Throughout their quest to enhance the aesthetic quality of the built environment, Landscape Architects frequently contour the land to serve a greater design purpose than its native state. The process of moving, placing, and shaping the earth can accomplish great things. It also carries with it engineering and environmental constraints that must be addressed as a part of the design process.

Call them hills, hillocks, mounds, or mountains, using berms of soil in the landscape serves many purposes:

• Screening views by creating a visual barrier

• Enhancing views by creating an elevated viewing platform

• Deflecting sound

• Diverting water (Dams, dykes, and wattles)

• Directing traffic (vehicular or pedestrian)

• Defining a space (Surrounding, depressing, or elevating land to invoke a sense of enclosure)

• Identifying a place (Vernacular shaping of land to create icons or imagery)

• Expression of a pure design element (Fanciful shapes as artistic expression)

The challenge is not in determining what berms can or cannot accomplish. The challenge is in finding ways to enhance and expand the roll of aesthetic land forming through innovative materials and construction methods.

A pile of loose soils in its natural state tends to form a cone. In nature, the steepness of the sides of the cone (the angle of repose) are determined by the texture, moisture content, and the relative compaction of the materials. At optimum moisture, select soils can be compacted sufficiently to form a vertical wall, or even a cantilevered ledge. Maintaining such a condition is another matter as the action of water and wind, and the effects gravity and other laws of physics, conspire to return the soil to a more natural state.

Further, if the intent is to landscape the mound, the water applied to irrigate the plantings may loosen the soil particles. At the same time, the compacted density required to maintain a radical mound shape may well conflict with the horticultural needs of the intended plant cover.

The challenge is to build earth forms that stay in place while allowing for the practical or artistic aspirations of designers, providing a suitable substrate for constructed hardscape elements, and a suitable growing medium for trees, shrubs, and groundcover. What follows is an examination of some of the means available to keep a berm in place and to expand the versatility of these landscape features by allowing more and varied uses.

Surface Protection

One of the most common approaches to stabilizing manufactured berms and mounds is to protect their surfaces from wind and water. Both of these elements interact on a microscopic level to loosen surface soil and begin the process of erosion. Water loosens the bonds that hold soil particles together and, combined with the physical force of water droplet impact, can quickly alter the shape of mounds or berms. Wind can accomplish the same effect through drying the surface and slowly pulling soil away. The net result is that the berm contours become flattened over time.

Stabilization is frequently accomplished by protecting the exposed mound surfaces with planting. Established planting provides both watershed and the reinforcing effect of roots within the soil. Until plants become established however, the integrity of the soil can be enhanced through the use of various fabric or mat materials that can be applied over the surface of the mound. Products for this purpose include:

• Jute netting

• Curled Aspen wood / fabric blankets

• Cocoa fiber / fabric blankets

• Plastic mono-filament matting

• Paper-backed geotextile fabrics

• Woven or non-woven geotextile fabrics

(or combination products)

• Hydraulically applied mulching fiber

(with or without stabilizing polymers)

These products are typically applied and secured with pins to the surface of the landform to protect the exposed surface from the impact of water, the drying and lifting effects of wind, and the lateral movement of fine soil particles as the soils become saturated and run-off occurs. The limitation of surface protection measures is that they themselves do not necessarily offer any structural support to the mound.

Structural Enhancement

The physical make-up of the berm can be enhanced through the use of various products that lock soil particles together. These products may simulate the fiberous "knitting" effect of roots or they may organize the loose soil materials into interlocked modules within a matrix of plastic and fiber. The "net" result is that those elements that would conspire to flatten the mound must now work on it as a whole rather than at the soil particle level. Structural enhancements can be divided into the following categories.

•Geotextile Fabrics

Woven and non-woven fabric products can be used in certain situations to effectively create an envelope around soil materials to hold them in place. A mound can be constructed, for example, by building the feature in layers or "lifts." Each lift is built upon a layer of geotextile fabric that is brought from under the layer, up and over the layer, to create a compacted pad for the next layer of soil and its fabric wrapper. The vertical load imposed by the upper layers of soil serves to restrain the fabric envelope of the lower tiers. The fabric is permeable and allows for the infiltration of water but protects the surface from the impact of rain or irrigation droplets and retains soil fine soil particles against the effects of erosion.

Depending upon the structural properties of the available soil, the thickness of the individual soil lifts can be varied to permit the construction of mounds to much steeper angles than would normally be possible without the fabric systems. The exposed surface of the fabric can be penetrated to permit planting of trees and shrubs. Depending upon the type of fabric and the selection of plant species, it is possible to over-seed the fabric-wrapped mound as well.

A wide range of geotextile products exist for use in reinforced land form construction.

•Geogrid Products

Geogrid products generally consist of open-mesh panels or roll stock constructed of high-tech mono-filament or fiber-reinforced extruded plastics. Geogrids have been primarily used to strengthen the load-holding capabilities of roadway soils by transferring vertical point loads across a larger area. The grids act to unite soil into a larger mass such that the point load cannot penetrate vertically without first moving a large area of surrounding soil horizontally in the process.

The role of geogrids in the construction of mounds, berms, or other landforms is much the same as that of geotextiles but is limited due to the large mesh size of typical geogrid products. Fine soil particles will easily pass through the mesh of a geogrid. Thus the geogrid is typically used in conjunction with a geotextile product. The combination serves to better distribute vertical loads and to reinforce the geotextile fabric against "blow-outs" if the enclosed soil mass becomes saturated with water and threatens to fail by means of liquifaction.

Geogrids find tremendous use in the construction of modular retaining wall systems which, though not the topic of this discussion, are frequently used in conjunction with mounding as a means of both practical and aesthetic land forming. The geogrids are secured to the wall and extended behind the wall into the "back-cut" area where they are integrated into the compacted backfill of the wall. As described above, each subsequent lift of compacted soil imposes a vertical load upon the geogrid which prevents it from being pulled laterally. The resistance serves to hold the wall in place against the weight of the retained soil.

•Geocell Products

Geocell products serve to segregate the greater mass of a landform into small modular "cells" while uniting these cells into a larger reinforced panel or mat. The isolation of soil into numerous small cells changes the dynamics of the mound’s soil mass. On a "micro" level, water and wind now act upon smaller units of soil where they can do less harm. On a "macro" level the cells are knitted together into large panels which distribute loads and hold the mound together in a manner similar to a geogrid.

Geocells are generally installed in two ways depending upon the characteristics of the soil and the role the landform is to serve.

1) Horizontal Mats in a Stepped Configuration:

In this method, the geocells are placed on a prepared sub-grade then filled and compacted. Additional layers of geocell are added to the stack, each filled and compacted in turn. To create sloped faces for the berm, each subsequent layer is set back from the edge of the layer below it. As the berm is constructed the front face slopes back in a stepped fashion. The exposed cells at the front edge form pockets suitable for seeding or planting with anything from cuttings to small container pots. As the plant material fills in, the stepped face of the berm recedes into the greenery.

The above methodology is particulary suited for extremely steep slopes and can be used in lieu of a wall for retaining embankments, as well as for defining freestanding mounds. Note that depending upon the soil, it may not be necessary for the entire footprint of the berm to be covered by the geocell. Rather, a consistent band about the edge may serve to retain the entire mound with compacted soil alone at the core (See sketches).

2) Surface Installation Following Slope Contours:

For less severe slopes or embankments, it may be feasible to use the geocell product as a single layer following the desired contours at the surface of the mound. The basic shape of the mound is constructed and compacted and the geocell panels are spread across the mound to follow the contours. The individual cells are filled with soil and planted similar to above.

For additional structural stability, the geocells may be anchored to the soil substrate with stakes driven through cells and into the compacted soil of the underlying mound. The depth of penetration and the on-center spacing of the stakes should be determined by a competent Soils Engineer.

Some geocell products provide for horizontal strapping that is integral to the geocell panel. The strapping is anchored to stakes at the top and bottom of a slope with supplemental retainer clips set along the straps. The strapping, stakes, and clips serve to hold the geocell from sliding down hill during soil placement or under saturated conditions until such time as the planted in-fill becomes sufficiently rooted to perform this task.

•Steel Mesh Retainers and Gabions

Developed for erosion control and storm water systems, gabions are steel mesh baskets that are filled with coarse aggregate and stone, sealed, stacked, and tied together to form the exposed edges or floors of flood control channels and embankments. The weight and durability of the rock holds up to rushing water while the porous and irregular surface allows for some degree of water percolation and some minor reduction in water velocity.

Gabions may not necessarily be a consideration for landscape mounding and berms per se, but may have applications for such land form uses as the "Ha Ha," a design application where grades are altered to drop walls or fences below one’s line of site. The result is the appearance of an unobstructed view from selected view points.

Evolving from the gabion concept are angled mesh elements constructed of heavy gauge steel rod. These elements are designed to be used in conjunction with geogrid and geotextile products in the creation of steep soil walls using reinforced fabric envelope. The angled steel mesh elements are assembled in lifts, lined with a geogrid and/or a geotextile, backfilled with soil, and compacted. The process repeats as the slope is constructed, with each subsequent lift set back slightly from the layer below it. As with the other systems noted above, the weight of the upper lifts holds the horizontal geogrid/geotextile reinforcement in place.

•Cribwall, Loffelstein Wall, and other Plantable Wall Systems

Generally associated with the retaining of slopes and wall embankments, plantable walls such as cribwall have been utilized experimentally to construct freestanding soundwalls. Two parallel runs of cribwall are built such that they contain compacted soil between them. The soil exposed in the gaps between the precast concrete lags and ties is seeded or planted with cuttings or small potted plants. The combination of the solid wall mass, the irregular surfaces created by the cells, and the texture and density of the planting create an effective sound barrier and planted visual backdrop.

•Loose Fiber or Mesh Elements

Another means of reinforcing the soil mass is the use of loose fibers or mesh elements. Fabricated from plastic, both products attempt to recreate the binding effect of roots by incorporating filament elements into the soil matrix. The filaments resist compaction and shear forces by distributing point loads over a wider area.

Loose fibers such as Stabilizer Solutions "Turf Grids" are incorporated into soil within the upper six to eight inches of the soil profile. The primary use has been in turfgrass areas as a means of resisting compaction in sports fields, pedestrian walkways, and planted emergency vehicle access lanes. The methodology is more suited for shallow slopes suitable for turfgrass, such as the berms that may be constructed adjacent to sports fields or amphitheaters for festival style seating, picnicing, and sunbathing.

A second method used in sports turf and fire lane applications with potential for slope stabilization is that of mesh elements. Products such as "Netlon Advance Turf" utilize small squares of plastic mesh distributed uniformly throughout the upper layer of the soil profile. In conjunction with plant roots, the mesh elements provide a high degree of compaction and shear resistance. Although primarily designed for use in horizontal applications, the incorporation of the mesh elements into shallow slopes will contribute to stability of the slope to the depth that they are present.

A few cautions must be reviewed in considering any of the above systems. The type of system should only be determined upon review of the physical properties of the soil to be sculpted and mounded. Things to consider are the physical parameters of the berm or mound, i. e., the size, shape, height, and steepness of the intended slopes, the type and density of proposed plantings and amount of rainfall and/or the type of irrigation to be used.

Mounds and berms of sufficient height or steepness to consider the use of geotextile, geogrid, or similar reinforcement systems should be reviewed by a competent Structural Engineer or Geotechnical Consultant to determine the most appropriate system, lift thickness, compaction requirements and construction specifications.