This technique can be used as a remedial measure to treat unstable natural soil slopes. This can also be used as a construction technique, allowing the safe over-steeping of new or existing soil slopes.

This technique involves the insertion of slender, threaded reinforcing bars (rebar) or hollow-system steel reinforcing elements into the slope. Solid bars are usually installed into pre-drilled holes and then grouted into place. Bars are usually fully grouted and installed at a slight downward inclination with bars installed at regularly spaced points across the slope face.

A rigid shotcrete facing or isolated soil nail head plates may be used at the surface.

Alternatively, a flexible reinforcing mesh may be held against the soil face beneath the head plates. Erosion control fabrics may be used in conjunction with flexible mesh facing for sloping faces or favorable, near-vertical soil conditions.

Micropiles are small diameter (typically less than 12 inch), drilled and grouted, non-displacement piles that are typically reinforced. Micropiles are typically installed using drilling and grouting methods and are typically reinforced with threaded bar and/or steel casing.

Micropiles are very useful where site access, either laterally or overhead, is restricted. Micropiles are commonly used to penetrate a layer of material (i.e. soft soils, or debris) that will not provide the necessary support for shallow foundations. Acting primarily in skin friction, micropiles develop high-compression and tension capacities. Micropiles can accommodate some lateral and moment loadings, which can be increased with the introduction of casing in the upper portions of the pile.

Micropile installation equipment produces very little ground vibration, which is an advantage when working adjacent to fragile structures or vibration-sensitive equipment.

Helical Piers consist of circular steel plates deformed in a helix configuration and welded to either a solid square bar or a pipe. The initial or lead section may include a single helix or multiple helix. Extensions can be added to advance the helices to a suitable bearing stratum. Extensions may or may not also include helices.

The helices act like circular spread footings and capacity is developed at each of the helices. Skin friction along the square bar or pipe is not considered and pile capacity is not gained between the helices.

Advantages to other foundation types are low noise; relatively small equipment can be installed and grouting is not required. It is not necessary to wait for grout to cure to apply loads as with other foundation types.

Disadvantages are that helical piers only increase significantly in capacity if soil conditions through which they are installed improve or more helices are added, since skin friction is not considered. Also, helical piers can hang up in hard layers like cobbles without achieving capacity.

Landslide stabilization can be achieved in a wide variety of ways. Although many landslides occur naturally, most that we have encountered are man made. A variety of factors can contribute to a landslide, including placing additional surcharge load on a slope, steepening a slope, or cutting the toe out of a slope, which essentially steepens the slope. Theoretically, once the driving force (i.e. weight of soil, surcharge) exceeds the resisting force (soil strength, weight of soil on toe), a slide could potentially occur. At the point of failure, the safety factor (resisting force/driving force) is about 1.0. The resisting force can be increased by placement of anchors or micropiles in the hillside, thus increasing the safety factor to acceptable levels.

If a landslide is presently occurring or suspected, it is often advantageous to have your geotechnical consultant perform a detailed site characterization such as borings taken through the slide material, installation of slope inclinometers to define the rate and location of ground movement, and a detailed site survey including the locations of tension cracks, borings, slope inclinometers, etc.

With this information, your geotechnical contractor can develop a plan to mitigate the movement and stabilize the slope. Anchorage might take the form of patterned ground anchors, which are tieback anchors attached to a steel or concrete plate that apply resisting force to the ground surface. Anchorage could also include soil nail walls or tieback walls. Further, micropiles can be installed in the slope and act in shear to increase slope stability. Micropiles are particularly useful if tieback anchors can not be installed in the lower portions of a slide, such as may occur if a slide is occurring from shore and extending out into a body of water.

Tieback anchors typically have an unbonded zone extending though the ground that is potentially moving and a bonded zone extending into and deriving support from the ground beyond the potential zone of movement. Tiebacks can extend through and be bonded in soil as well as rock. Tiebacks are usually installed by drilling and grouting methods. In rock, tremie grouting is sufficient. In soil, single or multiple post grouting after the initial tremie grouting can develop additional pullout capacity.

Tieback anchors can be used for a variety of purposes, such as soldier pile and tieback walls for shoring and patterned ground anchors for landslide stabilization. Essentially, tieback anchors extend through ground that is moving or has the potential to move and anchor to the ground beyond the potential zone of movement. Tieback anchors can be solid bar or strand in a wide variety of sizes to match the required strength.

Rock and ground anchors are similar to micropiles, yet they provide only tensile support and are typically constructed of grouted threadbar or strand anchors. Rock and ground anchors can be used to resist uplift loads from towers; retaining walls; tall, relatively thin structures; bridges; and any other applications where tensile loads need to be resisted by the underlying rock or ground.

Wurster Betterground is a partnership between Wurster Engineering and Construction and Betterground. Together, we’re able to offer design-build ground improvement solutions for a wide variety of structures in any type of soil conditions. Wurster Betterground uses state-of-the-art equipment and engineering techniques to provide cost-effective, engineered ground improvement solutions.