7-185 Section 718. Drilled Shafts
718.01 Description
This work consists of providing and constructing drilled shaft foundations.
718.02 Materials
Provide materials in accordance with the following sections : Steel Reinforcement ............................................................................. 905 Casing ................................................................................................... 919 Concrete, Grade 3500 ........................................................................ 1004 Provide steel reinforcement meeting the yield strength shown on the plans . Provide Concrete, Grade 3500 for Drilled Shafts . Provide slump for wet and dry placed drilled shafts in accordance with Table 1004-1. Provide concrete with natural aggregate with a maximum absorption of 2.50% in accordance with ASTM C127. Do not use slag aggregate.
718.03 Construction
Review available soil bori ng logs from subsurface investigations . If, during construction, actual subsurface conditions differ substantially from those reported on the boring logs, notify the Engineer in writing within 48 hours of identifying the discrepancy. The complete geotechnical report, outlining the subsurface exploration conducted during the design phase, is available for review before bidding. If desired, perform additional soil test borings and other exploratory procedures at no additional cost to the Department.
A.Drilled Shaft Installation Plan. Submit an installation plan to the Engineer for review and approval at least 21 days before beginning drilled shaft installation. The installation plan must include the following:
1.Proposed equipment, including cranes, drills, augers, core barrels, bailing buckets, final cleaning equipment, slurry pumps, tremie tubes or concrete pumps, and casings;
2.The construction sequence;
3.Shaft excavation methods, including proposed excavation methods through supporting and c aving soil layers;
4.Methods to mix, circulate, and de-sand slurry;
5.Methods to clean shaft excavation; MDOT Standard Specifications for Construction Section 718 7-186 6. Reinforcement placement, including support and centering methods;
7.Concrete placement, including free fall, tremie tube, or concrete pumping procedures;
8.Methods to prevent drilled shaft excavation spoils and slurry from entering waterways, wetlands , floodplains and sewers ; and
9.Other information shown on the plans or requested by the Engineer. The Engineer will evaluate the drilled shaft installation plan and notify the Contractor within 7 days of receiving the plan of additional required information and changes to meet contract requirements. The Engineer will reject unacceptable parts of the plan. Resubmit changes to the Engineer for reevaluation. Drilled shaf t installation plan procedures are subject to trials in the field. The Contractor must develop and follow a fall protection plan conforming to the MIOSHA Construction Safety Standards, including a rescue plan for shafts with a diameter of at least 30 inches and a depth of at least 6 feet. The Engineer ’s approval does not relieve the Contractor of completing the work and is not caus e for extra compensation if construction methods or equipment do not provide a satisfactorily drilled shaft. The Engineer ’s review and approval of the Contractor ’s drilled shaft installation plan will be made in accordance with subsection 104.02.
B.General M ethods and Equipment . Excavate for the drilled shaft to the dimensions and elevations shown on the plans through encountered materials . Use methods and equipment for the intended purpose and materials encountered. Control operations to prevent damage to existing structures and utilities . Use preventive measures including the selection of construction methods and procedures that prevent caving of the shaft excavation and monitoring and controlling the excavation depth. Repair damage to existing structures or utilities to the satisfaction of the Engineer, including engineering analysis and redesign, without extending the project completion dates and at no additional cost to the Department . Obtain the Engineer ’s approval for the selected general method.
1.Dry Construction Method . Use the dry construction method at sites where the groundwater table and site conditions allow shaft construction in dry excavation and where the sides and bottom of the shaft remain stable without caving, sloughing, squeezing, or swell ing. Make the shaft available to the Engineer to visually inspect before concrete placement. MDOT Standard Specifications for Construction Section 718 7-187 Excavate the drilled shaft hole, remove accumulated water and loose material, and place the shaft concrete in a dry excavation. Do not use the dry construction met hod if the flow of water into the excavation causes the height of water in the excavation to increase at a rate exceeding 12 inches per hour . Do not place the initial concrete if the depth of water in the bottom of the excavation exceeds 3 inches.
2.Wet Cons truction Method . Use the wet construction method at sites where dry excavation cannot be maintained during shaft concrete placement . Use water or slurry to contain seepage and groundwater movement and place concrete using a tremie tube or concrete pump. Maintain the stability of the excavation perimeter while advancing the excavation to the final depth, placing the steel reinforcement cage, and placing the shaft concrete . De-sand and clean slurry if used. Maintain a water or slurry fluid elevation higher than the static water table during drilling operations and inside drilled shafts not connected into the bedrock. Provide temporary surface casings for shaft alignment and position and to prevent sloughing of the top of the shaft excavation unless otherwise approved by the Engineer . Extend surface casings to an elevation in the shaft excavation that prevents sloughing of the surrounding soil.
3.Dry Temporary Casing Method . Use the dry temporary casing method where caving soils occur but casing can maintain a dry and stable excavation. Install a temporary casing in dry soil conditions through the caving soils to the bottom of shaft. I nstall a temporary casing to an impermeable stratum if groundwater is present . Remove excess water and soil from the casing. Advance the casing and excavation simultaneously . Do not drill outside or below the casing through caving soil layers. The bottom of the excavation must remain dry and stable until placement of the steel reinforcement and concrete. Withdraw the casing while the concrete is workable. Increase the level of fresh concrete in the casing before withdrawing casing to cause the upward displacement of fluid behind the casing.
4.Wet Temporary Casing Method . Use the wet temporary casing method where caving soils occur and a dry excavation cannot be maintained, the soil profile is permeable, and the groundwater elevation is higher than the bottom of the shaft elevation. Install the casing through caving soils to the required bottom of shaft elevation, and drill the excavat ion to the required dimensions . Advance the MDOT Standard Specifications for Construction Section 718 7-188 casing and excavation simultaneously . Do not drill outside or below the casing through caving soil layers. Maintain a positive pressure differential between the fluid level in the excavation and the groundwater elevation during drilling, excavation, and clean out . Place steel reinforcement and place concrete using a pump or tremie tube to the bottom of the excavation. Displace water inside the casing with concrete. Do not pump water out of the casing. The wet temporary casing method may include drilling slurry . Perform final cleaning of the entire bottom of excavation with a clean -out bucket . Increase the level of fresh concrete in the casing before and during casing removal to cause the upward displacement of flui d behind the casing without contaminating or displacing the shaft concrete.
5.Construction Method Log . Submit a construction method log to the Engineer each day during drilled shaft excavation and construction. Include the following information in the log fo r each drilled shaft:
a.Date (start date and completion date);
b.Drilled shaft identification number;
c.Location;
d.Actual top and bottom elevation of drilled shaft;
e.Shaft diameter;
f.Centerline location measured at the top of shaft ;
g.Variation of centerline from plumb ;
h.Top and bottom elevation and diameter of any casing;
i.Description of each soil and rock material encountered during excavating and the top and bottom depths or elevations;
j.Depth drilled into bearing stratum;
k.Top and bottom elevations of obstructions enc ountered;
l.Amount of obstruction time, if any ;
m.Depth or elevation of encountered seepage or groundwater;
n.Record of the actual volume of concrete placed with the theoretically calculated concrete volume to detect any large voids or intrusions of extraneous material; and
o.Remarks. MDOT Standard Specifications for Construction Section 718 7-189 C. Construction Tolerances . Drilled shafts constructed outside the following tolerances will be rejected. Correct unacceptable shaft excavations , reinforcement cages , and completed shafts to the Engineer ’s satisfaction. Complete corrections for out -of-tolerance drilled shafts, including engineering analysis and redesign, at no additional cost to the Department and with no extension to the project completion dates.
1.Horizontal Alignment . The drilled shaft top must be within 3 inches of the position shown on the plans measured at the centerline of the shaft.
2.Plumb . The out of plumb at the drilled shaft actual bottom elevations must be no more than 1% of the drilled shaft l ength, as measured horizontally from the actual center of the shaft at the shaft design top elevation.
3.Reinforcing Steel . Place the steel reinforcement cage to within 1 inch above and 3 inches below its planned location and secure in position.
4.Top of Shaf t Elevation . Finish t he top of the shaft to within 1 inch above and 3 inches below its planned elevation.
D.Casings. Case shaft excavations as shown on the plans . Provide smooth, watertight, metal casings capable of withstanding handling, installation, and t he pressure of concrete and surrounding earth materials . Provide a casing with an inside diameter at least the size of the shaft . Install casings such that intimate contact with the surrounding earth or rock is maintained. Cut off permanent and temporary c asings to be left in place at the elevation shown on the plans . Remove temporary casings. Attach fixtures to the top of the casings to aid in removing temporary casings . Remove temporary casings while the concrete remains workable. Complete concrete placement in the shaft before removing temporary casing. Extract casings slowly with the pull in-line with the shaft axis. Do not apply forces that induce moments in the shaft, detrimental to the concrete. If the top -of-shaft elevation is below gr ound during concrete placement, use a temporary casing to prevent material from caving into fresh concrete. Maintain a head of concrete above the bottom of the casing to overcome hydrostati c pressure for cased shafts . Monitor the concrete level in the casing during extraction. Stop the extraction and add concrete to maintain a full shaft upon casing removal.
E.Slurry . If using slurry in the drilling process , use a polymer -type slurry with viscosity and gel characteristics capable of transporting excavated MDOT Standard Specifications for Construction Section 718 7-190 material to a screening system or settling tank . The percentage and specific gravity of the material making the suspension must maintain the stability of the excavation and allow concrete placement . Maintain the height of the slurry capable of preventing the excavation sides from caving and the excavation bottom from heaving. Premix the slurry with clean, fresh water and allow time for hydration before introducing into the shaft excavation. Agitate, circulate, and adjust the properties of the slurry to prevent slurry from “setting up ” in the shaft excavation. Perform control tests using an apparatus on the slurry to determine density, viscosity, and pH in accordance with American Petroleum Institute (API) standards . The density, viscosity, and pH values must meet the ranges specified in Table 718- 1. Table 718- 1: Polymer Slurry Specifications Range of Values at 68°F Property Test Method Emulsified Polymer At Time of Slurry Introduction At Time of Concreting in Excavation Density, lb/ft 3 Mud weight ( density ) API 13B -1, Section 1 <63 <63 Viscosity, s/qt Marsh funnel c one and c up API 13B -1, Section 2.2 33–43 33–43 pH pH paper or meter 8–11 8–11 Maximum contact time (h our) — 72 72 Sand content API 13B-1, Section 5 <1% <1% If de-sanding is required, do not allow the sand content to exceed 1 % by volume in the shaft excavation, as determined by the API sand content test. Determine density, viscosity, and pH values before and during the shaft excavation to establish a consistent working pattern. Use a Department -approved slurry -sampling tool to take slurry samples from the bottom, and at mid- height, of the shaft before placing shaft concrete. Remove heavily contaminated slurry accumulated at the bottom of the shaft. Control and dispose of slurry exiting the excavation. Contain slurry in the excavation and remove as it becomes displaced by concrete placement . Do not allow slurry to enter waterways, wetlands, floodplains , or sewers . MDOT Standard Specifications for Construction Section 718 7-191 F. Excavation . Maintain the stability of the excavation sidew alls and extend the shaft excavation to a stratum accepted by the Engineer . Extend drilled shaft tip elevations if the Engineer determines the bearing stratum, encountered during excavation, is unsuitable or differs from the bearing stratum anticipated in the design of the drilled shaft. Provide the Engineer access to auger cuttings of the bearing material for additional analysis . Fill shaft over -excavation with concrete at no additional cost to the Department, unless the Engineer determines the bearing str atum encountered during excavation is unsuitable or differs from the bearing stratum anticipated in the design of the drilled shaft . Fill unauthorized shaft excavations extending below the required depths or elevations with concrete at no additional cost t o the Department. Dispose of excavated material in accordance with subsection 205.03.P . Keep excavated materials away from open shaft excavations . Direct surface water away from shaft excavations . Do not allow e xcavated material to enter waterways, wetlands, floodplains , or sewers.
1.Inspection . Provide equipment for checking the dimensions and alignment of each drilled shaft . Use the following methods to determine the dimensions and alignment with the Engineer ’s direction:
a.Check drilled shaft dimensions and alignment with reference stakes and plumb bob;
b.Check the dimensions and alignment of casing inserted in the excavation;
c.Insert a casing in shaft excavations temporarily; or
d.Insert a rigid rod assembly with several 90 degree offsets equal to the shaft diameter, into the shaft excavation. After shaft excavation, provide access, lighting capable of illuminating the entire shaft , and time for the Engineer to inspect the shaft. Reference the depth of the shaft during drilling to marks on the Kelly Bar. Meas ure final drilled shaft depths with a weighted tape after final cleaning. Clean each shaft so at least 50% of the base contains less than ½ inch of sediment . Do not leave sediment and/or debris on the base measuring more than 1½ inches in thickness . The Engineer will visually inspect dry excavations and inspect wet excavation by other methods to determine cleanliness. Inspect the drilled shaft excavation immediately before placing the concrete. MDOT Standard Specifications for Construction Section 718 7-192 Inspect wet method construction by probing and measuring.
2.Obstructions . Remove surface and subsurface obstructions with in the length of drilled shaft excavations. Obstructions may include, but are not limited to, old concrete foundati ons, abandoned utilities, and boulders . Use special procedures or tools, or both, if unable to advance excavations using augers fitted with soil or rock teeth, drilling buckets, or under -reaming tools . Special procedures and tools include:
a.Chisels ;
b.Boulder breakers;
c.Core barrels ;
d.Air tools ;
e.Hand excavation;
f.Temporary casing; and
g.Enlarging the hole diameter.
G.Placing Steel Reinforcement . Place steel reinforcement in accordance with subsection 706.03.E. Assemble the reinforcing steel cage and install it immedi ately after excavation inspection and immediately before concrete placement . If concrete is not placed immediately after steel reinforcement cage installation, the Engineer may direct removal of the cage before placing the concrete to verify the integrity of the excavated area and to determine whether removal of loose or soft material from the bottom of the excavation is necessary . If overhead obstacles prevent placing the cage as a single unit, connect individual segments with couplers or by lapping steel, as approved by the Engineer . Provide a fully assembled steel reinforcement cage for inspection a minimum of 2 days before the start of drilled shaft construction. Tie and support the steel reinforcement to meet the required tolerances . Tie spacers at quarter points around the cage perimeter and space at intervals no greater than 5 feet along the length of the cage. If the size of the longitudinal reinforcing steel equals or exceeds a diameter of 1 inch, the distance between spacing devices may be inc reased to 10 feet. Use non-corrosive spacers to maintain a minimum annular space of 3 inches between the outside of the cage and the side of the excavation or casing . Use at least one spacer per 30 inches of the outside MDOT Standard Specifications for Construction Section 718 7-193 circumference of the cage. Place at least three spacers at each level of the cage. Round plastic spacers may be used. Do not use concrete blocks, wood blocks, or metal chairs on the sides of the cage. Concrete blocks may be used on the bottom of the shaft to maintain cover to ends of the cage . Secure the cage to control vertical displacement during concrete placement and casing extraction. Use concentric support to prevent the steel reinforcement cage from racking and distorting. Check the elevation of the top of the steel reinforcement cage before and after concrete placement or after temporary casing extraction.
H.Concrete Placement . Do not place concrete in a drilled shaft excavation before the Engineer accepts the drilled shaft excavation. Inspect the drilled shaft excavation immediately bef ore placing the concrete. Provide lighting capable of illuminating the reinforcing steel cage and the sides and the bottom of the drilled shaft excavation for inspection. If the top -of-shaft elevation is below ground during concrete placement, use a casing to prevent material from caving into fresh concrete. Place concrete as soon as possible after completing excavation and placing reinforcing steel. For dry methods of construction, r emove loose material and accumulated water from the bottom of the ex cavation before placing concrete. Place concrete using underwater placement methods if unable to remove water . For wet methods of construction, place concrete in one continuous operation from bottom to top of the shaft . After the concrete reaches the top of the drilled shaft, continue pumping and remove contaminated concrete until the Engineer determines acceptable quality concrete appears at the top of the shaft . Continue placing concrete through the tremie pipe until contaminated concrete flows over the t op of the shaft. Do not vibrate concrete with a vibrator , except for the top 5 feet for concrete placed with dry methods . Remov e the casing in a manner that accounts for t he force of downward flowing concrete and does not deform the reinforcing steel cage. Place concrete by free -fall method, tremie tube, or pumping . Use a sump or other Department -approved methods to channel displaced fluid and concrete away from the shaft excavation. Recover and dispose of slurry as approved by the Engineer . Do not dis charge displaced fluids into waterways, wetlands , floodplains , or sewers . For concrete pours over water, provide a collar or other means to capture slurry and the top portion of the concrete overflowed from the shaft. MDOT Standard Specifications for Construction Section 718 7-194 1. Free Fall Concrete Placement . The free-fall method may be used in a dry shaft if concrete falls to the final position without striking the sides of the excavation, the reinforcing steel cage, or other obstructions . Use a centering drop chute at least 3 feet long if using the free-fall method. If concrete placement causes the shaft excavation to cave or slough or concrete strikes the rebar cage or sidewall, reduce the height of free fall, the rate of concrete flow ing into the excavation, or both. Do not use a shovel or other means to deflect the concrete discharged directly from the truck. Do not allow concrete to free fall more than 80 feet. If the Engineer determines that dewatering is impractical or concrete placement by free-fall method cannot be accomplished, place concrete using a tremie tube or a concrete pump.
2.Tremie Tubes. A gravity tremie may be used instead of a concrete pump for concrete placement in wet or dry ex cavations. Use tremie tubes with a length, weight, and diameter necessary to discharge concrete at the base of the shaft elevation. Use tremie tubes with an inside diameter of at least 10 inches. Use tremie tubes with smooth and clean inside and outside surfaces to allow flow of concrete and unimpeded tremie tube withdrawal during concrete placement operations . Use tremie tubes with thick walls to prevent crimping or sharp bends that restrict concrete placement. Use watertight tremie tubes for concrete plac ement . Do not begin underwater concrete placement until positioning the tremie tube to the shaft bottom elevation. Use valves, bottom plates, or plugs to begin concrete discharge within one tremie tube diameter of the base. Remove plugs from the excavation or use plugs of an Engineer -approved material that does not cause defects in the shaft if not removed. Fabricate the discharge end of the tremie tube to allow the free radial flow of concrete during placement operations . Immerse the tremie tube discharge end at least 10 feet in concrete after beginning the concrete flow. Keep the shaft full of concrete and the tremie tube submerged in placed concrete. Raise the tremie tube as necessary to maintain the free flow of concrete and casing stability , but maintain at least 10 feet of tremie embedment in concrete. If withdrawal of the submerged end of the tremie tube interrupts concrete placement, remove the tube, reseal it at the bottom, reinsert MDOT Standard Specifications for Construction Section 718 7-195 the tube into the placed concrete by at least 10 feet, and re charge before continuing the concrete placement. For uncased wet excavations, maintain the drilled shaft excavation full of slurry or water so water does not flow into the shaft excavation. For cased shafts, maintain a head of concrete above the bottom of the casing to overcome hydrostatic pressure. Extract casing at a slow, uniform rate with the pull in line with the shaft axis . Monitor the concrete level in the casing during extraction. Stop the extraction and add concrete to the casing to ensure a completely full excavation upon casing removal. The elapsed time from mixing the first concrete placed in the cased shaft to completion of casing extraction must not exceed the time concrete maintains a slump of over 4 inches . If the elapsed time is exceeded, modify the concrete mix, the construction procedures, or both for subsequent shafts.
3.Pumped Concrete . Pump concrete into wet or dry excavations using a pipe with a diameter of at least 4 inches and constructed with watertight joints . Arrange the concrete pum p equipment so vibrations do not damage fresh concrete . Arrange pipes carrying concrete from the pump to the shaft with minimal bends . Anchor pipes , conveying concrete to the bottom of the drilled shaft excavation, to the casing or other stationary objects to prevent the pipe from undulating during concrete placement . Do not begin concrete placement until the pipe orifice is positioned at the shaft bottom elevation. Operate the pump to produce a continuous stream of concrete without air pockets . To prevent contamination of concrete placed initially at the bottom of the shaft excavation, seal the pipe orifice with a diaphragm or a plug that gets flushed out when the hydrostatic pressure from the column of concrete exceeds that of the water in the shaft excavation. Control the initial rate of concrete placement to prevent lift or displacement of the steel reinforcement cage . Use a watertight conveying system, and maintain the outlet end at least 10 feet below the top of freshly placed concrete. When concrete reaches the top of the drilled shaft column, remove laitance. If withdrawal of the submerged end of the p ipe interrupts concrete placement, remove the pipe, reseal it at the bottom, reinsert it into the placed concrete by at least 10 feet, and recharge before continuing the concrete placement. For uncased wet excavations, maintain the drilled shaft excavation full of slurry or water so water does not flow into the shaft excavation. MDOT Standard Specifications for Construction Section 718 7-196 For cased excavations, maintain a head of concrete above the bot tom of the casing to overcome hydrostatic pressure. Extract casing at a slow, uniform rate with the pull in line with the shaft axis . Monitor the concrete level in the casing during extraction. Stop extraction and add concrete to the casing as necessary to ensure a completely full excavation upon casing removal. The elapsed time from mixing the first concrete placed in the cased shaft excavation to completion of casing extraction must not exceed the time concrete maintains a slump of over 5 inches . If the e lapsed time is exceeded, modify the concrete mix, the construction procedures, or both for subsequent shafts.
718.04 Measurement and Payment
Pay Item Pay Unit Drilled Shaft, __ inch .................................................................................. Foot Drilled Shaft Equipment, Furn (Structure Identification) ................... Lump Sum Temp Casing- Left in Place ......................................................................... Foot Permanent Casing ..................................................................................... Foot Obstruction Rem ...................................................................................... Dollar
A.Drilled Shafts . The unit price for Drilled Shaft, __ inch includes the cost of the following:
1.Drilled shaft excavation;
2.Temporary casings;
3.Slurry;
4.Shaft concrete;
5.Disposal of excavated material and slurry; and
6.Preventive measures for maintaining surface water or drains free of cuttings or slurry.
7.The Engineer will measure, and the Department will pay for , reinforcing steel as specified in subsection 706.04.
B.Casings
1.Temporary Casing -Left in Place . The unit price for Temporary Casing -Left in Place includes the cost of placing temporary casings left in place, as shown on the plans and cutting the casings to the elevation shown on the plans.
2.Permanent Casing . The unit price for Permanent Casing includes the cost of placing permanent casings as shown on the plans. MDOT Standard Specifications for Construction Section 718 7-197 C. Drilled Shaft Equipment . The unit price for Drilled Shaft Equipment, Furn includes the cost of providing and removing equipment for constructing the drilled shaft and providing and r emoving equipment for soil and rock excavation.
D.Obstruction Removal . The Department will pay for removing obstructions as Obstruction Rem if the Contractor uses the special procedures and tools specified in subsection 718.03.F.2. Notify the Engineer immediately when an impenetrable obstruction requiring the use of special procedures and tools is encountered. The Engineer will designate obst ructions for the Contractor ’s removal . The Department will establish a budget amount to pay for removing obstructions . If the Contractor and Engineer do not agree on a unit or lump sum price, the Engineer may order the work performed on a force account bas is in accordance with subsection 109.05.D.
Source: Michigan Standard Specifications for Construction, 2020 Edition. Pages 549–561 of 1,146.