Excavation and site work are an important cost category on
many construction jobs. Early in the job, they're also a major
unknown -- you can never be absolutely sure what's down there
until you start digging.
Those underground unknowns can also mean unexpected costs.
Installing drainage for a wet soil, excavating and replacing a
weak soil, or simply compacting a difficult soil can add days
to the schedule and strip big dollars off the bottom line. Soil
erosion that pollutes waterways or clogs storm drains can bring
on major penalties, or even shut down the job. And when you
think the job's long over, site work can come back to haunt
you: Problems and failures related to soil issues top the list
of expensive callbacks and claims.
Given all that, builders are glad to hear about any technology
that can take some of the uncertainty out of earth work. In
recent years, answers are coming from a new class of products
known as "geosynthetics." Tough plastic fabrics, grids, and
cells are helping to handle all kinds of soil-related concerns.
With the right product, you can reduce erosion, improve
drainage, stabilize slopes, and boost the bearing capacity of
soil -- often at a fraction of the cost of placing sand and
gravel or casting reinforced concrete.
There are now dozens -- maybe hundreds -- of geosynthetic
products on the market. Some are geared mainly to commercial or
industrial construction, although they're occasionally used by
residential builders as well. Others, like woven-poly silt
fencing and the filter fabric wrapped around gravel drains,
have become commonplace and are on the shelves at every
Problems and Solutions
Geosynthetics can solve a wide range of problems, but most of
those problems fall into a few broad areas (see Figure
Figure 1.The range of uses for geosynthetics
continues to expand as companies develop new materials and
customers find new applications. Most soil engineers now
recognize the value of geotextiles for separating gravel from
soft subsoil (top left) and holding soil in place against water
action (top right). Geogrids have proven their value as a
reinforcing element in earth-retaining walls (above left), and
geocomposite drainage systems are gaining popularity because of
their simplicity and effectiveness (above right).
Separation. When you mix
gravel and mud, you don't get more gravel -- you get more mud.
Fabrics placed between a gravel roadbed and soft underlying
soils keep traffic from driving the gravel down into the mud,
or "pumping" mud up into the gravel. This maintains the
gravel's drainability and strengthens the road or driveway,
prolonging its life.
soils have different properties, but no soil on earth has any
tensile strength to speak of. Synthetic fabrics or grids can
introduce a tensile component into the equation: They
distribute forces sideways through the soil and prevent soil
from slumping, slipping, or compressing. The result is a
stronger subgrade with less fill or compaction work, or a steep
stabilized bank with no expensive concrete work.
Filtration. Silt washing out
of fine soils can quickly clog foundation drains or even
municipal storm drains. Synthetic filter fabrics block the silt
and let the water through, protecting permanent drains and
keeping site runoff from polluting streams and ponds.
Drainage. All soils perform
best in a well-drained condition. Placed appropriately, a thin
synthetic sheet product weighing just ounces a yard can replace
tons of granular fill as a drainage medium. Composite products
-- filter fabrics wrapped or laminated over a free-draining
center -- make a fast-draining assembly that is cheap,
effective, and quick to install.
Erosion control. Federal and
state regulators are getting tough on silt-laden runoff from
construction sites. To comply with strict permit provisions,
builders need to use "Best Management Practices" (BMPs) that
can stand up to regulatory scrutiny. Natural and synthetic
fabrics are an important part of the strategy to keep soil from
washing off slopes, to trap silt that does run off, and to
protect seeds and help quickly revegetate exposed earth.
Geotextile fabrics were the first geosynthetic product, and
they're still the most common, at least in residential work.
They're also highly versatile. Geofabrics can be engineered to
provide solutions to all of the project categories just
mentioned, from separation of soil and fill to erosion control
(Figure 2). Some varieties are woven from spun yarn, some are
woven from "slit film" strips cut from sheets of plastic, and
some are nonwoven -- spun-bonded or laid down in a mass and
formed into a sheet with heat, pressure, or chemical action.
Some, especially the very open-weave fabrics made with heavy,
stiff fibers, are "needle-punched" -- tangled together
mechanically by barbed needles.
Figure 2.Geotextiles have become a commodity
product, with many companies competing to provide choices
across the whole range of strength, porosity, and other
properties. Silt fences (left) and filter wraps for trench
drains (right) are familiar uses to most builders.
Picking the right fabric.
Geotextiles are a commodity item -- most companies that have
geotech materials can offer the whole range of fabrics for any
need, and although comparable products from competing
manufacturers may be constructed differently, they have to live
up to the same industry standards. Companies publish values for
important traits like tensile strength, puncture resistance,
pore size, flow rate, and chemical resistance. Price and
availability may affect the final choice, but geotech
professionals generally try to match a product's
characteristics to the role it has to play on a case-by-case
For reinforcement under a road, for example, strength and
toughness are key considerations (Figure 3). For simple
filtration, on the other hand, a high flow rate and the correct
pore size may be more important. Many designers take the short
cut of following some sort of standard spec (state departments
of transportation publish lists of "prequalified" products that
are accepted for specific uses under given conditions). But the
choices aren't simple: The nature of the site soils as well as
the product can affect performance. If your application is at
all demanding, it's wise to consult an engineer.
Figure 3.For roads ranging from temporary access
roads to superhighways, tough woven fabrics isolate the gravel
road base from soft soil subgrades, stabilizing the road
structure and dramatically prolonging its life.
Beyond bales. Although
they're still useful in many applications, silt fences and hay
bales are no longer the state of the art in temporary erosion
control. And for permanent installations, rip-rap and concrete
culverts are old hat. Modern geosynthetics, and natural-fiber
erosion control blankets with or without synthetic
reinforcement, are doing the job more effectively and often at
less cost (Figure 4). The tougher geofabrics can serve as a
permeable undercourse to heavy concrete or stone armor to
protect shores against waves and currents; seawalls made with
this system can outlast rock-only systems by decades. Heavy
open-weave fabrics can anchor vegetation in swales to create
natural "living armor" that protects better than stone or
concrete, and also captures silt and other pollutants
Figure 4.Tangled open-weave fabrics made from
heavy plastic strands (left) provide a tough anchorage for
plants to resist stormwater erosion and trap silt. Wrapping
heavy fabrics around soil on an exposed river or lake bank can
hold the bank in place and help plants stay anchored
The more complex permanent measures are engineered
applications, but many temporary measures are simple. Any good
landscaping supply house can point you to natural or synthetic
products that will protect the area around your job site from
Heavier open meshes of tough plastic called geogrids provide
even greater strength and are used mostly to lock soils
together into a strong, stable mass. They're often used as soil
reinforcement under roads and behind segmental retaining walls
Figure 5.Tough, large-weave plastic geogrids are
used primarily for structural soil reinforcement. One important
use in residential work is to tie together the soil mass of a
segmental block retaining wall structure. Although the grids
typically interlock with the individual block (top left), they
function mainly to lock the soil into a stable mass, not to tie
back the block. With internal soil pressures managed by the
geogrid reinforcement, segmental block walls can readily reach
30 feet or more in height (right).