Chapter 6
Field Instrumentation and Monitoring

6.1 TYPES OF GEOTECHNICAL INSTRUMENTS

To control geotechnical problems within a manageable process, geotechnical instruments are deemed necessary, and the following should be measured during the process of reclamation and soil improvement.

Category A Measurement of ground behavior during construction to control the construction process.

Category B Monitoring of performance of ground during loading, unloading, and soil improvement process.

Before starting the instrumentation program, the first thing that needs to be done is procurement. Procurement of instruments can be made by various parties. Although the owner or consultant should have direct control over procurement, in practice it is left to the construction contractor. In that case, the responsibility for nonperformance will be on the contractor. The advantages and limitations of task assignment for procurement can be found in Dunnicliff (1988).

The specification of instrument can be written with specific brand name and model number or with only descriptive specification without model and number. On the other hand, the performance specification can be written. The advantages and limitations of various specifying methods can be found in Dunnicliff (1988). Normally, most designers prefer the first method because it is direct, and not much knowledge is needed to write specifications. However, performance specifications allow for maximum innovation.

Determination of price can be made based on least cost in competitive bidding and can be negotiated on preferred quality instrument in negotiation basis procurement. The advantages and limitations of the procedure for determining prices can be found in Dunnicliff (1988). In practice, negotiation methods are less time consuming, and it is easy to get quality products from preferred sources.

There are also various ways of contracting the instrument installation contractor. In practice, instrumentation bids are included in the construction contract with or without pre-qualification. However, the instrumentation specialist selected by the owner in conjunction with the contractor, and contracting with the contractor as an assigned subcontractor, can give quality products. The advantages and limitations of various contractual agreements for instrumentation can be found in Dunnicliff (1988).

6.1 TYPES OF GEOTECHNICAL INSTRUMENTS

There are two types of instruments. Some fulfil the Category A measurements, others the Category B measurements, and some both types of measurements.

Type A – Measurements of ground behavior during construction.

1. Settlement pad
2. Inclinometer
3. Pore pressure transducer
4. Earth pressure cell

Type B – Monitoring of performance of ground during loading, unloading, and soil improvement.

1. Settlement plate
2. Deep settlement gauge
3. Multilevel settlement gauge
4. Pore pressure transducer
5. Earth pressure cell
6. Inclinometer

In reclamation projects, the instrument can be installed at the platform level where the vertical drain is going to be installed or on

the special platform erected offshore (Fig. 6.1). Most of the instruments are installed at platform level during or before installation of the vertical drain because the settlement occurring before vertical drains are of small magnitude, and installation and protection of instruments at offshore condition are difficult and costly. Only very few instrument clusters are installed in offshore condition with proper protection to monitor the settlement occurring during reclamation. Even instruments installed on land condition require protection during surcharge placement (Fig. 6.2). Normally, instruments are installed in the form of instrument clusters, and Type B instruments are included in a cluster. Details of geotechnical instrumentation for land reclamation projects can be found in Bo et al. (1998c) and Bo and Choa (2002b).

6.1.1 Deep Reference Point

Deep reference points are installed to be used as a benchmark for survey. The normal benchmark provided by the surveyor may not provide the required accuracy, if the benchmark is far from the site, especially if it is installed on the pile which is driven through unstable settling ground or sits on the formation above the groundwater aquifer being exploited. In the first case, the benchmark can be settled due to down drag on the pile, and in the latter case it

can be moved down due to land subsidence caused by groundwater extraction. This type of benchmark movement can be found in Jakarta bay area. Unstable condition can occur owing to lowering of groundwater and seasonal thermal effects.

Therefore, only a few deep reference points should be installed at sites that should be anchored on the bedrock and installed with necessary negative friction reducer. The typical design of deep reference points is shown in Fig. 6.3. There are some records of data showing heave-up of deep reference points relative to the benchmark that were checked against some years after installation. Actually it is not heaving up of the deep reference points, and it shows settlement of the benchmark after a few years.

6.1.2 Settlement Plate

The settlement plate can be installed on the seabed with some protection so that it is not damaged during filling. The required extensions are made whenever necessary during filling. Most of the settlement plates are installed at the platform level where vertical drains are installed. Settlement rods are required to extend when the fill level is raised to the surcharge level. The riser rod should be protected with a friction reducer sleeve pipe. Protection during

surcharging is necessary to prevent damage during filling. Measurements are made by survey methods. The typical design of a settlement plate is shown in Fig. 6.4.

6.1.3 Liquid Settlement Gauge

The liquid settlement gauge is used with a pneumatic pressure indicator or vibrating wire pore pressure transducer to monitor settlement of foundation. Liquid settlement gauges can be read from a central location, and the vibrating wire type is particularly useful when automatic recordings are required. It can be installed before filling or in a borehole. Measuring the change in differential elevation

between the pressure sensor and the reference reservoir provides settlement records. The diagram of liquid settlement gauge and principle of measurement are shown in Figs. 6.5 and 6.6. The typical installation design of a liquid settlement gauge in the borehole is shown in Fig. 6.7.

6.1.4 Deep Settlement Gauge

The deep settlement gauge can be installed at various levels to monitor the settlement of various sublayers. There are two major types of deep settlement gauges: the screw plate deep settlement gauge and the Borros screw deep settlement gauge. Both settlement gauges are installed in a borehole with necessary friction reducers. Friction reducers are required to install from a sufficient distance above the screw plate by allowing for the settlement of friction pipe caused by down drag. Otherwise the friction reducer will push down the settlement plate, resulting in overestimation of settlement of sublayers.

The typical designs of deep settlement gauges are shown in Figs. 6.8 and 6.9.

6.1.5 Multilevel Settlement Gauge

Multilevel settlement gauges are made up of a series of spider metal rings. An access tube is installed in a borehole that is drilled up to hard formation. Therefore, the tube is anchored at hard formation where no settlement can occur. The datum magnet is installed at an anchored location, and the spider rings are installed along the access tube at predetermined locations, and the spider arms are released to anchor into the formation. Measurements are made with the help of a magnetic probe, and the probe detects the location of the spider rings relative to the datum magnet. The spider rings settle together with the soil mass during consolidation settlement. To get a recent elevation of spider ring with reference to the certain datum, the monitoring is carried out together with the elevation survey of the top of the access tube. The typical installation design of multilevel settlement gauges and various types of magnetic rings are shown in Figs. 6.10 and 6.11, respectively.

In some cases, the multilevel settlement gauges will measure lower magnitude of settlement of the sublayer than the screw type deep settlement gauges. The Fig. 6.12 shows a comparison of the measurement of magnitude of settlement of sublayer at the same elevation by screw plate deep settlement gauges and multilevel settlement gauges. It can be seen that the multilevel settlement gauges measure the smaller magnitude of settlement. There are various reasons for measuring lower settlement.

1. Grout is not deformed.
2. Spider ring does not follow with soil mass owing to jam between access tube and ring.
3. Datum magnet moves down due to down drag on access tube and measures lower relative movement in the case where the elevation of the top of the access tube is not surveyed.
4. The deflection of the access tube due to lateral stress and movement.
   
   
5. No sufficient gap between the initial location of the spider ring and the coupling.

Therefore, interpretation from multilevel settlement gauges should be carried out with great care.

6.1.6 Earth Pressure Cell

Earth pressure cells are designed to measure the total pressure of earth and water imposed on the cell. Together with static water pressure measurement from the water stand pipe, the effective pressure caused by fill and surcharge can be measured. The earth pressure cell can be installed on the foundation before filling, or it can be installed in a borehole. The earth pressure cell can be installed in two positions depending on the situation. One is a sensitive side down and the other is a sensitive side up. If pressure cell is to be installed on the rigid foundation or rigid structure, it should be installed with the sensitive side facing the rigid foundation surface. If it is to be installed on the flexible surface, the sensitive side should face the filling soil. The pressure cell will give different measured data on the sensitive side up and down even in laboratory loading. The comparative plot of earth pressure measurement from the sensitive side up and down condition in the field is shown in the Fig. 6.13. In this

case, the earth pressure installed sensitive side up measures the expected pressure. Underestimation of the earth pressure cell installed under the granular fill is mostly due to arching of the earth fill on the pressure cell.

6.1.7 Piezometers

Three types of piezometers are used in reclamation projects:

– Pneumatic piezometer,

– Electric vibrating wire piezometer and

– Casagrande open type piezometer

Piezometers are installed in a borehole. One piezometer should be installed in each borehole at a predetermined elevation. Pneumatic and vibrating wire piezometers should be calibrated in the local environment before installation. Calibration can be carried out in a large diameter tubewell and measured as pressure against actual water column pressure on the piezometer. Figure 6.14 shows on-site calibration of a piezometer in the large diameter cell, and Fig 6.15 shows typical on-site calibration data of the piezometer. In the case of the vibrating wire piezometer, calibration measurement is as frequency against actual water pressure.

SITE CALIBRATION RESULT OF PNEUMATIC PIEZOMETER

S/N : 48775 (PP-203) DATE : 12-January-98

CLUSTER NO. : A4S-07 (A4 Area)

1.SITE CALIBRATION DATA

Piezometers are packed in a sand bag and saturated in the water at least twenty-four hours before installation. After installation in a borehole, sand should be placed again to a certain limit, and a bentonite seal should be placed on top of the sand column. The bentonite should be suitable for marine condition, and upon reaction with seawater, sufficient swelling and reduction of permeability

must be achieved. On top of bentonite plug, a borehole should be backfilled up to the original seabed, preferably with original soil. Or else, it should be backfilled with a good mixture of bentonite cement whose permeability is equivalent to or lower than natural soil. Backfilling with sand will lead to lower excess pore pressure at measured location owing to rapid dissipation of pore pressure along the sand ll column above the piezometer. The typical installation designs of piezometers are shown in Fig. 6.16. Most engineers assume a static water pressure profile as a straight line pressure– depth relationship. Sometimes it leads to misinterpretation of the excess pore pressure because the natural pore-pressure profile can vary from a straight line relationship. Schiffman et al. (1994) explained the deviation of pore pressure profile from static condition due to the hydrogeologic boundary condition (Fig. 6.17). Similar cases can be found at Bangkok clay where groundwater extraction from the aquifer is high. The reverse condition can be encountered at locations where an artesian aquifer lies below the compressible layer. At such locations, the natural pore pressure can be higher

than static. Examples of such locations can be found at Changi southeast location. Basically, piezometers are installed to monitor the dissipation of excess pore pressure. However, some should be installed before reclamation to check the natural variation of pore pressure profile.

Problems of high damage rate can be encountered especially on the pneumatic piezometer installed in highly compressible soft clay. Because of the high strain, the pneumatic cable can be stretched and pinched. This can be overcome by installing the pneumatic piezometer in a protected guard cell, and the pneumatic cable can be protected in a steel pipe. The design of a specially protected pneumatic piezometer is shown in Fig. 6.18. The comparison of damage rate with and without protection is shown in Table 6.1. Normally, pneumatic piezometers and vibrating wire piezometers give similar readings. However, care should be taken in analyzing piezometer results. Piezometer reading should be corrected by taking into account the piezometer tip settlement. Uncorrected piezometer monitoring data will lead to underestimation of the degree of dissipation.

A comparison of the corrected and uncorrected excess pore pressure data is shown in Fig. 6.19.

6.1.8 Casagrande Open Type Piezometer

Open type piezometers are installed in the more permeable formation where the drainage condition needs to be checked. Open type piezometers are installed in the same manner as pneumatic piezometers. Instead of a pneumatic cable and a water pressure cable, it has an extruding open pipe for water to be floated in the pipe. The typical installation design of open type piezometer is shown in Fig. 6.20.

Water heads are measured with the help of the water level indicator. Sometimes water can overflow through the pipe owing to extremely high artesian pressure in the aquifer below the compressible layer. So a pressure gauge should be installed to measure the water head (Fig. 6.21).

6.1.9 Water Stand Pipe

Water stand pipes are installed to measure the static water pressure of groundwater during and upon completion of filling. Some water stand pipes installed before filling should provide a water intake open slot above the seabed that would be in the granular ll after filling. If not, measured water levels from the water stand pipe may not be representative of the groundwater level in the ll. A sufficient open area normally more than 11% should be provided to reduce the hydrodynamic time-lag. On the other hand, the opening slot must be small enough to retain the surrounding soil. In normal practice, nylon wire meshes are wrapped round the slotted area to retain the surrounding soil. The typical installation design of a water stand pipe is shown in Fig. 6.22 .

6.1.10 Inclinometer

Inclinometers are installed to measure the lateral movement during lling or during consolidation. To measure the lateral movements during lling, inclinometers are installed at offshore cluster. To monitor the lateral movement during surcharge lling, inclinometers are installed at or near the edge of the surcharge embankment. Thus the vertical displacements caused by consolidation and lateral displacement can be differentiated.

Inclinometers should be anchored at hard formation where there is no lateral movement. Since inclinometers measure the relative movement to the toe, any movement at toe will lead to the underestimation of lateral displacement. The typical installation design of an inclinometer is shown in Fig. 6.23. A comparison of

two displacement measurements from inclinometers where one toe is xed at nonmovement formation and another at laterally displacing formation is shown in Fig. 6.24. It should be noted that sometimes the formation having SPT blow of 30–40 can move laterally.

6.1.11 Automatic Monitoring Instruments

Sometimes it is necessary to install instruments to monitor from a remote location. Such instruments should be able to record readings automatically. For the settlement gauges, an instrument like a liquid settlement gauge is used because it can be installed at various elevations, and the measured data can be autologged. For static water level measurement, either a water stand pipe with an automatic water level recorder or a low air entry vibrating wire piezometer with an autologger is used. For the piezometer, an electric vibrating wire piezometer is normally used. All the connection cables are

connected to the one instrument monitoring hut where the autologger is located. Power supply is normally provided by a battery charged by a solar panel. Data transmission can be through the telecommunication system. An example of an automatic remote monitoring instrument cluster is shown in Figs. 6.25 through 6.28. The typical pore pressure and settlement measurement from an automatic remote instrument cluster is shown in Fig. 6.29.