Rev. 7-1-95

PIEZOMETERS

204-R-049

The Standard Specifications are revised as follows:

SECTION 204, AFTER LINE 80, DELETE AND INSERT AS FOLLOWS:

204.04 Piezometers. Piezometers shall be furnished, placed, and maintained as shown on the plans. The necessary metering device shall be furnished for making pore pressure readings. Unless otherwise permitted or directed, the piezometers shall be installed prior to placing the first lift of embankment. All piezometers shall be installed within 1.5 m (5 ft) of a settlement plate.

(a) Installation. Unless otherwise directed or permitted, installation of piezometers shall be as described below.

A steel pipe casing of 100 mm (4 in.) nominal diameter shall be driven or pushed to a depth of 1 m (3.25 ft) below the required elevation of the bottom of the deepest piezometer tip. The casing shall be in 1.5 m (5 ft) sections. The lower end shall not be provided with a drive shoe. Jetting or washing shall not be done to advance the casing as it is driven.

After the casing is at the required elevation, it shall be cleaned and washed inside for its full length. The wash water shall be replaced with clean water.

The casing shall be withdrawn 0.6 m (2 ft) by means of jacking or other steady pull. The hole shall be filled to the bottom of the casing with saturated standard Ottawa sand. This layer shall be tamped with a annular tamping hammer.

The porous stone piezometer tip shall be attached by means of a leakproof joint to a length of 10 mm (3/8 in.) inside diameter polyvinylchloride plastic standpipe tube. The bottom end of the tip shall be plugged. The porous stone shall be soaked in water for several hours before it is installed. The tip and standpipe tube shall be filled with clean water. The tip shall be lowered into the casing until it rests on the sand at the bottom. A small excess head shall be maintained in the tube during lowering to ensure that a small amount of water flows out of the point.

The casing shall then be pulled or jacked up an additional 0.6 m (2 ft) to the top of the porous stone. Saturated Ottawa sand fill shall be placed to the bottom of the casing. Such layer of sand shall not be tamped.

The casing shall then be withdrawn 0.6 m (2 ft) more and filled with a 0.3 m (1 ft) layer of untamped saturated Ottawa sand, and a 0.3 m (1 ft) layer of tamped No. 14 sand.

The casing shall then be withdrawn 0.9 m (3 ft) more. The second bentonite seal shall then be placed in 5 layers, each 75 mm (3 in.) thick. An 0.45 m (18 in.) layer of tamped No. 14 sand shall be place on the seal.

The casing shall then be withdrawn 0.3 m (1 ft) more. The third bentonite seal shall be placed in the same manner as the first. This step shall complete the installation of the deepest piezometer at each location.

If more than one piezometer is installed in the same casing, No. 14 sand shall be added to a predetermined depth to permit installation of the upper piezometers at the specified elevations. Such tamped sand shall be added between the upper bentonite seal of the next lower piezometer and the lower seal of the next upper piezometer. The tip of the protective casing shall be placed at least 0.6 m (2 ft) below the original ground line. A suitable identification mark shall be placed at the top of each plastic standpipe, which shall clearly show the depth of the porous point.

During filling and tamping operations, the plastic tubing shall be kept in tension. The tamping hammer shall be smooth and rounded.

As the fill is advanced, the casing and plastic standpipe shall be extended by adding extra lengths of 1.5 m (5 ft) or less, with waterproof joints. The top of the casing shall be at least 0.6 m (2 ft) above the grade of the new embankment at all times. Each time the casing and standpipe are extended, the casing shall be filled with No. 14 sand. The last extension of pipe shall be of such length that it extends 0.3 m (1 ft) above subgrade. It shall be filled with No. 14 sand to within 230 mm (9 in.) of the top of the casing. A 75 mm (3 in.) mortar seal shall be placed on the sand. A steel cap shall be furnished.

The piezometer shall be protected from damage due to construction operations by means of erecting stout barricades, or by other suitable means. B borrow shall be used as embankment material around the installations as specified for settlement plate installations through rock embankment construction.

(b) Readings and Maintenance of Piezometer. The elevation of the water in the plastic standpipe shall be obtained by direct reading where possible.

When the water level is below the elevation permitting direct reading, it shall be read by means of the electrical device described below. A pair of No. 22 waterproof insulated stranded wires shall be taped together and marked for measuring. Several pieces of sheet lead shall be wrapped around the wires at intervals near the end to keep the wire taut. The contact point shall be made by removing the insulation at the ends for about 6 mm (1/4 in.), separating the 2 ends by 6 mm (1/4 in.), and forming a plug of sealing wax just above the bare ends to keep the points separated and in shape. Such sounding wire shall be connected to a suitable ohmmeter. The point shall be lowered to the water surface. When the bare ends touch the water, the hand on the ohmmeter shall kick over, to indicate that the circuit has been closed.

If the pore pressure becomes so great that the water is forced out of the top of the standpipe, the excess pressure shall be measured by means of a Bourdon gauge installed as described below.

When the water level is at the top of the plastic standpipe, a 0.6 m (2 ft) length of 10 mm (3/8 in.) inside diameter rubber pressure tubing shall be attached to the end of the standpipe. The threads of the connection to the Bourdon gauge shall be coated with an adhesive such as rubber cement to prevent leakage. The other end of rubber tubing shall be forced over this connection. The ends of the rubber tubing shall be tightly wrapped with soft copper wire to make the joints leakproof. The Bourdon gauge shall be mounted so that it is not higher than the top of the standpipe. If the pressure in the gauge drops to zero, it shall be detached. Readings shall be made directly or by means of the electrical measuring device.

The pore pressure data will be used to evaluate the rates of settlement. If it is determined that pore-water pressures are not sufficiently dissipated, the settlement time shall be extended as directed, even though the settlements may have decreased to the rates as described in 204.03.

204.04 204.05 Method of Measurement. Steel settlement plates with pipe extensions, and piezometers will be measured by the number of units installed.

204.05 204.06 Basis of Payment. Steel settlement plates with pipe extensions, and piezometers will be paid for at the contract unit price per each.

SECTION 204, AFTER LINE 93, INSERT AS FOLLOWS:

Piezometer .....................................................................EACH

SECTION 204, AFTER LINE 101, INSERT AS FOLLOWS:

The costs of furnishing all necessary materials and equipment including water elevation measuring equipment; installing, maintaining, and protecting the piezometer installation; B borrow used as embankment; and all necessary incidentals shall be included in the cost of piezometer.

Piezometer installations which are damaged or destroyed shall be replaced with no additional payment.