Micropile installation

The followings are some suggestions and steps that may result in a better quality of micropile construction. 

Type A The grout eolumn is not pressurized. Sand-eement mortars as well as neat cement grout is plaeed under gravity head only. 

Type B As the temporary steel drill-hole casing is withdrawn, the neat cement grout is placed under pressure in the range 0.5 to 1.0 MPa to maintain a seal around the casing during its withdrawal and is limited to avoid excessive grout takes. 

Type D This is similar to the 2-step grouting as in Type C with some modification. Hardening of the primary grout is allowed before the secondary grouting is performed. The additional grout is injected via tube-a-machette under high pressure of 2.0 to 8.0 MPa. A packer may be used so that specific levels can be treated several times, if necessary. 

An old property in Boston needed additional support due to increased loading from redevelopment and refurbishing of its six-storey structure (Bruce Nicholson 1988). The fill consisted of saturated loose-gray brown fine sand and silt, and overlaid soft gray organic silt with traces of shells, sand 

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Groups of micropiles installed at relatively close centers interact with the surrounding soil to form a composite soil pile retaining structure to stabilize natural and constructed fill slopes. In such cases, micropiles can also be used more directly as soil nails to reinforce soil in earth retention and instability in excavations and slopes as illustrated in Figure 9.1(f). 

Figure 9.7 shows the construction stages of the micropile installation for this project. The installation took 3 months and the major structural rebuilding took 8 months. The performance of the piles was very satisfactory. 

Bridge piers and abutments - micropiles may be installed through existing bridge piers and abutments with the minimum vibration, thus achieving a direct, positive connection between pile and structure as illustrated in Figure 9.1(a). 

Retaining walls - micropiles systems may be installed to support compressive and tension loads associated with retaining wall instability problems. As the piles are not visible after installation as in Figure 9.1(b), the appearance of the structure is unaltered. 

Stitch piling - micropiles are installed through existing walls and foundations to restore or increase the factor of safety. Pile spacing and loads are dependent upon the nature of the existing structure as indicated in Figure 9.1(c). 

As micropiles are essentially cast-in-place piles, the soil/pile interaction depends on the mechanical characteristics of the soil, specifically, the outer surface of the pile as well as on the installation procedures. Depending on the type and method of installation, certain type of micropiles can also be classified into displacement micropiles as well as non-displacement micropiles (Lizzi 2000). 

In this case, micropiles were used to support part of the 1-78 dual highway, which crosses the Delaware River between Pennsylvania and New Jersey. It was thought that the New Jersey pier was to be founded on solid rock but excavation revealed that the solid rock was nonexistent. So two alternatives were considered the installation of six 36 in. caissons, each with a working load of360 tons (3,203 kN) and the placement of 24 minipiles, each of nominal working load of 100 tons (890 kN). 

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