to deform and hinder the transducer movement in the casing. Larger diameter wall tubes allow monitoring with larger displacements. Always use the largest diameter wall pipe possible.

- The materials of the wall pipes also affect the installation life. Wall pipes made of ABS plastic are suitable for long-term contact with soils, mortars and groundwater. It does not corrode and adapts to soil movements. Aluminum walled pipes can corrode. When installing aluminum siding, be careful not to scratch its protective epoxy adhesive.

b . Direction of wall pipe

- The borehole should be almost vertical. The error in tilt measurement increases with the inclination of the tube.

- When installing the inclined measuring tube, it should be noted that the direction A(+) - A(-) must be parallel to the direction of displacement. In the deep excavation work, it is necessary to place as shown below as follows:

Figure 7. Wall pipe orientation

c. Connecting pipes

- ABS glue used on connecting pipes requires 24 hours to reach full strength. Wall pipes are usually assembled and installed much more quickly. In water-filled pits, buoyancy reduces the weight of the bulkhead so that the strength of the glue and the fitting does not matter.

- However, when the bulkhead is installed in a dry borehole, the fitting must hold the full weight of the bulkhead. In this case, secure the fitting with studs. When connecting pipes, glue entering the wall pipe can cause blockage, inaccurate guide grooves and damage to the transducer wheels.

d. Pour grout

- Cement mortar is mixed to obtain a strength similar to that of the soil. The density of the grout creates a buoyant force, so measures should be taken to keep the casing pipe in the borehole from being pushed up during installation.

- Do not press down the casing with a drill as this method may cause the casing to twist in the bore. Mortar is poured many times, after each set of hardening, more mortar is usually poured into the borehole from 1 to 2 times until the pipe is full.

4.2 Installation of inclined gauge pipe

When installing wall pipes, there are 2 common cases: installation inside the diaphragm wall with a waiting steel pipe and installation in the natural ground.

In case of installation in natural soil:

Figure 9: Tilt gauge installation.

- Step 1: Clean the drilling fluid and debris in the borehole. Attach the cap to the end of the bulkhead pipe.

- Step 2: Install the wall pipe in the borehole, add the wall pipe when the required depth is reached.

- Step 3: Drop the grout hose to the bottom of the borehole and inject some of the mortar.

- Step 4: Wait for the mortar to harden, fill the hole with mortar and install the protective box.

When installing in a diaphragm wall with a waiting steel pipe, we must use a submersible electric pump to clean the mud in the steel pipe before proceeding with the above steps.

4.3 Test measurement and first cycle measurement

After the installation is complete, it is necessary to check the functions of the device. The first reading will be taken as a reference for the calculation of subsequent measurement cycles. Measure the first cycle only after the installation is completed at least 2 days. In the process of measuring and checking, performed three times, if there is a significant difference, it must be evaluated and selected data to be used as a standard data.

5. Principle of measurement and data processing

Drop the probe to the bottom of the tube, the accelerations inside the probe will determine the angle of inclination of each 0.5m segment, pull the probe up 0.5m to measure the next pipe segment. The measurement process is repeated until the end of the pipe length.

Figure 11. Horizontal deviation chart

Each measurement cycle we determine the inclination of the pipe, compare the inclination of the current cycle with the original data, we get the horizontal displacement value.