PART 1 – GENERAL INFORMATION ABOUT SLURRY WALLS / CUTT-OFF WALLS / SLURRY TRENCHES

1. Bentonite. An ultra fine natural clay whose principal mineral constituent is premium-grade sodium cation montmorillonite.
2. Groundwater Level. The surface elevation of naturally occurring groundwater when subjected to atmospheric pressure. Water may be perched in discrete areas at elevations higher than the general groundwater level.
3. Fines. Fine grained soil particles, smaller than No. 200 U.S. Standard Sieve size.
4. Probing for Inspection. The lowering of an approved sounding instrument or appropriately marked device to demonstrate the depth of the trench and the trench bottom conditions.
5. Remote Mixing Area. A temporary working surface constructed at a remote location from the slurry wall alignment in which the soil bentonite backfill will be proportioned and mixed.
6. Slurry. A stable colloidal suspension of hydrated bentonite in water.
7. Slurry Ponds. Ponds constructed for the purpose of storing mixed slurry. Earthwork for slurry ponds must consist of fill only - excavation into the existing surface will not be permitted, unless approved on a case-by-case basis by the Owner/Engineer.
8. Slurry Trench. A vertical trench excavation, at least 3 feet wide, full of bentonite slurry to support the trench sidewalls.
9. Slurry Wall. A semi-impervious barrier of blended soil and bentonite, at least 2 or 3 feet wide, with a hydraulic conductivity of 1 x 10-6 centimeters per second (cm/sec) or less, constructed using a slurry trench during excavation and backfilling.
10. Working Surface. The prepared temporary working surface (pad) from which the slurry trench excavation, backfill mixing, and backfilling operations are performed.

1. Preconstruction. The Contractor shall submit the following items for approval to the Owner/Engineer at least 10 working days prior to commencing the Work:
   1. Bentonite Certification. A Certification of Compliance including test results from the bentonite supplier that each shipment of the bentonite meets the requirements of the Specifications and is in compliance with American Petroleum Institute (API) Standard 13A. The sources for all bentonite shall be included with the Certificate of Compliance.
   2. Sample. Five pounds of the proposed powdered bentonite.
   3. Identification of Water Source. Name of water source and provide chemical analysis, if source is other than potable water collected from a municipal hydrant.
   4. Detailed Work Plan. The detailed work plan shall include or describe, at least, the following:
      1. List of major equipment to be used.
      2. Specifications of the batch plant and slurry mixing including tanks, pumps, valves, hoses and supply lines.
      3. Layouts showing locations of equipment, ponds, remote mixing areas, access and haul roads.
      4. Material and equipment storage methods and locations.
      5. Procedures for water-bentonite slurry mixing, transportation, and recirculation.
      6. Procedures for control of drainage, spills, wastes, etc.
      7. Sequence and schedule for temporary working surfaces, haul roads, remote mixing and backfill transportation, as necessary, working surface removal, and restoration.
      8. Procedures for trench excavation, slurry placement, backfill mixing and backfill placement.
      9. Material properties (gradation and moisture content) and sources for materials to be used to augment the fines content of the backfill mixture.
      10. Disposal procedures, disposal facilities and haulers for excess slurry.
   5. Quality Control Program. Details of the Contractor's quality control program, including personnel, organization, responsibilities, sampling and testing equipment and frequencies, inspections and the quality control report forms.
   6. Bar-Chart Construction Schedule. The Work shall not commence prior to approval of the preconstruction SUBMITTALS by the Owner/Engineer.
2. During Construction. The Contractor shall submit for approval during construction:
1. The Contractor shall prepare and submit Quality Control Test Results of field quality control tests and inspections within 3 calendar days of performance, unless otherwise directed by the Owner/Engineer. The results shall be reported on forms accepted by submittal.
2. A Certification of Compliance from the supplier shall accompany each shipment of bentonite to the site.
3. Requests for revisions to the bentonite source, water source, work plan, quality control plan, or bar chart schedule shall be submitted to the Owner/Engineer. Approval must be received from the Owner/Engineer prior to implementing any of the revisions described above.
4. For any approved material for the backfill mixture, the Contractor shall submit:
1. Gradation test results in accordance with ASTM D422, at a frequency of 1 test per 4,000 cubic yards and if the material characteristics change, and
2. Atterberg limits test results in accordance with ASTM D4318, at a frequency of 1 test per 4,000 cubic yards and if the material characteristics change, and
3. Moisture content test results in accordance with ASTM D-2216, at a frequency of 1 test per 4,000 cubic yards and if the material characteristics change.
5. The Contractor shall maintain quality control and construction SUBMITTALS up to date. If the Contractor for whatever reason does not submit acceptable quality control data in a timely manner, the Owner/Engineer may stop the associated work without a contract extension.

1. General. The Contractor shall establish and maintain records of quality control for all slurry wall construction operations to assure compliance with contract requirements. The materials to be tested, test methods, sample and/or test locations, frequencies, test specifications and failure response actions for the quality control program are summarized in Table _____. The test procedures for the Modified Filter Press test are included in Attachment _____in Appendix ___.
2. Sample and Test Location References. All quality control samples and testing shall be referenced to the trench station (or baseline station) and depth at which the sample was obtained or the test was conducted.
3. Surveillance of Excavation. The Owner/Engineer will be present, whenever trench excavation is near the design depth of the wall, to visually check and classify soil excavated from the proposed maximum depth of trench excavation. Cuttings from the bottom of the trench shall be examined on a continuous basis to ensure continuity. The Owner/Engineer shall document the soil classification of samples obtained from the bottom of trench. The Contractor shall provide sufficient notice of excavation and allow sufficient time during the excavation cycle for observation and sampling of the soil as it is removed from the trench.
4. Trench Depth. The Contractor, in the presence of the Owner/Engineer, shall provide and place a 36-inch minimum length wood lath parallel to the trench. Sound or probe the trench with a measuring devise with incremental markings at 0.25-foot increments. The trench depth measurement shall be rounded (up or down from the measurement) to the nearest 0.5 foot, and record the depth every 20 feet along the trench centerline. The Contractor shall measure and record the depth to the bottom of the trench excavation immediately after excavating and immediately before backfilling to document the depth and silt conditions at the bottom of the trench.
5. Assistance from Contractor. The Owner/Engineer reserves the right to make reasonable measurements as deemed necessary to inspect and evaluate the as-built condition of the wall. The Contractor shall cooperate with the Owner/Engineer during performance of these measurements, without claims for extras or delays. The Contractor shall provide the Owner/Engineer, at no additional cost, with the following:
   1. Labor and materials to measure the lines and grades of the temporary working surfaces and the bottom of the trench excavation under the observation of the Owner/Engineer.
   2. Labor and materials required to collect samples and test samples for quality assurance tests.
6. Equipment Calibration. The test equipment shall be calibrated in accordance with applicable API and ASTM standards.

PART 2 – MATERIALS AND EQUIPMENT USED WHEN CREATING A SLURRY WALL / CUTT-OFF WALL / SLURRY TRENCH

1. Bentonite
   1. The bentonite for use in the slurry trench shall be unadulterated powdered premium-grade Wyoming sodium montmorillonite, or equal, as approved by the Owner/Engineer.
   2. The bentonite, to be delivered, shall be tested by the Manufacturer and shall meet the following requirements as determined from a mixture of bentonite and distilled water prepared in accordance with API Standard Specifications, 13A, latest revision, and tested in accordance with API Code RP 13B, latest revision:
      1. Apparent viscosity: 15 centipoise minimum @ 68 degrees Fahrenheit
      2. Filtrate loss: 20 cc max in 30 minutes @ 100 psi
      3. Yield: 90 Barrels minimum
   3. The use of chemically treated bentonite will not be permitted.
2. Water
   1. Water shall be free of excessive amounts of deleterious substances, as determined by the Owner/Engineer, that could adversely affect the properties of the slurry or backfill.
   2. The water shall comply with the following:
      1. pH 6 to 8
      2. Hardness <50 ppm
      3. Low dissolved salts, oil and total organic residue to allow proper hydration of bentonite
3. Initial Slurry Mixture
   1. At the time of introducing slurry into the excavations, the slurry mixture shall consist of:
      1. A minimum of 5.5 % bentonite per unit weight of water (range 5.5 to 7%) with a minimum unit weight of 64 poundsper cubic foot;
      2. An apparent viscosity not be less than 12.5 centipoise (36 Marsh seconds) at 68 degrees Fahrenheit as measured by the direct indicating viscometer; and
      3. A filtrate loss not be greater than 20 cubic centimeters in 30 minutes at 100 psi as measured by the filter press.
   2. The Contractor shall add additional bentonite to make the slurry denser or more viscous than the limits specified above, if deemed necessary by the Owner/Engineer.
   3. Admixtures to alter the characteristics of the slurry in the trenches, including but not limited to softening agents, dispersants, retarders or plugging or bridging agents, shall not be permitted unless approved by the Owner/Engineer.
4. Slurry Mixture in Trench.The slurry mixture in the trench shall meet all of the following requirements:
   1. A unit weight greater than 64 pounds per cubic foot;
   2. A unit weight at least 15 pounds per cubic foot less than the backfill unit weight;
   3. A unit weight less than 85 pounds per cubic foot, or as approved by the Owner/Engineer;
   4. A filtrate loss not greater than 30 cubic centimeters in 30 minutes at 100 psi as measured by the filter press;
   5. An apparent viscosity not less than 15 centipoise (40 Marsh seconds) at 68 degrees Fahrenheit as measured by the direct-indicating viscometer; and
   6. A sand content less than 15-20 percent as measured by API RP-13B, Sand Content Kit.
5. Off Site Materials for Slurry Wall Backfill
   1. Materials shall be from a source approved by the Owner/Engineer.
   2. Materials shall be free of roots, organic soil, lumps, trash and debris.
   3. Materials needed to increase the fines content of the backfill mixture shall contain at least 65 percent fines (minus 200 sieve) by weight, a Plasticity Index of or greater than 10 and have a moisture content less than 15 percent, measured prior to loading in the truck at the borrow pit, unless approved by the Owner/Engineer on a case-by-case basis.
6. Backfill
   1. Materials for the backfill mix shall consist of slurry, dry bentonite, if necessary excavated trench materials, and approved off-site soil.
   2. Backfill shall be free of roots, organic soil, lumps, trash and debris.
   3. Frozen materials shall not be used.
   4. The backfill mixture shall have the following gradation limits:

      % U.S. STD	Passing Sieve No.
      100%	3"
      70-100%	#4
      >30 %	#200

      However, occasional lumps of up to 5 inches in their largest dimensions may be permitted provided that the lumps are not close together and, in the Owner/Engineer's opinion, will not affect the performance of the slurrywall.
   5. If material from off site is needed to increase the fines content of the backfill mixture, the fines shall not exceed 45 percent, unless approved by the Owner/Engineer on a case-by-case basis.
   6. The backfill mixture shall have a consistency that has the appearance of a wet concrete and a slump of 3 to 5 inches, just prior to placing.
   7. The density of the backfill shall be a minimum of 15 pounds per cubic foot more than the slurry at any depth in the trench.
   8. The five test running average hydraulic conductivity of the backfill shall be 5 x 10-7 cm/sec, or less. The maximum permissible value shall be 1 x 10-6 cm/sec as measured by the modified filter press test.
   9. The Contractor shall vary the bentonite content and mixing operations periodically, based on varying in-situ soil properties, to achieve the specified hydraulic conductivity value.

1. Trench Excavation. Equipment for excavating the slurry trenches shall consist of a cable clam shell and/or hydraulic excavator. The equipment shall be capable of excavating the minimum required trench width of 3 (or 2) feet in a single pass of the excavating tool. The equipment shall be capable of excavating at least 5 feet deeper than the maximum depth shown on the drawings. If used, dragline buckets shall be free of protrusions.
2. Slurry Mixing. The Contractor's slurry plant shall include a suitable mixer capable of producing a colloidal suspension of bentonite in water, plus necessary pumps, sumps, valves, lines, hoses and storage tanks or ponds to provide a continuous supply of slurry.
3. Cleaning Slurry. The method of cleaning used for removal of suspended solids and bottom sediments from the slurry in the trench shall be capable of maintaining the slurry density within the specified limits. Equipment shall include clamshell, air lift pumps, mud shakers, settlement ponds, pipes hoses, and fittings, or other equipment approved by Owner/Engineer.
4. Mixing and Placing Backfill. Equipment for mixing and placing the backfill shall be any type of earthmoving or grading equipment, such as bulldozers, disk harrows, and blade graders, or blenders and pug mills that are capable of thoroughly mixing the backfill material (soil and bentonite/bentonite slurry) into a homogeneous mixture as specified. Placement of initial backfill in the trench may require a clamshell bucket.
5. Quality Control. The Contractor shall furnish and maintain the following minimum equipment (one half of the amount of each type of equipment is for exclusive use by the Owner/Engineer). The equipment shall be in accordance with applicable API Standard Specifications. All equipment used by the Owner/Engineer shall be returned to the Contractor upon completion of the slurry trench Work.
   
   1. Marsh Funnel Sets (2)
   
   2. Direct Indicating Viscometer, hand crank (2)
   
   3. Standard Filter Presses (4) with the following modifications for hydraulic conductivity tests:

a. Regulators and gages which have a range of 1 to 10 psi for application of low pressures.

b. A trimming jig to control thickness of bentonite paste to be applied to the inside diameter of the rigid wall test cylinder.

c. Porous stone filters and filter papers (adequate supply).

d. A 20 cc graduated cylinder with accuracy of 1/2 cc.

4. Regulators and gages which have a range of 100 psi +/- which may be adapted to the Standard Filter Press for Slurry Filtrate Loss testing. (2)

5. Mud Balances (direct reading of density) (2)

6. Sand Content Sets (2)

7. Slurry Samplers (2)

8. Sets of U.S. Standard Sieves (No. 4, 16, 40, 70, 200 and 3" size) (2)

9. Pressure sources and feed lines (2)

10. Dry bentonite and Ottawa sand (adequate supply)

11. Analytical scales (accuracy: 0.01 grams) (2)

12. Drying ovens (2)

The Contractor shall also furnish a measuring devise with 0.25-foot incremental markings, or approved equivalent, for determining the depth of trench and sediment accumulations.

PART 3 – EXECUTING THE CONSTRUCTION OF A SLURRY WALL / CUTT-OFF WALL / SLURRY TRENCH

1. General. The Contractor shall prepare the temporary working surface in accordance with Section ____ - Excavation, and Section ____ - Fill, of the Specifications. The Work shall include stripping topsoil and organic materials from the area of the temporary working surface, clearing trees, excavating soil, salvaging and stockpiling of selected materials, disposing of surplus and/or unsuitable material, furnishing and placing embankment or fill materials, trimming the earth grade and maintaining the Work as required.
2. Grade. The longitudinal grade of the working surface shall not exceed 1 percent, unless approved by the Owner/Engineer. The level of the temporary working surface shall be at least 3 feet above the measured groundwater level. The temporary working surface shall be graded to ensure excavation equipment produces a vertical trench. The Contractor shall provide and maintain gravity drainage control along the temporary working surface.
3. Maintenance. The Contractor shall maintain the working surface free of excessive amounts of debris and slurry.
4. Remote Mixing Area. The Contractor shall prepare and maintain, as needed, a temporary working surface at a remote location from the slurry wall alignment in which the soil bentonite backfill will be proportioned and mixed. The areas available for remote mixing are identified on the Drawings. The Contractor may request to the Owner/Engineer for additional areas. The request for additional areas may or may not be approved by the Owner/Engineer.

1. Slurry Preparation. The Contractor shall prepare slurry using a flash or paddle mixer to provide stable colloidal suspension of bentonite in water. Freshly mixed slurry shall be allowed to hydrate initially in storage ponds or tanks. The slurry shall be occasionally agitated or recirculated in the storage ponds until bentonite is fully hydrated, based on stabilized Marsh Funnel viscosity readings, and until the slurry appears homogeneous. No slurry is to be made in the trench. The slurry shall meet the requirements set forth in Paragraph ________, Initial Slurry Mixture, before the slurry is introduced into the trench.
2. Slurry Placement. Slurry shall be introduced into the trench at the time excavation begins. The level of the slurry in the open trench shall be maintained at all times at least 3 feet above the groundwater level and no more than 2 foot below the temporary working surface, unless approved by the Owner/Engineer. Dilution of the slurry by surface water shall be minimized. The slurry shall be maintained at all times in a condition which meets the requirements set forth in Paragraph _____, Slurry -Mixture in Trench. The Contractor shall maintain the level of the slurry in the trench at all times, including weekends, nights and holidays.
3. Slurry Cleaning. The Contractor shall remove, from the trench, any slurry which contains excessive suspended solids as indicated by a slurry unit weight exceeding 85 pounds per cubic foot or within 15 pounds per cubic foot of the unit weight of the backfill mixture. The excessive suspended solids may be separated and removed from the slurry, and the "desanded" slurry may be re-introduced into the trench; or the slurry with the excessive suspended solids may be used for the preparation of the backfill mixture.
   
   If the density of the slurry becomes greater than 85 pounds per cubic foot, the slurry shall be removed by methods approved by the Owner/Engineer and the excess ,solids shall be removed by screening or by a centrifugal-type desander.

1. The Contractor shall take all precautions in conduct of work as may be necessary to avoid disturbance or damage to existing utilities and other structures.
2. The excavation equipment shall remain at least 20 feet away from overhead utilities.
3. The trench excavation shall be at least 2 feet wide.
4. The trench excavation shall start at the _______________ comer of the site and proceed from the station _______ to the ___________station.
5. Excavation of a lead-in trench is required if placement of the initial slurry is to be accomplished without the use of a clamshell. The lead-in trench shall commence far enough away from the cutoff wall and ramp down to the full depth of the cutoff wall at a slope of 1.5 horizontal to 1 vertical to avoid segregation of backfill particles.
6. The trench excavation shall be continuous to the depths indicated on the Plans or as directed by the Owner/Engineer. Actual depth of the wall will be determined based on visual observation of the trench cuttings. The wall shall be keyed into the dense base layer material at least 1 foot or as directed by the Owner/Engineer.
7. The entire depth of excavation shall be carried along the trench line. Prior to backfilling any portion, and before the excavator starts his next cut, the Contractor shall pass the excavating tool along the completed section of the trench excavation to confirm continuity.
8. The toe of the slope of the trench excavation shall not advance beyond the backfill slope by more than 200 feet.
9. The excavation shall be continuous from one end to the other, unless approved by the Owner/Engineer. If for some reason it becomes necessary for the slurry wall to be constructed in phased segments, some re-excavation of the previously constructed phase of the slurry wall will be necessary. This re-excavation shall consist of backfill removal and reconstruction of a minimum horizontal length of 10 feet of slurry wall over the entire depth of the wall.
10. The Contractor shall maintain trench stability to its full depth at all times. Sloughs, slides or any instability shall be re-excavated and/or repaired as approved by the Owner/Engineer.
11. Stockpiling. Material excavated from slurry trenches that is suitable for the backfill mixture may be stockpiled adjacent to the trench for subsequent processing.
12. Cobble and Boulder Removal. The Contractor shall excavate all cobbles and boulders as necessary along the slurry wall alignment and dispose of them at an on-site location -as directed by the Owner/Engineer.

1. Mixing
   1. Mixing and blending shall be performed in such a manner as to produce the required backfill as specified in Paragraph _____, Backfill.
   2. The-Contractor shall make every effort to cooperate with the Owner/Engineer in developing and enacting a mixing technique to satisfy the hydraulic conductivity criterion.. The Contractor shall, if directed by the Owner/Engineer, employ one or more of the following techniques to satisfy the hydraulic conductivity criterion:
      1. Use a higher percentage of clay and silt materials in the backfill by separating and spoiling the excavated sandy layers.
      2. Add dry bentonite, up to 3.5 percent by dry weight, to the backfill. Bentonite shall be spread over 6 to 8 inch thick layers of soil backfill material and mixed thoroughly.
      3. Add more bentonite to the slurry mixture.
      4. Use material from an approved off-site area with a high percentage of clay and/or silt in the backfill by importing material.
   3. Stockpiled material from excavation and off-site materials for slurry wall backfill shall be thoroughly mixed and blended into a homogeneous mass, free from large lumps or pockets of fines, sand, or gravel, by windrowing, disk harrowing, bulldozing, blading, or by other methods such as a pug mill, as approved by the Owner/Engineer.
   4. Slurry may be sluiced into the backfill mixture during blending operations to obtain the required consistency and slump. Sluicing with water will not be permitted.
   5. Excess slurry that may drain away from the mixing operations shall be allowed to drain back into the trench or shall be collected if mixing is done at a remote location. In no case should excess slurry flow outside of the construction limits.
   6. The Contractor may be required to add dry bentonite to the backfill material by broadcasting, pug mill or other methods approved by the Owner/Engineer to achieve the desired hydraulic conductivity value.
   7. If the backfill material is mixed adjacent to the trench, the Contractor shall construct a small dike (0.5-1 foot high) parallel to the slurry trench, if required by the Owner/Engineer, in order to keep the backfill from flowing uncontrolled into the trenches as a result of wave action created by the dozer mixing the backfill. Intermittent openings in the dike will be allowed so that excess slurry may flow back into the trenches.
   8. The Contractor shall mix backfill at a remote mixing area if conditions are not acceptable to the Owner/Engineer along side of the trench.
   9. No mixing of backfill material shall be performed when the air temperature is continuous below 25 degrees Fahrenheit unless otherwise approved by the Owner/Engineer.
   10. Mix only the quantity of backfill that is needed to construct the wall.
2. Placing
   1. The backfill mixture shall be in compliance with the backfill requirements, immediately prior to introduction into the trench.
   2. The backfill shall be placed so that no pockets of slurry are present in the completed slurry wall. The backfill shall not be deposited in any manner that will cause segregation. Dropping of backfill material into the slurry will not be permitted.
   3. The Contractor shall backfill continuously in the direction of the excavation from the beginning of the trench to the end of the trench, unless approved by the Owner/Engineer.
   4. If a lead-in trench is not excavated, the initial backfill shall be placed by lowering the material to the bottom of the trench by means of a clamshell bucket or other approved equipment until the backfill emerges from below the slurry surface and achieves its natural angle of repose from the bottom of the trench to the surface.
   5. Placing operations shall proceed in such fashion that the top of the backfill below the surface of the slurry shall follow a reasonably uniform grade and shall not have hollows that may trap pockets of slurry during subsequent backfilling. The bulldozer operator shall pile sufficient backfill on the edge of the existing backfill to cause the backfill to enter the trench by sliding down the forward face of the previously placed backfill.
   6. The toe of the backfill slope shall not be less than 30 feet following the toe of the excavation to permit proper cleaning.
   7. Additional backfill material shall be placed on top of the trench to accommodate settlement.
   8. No placing of backfill material shall be performed when the air temperature is continuously below 30 degrees Fahrenheit unless authorized by the Owner/Engineer.

1. Temporary Protective Layer. A 2 foot temporary protective layer of excess slurry shall be placed over the top of the backfill mixture, after the backfill reaches the top of the slurry trench and before drying out of the backfill can occur.
2. Final Protective Layer. The Contractor shall not cap the slurry wall until approved by the Owner/Engineer. Excavation for the final slurry trench shall not be accomplished until that portion of the completed trench has been in place a minimum of 7 days, except for the areas where traffic access must cross the wall.

PART 4 - MEASUREMENT AND PAYMENT FOR THE SLURRY WALL / CUTT-OFF WALL/ SLURRY TRENCH