2.Installation

2.1Preliminary Tests

It is always wise, before installation commences, to check the cells for proper functioning. Each cell is supplied with a calibration sheet, which shows the relationship between readout digits and pressure, as well as the initial no load zero reading. (Figure 19 in Section 4 shows a typical calibration sheet.)The cell electrical leads (usually the red and black leads) are connected to a readout box (see Section 3) and the zero reading given on the calibration sheet is compared to the current zero reading. The two readings should not differ by more than 50 digits after due regard to corrections made for different temperatures, barometric pressures and height above sea level and actual cell position (whether standing up or laying down).

By pressing on the cell, it should be possible to change the readout digits, causing them to fall as the pressure is increased.

Checks of electrical continuity can also be made using an ohmmeter. Resistance between the gauge leads should be approximately 180 ohms, ± 5%. Check the resistance between the two thermistor wires (usually white and green). Using Table 5 in Appendix B, convert the resistance to temperature. Compare the result to the current ambient temperature. Resistance between any conductor and the shield should exceed 20 megohms. Remember to add cable resistance when checking (22 AWG stranded copper leads are approximately 14.7Ω per 1,000 feet (48.5Ω per km), multiply by two for both directions).

2.2Pressure Cell Installation

2.2.1Inside Fills and Embankments

Earth pressure cells are normally installed with the flat surfaces horizontal to measure vertical stresses. However, they can be placed at other orientations, inside the fill, to measure stresses in other directions e.g., a cell placed with the flat surfaces vertical will measure horizontal stresses in a direction perpendicular to the plates of the cell. They are sometimes placed at angles of 45 degrees.

Experience has shown that attempts to measure earth pressures in fills frequently meets with failure. The problem is twofold. First, the stress distribution in the fill can be inherently variable due to varying properties of the ground and varying degrees of compaction of the ground. Thus, the soil stress at one location may not be typical of the surrounding locations. Secondly, a cell installed directly in the fill could result in the creation of an anomalous zone immediately around the cell where there may be a different, more fine-grained material, under less compaction. (The material around the cell may be poorly compacted because of the need to avoid damage to the cell.)

In an earth fill, this zone of poor compaction would not be expected to be a problem since the earth above might be expected to move downwards to fill the voids and consolidate the ground. However, under the influence of rainwater and vibration, any spaces in the soil immediately around, and especially under, the cell may grow, causing the cell to become completely decoupled from the soil around it. In such situations, the internal soil stresses go around the cell instead of through it. The cell will then register only a very low pressure, which does not change much as the loads increase. This situation occurs frequently.

Weak Grout Method

One way to avoid the problem is to cast the cell inside a weak grout. A method used successfully in South Africa, by Oosthuizen et al, essentially uses the techniques similar to the one described in Section 2.2.5. Installation of the cells begins when the fill has reached a height of one meter above the instrument level. The Instrument location and the cable trenches are excavated one meter deep, the instrument pocket, with 45° sloping sides (see Figure 10).

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Figure 10: Model 4800 Earth Pressure Cell Installation

The cells (Model 4800-1-1P, complete with pinch tubes and lugs) are positioned on a thin layer of non-shrink, sand cement grout, and are nailed in position using the lugs on the cells provided for this purpose. The excavated pocket is then backfilled to a depth of 300 mm with a weak concrete in 100 mm layers, vibrated with a poker vibrator. After 24 hours, the cells are pressurized by pinching the pinch tubes until the pressure in the cell, displayed on a connected readout box, starts to change.

The instrument location containing the grouted cells and the cable trench is then backfilled in 250 mm layers, using the same material as the main fill placed by hand and compacted with pneumatic or gasoline backfill tampers, or vibratory trench rollers. After this, standard construction filling and compaction practices can continue.

Earth pressure cell clusters, placed according to the methods outlined above, may be installed either in trenches, below the temporary embankment grade, or in ramps above the temporary embankment grade. In dams, for example, it is usually convenient to install in trenches in the impervious rolled fill core, and in ramps in the filter zones and compacted rockfill shell zones. In earth embankments, it is convenient to install in trenches. By doing so, adequate degrees of compaction of the backfill can be more easily obtained without damage to the cell clusters or cable arrays. As the cells are being covered and compacted, repeated readings should be taken to ensure that the cells are continuing to function properly.

See Section 2.3 for cable installation and protection.

table 1: Ratios for Two Grout Mixes.

Alternative Method

In this method, the pressure cell used to monitor vertical earth pressures is placed directly in the fill. The procedures are similar to those in the Weak Grout Method section above, except that the pressure cell does not have a pinch tube and the layer of weak grout is dispensed with. Instead, the cell is placed on a pad of quick-setting mortar. This is done to ensure uniform contact with the soil at the bottom of the trench. The cell is then covered by soil placed in 300 mm layers and compacted as before.

2.2.2Installation of Model 4810 Contact ("Fat Back") Pressure Cell

This section details installation instructions for Model 4810 earth pressure cells, which are used for the measurement of earth pressures on structures. In backfills for piers, piles, bridge abutments, retaining walls, culverts and other structures the cells may be installed either inside a concrete structure being poured or directly on the surface of an existing structure. For slurry walls, the Model 4820 earth pressure cell is used as described in Section 2.2.4.

Installation in Poured Concrete

When pouring concrete, the cells can be held to the forms using nails through the lugs welded to the edge of the cell. Position the cell so that the thin pressure sensitive plate is directly against the concrete form. Nail the plates to the form lightly in such a manner that they engage the concrete sufficiently and will not pull out of the concrete when the forms are removed. Route the cable inside the concrete to a convenient readout location or to a block out inside where excess cable can be coiled. Protect the cable from damage during concrete placement and vibration by tying it to adjacent rebar. See Figure 11.

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Figure 11: Attachment of Model 4810 to Concrete Form

Installation on Existing Structures

The lugs welded to the edge of the cell can be used to hold the cell against the structure using nails, lag bolts, tie wire, etc. Even if the surface is smooth, but especially when the surface is rough or irregular, a mortar pad between the cell and the structure is required. See Figure 12 below.

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Figure 12: Model 4810 Contact Pressure Cell Installation

Use the lugs on the cell as a template to locate the position for drilling holes for the installation of expanding anchors or install the anchors nearby and use wire to hold the cells in place. Alternately, the cell may be nailed in place using the lugs as a guide.

Mix up some quick-setting cement mortar or epoxy cement. Trowel this onto the surface then push the cell into the cement so that the excess cement extrudes out of the edges of the cell. Hold the cell in place while the cement sets up then complete the installation by adding the lag bolts (using the expansion anchors) and tightening or nailing the cell in place. Protect the cell, transducer housing, and cable from direct contact with large chunks of rock by covering them with a fine-grained fill material from which all pieces larger than about 10 mm (0.5") have been removed. This material is kept near the cell and cable as the fill is placed. Additional cable protection can be achieved by using metal conduit strapped to the surface of the structure.

2.2.3Installation of Model 4815 Hydraulic Load Cell

A particular installation, shown in Section 13, used the Model 4815 Hydraulic Load Cell to measure the concentrated load on a tunnel lining from an existing wooden pile (supporting a building above) that had been cut short by the tunnel excavation in frozen ground. The load cell was designed to measure any increase of load on the tunnel lining that might occur when, at the end of tunnel construction, the ground was allowed to thaw out. The load cell was positioned below the bottom of the pile and temporarily held in place with lugs and a mortar pad until the shotcrete tunnel lining was sprayed.

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Figure 13: Model 4815 Hydraulic Load Cell Measuring Tunnel Lining Loads

2.2.4Installation of Model 4820 Jackout Pressure Cell in Slurry Trenches

The Jackout Pressure Cell (JOPC) first needs to be assembled into the Jackout frame (geokon part #4820-5 or 4820-6). The assembly is shown in Figure 14. The support plate has a circular hole cut in it and bolt holes to fit the jackout pressure cell and is connected to one end of a double-acting hydraulic jack by means of steel struts. The support plate and reaction plate are cambered top and bottom to prevent them from snagging on the sides of the slurry trench. The reaction plate is attached to the other side of the double-acting hydraulic jack. The jack is attached firmly to the rebar cable and arranged so that the plates are free to move outwards. The hydraulic line and signal cable are tied off to one of the rebar at intervals of one meter (~ three feet).

When the rebar cage has been lowered to its proper depth, the jack is activated, forcing the two plates out against the trench walls.

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Figure 14: Model 4820 Jackout Pressure Cell Installation

Observation of the pressure indicated by the JOPC (see Section 3 for readout instructions) will indicate when the cell has contacted the wall. Pump up the jack until the JOPC reading indicates a pressure roughly 70 KPa (10 psi) greater than the slurry pressure at JOPC depth. This ensures that the cell is bearing against the walls of the trench, and that the concrete grout pressure will not close the jack, which could allow the reaction plates to move away from the trench walls. Check the JOPC reading from time to time, because the pressure might bleed away if the walls of the trench are soft and yielding. Repressurize as needed. Leave the jack pressurized until the grout has set up.

2.2.5Installation of Cells to Measure Earth Pressure at the Base of Footings, Floor Slabs, Pavements, Etc.

Experience has shown that attempts to measure contact earth pressures on the base of footings, floor slabs, pavements, etc., frequently meets with failure. The problem is twofold. First, the contact stress distribution can be inherently variable due to varying properties of the ground and varying degrees of compaction of the ground. Thus, the contact stress at one location may not be typical of the surrounding locations. Secondly, a cell installed as described in Section 2.2.1 could result in the creation of an anomalous zone immediately around the cell where there may be a different, finer grained material, under less compaction. The material around the cell may be poorly compacted because of the need to avoid damage to the cell.

In an earth fill, this zone of poor compaction would not be a problem, since the earth above would move downwards to fill the voids and consolidate the ground. However, where there is a concrete slab immediately above the cell, this consolidation may not take place. In fact, under the influence of rainwater and vibration, the spaces around the cell may grow, causing the cell to become completely decoupled from the concrete above. In such a situation, the concrete slab bridges over the gap and the loads in the concrete go around the cell instead of through it. The cell registers only a very low pressure, which does not change as the loads increase.

The best way to avoid the problem is to cast the cell inside the concrete if possible. This can often be done when the initial concrete bonding layer is spread over the surface of the ground. At this time a Model 4800-1-1P Earth Pressure Cell with a pinch tube, is pressed into the bonding layer so that it rests against the ground below. A weighted tripod can be used to hold the stress cell in place until the concrete hardens. The pinch tube is arranged to protrude above the bonding layer and, when the concrete has hardened, it is used to pressurize the cell and ensure good contact between the cell and the surrounding concrete. See Figure 15. The advantage of this method is its simplicity and that it permits the ground below the concrete to be completely compacted in the normal way.

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Figure 15: Model 4800-1-1P Earth Pressure Cell Installation

2.2.6Installation of Push-In Pressure Cells to Measure Lateral Earth Pressures

The Model 4830 is designed to be pushed into soft soils using available drill rods, usually AW. Unless the ground is very soft, it is recommended that a borehole be drilled to within about two feet of the desired location, and then push the cell the rest of the way. A few things to note and be aware of include:

Temperature effects

This pressure cell is relatively stiff due to the geometry and the need for a robust construction for pushing into the ground. It is always advisable to obtain the pre-installation zero pressure readings in the borehole at the borehole temperature. It may take a significant amount of time for the sensor to come to thermal equilibrium, but this is an important measurement and if it is not possible to take this reading in the borehole, it may be possible to take the reading in a bucket of water that is at the ground temperature.

Piezometer Saturation

The piezometer filter and sensor are saturated at the factory and sealed with Mylar tape. Do not remove the tape until just before the sensor is installed in the ground. The filter is saturated by drawing a vacuum on the sensor and then allowing water to flow into the sensor when the vacuum is released. If the sensor is to be installed and then removed for use at other sites, the saturation process should be performed at each installation. geokon can supply the necessary portable equipment to accomplish this.

Overpressure

When pushing the cell into the ground it is possible that pressures in excess of the sensors full-scale range can be generated causing the sensor to experience a zero shift or even permanent damage. To prevent this, readings should be taken as the sensor is pushed. When the indicated pressure approaches 150% of full scale the pushing operation should be terminated until the sensor output comes back within its calibrated range.

Sensor Wiring

table 2: 4830 Wiring Chart

2.3Cable Installation and Splicing

Cable placement procedures vary with individual installations. In general, however, all installations have in common the following requirements:

1.The cable must be protected from damage by angular particles of the material in which the cable is embedded.

2.The cable must be protected from damage by compaction equipment.

3.In earth and rock embankments and backfills, the cable must be protected from stretching as a result of differential compaction of the embankment.

4.In concrete structures, the cable must be protected from damage during placement and vibration of the concrete.

In embankments, cables may be embedded in a protective covering of sand or selected fine embankment materials. A typical installation might, for example, comprise the positioning of a series of cables on a prepared layer consisting of not less than 200 mm (8") of compacted selected fine material. To establish an acceptable grade without undue interference with construction operations, the prepared layer may be located either in a trench or on an exposed ramp. In rockfill dams with earth fill cores, for example, it is frequently convenient to install cable in trenches in the core and fine filter zones, and in ramps in the coarse filter and compacted rockfill shell zones. Individual cables should be spaced not less than 12 mm (0.5") apart, and no cable should be closer than 150 mm (6") to the edge of the prepared layer. In instances in which cables must cross each other, or in which more than one layer of cables must be placed in a given array, the cables should be separated from each other by a vertical interval of not less than 50 mm (2") of hand compacted sand or selected fine embankment material. Since the elongation capability of electrical cable is quite substantial, it is not necessary to install the cable with any "S" shaped meanders.

During the backfill of trenches in earth dams, a plug, approximately half a meter (two feet) in width, made of a mixture of 5% bentonite (by volume) from an approved source and exhibiting a free swell factor of approximately 600%, and 95% embankment material, can be placed in the trenches at intervals of not greater than 20 meters (50 feet). The purpose of the bentonite plugs is to reduce the possibility of water seepage through the embankment core along the backfilled trenches.

The cable may be marked by using a Mylar cable labels. For an individual cable, the identification number should be taped near the end of the cable. Additional cable labels can be specified to aid in identification if cables need to be dug up for splicing, etc.

Splice kits recommended by geokon incorporate casts, which are placed around the splice and are then filled with epoxy to waterproof the connections. When properly made, this type of splice is equal or superior to the cable in strength and electrical properties. Contact geokon for splicing materials and additional cable splicing instructions.

Cables may be terminated by stripping and tinning the individual conductors and then connecting them to the patch cord of a readout box. Alternatively, a connector may be used which will plug directly into the readout box or to a receptacle on a special patch cord.

2.4Electrical Noise

Care should be exercised when installing instrument cables to keep them as far away as possible from sources of electrical interference such as power lines, generators, motors, transformers, arc welders, etc. Cables should never be buried or run with AC power lines. The instrument cables will pick up the 50 or 60 Hz (or other frequency) noise from the power cable and this will likely cause a problem obtaining a stable reading. Contact the factory concerning filtering options available for use with the geokon dataloggers and readouts should difficulties arise.

2.5Initial Readings

Initial readings must be taken and carefully recorded along with the barometric pressure and temperature at the time of installation. Take the initial readings while the cell is in position, prior to covering it with fill and pouring the concrete. Again, it is imperative that initial readings at zero load are taken!