Reinforcement (Rebar Placement) in Micropile Construction

Steel Reinforcement Amount and Selection

The amount of steel reinforcement placed in a micropile (or minipile) is determined based on the bearing loads and the stiffness required to limit elastic displacement. The reinforcement system may consist of a single rebar, a group of rebars, a steel casing tube, or rolled structural steel.

Placement of Reinforcement

Rebars can be positioned before grout injection or inserted into the grout inside the borehole after pouring and before withdrawing any temporary casing (if used). The rebar surface must be free of harmful materials such as soil and mud, which can contaminate the grout or the rebar and impair the grout-soil composite quality. Rebars should be carefully centered in the borehole to ensure sufficient grout cover protection.

Types of Reinforcement

Steel Reinforcing Bars (Rebar)

Typically, standard reinforcing steel bars (see Table 1) conforming to ASTM A615/AASHTO M31 and ASTM A706, with tensile strengths ranging from 240 to 520 MPa, are used. Bar diameters range from 25 mm to 63 mm. Usually, a single bar is used, but grouped bars are also possible. In grouped rebars, individual bars can be separated with spacers or tied with spiral reinforcement to ensure grout flow between bars and the required grout volume (see Figure 1). Bars can be bundled if sufficient development length for each bundle is provided. Where working height is limited, making full-length placement impossible, couplers can be employed.

Continuous Deformed Steel Bars

Deformed steel rebars, such as the DYWIDAG system “DSI” or William deformed bars, are common. The DYWIDAG (GEWI) bars, widely used worldwide for micropile reinforcement, have sharp-edged threads rolled continuously during production (see Figure 2 and Table 2). These bars are available in diameters from 19 mm to 63 mm, made of steel conforming to ASTM A615/AASHTO M31 with yield strengths of 520 to 550 MPa. Diameters 44 mm to 63 mm are most common. High-strength steel bars conforming to ASTM A722/AASHTO M275 with ultimate strengths of 1035 MPa are available in 26, 32, and 36 mm diameters.

William deformed bars come in diameters from 20 mm to 53 mm, conforming to ASTM A615/AASHTO M31, with some sizes offering 1035 MPa ultimate strength. Their thread pitch is less than DYWIDAG bars. The deformation ensures grout-to-steel bonding and allows bars to be coupled for tensile and compressive load capacity. Continuous threads facilitate pile-to-structure connection, where the bar is connected to a bearing plate via a hex nut.

Hollow Full-Deformed Steel Bars

Hollow full-threaded reinforcing bars include types like DYWIDAG MAI, Ischebeck Titan, and Chance IBO Injection Boring Rods. These bars combine the advantage of threading with a hollow core, allowing drilling through the bar itself. A drill bit is attached to the bar’s end, and grout is pumped through the bar’s interior to the drill head during drilling. Fluids such as water or air can later be used to convey grout through the bar after drilling to the final depth.

Continuous threading permits cutting and coupling and allows the use of hex nuts to connect the pile head. The main drawback is the high cost. Table 3 shows dimensions and capacities of MAI, Titan, and IBO bars.

Steel Casing for Micropiles

To support higher loads with minimal displacements and to resist lateral loads, steel casing reinforcement became common. This casing increases pile stiffness and provides reasonable shear and bending capacity against lateral forces.

This reinforcement may use the drilling casing as a permanent reinforcement or by placing a smaller permanent pipe inside the drilling casing. Using drilling casing as permanent reinforcement is common for micropiles installed in rock, where withdrawing the casing for pressure grouting is unnecessary. The length of pile sections is determined by the drilling rig capacity and workspace height. Casing sections are joined by threaded, polished connections. The reduced cross-section at threaded connections must be considered in structural design, especially for tensile and bending capacities. Reinforcement methods exist to restore connection strength to the full pipe capacity.

Casing pipes typically conform to ASTM standards A53, A519, A252, A106 with yield strength around 241 MPa. The steel type can dictate pipe selection. The main disadvantage is the relatively low yield strength and high cost per meter.

API 5CT or 5L casing pipes are also used. Their high yield strength (~551 MPa) enables higher load capacity and improved threaded connection strength. Rolled pipes with reasonable prices are available. Most high-capacity micropiles in the U.S. use N-80 casing. N-80 requires quality control by mechanical and chemical testing rather than mill certificates, which are often unavailable.

Due to the chemical composition and high strength of API N-80, special welding procedures are required, and approval must be obtained from the client before welding. Table 4 presents dimensions and yield strengths of common casing types.

Composite Reinforcement System

For micropiles with permanent casing of short length (types C1, C2, and C4), it is common to reinforce the lower part of the pile with a steel rebar, creating a composite reinforced pile (see Figure 3). To resist tensile loads, the reinforcement may extend to the top of the micropile. The composite system adds complexity to structural pile analysis.