Piezometers

(a) Locations. The M, R, S, and T piezometers shall be installed to measure the groundwater table in the deep sand formation beneath the excavation, be­tween the dewatering wells, and on three lines out from the excavation at the approximate locations shown on the drawings. The N piezometers shall be in­stalled to measure the groundwater table in semiper­vious strata in the bottom of the excavation and mid­way between jet-eductor wells at the approximate loca­tions. The Contractor shall stake the piezometers at designated locations. The tips of M, R, S, and T pie­zometers for measuring the groundwater table in the deep sand formation shall be set in clean sand at eleva­tion -80.0 feet or below as necessary; the tips of the N piezometers for measuring the groundwater table in semipervious strata in the top stratum shall be set at the bottom of the semipervious strata.

(b) Piezometer materials. The N piezometers shall consist of a 1.50-inch I. D. (Schedule 80) PVC screen with 0.025-inch slots connected to a 1.50-inch

I. D. (Schedule 80) PVC riser pipe. The screens shall be 10 feet long. The joints of the screen and riser shall be flush (inside and outside) and shall be glued together with PVC pipe cement. The filter sand (B) shall meet the specifications set forth for jet-eductor wells. De­pending upon the method of installation, the riser and screens for the M, R, S, and T piezometers shall be as specified above, or 1.5-inch galvanized iron riser pipe connected to a 1.5- by 30-inch self-jetting wellpoint with a 30- to 40-mesh stainless steel screen.

(c) Installation of piezometers. Holes for pie­zometers may be advanced by either: using an S-inch O. D. continuous flight auger with a 3%-inch I. D. hol­low stem with the hollow stem plugged at the bottom with a removable plug; augering and more or less simultaneous installation of a 6-inch casing; or using a rotary wash drilling procedure (6-inch diameter) and an organic drilling fluid, such as Revert, if necessary, to keep the drilled hole open. The tip of the piezom­eters shall be installed at approximate depths or eleva­tions as approved by the C. O.R.; piezometers shall also be installed as instructed. The hole for a piezometer shall be kept filled with water or an approved organic drilling fluid at all times. Bentonitic drilling mud shall not be used. Any auger used in advancing the hole shall be withdrawn slowly from the hole so as to mini­mize any suction effect caused by withdrawing the auger. (Hollow-stem augers shall be filled with drilling fluid before pulling the plug in the bottom of the auger.) Drilling and installation procedures shall be as specified below and shall be in accordance with ac­cepted practice and to the satisfaction of the C. O.R.

Method 1. Hollow-stem auger. After advancing the hole for the piezometer to grade (1 to 2 feet below the piezometer tip), or after taking the last sample in a hole to receive a piezometer, the hollow-stem auger shall be flushed clean with water and the plug rein­serted at the bottom of the auger. The auger shall then be slowly raised to the elevation that the piezometer tip is to be installed. At this elevation the hollow stem shall be filled with clean water and the plug removed. Water shall be added to keep the stem full of water during withdrawal of the plug. The hole shall 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 shall be cleaned by flushing with clear water, or clean Revert 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 shall then be lowered to the proper depth inside the hollow stem and the filter sand placed. (A wire spider of design ap­proved by the C. O.R. shall be attached to the bottom of the piezometer screen so as to center the piezometer screen in the hole in which it is to be placed. Use two crossed wires just above the plug in the tip.) Filter sand shall be poured down the hollow stem around the riser at a rate (determined in the field) that will ensure continuous filter sand flow down the hollow stem around the riser and piezometer, and will keep the 8- inch hole below the auger filled with filter sand as the auger is withdrawn. Withdrawal of the auger and fill­ing the space around the piezometer tip and riser with filter sand shall continue until the hole is filled to a point about 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 with less than 5 percent passing a No. 100 U. S. Stand­ard sieve up to within 20 feet of the ground or exca­vated surface. An impervious grout seal shall be placed around the top 20 feet of the ground or excavated sur­face. An impervious grout seal shall be placed around the top 20 feet of the hole for the M, R, S, and T pie-

zometers, and 5 feet for the N piezometers.

Method 2. Casing. The hole for a piezometer may be formed by setting a 6-inch 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 shall 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 shall be flushed with water or (clean) drilling fluid until clear of any sand. The piezometer tip and riser pipe shall then be installed and the filter sand poured in around the riser at a rate (to be determined in the field) which will insure a continuous flow of filter sand down the casing that will keep the hole around the riser pipe and below the casing filled with filter sand as the casing is withdrawn without “sand-locking” the casing and 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 sand shall continue until the hole is filled to a point about 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 and the hole grouted as specified in Method 1 above.

Method 3. Rotary. The hole for a piezometer may be advanced by the hydraulic rotary method using water or an organic drilling fluid. The hole shall have a minimum diameter of 6 inches. After the hole has been advanced to a depth of 1 or 2 feet below the pie­zometer tip elevation, it shall be flushed with clear water or clean drilling fluid, and the piezometer, filter sand, sand backfill, and grout placed as specified in Method 2 above, except there will be no casing to pull.

Method 4. Self-jetting. The M, R, S, and T piezom­eters may be drilled to within 4 feet of planned total depth, then clean water used to advance the self-jet­ting wellpoint to the design grade without the use of filter sand around the wellpoint. The seal and backfill for the piezometers shall consist of a pumpable cementbentonite grout, with a ratio of 1 gallon of ben­tonite per bag of cement, or equivalent cement grout approved by the C. O.R. (Only enough water shall be added to make the grout pumpable.) The tops of the risers shall be cut off 36 inches above the ground sur­face. The upper part of the piezometer shall be pro­tected by installing a 6-inch I. D. PVC or corrugated metal pipe around the riser cemented in to a depth of 3 feet below the ground surface. The number of the pie­zometer shall be marked with 3-inch-high black letters on the pipe guard around the riser pipe.

Method 5. Sampling. Split-barrel samples shall be obtained at 5-foot intervals and at every strata change for the N piezometers. Samples shall be obtained using a 1%-inch (minimum) I. D. split-barrel or З-inch Shelby tube sampler by driving or pushing. The length of drive or push shall not be less than 3 inches. If there is insufficient sample recovery to identify the soil prop­erly, another sample shall be obtained immediately be­low the missed sample. If desired, the sampler may be advanced using driving jars on a wireline.

(d) Development and testing. After each pie­zometer is installed, it shall be promptly flushed with clean water, developed, and pumped to determine if it is functioning properly. (If an organic drilling fluid has been used, Johnson’s Fast Break, or equivalent, shall be added in accordance with the manufacturer’s rec­ommendations to break down the drilling fluid,) A 10- foot minimum positive head shall be maintained in the piezometer following addition of the Fast Break. After at least 30 minutes has elapsed, the piezometer shall be flushed with clear water and pumped. The piezom­eter may be pumped with either a suction pump or by means of compressed air. The approximate rate of pumping during development shall be measured, Pie­zometers installed in the deep sand formation will be considered acceptable if they will pump at a rate of 2 to 5 gallons per minute or more. Piezometers installed in the semipervious strata within the top strata will be considered acceptable if they will pump at a rate of at least 0.5 gallon per minute, or when the piezometer is filled with water, the water level falls approximately half the distance of the groundwater table in a time less than the time given below for various types of soil:

Approximate

Type of soil in Period of time of

which piezometer observation 5Opercent fall

screen is set minutes minutes

Sandy silt (> 50% silt) 30 30

Silty sand (< 50% silt, > 12% silt) 10 5

Fine sand (<12% silt) 5 1

If the piezometer does not function properly, it shall be developed by a combination of air surging and pump­ing with air as necessary to make it perform properly. If the piezometer still will not perform properly, it shall be reinstalled at a nearby location selected by the C. O.R.

(e) Monitoring groundwater table. The Contrac­tor shall read all M and N piezometers at least once a week, selected piezometers at least twice a week, some of the piezometers in the deep sand formation daily, and the water level in the dewatering and the jet-educ­tor wells at least once a week for his or her informa­tion and use in operation of the dewatering systems and control of groundwater as specified. The Contrac­tor shall record what deep well and jet-eductor wells are being pumped when he takes his piezometer or water level readings. The C. O.R. will also read the M

and N piezometers on a schedule similar to the above for his own check and evaluation purposes.

f) Records, The Contractor shall furnish copies of all piezometer and water level readings to the C. O.R. within 24 hours of being taken. Copies of pie­zometer, water level, and flow measurements made by the C. O.R. will be furnished to the Contractor within 24 hours.