Which foundation
options work on peat?
In practice, architects and designers have developed several
approaches to solving the problem of building on peat soils.
•
Complete peat excavation. If the peat is up to
1.5 meters deep, it is completely removed and replaced with a sand and gravel
bed. A strip foundation is laid on this base. This is reliable, but expensive
and labor-intensive—tens, even hundreds, of cubic meters of soil must be
removed by specialized vehicles, and the same amount of high-quality sand must
be brought in and compacted.
•
Partial peat excavation. If the peat depth is
1.5-2 meters, trenches are dug to firm ground and a strip or columnar
foundation is installed. Problem: excavation work in marshy areas is a battle
against constantly incoming groundwater and collapsing wet walls.
•
Pile foundation. If the peat is more than 2-3
meters thick, this is the only reasonable option. Piles penetrate weak layers
and rest on dense soil. There are options: driven reinforced concrete, bored
piles, or screw piles.
Driven piles require a heavy piling machine—it's simply
impossible to drive one into a swampy area; it will get stuck within the first
meter. Bored piles in peat quickly fill with water, and the borehole walls
collapse under the pressure. Screw piles are the optimal solution: they are
driven in like screws, require no excavation, and can reach any depth using a
self-propelled mechanism.
Why screw piles for
peat?
•
No excavation work. The piles are screwed into
the ground like screws into a board. There's no need to dig, pump out water, or
reinforce the pit walls—all of which are a nightmare in marshy areas and
distract the construction crew from their main work.
•
Compact
equipment or completely manual installation. A lightweight drilling rig
or even four people with metal levers and labor is sufficient. Heavy equipment
won't get stuck in peat.
•
Unlimited
extension for any depth. The standard pile is 3 meters. If deeper is
needed, an extension is welded on and continued screwing. In practice, screw
piles in peat bogs reach 6-8 meters or more, solving even the most complex
geological problems.
•
Frost heave protection. The blades are anchored
below the frost line in dense soil. A smooth shaft prevents the soil from
pushing the pile upward, as shear forces do not act on the smooth shaft.
•
Work
can be carried out at any time of year. In winter, the work can be even
easier—the frozen crust facilitates movement around the site and reduces
equipment sinking.
•
Quick installation. A screw pile foundation is
installed several times faster than a strip foundation, reducing construction
time and cost.
Preliminary Work and
Design
Site Geology – the Foundation of the Solution
First and foremost, geotechnical surveys with test drilling
are required. This reveals the thickness of the peat layer, the type and
density of the supporting soil, the groundwater level, and the chemical
aggressiveness of the environment. Without this data, installation is a gamble.
The geotechnical survey process includes drilling boreholes
at various points on the site (at least 3-5 points for a house), collecting
soil samples from different depths, laboratory analysis of the samples, and
compiling a geological cross-section. Based on this, the designer obtains an
accurate picture of the soil distribution.
Calculation of Loads and Pile Parameters
The engineer determines the total building load (the weight
of walls, floors, roof, snow, and wind), the estimated load-bearing capacity of
a single pile in dense soil, the required number of piles, and the optimal
arrangement of the piles. In peat, the pile's load-bearing capacity depends
almost entirely on the blade's performance in dense soil; lateral friction
along the shaft in soft peat is not taken into account.
Choosing the Diameter and Type of Pile
For a residential building on peat, the minimum shaft
diameter is 89 mm, while the optimal diameter is 108 mm. Blades should be
wide—the blade diameter should be 1.5 times or more greater than the shaft
diameter. This design ensures good penetration into dense soil and prevents the
pile from sinking.
Drainage and Site
Preparation Before Piling
On peaty and waterlogged soils, site drainage is a key step.
Even if piles can guarantee a secure foundation, high groundwater levels and
seasonal waterlogging will create difficulties for the operation of the
building and basements, as well as for installing utilities.
Important Considerations:
•
If
natural water flow is weak, a ring or wall drainage system is installed around
the future building—a system of perforated pipes located just below the base of
the future grillage or basement floor.
•
For
small areas, a drainage layer of sand or a mixture of sand and crushed stone
within the building footprint may be sufficient; it allows water to pass
through and reduces waterlogging of the upper part of the foundation.
•
All
drainage systems are routed to a well or drainage ditch to ensure uninterrupted
drainage even after heavy rainfall or snowmelt.
Why do this:
•
The
water level in the ground not only affects the corrosion of the pile metal but
also the long-term stability of adjacent structures and garden paths.
•
Modern
projects often combine peat pile foundations with artificial embankments,
geotextiles, and groundwater level reduction systems—these measures improve
living comfort on problematic soils.
•
Drainage
not only benefits the foundation but also prevents waterlogging and erosion on
the site, extending the life of lawns, pavements, and drainage pumps.
The process of
installing screw piles
Marking and pits
On the site, the axes of the future building are marked out
using surveying instruments. Pits 20-30 cm deep are dug at the pile
installation locations to facilitate initial pile positioning.
Screwing into soft soil
The pile is installed vertically in the pit and screwing
begins. On soft peat, the first 1-2 meters sink easily—almost under their own
weight. It is critically important at this stage to check the verticality with
a level. The deviation should not exceed 2 degrees, otherwise the pile will not
perform properly and the load will be unevenly distributed.
Reaching dense soil
As the pile depth increases, the torsional resistance
increases. When the blades reach dense soil (sand, loam, clay), the torque
increases sharply. This is the moment when the break in the pile's performance
becomes noticeable. An experienced installer can sense this transition by the
pile's behavior, the change in the sound of the work, and the effort of the
workers. Extension and Deepening
If the standard 3-meter length is insufficient, an extension
is performed. A process coupling (a larger-diameter pipe) is attached to the
top end of the pile, an extension (usually 2 meters long) is welded to it, all
welds are cleaned and treated with an anti-corrosion compound. Then, the drive
continues with the new extension.
When working in peat, fire safety regulations must be
observed—organic material is easily ignited by welding sparks.
Determining the completion of the drive
The main indicator is a sharp increase in torque. When four
workers with 3-meter levers cannot drive the pile further, this means the
blades are securely anchored in the dense soil.
Concreting the Cavity
After installation, all piles are cut to a uniform level.
The cavities are filled with a cement-sand mixture (in a 1:3 ratio) or a
ready-mixed dry concrete mix. This is critically important:
•
protects
the inner surface from corrosion
•
prevents
condensation accumulation and freezing of water
•
increases
the rigidity of the structure by 25-30%
•
prevents
pipe rupture due to frost heaving
Without concrete in winter, the water inside will freeze,
expand with tremendous force, and rupture the pipe. This is a common cause of
pile failure in marshy areas.
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