Hollow-stem auger method

(a) After the hole for the piezometer is advanced to grade, 1 to 2 feet below the piezometer tip, or after the last sample is taken in a hole to receive a piezomet­er, the hollow-stem auger should be flushed clean with water and the plug reinserted at the bottom of the au­ger. The auger should then be slowly raised to the ele­vation that the piezometer tip is to be installed. At this elevation, the hollow stem should be filled with clean water and the plug removed. Water should be added to keep the stem full of water during withdrawal of the plug. The hole should then be sounded to determine whether or not the hollow stem is open to the bottom of the auger. If material has entered the hollow stem of the auger, the hollow stem should be cleaned by flushing with clear water, or fresh Johnson’s Revert or equivalent drilling fluid if necessary to stabilize the bottom of the hole, through a bit designed to deflect the flow of water upward, until the discharge is free of soil particles. The piezometer screen and riser should then be lowered to the proper depth inside the hollow stem and the filter sand placed. A wire spider should be attached to the bottom of the piezometer screen to center the piezometer screen in the hole in which it is to be placed.

(b) The filter sand should be poured down the hollow stem around the riser at a rate (to be deter­mined in the field) which will ensure a continuous flow of filter sand that will keep the hole below the auger filled as the auger is withdrawn, Withdrawal of the au­ger and filling the space around the piezometer tip and riser with filter sand should continue until the hole is filled to a point 2 to 5 feet above the top of the piezom­eter screen. Above this elevation, the space around the riser pipe may be filled with any clean uniform sand up to the top of the particular sand stratum in which the piezometer is being installed but not closer than 10 feet of the ground surface. An impervious grout seal should then be placed from the top of the sand backfill to the ground surface.

(1) Casing method. The hole for a piezometer may be formed by setting the casing to an elevation 1 to 2 feet deeper than the elevation of the piezometer tip. The casing may be set by a combination of rotary drill­ing and driving the casing. The casing should be kept filled with water, or organic drilling fluid, if neces­sary, to keep the bottom of the hole from ‘blowing.” After the casing has been set to grade, it should be flushed with water or fresh drilling fluid until clear of any sand. The piezometer tip and riser pipe should then be installed and a filter sand, conforming to that specified previously, poured in around the riser at a rate (to be determined in the field) which will ensure a continuous flow of filter sand that will keep the space around the riser pipe and below the casing filled as the casing is withdrawn without “sand-locking” the casing and the riser pipe. Placement of the filter sand and withdrawal of the casing may be accomplished in steps as long as the top of the filter sand is maintained above the bottom of the casing but not so much as to “sand – lock” the riser pipe and casing. Filling the space around the piezometer tip and riser with filter and should continue until the hole is filled to a point 2 to 5 feet above the top of the piezometer screen. An imper­vious grout seal should then be placed from the top of the sand backfill to the ground surface.

(2) Rotary method. The hole for a piezometer may be advanced by the hydraulic rotary method using water or an organic drilling fluid. After the hole has been advanced to a depth of 1 or 2 feet below the pie­zometer tip elevation, it should be flushed with clear water or clean drilling fluid, and the piezometer, filter sand, sand backfill, and grout placed as specified above for the casing method, except there will be no casing to pull.

b. Development and testing. The piezometer should be flushed with clear water and pumped after installa­tion and then checked to determine if it is functioning

properly by filling with water and observing the rate of fall. A lo-foot minimum positive head should be maintained in the piezometer following breakdown of the drilling fluid. After at least 30 minutes have elapsed, the piezometer should be flushed with clear water and pumped. For the piezometer to be consid­ered acceptable, it should pump at a rate of at least 2 gallons per minute, or when the piezometer is filled with water, the water level should fall approximately half the distance to the groundwater table in a time slightly less than the time given below for various

types of soil:

Type of Soil in Which Piezometer Screen Is Set

Period of Observation minutes

Approximate Time of 50 Percent Fall

minutes

Sandy silt (>50% silt)

30

30

Silty sand (<50% silt,

>12% silt)

10

5

Fine sand

5

1

If the piezometer does not function properly, it will be developed by air surging or pumping with air if neces­sary to make it perform properly.