Wellpoint and pressure relief systems

(1) General. The jet-eductor wellpoint system specified is to lower the groundwater table adjacent to the excavation and intercept seepage from the sandy and gravelly soil above the Cockfield formation (at about el 253.0 feet) so that excavation and placement of the wood sheeting and pea gravel backpack can be accomplished with minimum difficulty, and with little if any sloughing of the soil from the cut slope. The wellpoint system combined with a sump ditch between the sheeting and edge of structural mat when properly installed, maintained, and pumped should permit plac­ing the “mud” mat on foundation soils free of any sur­face water. If the Contractor considers the spacing of the wellpoints inadequate to accomplish the excava­tion and placement of the wood sheeting, as specified, he or she should space the wellpoints closer. The well­point system shall remain in continuous operation un­til the excavation has been dug to grade and the “mud” mat poured. After the “mud” mat has been placed, the wellpoint system may be turned off and any seepage through the wood sheeting removed by sump pump­ing. After the “A” piezometers for monitoring the groundwater table above the Cockfield formation and the wellpoint system have been installed, the system shall be tested for flow and groundwater lowering pri­or to starting any excavation. The well system is to re­lieve excess hydrostatic pressure in the semipervious stratum below the excavation to insure no heave of the soil strata above this stratum. It is imperative that no heave of the soil strata above the semipervious stra­tum occur inasmuch as the building will be founded di­rectly on the foundation soils above this stratum.

(2) Control of water. The Contractor shall control all surface water, any seepage into the excavation, and hydrostatic pressure in the semipervious stratum be­neath the excavation, regardless of source. Any open­ing in the wood lagging, through which seepage is car­rying any soil, shall be promptly caulked. Any water seeping, falling, or running into the excavation as it is dug shall be promptly pumped out. The entire periph­ery of the excavation area shall be suitably diked, and the dike maintained, to prevent any surface water from running into the excavation. The Contractor shall be fully responsible for disposal of all water from the excavation and from the wellpoint and relief well systems in an approved manner at no additional cost to the Government.

(3) Wellpoint and pressure relief system data. The Contractor shall submit for approval by the Contract­ing Officer, within 10 calendar days after receipt of Notice to Proceed, complete information regarding motor generator, pumps, wellpoints, jet-eductors, well screens, and any other equipment that he or she pro­poses to utilize in dewatering and relieving hydrostatic pressure, and in maintaining the excavation in a dewa­tered and hydrostatically relieved condition. The data to be submitted shall include, but not necessarily be limited to, the following:

(a) Characteristics of pumps and motor gener­ator, and types and pertinent features of well screens, wellpoints, and jet-eductor pumps.

(b) Plans for operating, maintaining, and moni­toring the wellpoint and relief well systems.

(4) Jet-eductor wellpoint system.

(a) The soils at the site consist essentially of fill, clays and silts, and some silty sand and clay sands with occasionally pea gravel above the Cockfield formation. The jet-eductor wellpoint system has been designed to lower the groundwater table and intercept most of the seepage that will flow toward the excavation. Because of the impervious Cockfield formation at about the bottom of the excavation, it is not possible to lower the groundwater table and intercept most of the seepage that will flow toward the excavation as it is dug. Be­cause of the impervious Cockfield formation at about the bottom of the excavation, it is not possible to lower the groundwater table completely down to the top of this stratum or to intercept all seepage above it. By proper installation, the jet-eductor wellpoint system should intercept most of the seepage and stabilize the soil sufficiently to install the wood sheeting subse­quently specified without any significant sloughing of the soil behind the sheeting prior to placement of the pea gravel backpack. The fact that some minor seepage may bypass the wellpoints should be anticipated by the Contractor. The tips for the wellpoints shall be in­stalled approximately 12 inches below the top of the

Cockfield formation as encountered during installa­tion of the system.

(b) The wellpoints shall have a minimum screen or slotted length of 30 inches and shall be of the “bot­tom suction” type. The wellpoint screens shall be of30- mesh cloth or have #25 slots. The wellpoints shall be of a high-capacity type with a diameter of approximately 2% inches. The jet-eductor pumps and pressure and re­turn riser pipes shall have a capacity and be operated at a pressure which will produce a minimum flow of 2 gallons per minute with a static lift of 25 feet. If the Contractor uses jet-eductor pumps with a yield capac­ity greater than 3 gallons per minute, it will be neces­sary for him to redesign the pressure and return head­er lines. The pressure pumps and recirculation tank shall be designed by the Contractor. The pumps shall have adequate head and pumping characteristics to match the number and capacity of the jet-eductors be­ing used. Both the pressure and return header lines shall be provided with pressure gages at the pumps and across the excavation from the pumps. Overflow from the recirculation tank shall be arranged so that the flow from the wellpoint system can ye readily measured.

(c) The fdter sand to be placed around the well – points and up to within 10 feet of the ground surface shall be a clean, washed sand (A) having a gradation falling within the following range:

U. S. Standard

Filter Sand A

Sieve Size

Percent Passing

10

78-100

16

35-82

20

15-58

30

3-32

40

0-13

50

0-3

(d) The wellpoints shall be installed by a combi­nation of driving and jetting (with a hole puncher) a 12-inch “sanding” casing to approximately 1 foot be­low the top of the Cockfield formation, rinsing the “sanding” casing until there is no more sediment in the casing, centering the wellpoint and lowering it to the bottom of the hole, placing Filter Sand A in the casing around the wellpoint in a heavy continuous stream up to within 10 feet of the ground surface, and then fill­ing the remainder of the hole with a thick bentonitic grout or by pouring in a mixture of dry sand contain­ing 10 percent granular bentonite, up to the ground surface. Before the working day is over, all wellpoints installed that day shall be pumped with a small centri­fugal pump, capable of producing a 25-foot vacuum, until the effluent becomes clear. Jet water used for in­stalling the wellpoints shall be clear polished so that return of jet water up through the sanding casing is achieved before the casing has been driven to a depth of more than about 10 feet from the surface. It may be necessary or expeditious to prebore the holes for the

Wellpoint and pressure relief systems

wellpoints with a lo-inch auger prior to driving and jetting the 12-inch sanding casing to grade, Any such predrilling should be accomplished in a manner which will not cause any caving of the hole as it is drilled. In jetting the sanding casing for the wellpoints, provision shall be made to prevent jet water from spraying over sidewalks and streets being used for pedestrians and/or vehicular traffic, and from running over or into streets being used by such. The Contractor will be re­sponsible for disposal of jet water in a manner satisfac­tory to the City and the Construction Manager.

(e) Each jet-eductor wellpoint shall be provided with a suitable strainer at the top of the pressure riser pipe to prevent the entrance of any particles, which might clog the jet-eductor nozzle, and a stopcock or valves, which will permit isolating the wellpoint to permit pressure testing and pumping the wellpoint separately if desired,

(5) Pressure relief wells and pumps. The pressure relief wells shall be installed at the locations specified, These wells are to reduce the hydrostatic head in the semipervious stratum below the excavation to eleva­tion 250.0 feet or lower as measured by the B piezome­ters installed in this same stratum at the locations, as subsequently specified.

(a) The screens for the relief wells shall be 10 feet long and shall be installed in the semipervious stratum at about elevation 235±1 foot. The screens shall have a nominal diameter of 4 inches. They may be manufactured of slotted Schedule 40 PVC pipe, Johnson wire-wrapped screen, or other approved screen. The screens shall be covered with 30-mesh brass or stainless steel gauze or slotted with #20 slots with a minimum open area of 5 percent. The riser pipe shall also have a 4-inch nominal diameter; it may be either PVC plastic pipe or steel. The screen portion of the wells and up for a distance of 5 feet above the top of screen shall be surrounded with a clean, washed uni­form filter sand (B) with the following gradation:

U. S. Standard Filter Sand B

SieveSize Percent Passing

10 95-100

20 58-90

30 30-73

40 5-50

50 0-25

70 0-8

Suppliers who can furnish Filter Sands A and В are as follows:

Filter Sand Filter Sand

Supplier A B

(b) Pumps for the relief wells shall be submersi­ble pumps suitable for installation in 4-inch nominal diameter wells with a capacity of 3 to 10 gallons per minute at a total dynamic head (TDH) equal to 50 feet. The Contractor shall be responsible for designing and installing the electrical system for powering the sub­mersible pumps in the relief wells. The electrical sys­tem shall meet the local electrical code and be ap­proved by the Contracting Officer. Each pump shall be provided with a starter and fuse disconnect with a clearly visible red light mounted on the control panel that shows red when the pump is running. A motor generator with a capacity capable of starting all of the submersible pumps in the relief wells at one time shall be provided and hooked into the electrical system at all times. This generator shall be operated to keep the pumps in the relief wells running in event of power failure. It shall be maintained in first class condition, protected from the weather, and started at least twice a week to check its operating condition.

(c) The bottom of the screens for the relief wells shall be installed at or slightly below the bottom of the semipervious stratum as encountered during installa­tion. They shall be installed using the same procedure as specified for the wellpoints except that in all proba­bility it will be necessary to predrill the holes to grade with a lo-inch auger because of the difficulty in jetting through some of the clay strata to be penetrated. After the (12-inch) sanding casing has been driven and jetted to grade, the well screen and riser pipe shall be cen­tered accurately and lowered to within 1 foot of the bottom of the casing. Filter Sand B poured into the sanding casing around the well riser pipe in a heavy steady stream until the filter sand is at least 5 feet above the top of the screen; the remainder of the hole may be filled with clean medium sand. The sanding casing shall then be immediately pulled. After the well has been installed, it shall be promptly pumped with a centrifugal pump capable of producing a 25-foot va­cuum until the effluent is clear. The submersible pump then shall be lowered and suspended in the well with the intake of the pump at about the top of the well screen. The pump or discharge pipe shall be provided with a check valve, and the connection between the discharge pipe from the pump to header pipe shall be provided with a gate valve which will permit removal of the pump if necessary. The top of the well shall be sealed with a suitable well cap or plate provided with a hole through which a sounding line may be lowered to determine the water level in the well when testing and monitoring the system, A full-scale pumping test shall be run on the completed system to determine its ade­quacy and pumping rate. This test shall be run for a minimum of 6 hours. The water level in the B piezome­ters and in the wells and flow from the system shall be measured at the following intervals after the start of pumping: 10 minutes, 30 minutes, 1 hour, 2 hours, 3 hours, 4 hours, and 6 hours.

(d) All of the submersible pumps shall be new,

and one new spare pump and controls shall be at the job site at all times. The electrical system and controls shall be designed so that failure of any one pump, or the need to disconnect and replace a pump, does not adversely affect operation of any other pump.

(e) The Contractor shall be responsible for re­cording the results of the pumping tests on the well system and furnishing them to the Construction Man­ager. He shall also advise the Construction Manager at least three days in advance of making this test so that a representative of the Government can observe the test.

(/) Because of the importance of preventing any significant artesian head in the semipervious stratum above the bottom of the excavation, tees shall be in­stalled in the riser pipe for the deep wells to provide relief of this artesian pressure in event of any pump or electrical failure. These tees shall be provided with plugs during installation of the wells; as the excava­tion is carried down to each tee, the plug in that tee shall be removed and a Z-inch pipe with brass check valve inserted in the tee so that water can flow into the excavation. As each tee is connected into the excava­tion, the tee above shall be sealed.

(g) Installation of the wellpoint and the relief well systems shall be supervised by someone with at least 5 years actual experience in installing such sys­tems. A log of each relief well shall be maintained by the Contractor; forms for logging installation of the wells will be furnished by the Contracting Officer.

(6) Piezometers. Piezometers shall be installed at the locations specified to measure the groundwater ta­ble in the line of wellpoints and in the excavation (A piezometers), and to measure the hydrostatic water level in the semipervious stratum beneath the excava­tion (B piezometers). The tips of the A piezometers shall be set at the top of the Cockfield formation; the tips of the B piezometers shall be set in the middle of the semipervious stratum. The A piezometers shall be surrounded with Filter Sand A; the tips of the В pie­zometers shall be surrounded with Filter Sand B. The A piezometers shall be installed using the same equip­ment and procedure specified for installing the well – points; the B piezometers shall be sealed with an ex­panding cement bentonite grout up to at least eleva­tion 252.0 feet. After sealing around a B piezometer, the sanding casing shall be checked to see if there is any bentonite or cement in the casing; and if such ex­ists, the casing shall be thoroughly washed prior to in­stallation of the next piezometer.

(a) The piezometers shall consist of a 1.50-inch inside diameter (I. D.) (Sch 40 or 80) PVC screen with 0.025- to 0.030-inch slots connected to 1.50-inch I. D. (Sch 40 or 80) PVC riser pipe. Screens shall be 5 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 top of the riser shall be cut off 30 inches above the ground surface and provided with a threaded cap with a ‘&inch hole in the side. The up­per part of the piezometer shall be protected by install­ing a 6-inch I. D. cardboard casing around the riser em­bedded 2 feet below the ground surface and filling the casing with concrete. After removal of the cardboard casing, the top 30 inches of each piezometer shall be painted day-glo orange, and the piezometer number marked in 3-inch-high black characters.

Ф) Each piezometer shall be pumped after in­stallation and then checked to determine if it is func­tioning properly by filling with water and observing the rate of fall. For the piezometer to be considered ac­ceptable it shall pump at a rate of at least 0.5 gallon per minute, or when the piezometer is filled with wa­ter, the water level shall fall approximately half the distance to the groundwater table in a time less than the time given below for various types of soil:

Type of Soil Period of Approximate time

in which piezometer observation of 5Opercent

screen is wet minutes fall, 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 air surging or pumping with air if neces­sary to make it perform properly.

c. Availalble soil data. Generalized soil profiles and logs of boring were made for Government. Samples of

the soils from the borings made by_____________ are

available for inspection by bidders at____________

d. Damages. The Contractor shall be responsible and shall repair any damage to the excavation, including damage to the bottom due to heave, that may result from his or her negligence, improper operation of the dewatering and pressure relief systems, and any me­chanical or electrical failure of the systems.

e. Maintaining excavation unwatered and pressure relieved. Subsequent to completion and acceptance of all work in the excavated area, the Contractor shall maintain the excavation unwatered and the water lev­el in the B piezometers at or below elevation 250.0 feet until placement of the structural mat is complete and the backfill has been placed to elevation 256.0 feet, and a written directive to cease pumping has been re­ceived from the Contracting Officer. The Contractor shall be responsible for maintaining the excavation un­watered and pressure relieved during this period as set for above except for operation of the wellpoint system.

f. Removal of wellpoin tsys tern. After excavation to grade, installation of drainage ditch, sump pump, and placement of the “mud” mat, the wellpoint system may be removed, Removal of the wellpoint dewatering sys-

tern shall include pulling all of the wellpoints and re­lated header pipes, pumps, recirculation tank, and temporary electrical secondary, as approved by the Contracting Officer. Any holes remaining after pulling the wellpoints shall be filled with sand. Sump pump­ing of seepage and surface water thereafter shall con­tinue as required to keep the excavation in an unwa­tered condition until all structural concrete work and backfill are complete, and the succeeding Contractor has assumed responsibility for maintaining the exca­vation in an unwatered condition.

g. Method of measurement. Dewatering, as speci­fied in/above, to be paid for will be determined by the number of calendar days (24 hours), counted on a day – to-day basis, the excavation is maintained in an unwa­tered condition and pressure relieved, from completion and final acceptance of the concrete mat to the date on which a written directive to cease pumping operations is received from the Contracting Officer.

h. Unit price. Any pressure relief wells and related apurtenances required in addition to the specified pressure relief system shall be paid for at the unit price for “additional pressure relief wells.”