Application of Geotechnical Instruments in Reclamation and Soil Improvement Projects
Application of Geotechnical Instruments in Reclamation and Soil Improvement Projects
13.1 TYPES OF POSSIBLE GEOTECHNICAL PROBLEMS IN LAND RECLAMATION
13.2 TYPES OF MEASUREMENT
13.3 SELECTION OF LOCATION FOR GEOTECHNICAL INSTRUMENTS
13.4 TYPES OF GEOTECHNICAL INSTRUMENTS
13.5 MONITORING PROGRAM AND FREQUENCY
13.6 ASSESSING THE RELIABILITY OF MEASURED DATA
Deformation of soil involves the combined effects of elastic, plastic and viscosity. Therefore, the deformation behavior is somewhat complex. In addition to the complex nature of soil deformation, the additional stresses imposed on the soils vary not only in terms of magnitude but also in directions. This makes it difficult to predict the deformation of soil. However, geotechnical engineers have been predicting ground behaviour in advance with the help of finite difference or finite element computer modeling. Nevertheless, most of the cases, performances were far from the predictions because of the complexibility of the soil profile, parameters, and hence loading conditions.
Geotechnical instrumentation fulfills the gap between prediction and performance and prevents soil mass failure as well as damage to the structure on the ground. Geotechnical instrumentation can provide construction control as well as performance monitoring. First, it will provide safe construction of earth as infrastructure on the soil, and secondly, it will provide back analysed in-situ soil parameters for future analysis. In a way, a more economical and safer design can be established.
Numerous geotechnical problems can arise in reclamation on soft clay. The followings are some of the problems that may be encountered.
- Slope stability
- Stability of the retaining structure
- Consolidation settlement of soft clay
- Immediate settlement of granular soil
- Liquefaction. In order to manage the situation, prediction, instrumentation, construction control, and performance monitoring become essential.
However, the discussion in this chapter will only emphasize the first three problems.
In order to control the geotechnical problems arising from the 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 behaviour during construction in order to control the construction process.
Category B: Monitoring of performance of the ground during loading, unloading and soil improvement process.
The selection of locations for the instruments depends upon the type of measurement. To measure the total settlement of the ground, surface settlement plates are usually installed in a certain grid pattern. Generally, a square grid pattern with 50 meters by 50 meters spacing is necessary for soil improvement works on a few hundred hectares of land. Grid spacing can be varied depending upon the variation of the sub-surface compressible soil profile. The grid spacing can be wider where less variation of the subsurface layer is evident.
On top of the settlement plates, other major instruments are deep settlement gauges and piezometers which form an instrument cluster. Instrument clusters are also spread throughout the area at certain intervals, but more clusters are installed in critical areas and those areas with the thickest layer of compressible soil. Deep settlement gauges are usually installed on top of a sub-layer whereas piezometers are installed at the center of a sub-layer. With this arrangement, settlement and effective stress of the sub-layer can be correlated. A typical arrangement of an instrument cluster in the Changi East reclamation projects, where three types of soil layers existed, is shown in Figure 13.1.
Inclinometers are installed to monitor the lateral movement of the ground at the edge of the reclamation area or shore protection works. The locations are selected based on slope stability analysis. Generally, an inclinometer is installed at the trough of a possible slip circle line. One each could be installed on the crest and the toe. The typical arrangement
for inclinometer installation at the shore protection structure is shown in Figure 13.2. Settlement plates are also installed together in order to relate with vertical displacement. For a vertical wall, inclinometers are attached to the inner side of the wall.
There are two types of geotechnical instruments. One measures ground behaviour during construction whereas the other measures the performance of the ground during loading, unloading, and soil improvement.
TYPE A: Measurement of ground behaviour during construction.
- Settlement pad
- Pore pressure transducers
- Earth pressure cell
TYPE B: Monitoring of ground behavior during loading, unloading, and soil improvement.
- Settlement plate
- Deep settlement gauges
- Multi-level settlement gauges
- Pore pressure transducers
- Earth pressure cell
Instruments can be installed on the reclaimed platform where vertical drains will be installed or on a special platform erected offshore. Instruments are usually installed on the platform during or before installation of vertical drains since settlement that occurs before installation of vertical drains is insignificant and installation and protection of instruments offshore is difficult and costly.
Some instrument clusters are installed offshore with proper protection to monitor settlement occurring during reclamation. The typical protection used in the Changi East reclamation project is shown in Figure 13.3. The offshore and on-land instrument clusters with proper protection during filling are shown in Figures 13.4 and 13.5 respectively. Some contractors use a sand mound to surround the instruments as protection. Normally, instruments are installed in a cluster and the following instruments are included in a cluster.
- * Deep reference point
- * Settlement plate
- * Liquid settlement gauge
- * Multi-level settlement gauge
- * Piezometers
- * Open type piezometer
- * Water standpipe
- * Total earth pressure cell and
- * Inclinometer
13.4.1 Deep reference point
Deep reference points are installed to be used as benchmarks for survey purposes. A normal benchmark set by the surveyor may not provide the required accuracy especially when the benchmarks are far from the site or when it is installed on a pile which is driven through unstable settling ground or sit on the formation above a groundwater aquifer which is being exploited. In the first case, the benchmark can settle because of the pile being dragged down and in the later case it could be moved down owing to land subsidence caused by groundwater extraction. Unstable conditions can occur because of the lowering of groundwater or seasonal thermal effects.
Therefore, some deep reference points should be installed at the site, anchored on the bedrock and installed with a negative friction reducer. A typical design of a deep reference point is shown in Figure 13.6. There are records of data showing heaving of deep reference points relative to the benchmark when checked some years after installation. It is not really the heaving of the deep reference point but the settlement of the benchmarks after a few years.
13.4.2 Settlement plate
A settlement plate can be installed on the seabed with some protection against damage during filling. The required extensions are made whenever necessary during filling. Settlement plates are mostly installed on the platform where vertical drains are installed. The settlement rods need to be extended 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 avoid damage during filling. Measurements are taken using survey methods. A typical design of a settlement plate is shown in Figure 13.7. These surface settlement plates can measure the total settlement of the ground. Figure 13.8 shows a photograph of a settlement plate, and Figure 13.9 shows a settlement plate being installed. Generally, the top of the settlement rod is measured by the survey method to monitor the settlement of the ground. Table 13.1 shows the monitoring data during settlement, and Figure 13.10 shows a typical monitoring record in graphical
form. Settlement records are usually shown together with the soil profile and record of construction activities, as in Figure 13.10. If settlement plates are installed with certain grids, the isoline of settlement can be obtained (Figure 13.11).
13.4.3 Liquid settlement gauge
A liquid settlement gauge is used with a pneumatic pressure indicator to monitor the settlement of the foundation. Liquid settlement gauges can be read from a central location and the vibrating wire types are particularly useful where automatic recordings are required. It can be installed before filling or in a borehole. By measuring the change in differential elevation between the pressure sensor and the reference reservoir, settlement records are taken. The diagram of a liquid settlement gauge and the principle of measurement are shown in Figures 13.12 and 13.13. Table 13.2 shows the processing of measured data to obtain settlement records from a liquid settlement gauge. However, liquid settlement gauges are only suitable for measuring the total surface settlement. Using liquid settlement gauges in deep-seated locations below groundwater level is not advisable. Figure 13.14 shows the monitoring data in graphical form. High capacity pressure cells are required for deep-seated locations and hence the measurements are less accurate. Table 13.3 shows the range of pressure cells available for liquid settlement gauges and their accuracy.
|Table 13.3 Range of pressure cells available for liquid settlement gauge.|
|Cell Type (psi)||Resolution (mm)||Cell Diameter (mm)||Cell Height (mm)|
13.4.4 Deep settlement gauge
A deep settlement gauge can be installed at various levels in order to monitor the settlement of various sub-layers. There are two major types of deep settlement gauges: screw plate deep settlement gauge, and Borros anchor deep settlement gauge. Both types of settlement gauges are installed in a borehole with the necessary friction reducer. Friction reducers must be installed at a sufficient distance above the screw plate to allow settlement of the friction pipe caused by the down drag, otherwise the friction reducer will push down the settlement plate and an overestimation of settlement of the sub-layers will result. The typical designs of deep settlement gauges are shown in Figures 13.15 and 13.16. Figure 13.17 shows a photograph of a deep settlement gauge. Deep settlement gauges are usually installed on top of each sub-layer to monitor the magnitude of the settlement of sublayers. Measurements are usually taken using the survey method, and
Table 13.4 shows data obtained from deep settlement gauges. Figure 13.18 shows a graphical presentation of settlement measurements from deep settlement gauges. The measurements are usually shown together with the soil profile, level of installation, and construction stages.
13.4.5 Multi-level settlement guage
Multi-level settlement gauges are made up of a series of spider magnetic rings. An access tube is installed in a borehole and drilled to the hard formation. The tube is anchored on the hard formation where no settlement can occur. A datum magnet is installed on the anchored location and the spider rings are installed along the access tube at predetermined intervals and spider arms are then released to anchor them to the formation. The measurements are taken with the help of a magnetic probe which 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 obtain the latest elevation of the spider ring with reference to the datum magnet, monitoring is carried out together with an elevation survey at the top of the access tube. The various types of multi-level settlement gauges and the typical installation arrangement of multi-level settlement gauges is shown in Figure 13.19 (a and b). Table 13.5 shows processed data from multi-level settlement gauges and Figure 13.20 is a graphical presentation of monitoring data from multi-level settlement gauges. Readout used in monitoring multi-level settlement gauges is shown Figure 13.21.
In some cases, the multi-level settlement gauges measure lower rates of settlement of the sub-layer than the screw type deep settlement gauges. Figure 13.22 shows a comparison of measurements of the rate of settlement of the sub-layers at the same elevation by screw plate deep settlement gauges and multi-level settlement gauges in one particular case. It can be seen that multi-level settlement gauges underestimate the settlement. There are various reasons for this under- or over-measurement of settlement:
- The grout is not deforming.
- The spider rings do not follow soil mass because of a jam between the access tube and the ring.
- The datum magnet moves down because of the down drag on the access tube, thus underestimating the relative movement when the top of the excess tube is not surveyed.
- . The deflection of the access tube or riser pipes because of lateral stress and movement.
- . The gap between the spider rings and the coupling is not sufficient.
- . Settlement caused by the dead weight of the screw type settlement gauge and riser pipes.
- . Kink in the riser pipe of the deep settlement plate because of lateral soil movement, resulting in measurement of the settlement at the kink rather than at the plate.
Note: Problems 2 and 3 can be minimized by using telescopic coupling for the access tube.
Therefore, interpretation of data from multi-level settlement gauges should be done with care.
13.4.6 ■ Earth pressure cell (EPC)
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 a water standpipe, the effective pressure caused by the 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 upon the situation. One is with the sensitive side down and the other is with the sensitive side up. If the pressure cell is to be installed on a rigid foundation or structure, it should be installed with the sensitive side facing the rigid foundation surface. If it is to be installed on a flexible surface, the sensitive side should face the filling soil.
The pressure cell will give different measurement data depending upon whether the sensitive side is up or down, even in laboratory loading. A comparative graph of earth pressure measurements with the sensitive side up and down is shown in Figure 13.23. An underestimation by the earth pressure cells installed under a granular fill is mostly due to arching of the earth fill on the pressure cell. The EPC should be installed with its sensitive surface in direct contact with the soil. Both surfaces of the EPC must be in full-face contact with the soil or the rigid structure.
A point load on the surfaces of the EPC will result in over measurement. The EPC should be installed on rigid structures measuring 1000mm x 1000mm (150mm thick concrete or 12mm thick steel) to minimize arching problems which can result in under-measurement of the fill. Pressure cells must be calibrated before usage. On-site calibration is possible when a large diameter tube well is available.
The measured data can be calibrated against the actual water pressure on the cell. Table 13.6 shows processed data from earth pressure cell measurements. Figure 13.24 shows the various types of earth pressure cells and readout unit. Monitored data are also shown together with construction activities and static water level, as in Figure 13.25.
Three types of piezometers are usually used in reclamation projects.
- Pneumatic piezometer
- Vibrating wire piezometer and
- Casagrande open type piezometer
Piezometers are installed in a borehole. Each piezometer should be installed in a borehole at a predetermined elevation. Pneumatic and vibrating wire piezometers should be calibrated for the local environment before installation. On-site calibration can be carried out in a large diameter tube
well and pressure measured against the actual water column pressure on the piezometer. In the case of the vibrating wire piezometer, calibration is frequently done against the actual water pressure. An example of a site calibration is shown in Figure 13.26.
Piezometers are packed in a sand bag and saturated in water at least twenty-four hours before installation. After installation in a borehole, sand should be filled around it to a certain limit and a bentonite seal placed on top of the sand column. The bentonite should be suitable for marine conditions and upon reaction with seawater sufficient swelling and reduction of permeability must be achieved. On top of the bentonite plug, the borehole should be backfilled up to the original seabed level, preferably with original soil. If not, it should be backfilled with a good mixture of bentonite cement with permeability equivalent to or lower than the natural soil. Backfilling with sand will lead to underestimation of the pore pressure at the measured location because of rapid dissipation of pore pressure along the sand fill column above the piezometer. A typical installation of a piezometer is shown in Figure 13.27. Figure 13.28 shows a photograph of a pneumatic and vibrating wire piezometer.
Piezometers generally measure pressure or water head above the measured level. The measured values are generally translated into piezometric head or excess pore pressure. Data are usually presented together with construction stages and activities. However, care should be taken in analyzing piezometer results. Piezometer readings should be corrected by taking into account piezometer tip settlement. Uncorrected piezometer monitoring data would lead to an under-estimation of the degree of dissipation (Bo et al. 1999b). Table 13.7 shows measured and processed data, and Figure 13.29 shows data presented in terms of pressure head, piezometric elevation, and excess pore pressure.
A comparison of corrected and uncorrected excess pore pressure data is shown in Figure 13.30. Normally, a pneumatic piezometer and a vibrating wire piezometer will produce similar results.
Basically, piezometers are installed to monitor the dissipation of excess pore pressure. However, some are installed prior to reclamation to check the natural variation of pore pressures in the soil. Sometimes, natural pore pressures in the soil vary from the static condition because of hydrogeologic boundary conditions at the drainage layer. As such, it would mislead the interpretation of excess pore pressure on the piezometer head.
13.4.8 Casagrande open type piezometer
Open type piezometers are installed in more permeable formations where the drainage condition needs to be checked constantly. 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 flow. Water depths are measured with the help of a water level indicator. Sometimes, water can overflow through the pipe because of extremely high artesian pressure at the aquifer below the compressible layer. As such, a pressure gauge should be installed to measure the water head (Figure 13.31b).
A typical installation design is shown in Figure 13.31a. Figure 13.32 shows a water level indicator of an open type piezometer used in monitoring. Table 13.8 shows measured and processed data. Figure 13.33 shows a graphical presentation of processed data. Data are generally presented at the piezometric head or elevation. Figure 13.34 shows a photograph of a Casagrande open type piezometer.
13.4.9 Water standpipe
Water standpipes are installed to measure the static pressure of ground water during and upon completion of filling. Some water standpipes installed prior to filling should provide an open slot above the seabed so that water intake from the granular fill is possible after filling. If not, the water levels from the standpipe may not be representative of the ground water level in the fill. Sufficient open area, normally greater 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, geotextile is wrapped around the slotted area in order to retain the surrounding soil. A typical installation design of a water standpipe is shown in Figure 13.35. Figure 13.36 shows a photograph of a water standpipe. Measurement, data processing and presentation are the same as for the
open type piezometer. Table 13.9 shows processed data, and Figure 13.37 shows an example of data presentation.
Inclinometers are installed to measure the lateral movement during filling or during consolidation. To measure the lateral movements during filling, inclinometers are installed in an offshore cluster.
To monitor the lateral movement during a surcharge filling, inclinometers are installed at or near the edge of the surcharge embankment. As such, vertical displacement caused by consolidation and lateral displacement can be differentiated.
An inclinometer should be anchored on a hard formation where there is no lateral movement. Since an inclinometer measures relative movement at the toe, any movement at the toe would lead to an underestimation of lateral displacement.
A typical installation design of an inclinometer is shown in Figure 13.38. The probe generally measures the degree of inclination between two points, and lateral displacement is calculated using the following simple equation:
Lateral displacement (D) = L Sin θ
where L is length of probe, θ is angle measured.
Cumulative displacements are calculated from this measurement. Table 13.10 shows an example of processed data, and Figure 13.39 shows an example of data presentation. Figure 13.40 shows a photograph of various
types of couplings and casings used in inclinometer installation. Figure 13.41 shows the inclinometer probe used for monitoring, and Figure 13.42 shows the inclinometer spiral sensor for checking casing twist.
Generally, the stage construction technique is applied in shore protection and surcharging along the edge of the reclamation. The three types of instruments which can be used to ensure the safe construction of slopes are inclinometers, settlement pads, and plates. Inclinometers provide absolute measurement of lateral movements. Settlement pads are placed along the side of the slope. In this case, surveying of pads along the line at close intervals is essential.
The factor which causes slope failure is not only the magnitude of the lateral movement but also the rate of movement. Therefore, a sudden increase in the rate of movement at the maximum displacement point is an indication of danger to slope stability. Figure 13.43 shows construction control using inclinometer monitoring data.
Alternatively, slope construction can be controlled by monitoring the settlement at the crest with a settlement plate, and the lateral movement with an inclinometer. The ratio of settlement caused by lateral movement is a good indication of slope stability.
13.4.11 Automatic monitoring instrument
Sometimes it is necessary to install automatic monitoring instruments to monitor the deformation behaviour of the foundation soil at a centralized location. The instruments selected should be able to record the readings automatically. For the monitoring of settlement, instruments such as liquid settlement gauges are used since these can be installed at various elevations, and the measured data can be auto-logged.
For static water level measurement, either a water standpipe with an automatic water level recorder or low air entry vibrating wire piezometer
with auto-logger is used. For the piezometer, a vibrating wire piezometer is normally used. All the signal cables are connected to one instrument monitoring hut where all the auto-loggers are located. Power supply is normally provided by batteries that are charged by a solar panel. Figure 13.44 shows the data logging station for long-term monitoring.
After the instruments have been installed, the second step is to make a schedule of the monitoring program. The monitoring program should be scheduled according to the purpose of monitoring. It should be carried out at the right time, otherwise the recorded data may not be meaningful and useful.
For example, monitoring of the consolidation process should be planned such that the duration is long enough for a high degree of consolidation to take place, and closer intervals should be planned at the initial stage and longer intervals at the later stage.
To monitor the construction process, such as slope stability control, forces and load measurements should be carried out prior to the application of the load to serve as baseline data, and close-interval measurements of lateral and vertical movements during the application of the load should be monitored. Otherwise, it may not be possible to control the situation. Table 13.11 shows the typical monitoring program adopted in the Changi East reclamation projects. Details of the application of geotechnical, instrumentation in land reclamation projects can be found in Bo et al. (1998c) and Bo and Choa (2002b).
|TABLE 13.11 Typical monitoring program adopted in the Changi East reclamation project.|
|Type of Instrument||Frequency||1st month after filling||2nd and 3rd month after filling||4th month after filling|
|Settlement Monitoring & Datum Point||Settlement Plate (SP)||Once/3 days||Once/1 week||Once/1 week|
|Deep Settlement gauge (DS)||Once/3 days||Once/1 week||Once/1 week|
|Deep Reference Point (DR)||Once/1 day - once/3 days||Once/1 week||Once/1 week|
|Pore Pressure & Water Level||Pneumatic Piezometer (PP)||Once/1 day - once/3 days||Once/1 week||Once/1 week|
|Open type Piezometer (OP)||Once/1 day - once/3 days||Once/1 week||Once/1 week|
|Water Standpipe (WS)||Once/1 day - once/3 days||Once/1 week||Once/1 week|
|Horizontal Displacement||Inclinometer (IN, IV)||Once/1 day - once/3 days||Once/1 week||Once/1 month|
There are two important geotechnical instruments that are needed for land reclamation and soil improvement projects. One is the settlement gauge and the other is the piezometer. Settlement monitoring data rely totally on survey measurements whereas accurate measurement of pore pressure requires a combination of piezometric head measurements using a pressure transducer and elevation adjustment by applying survey measurements. Although the accuracy and precision of instruments are selected to suit the local environment, it is still necessary to recalibrate the instruments prior to installation. A function test of the piezometer is essential before sealing the borehole.
In the case of deviation of instrument readings from the linearity, the instrument should be rejected and replaced. Even when the instrument has been properly selected, calibrated and installed, it is still necessary to check the reliability of the instruments in the ground after installation. This can be done if the initial condition of the pore pressure is known, as in the case of the piezometer.
As shown in Figure 13.45, a piezometer installed at 44 meters below the ground water level under static pore pressure conditions should give an approximate reading of 440 kN/m2 pore pressure magnitude. If the measured pore pressure deviates from the expected pore pressure, there are a few possible reasons:
- malfunction of the piezometer.
- in-situ pore pressure is lower or higher than the static water pressure.
- the installed depth is shallower than planned because of borehole inclination.
In such a case, a dispute may arise among the instrument supplier, the owner, and installation contractor over the reliability of the instrument, the mode of installation, and in-situ conditions. The functionability of the instrument can be checked by applying or reducing the load. In-situ pore pressure can be checked by other means, such as a CPT holding test, using an alternative piezometer, such as the open type piezometer and water standpipe.
However, during the installation of counter-checking instruments, special care has to be taken to install the piezometer at the correct depth by creating a straight vertical hole. Failure to drill a straight and vertical borehole could result in installation of the piezometer at a shallower depth, which would give a lower pore pressure than expected. This sort of situation can be encountered at locations where an intermediate stiff layer exists or the operation is carried out by an unskilled driller using inappropriate drilling equipment. This scenario is briefly explained in Figure 13.46. It is difficult to assess the settlement monitoring data since measurements are simple and purely based on the survey method. Nevertheless, this can be done when the data are accumulated. Figure 13.47 shows a good spread of settlement monitoring data obtained from a special pilot test area. It can be seen that the settlement data are hyperbolic and the settlement rate reduces
over time. This agree with consolidation theory. In contrast, there is another set of settlement data (Figure 13.48). It also shows hyperbolic and settlement rate reducing over time. However, if the data are processed with the settlement rate, the two sets of data are very different. The first one shows a decreasing rate over time whereas the second one shows significant fluctuation. Therefore, it can be concluded that the first set of data is more accurate than the second one.