Section 259 — Soil Nail Retaining Walls
Description
259.01This work consists of constructing soil nail retaining walls.
259.02Definition. For soil nail retaining walls, a lift is defined as a stable height of unsupported excavation before application of shotcrete, typically midpoint between rows of soil nails.
Material
259.03Conform to the following Subsections: Bolts and nuts 717.01(d) Centralizers and spacers 722.02(e) Geocomposite drain 714.02 Neat hydraulic cement grout 701.04(a) Soil nails 722.04 Soil and soil- aggregate materials 704.01 Structural carbon steel (bearing plates) 717.01(a) (4) Welded stud shear connectors 717.05
Construction Requirements
259.04Qualifications. Provide a professional engineer, an on- site supervisor, and installation personnel with experience installing and testing soil nails. At least 30 days before starting soil nail retaining wall work , submit a résumé and copies of applicable certifications for each individual describing their experience on at least five soil nail retaining wall projects of similar complexity within the past 5 years for approval. Include project names, locations, and con tact information for project owners.
259.05Submittal s. At least 30 days before starting soil nail retaining wall work, submit the following according to Subsection 104.06:
(a)Start date;
(b)Method of excavation to ensure wall and slope stability;
(c)Retaining wall construction sequence;
(d)Soil nail types, sizes, spacings, depths, and installation angles, bond zone lengths, and ultimate tendon strengths for the range of material to be encountered; (e) Tendons, couplers, bearing plates, facing items, and additional hardware with the manufacturer’s product technical data sheets, specifications, catalog cuts, and mill certificates;
(f)Manufacturer’s recommendations for tendon and hardware handling, storing, assembly, and working temperatur e ranges;
(g)Grout type, mix design, mixing equipment, placement procedures, and 3 -day and 28 -day grout compressive strength test results;
(h)Procedures and material for repairing corrosion protection coatings in the field and for applying epoxy finish coatings on end hardware;
(i)Drilling methods and equipment;
(j)Drill hole diameter to achieve the specified pullout resistance;
(k)Alternative drilling and grouting methods (such as grout additives);
(l)Additional material needed to achieve required bond capacities (such as g rout socks);
(m)Methods to ensure borehole stability during excavation and grout placement;
(n)Soil nail testing methods and equipment. Include type and capacity of reaction load system, load test increments, drawings, and supporting calculations for structural components of the soil nail load test apparatus; and
(o)Identification number and calibration test certification for each jack, pressure gauge, and electronic load cell. Clearly indicate the serial number of each component of the testing assembly on calibrati on graphs. Submit results from calibration tests conducted by an independent testing laboratory within the previous 60 days.
259.06General. Conform to Section 257 for Contractor- designed soil nail retaining walls. Survey according to Section 152 and verify the limits of wall installation. Clear the work area of vegetation and obstructions according to Sections 201 and 203. Excavate according to Section 209 for the wall in staged lifts concurrent with soil nail installation and shotcrete placement. Do not allow the exposed unsupported final excavation face cut height to exceed the vertical soil nail spacing plus the required reinforcing lap or the short- term stand -up height of the ground, whichever is less. Do not excavate to the next lower lift until soil nail installation, reinforced shotcrete placement, attachment of bearing plates and n uts, and soil nail testing have been completed and approved for the current lift. Cure grout and shotcrete at least 72 hours or attain the specified 3- day compressive strength before excavating the next underlying lift. Application of the shotcrete may be delayed up to 24 hours if it can be demonstrated the delay will not adversely affect the excavation face stability. Handle and store material according to Subsection 256.06.
259.07Tendon Fabrication. Provide tendons designed to carry loads consistent with the approved drawings. Provide threaded tendon ends so they can withstand the approved test loads in addition to accommodating the attachment of the bearing plate, beveled washer, nut, or other appurtenances. Limit coupling of tendons to two per installation. Use couplers manufactured with a center stop to ensure equal length of thread connects each section. Do not use couplers that interfere with the flow of grout. Protect couplings from corrosion using the same corrosion protection used on the soil nail tendon assembly. Use centralizers to position the tendon within 1 inch of the center of the drill hole. Use centralizers that do not impede the free flow of grout into the drill hole. Position centralizers within 24 inche s from the top and bottom of the tendon and with center -to-center spacing of 10 feet or less. Secure centralizers to the tendon to prevent shift during handling or inserting into the drill hole.
259.08Installation.
(a)Drilling. Drill soil nail holes at the required locations and orientations. Use drilling equipment and methods suitable for the ground conditions. Do not use water, drilling slurry, or other fluids for drilling or removing cuttings. Insert the soil nail tendon into the hole. Clean or redrill holes if the tendon with centralizers cannot be completely inserted without forcing or driving.
(b)Grouting. Use a positive displacement grout pump according to Subsection 256.07(b)(1) . Grout tendons into holes using a neat hydr aulic cement grout. Mix grout as recommended by the cement supplier at the specified water -to-cementitious material ratio. Inject grout within 45 minutes of adding the cement to the water or within 15 minutes after mixing when the ambient temperature is 90 °F or higher. Do not allow the grout temperature to exceed 90 °F. Grout the hole within 2 hours of completing drilling. Inject the grout at the lowest point of each hole. Place primary and secondary grout stages in one continuous operation. Keep the outlet end of the grout tube below the surface of the grout as the tube is withdrawn to prevent voids. Control grout pressures to avoid ground heaving or fracturing. Record the quantity of injected grout and the grout pressure for each soil nail installation. A fter grouting is complete, fill the grout tube with grout if it will remain in the hole. Maintain the temporary unbonded length of proof test soil nails open for later grouting. If the unbonded test length of production proof test soil nails cannot be satisfactorily grouted after testing, install a new soil nail in its place.
259.09Testing and Stressing.
(a)Testing equipment. Provide equipment conforming to Subsection 256.08(a) .
(b)Stressing. Place stressing equipment over the soil na il so that the jack, bearing plates, load cells, and stressing assembly are axially aligned with the tendon and the tendon is centered within the equipment. Do not apply loads greater than 80 percent of the minimum guaranteed ultimate tensile strength of t he tendon for Grade 150 bars , or 90 percent of the minimum guaranteed ultimate tensile strength of the tendon for Grade 60 or 75 bars. Do not test soil nails until the soil nail grout and shotcrete facing have cured for at least 72 hours and attain ed the specified 3 -day compressive strength. Place the reference pressure gauge in series with the pressure gauge and jack so they need not be unloaded and repositioned during a test. Raise the load from one increment to another. Hold the load for the required time starting immediately after the load is applied and record the soil nail head movement to the nearest 0.001 inch with respect to an independent fixed reference point. Repump the jack as necessary to maintain a constant load. Monitor the load with a pressu re gauge. If the load measured by the pressure gauge and the load measured by the reference pressure gauge differ by more than 10 percent, recalibrate the jack, pressure gauge, and reference pressure gauge.
(1)Verification test. Perform sacrificial verification tests at locations shown in the plans or as directed. Provide verification test soil nails with a minimum unbonded length of 3 feet and a minimum bonded length of 10 feet. Grout with the same methods, equipment, and materials as will be used for the production nails. Use the following formula to determine the maximum bonded length for Grades 60 or 75 steel: LB VT max =At × f𝑦𝑦 × CRTY
RPO
Use the following formula to determine the maximum bonded length for Grade 150 steel: LB VT max =At × fu × CRTU
RPO
where: LB VTmax = Maximum verification test soil nail bonded length (ft ) CRTY = Reduction coefficient for mild grade steel = 0.9 for grades 60 or 75 bars CRTU = Reduction coefficient for high strength steel = 0.8 for grade 150 bars fy = Yield strength of test tendon for mild steel (psi ) fu = Ultimate tensile strength of test tendon for high -strength steel (psi) At = Cross -sectional area of the test tendon ( in2) RPO = Nominal pullout resistance (per unit length) of soil nail = π × q u × D dh Determin e the design test load by the following equation: VTL = L B VT × R PO where: VTL = Verification test load ( lb) LB VT = Bonded test length ( ft) as determined from test residual elongation Provide a larger bar size if necessary for safety. Test according to Table 259-1 and see Table 259-5 for acceptance criteria. Table 259-1
Verification Test Load Schedule
Test Load Increment Hold Time
(minutes)(2) AL(1) 1 (Record at 0, 1) 0.10VTL 10 (Record at 0, 1, 2, 4, 5, 10) 0.20VTL 10 (Record at 0, 1, 2, 4, 5, 10) 0.30VTL 10 (Record at 0, 1, 2, 4, 5, 10) 0.40VTL 10 (Record at 0, 1, 2, 4, 5, 10) 0.50VTL 10 (Record at 0, 1, 2, 4, 5, 10) 0.60VTL 10 (Record at 0, 1, 2, 4, 5, 10) 0.70VTL 10 (Record at 0, 1, 2, 4, 5, 10) 0.80VTL (Creep Test) 60 (Record at 0, 1, 2, 4, 5, 10, 20, 30, 50, 60) 0.90VTL or failure 10 (Record at 0, 1, 2, 4, 5, 10) 1.00 VTL or failure (Maximum acceptance load) 10 (Record at 0, 1, 2, 4, 5, 10) AL(1) 1 (Record at 0, 1)
(1)No greater than 2.5 percent of VTL (0.025VTL) applied to the soil nail before setting the movement recording devices. Zero dial gauges after the first setting of AL.
(2)Hold the load to within 2 percent of the test load increment and measure. Provide a written report confirming soil nail geometry, construction, testing details, and verification test results for approval before installing production soil nails.
(2)Proof t ests. The CO will designate production soil nails for proof testing, including 5 percent of the nails in a row and at least one per each day of production. Provide production proof test soil nails with a minimum temporary unbonded length of 3 feet and a minimum bonded length of 10 feet. Use the following formula to determine the maximum bonded length for Grades 60 or 75 steel: LB PT max =At × f𝑦𝑦 × CRTY 𝑅𝑅PO× 0.75 Use the following formula to determine the maximum bonded length for Grade 150 steel: LB PT max =At × fu × CRTU 𝑅𝑅PO× 0.75 where: LB PTmax = Maximum verification test soil nail bonded length (ft ) CRTY = Reduction coefficient for mild grade steel = 0.9 for grades 60 or 75 bars CRTU = Reduction coefficient for high strength steel = 0.8 for grade 150 bars fy = Yield strength of test tendon for mild steel (psi ) fu = Ultimate tensile strength of test tendon for high- strength steel (psi) At = Cross -sectional area of the test tendon ( in2) RPO = Nominal pullout resistance (per unit length) of soil nail = π × q u × D DH qu = Bond strength DDH = Drill hole diameter Determine the design test load by the following equation: PTL = L B PT × R PO × 0.75 where: PTL = Proof test load (lb ) LB PT = Bonded test length ( ft) as determined from test residual elongation Test according to Table 259-2 and see Table 259-6 for acceptance criteria. Table 259-2
Proof Test Load Schedule
Test Load Increment Hold Time
(minutes) AL(1) 1 (Record at 0, 1) 0.10PTL(2) Until movement stabilizes(3) 0.30PTL(2) " 0.50PTL(2) " 0.70PTL(2) " 0.80PTL(2) " 1.00PTL (Creep Test) 60(4) AL(1) 1 (Record at 0, 1)
(1)No greater than 2.5 percent of PTL (0.025PTL) applied to the soil nail before setting the movement recording devices. Zero dial gauges after the first setting of AL
(2)Times are measured at the and after the target load value has been achieved in each increment.
(3)Hold each load increment for 10 minutes and to record the soil nail movement at 1, 2, 5, and 10 minutes if the movement does not stabilize after 1 minute.
(4)Hold the load to within 2 percent and measure and record soil nail movement at 1, 2, 3, 5, 6, and 10 minutes. When the soil nail movement between 1 and 10 minutes exceeds 0.04 inch, continue measuring and recording soil nail movement at 20, 30, 50, and 60 minutes.
(c)Test results and reporting. Provide prel iminary results to the CO for each soil nail tested before testing personnel leave the site. Submit detailed verification and proof test load and deflection data in a tabular format. Submit a graph that plots total soil nail movement versus load, the A -line, and the B-line. The A -line is defined as 0.8 multiplied by the theoretical free test length elastic elongation. The B -line is defined as the theoretical free test length elastic elongation plus 0.50 multiplied by the theoretical bond length elastic elon gation. Allow 5 days for the CO to conduct a review of the data and approve ground anchor installation.
259.10Wall Drainage Network. Install required elements of the wall drainage network before applying shotcrete to each lift. Install geocomposite sheet drain system according to Subsection 605.05. Center geocomposite sheet drains between soil nail columns with the geotextile filter side against the ground. Install additional sheet drains at wet locations and as requested. Secure sheet drains to the excavated face to prevent shotcrete from contamina ting the ground side of the geotextile. Construct sheet drain splices according to the manufacturer’s recommendations. Install underdrains at the bottom of the wall according to Section 605.
259.11Wall Construction. Place welded wire steel mesh and reinforcing steel according to Section 554. Construct a shotcrete construction facing according to Section 566. Completely fill ungrouted zones of soil nail drill holes or other voids with shotcrete. Attach a bearing plate and nut to each soil nail head. Uniformly seat the plate by tightening the nut with a hand wrench while the shotcrete is still plastic. Where uniform contact between the plate and the shotcr ete cannot be provided, set the plate in a bed of grout and tighten the nut with a hand wrench after the grout has set for 24 hours. Construct wall elements to the tolerances shown in Table 259-3. Table 259-3
Wall Element Construc tion Tolerances
Wall Element Tolerance
Horizontal location of headed studs, from plan location ⅜ inch Location of headed studs on bearing plate, from plan location ¼ inch Soil nail head bearing plate, deviation from parallel to wall face 10 degrees
259.12Permanent Wall Facing. Construct the permanent wall facing according to the applicable Section below.
(a)Shotcrete -faced walls. Construct according to Section 566 and to the construction tolerances shown in Table 259-4.
(b)Concrete- faced walls. Construct according to Section 258.
(c)Simulated stone masonry. Construct according to Section 613.
(d)Stone masonry. Construct according to Section 620.
(e)Precast concrete panels. Construct according to Section 578. Use concrete class A(AE). Erect panels by lifting devices connected to the upper edge of the panel. Construct joints between panels according to the manufacturer’s recommendations. Table 259-4 Permanent Shotcrete Facing Construction Tolerances
Facing Finish Tolerance , inch
Complete thickness of shotcrete, from plan dimension: Troweled or screeded finish ±⅝ Shot finish ±1⅛ Planeness of finish face, surface gap under a 10 -foot straightedge: Troweled or screeded finish ⅝ Shot finish 1⅛
259.13Backfilling Behind Wall Facing Upper Cantilever Section. Backfill behind the wall with structural backfill according to Subsection 209.09. Use light mechanical tamper to compact within 36 inches behind the wall facing section. Do not compact behind cantilever until permanent facing concrete or shotcrete has cured at least 7 days.
259.14Acceptance. See Table 259-7 for sampling, testing, and acceptance requirements. Material for the soil nails will be evaluated under Subsections 106.03 and 106.04. Submit a production certification with each shipment of soil nails. Construction of soil nail retaining walls will be evaluated under Subsections 106.02 and 106.04. Clearing of vegetation will be evaluated under Section 201. Reinforcing steel will be evaluated under Section 554. Removal of obstructions will be evaluated under Section 203. Excavation and backfill will be evaluated unde r Section 209. Survey will be evaluated under Section 152. Underdrains will be evaluated under Section 605. Installed soil nails will be evaluated based on the criteria in Table 259-6. Verification test soil nails will be evaluated based on the criteria in Table 259-5. Table 259-5
Verification Test Soil Nail Criteria
Total Load -Test
Hold Movement
at 0.80 VTL Total Movement at
Maximum Load Pullout
Failure(2) < 0.080 inch between 6 and 60 minutes(1) A-line criteria: Greater than 80% of the apparent elastic elongation of the test soil nail unbonded length. No pull- out at 1.00 VTL B-line criteria: Less than the apparent elastic elongation of the test soil nail unbonded length and half of the bond length.
(1)And the rate of movement is linear or decreasing throughout the load- hold period.
(2)Pullout failure is the inability to further increase the test load while there is continued pullout movement of the test soil nail. Record the pullout failure load as part of the test data. If the soil nail verification is unacceptable, establish the cause and make design or construction modifications. Submit modifications requiring changes to the structure and retest the new system as directed. Table 259-6
Proof Soil Nail Acceptance Criteria
Total Load -Hold
Movement
at PTL Total Movement at
Maximum Load Pullout
Failure(2) < 0.040 inch between 1 & 10 minutes or < 0.080 inch between 6 & 60 minutes(1) A-line criteria: Greater than 80% of the apparent elastic elongation of the test soil nail unbonded length. No pull- out at 1.00 PTL B-line criteria: Less than the apparent elastic elongation of the test soil nail unbonded length & half of the bond length.
(1)And the rate of movement is linear or decreasing throughout the load- hold period.
(2)Pullout failure is the inability to further increase the test load while there is continued pullout movement of the test soil nail. Record the pullout failure load as part of the test data. If a proof -tested soil nail is unacceptable, replace some or all the installed production soil nails between the unacceptable proof test soil nail and the next proof test soil nail in the row as directed. Alternatively, install additional proof test soil nails within this area to ensure that the acceptance criteria are met within this area. Pr opose alternative installation and testing methods before installing additional soil nails. If ground conditions, soil nail installation methods, or testing procedures change, conduct additional verification testing before proceeding with production soil nail installation.
Measurement
259.15Measure the Section 259 pay items listed in the bid schedule according to Subsection 109.02 and the following as applicable: When measuring soil nail re taining walls by the square foot, measure the front wall face. When measuring verification test soil nails by the each, do not measure failed verification test soil nails or additional verification test soil nails installed to verify alternative soil nail installation methods proposed by the Contractor. When measuring production soil nails by the linear foot, measure along bar centerline from the line of the wall excavation face to the tip of the soil nail.
Payment
259.16The accepted quantities will be paid at the contract price per unit of measurement for the Section 259 pay items listed in the bid schedule. Payment will be full compensation for the work prescribed in this Section. See Subsection 109.05 . Table 259-7 Sampling, Testing, and Acceptance Requirements
Material or
Product
(Subsection) Type of
Acceptance
(Subsection) Characteristic Category Test Met hods
Specifications Sampling
Frequency Point of
Sampling Split
Sample Reporting
Time Remarks
Mix Design
Neat
hydraulic cement grout (701.04(a) ) Measured & tested for conformance (106.04 ) Compressive Strength(1) − AASHTO T 106 1 per mix design Source of material Yes 30 days before production −
Production
Neat
hydraulic cement grout (701.04(a) ) Measured & tested for conformance (106.04 ) Compressive Strength(1) − AASHTO T 106 1 per row or day(2) Source of material Yes 28 days −
Verification
test soil nail Measured & tested for conformance (106.04) Performance − Subsection
259.09(b)(1) Each soil nail Installed No 7 days −
Proof test
soil nail " " − Subsection
259.09(b)(2) " " " 5 days −
(1)At 3-day and 28 -day.
(2)If three successive samples are tested and compliance to the specifications is indicated, screening tests may be reduced to a n approved frequency.