1 What
causes land drainage problems – the principles of water movement in soils
Whilst
the effect of a functioning land drainage scheme can be observed by all, a
basic understanding of the physical principles which govern this process
enables the observer to both assess the effectiveness of a land-drainage scheme
and also to appreciate why inappropriately or poorly designed drainage schemes
fail to work.
Figure
1 A diagram depicting a microscopic view of soil
The
key to all of the following discussion is to understand what makes up a
rootzone. The key ingredients are:
1.Mineral particles (sand,silt,clay)
2.Water
3.Air
4. Organic matter
Consider
for example the soil rootzone represented in Figure 1, mineral and organic
particles are locked together to form the solid fraction of the soil, whilst
the pores (the spaces between the solid particles) are occupied by either soil
water or soil air. The combination of solids, water and air is critical for not
only plant growth, turfgrasses need all three components,
but also surface strength.
1.1
Sports surface strength and soil moisture content
The
resistance of a football pitch to wear by players and the ability of a pitch to
provide a stable platform for quality football is highly dependent upon the
turfgrass and the soil in which it grows. The grass plant provides a cushioned
surface, with natural lubrication to reduce skin abrasion – its root network
also provides a core structural function. The soil not only provides an
anchorage for the grass plant but it is also the key load bearing component.
The two components combined (grass and soil), must be of sufficient strength to
resist damage from repeated use – both from running and sliding, but not of
excessive strength so as to cause impact or musculoskeletal damage.
Figure
2 The effect of soil moisture content and bulk density
on rootzone strength. As moisture content increases, surface strength initially
increases but then rapidly decreases. As bulk density increases from Bulk
density 1 to Bulk density 2, the rootzone strength increases
The
strength of a turfgrass plant is a function of agronomic factors related to
plant health and growing environment. The strength of a natural soil or sand
rootzone is a question of engineering and is closely related to the bulk
density of the soil (how well packed the soil is) and its moisture content (how
wet the soil is). Typically, as bulk density increases – soil strength
increases and as moisture content increases – soil strength decreases (see
Figure 2).
The
principal target for groundstaff is to prepare surfaces that have a
sufficiently high bulk density so as to provide strength to resist wear. The
problem is that as bulk density increases, porosity (the amount of pores in a
soil) decreases and restricts the amount of air in the soil available for the
turfgrass; a balance is required.
1.2
Water movement and retention in soils
As is
shown in Figure 1, water occupies the pore space in a soil; it also illustrates
the range of pore sizes in a soil. Water is held in a pore by capillary action,
effectively sucking water into the pore – this is the same way in which a
sponge absorbs a liquid. Large pores create a low amount of suction; small
pores create high amounts of suction – therefore the size of pores is critical
in water movement in soils.
Figure
3 Unsaturated (a) and saturated (b) rootzones. In the saturated condition all
the pores are full of water
All
the water in a soil is being pulled downwards by gravity,
large pores will drain because there is not enough suction to hold the water in
the pore against gravity, meanwhile small pores, where the suction is greater
than the pull of gravity do not drain. This is why sandy soils, where the
particles are large, and hence the pores are large, drain easily under gravity,
but clay soils, where the particles and pores are very small will hardly drain
under gravity and require land drainage schemes. In fact in clay soils, it is
very difficult to pull water out – it must be pushed out, this happens when the
soil is saturated (see Figure 3).
The
same physical property also means that water is pulled upwards (and sideways)
to occupy dry pores – a phenomenon known as capillary rise.
Saturation
in a soil is defined as when all the
pores are full of water. Note that any water applied to the top of the soil in
Figure 3b will displace water out of the bottom of the soil by pushing it out
as a head of water builds (see Figure 4a). The rate at which the water moves
through and out of the soil is known as the saturated hydraulic conductivity.
In a coarse grained soil such as a sand rootzone, the hydraulic conductivity is
high; in finer clay soils, the hydraulic conductivity is orders of magnitude
lower.
Figure
4 Flooding due to high water table; (a) water added to the top of the saturated
soil displaces water from the freely draining base of the profile; (b) when the
water table is high, water cannot flow out of the soil and accumulates at the
surface – flooding the pitch
If there
is no exit for the water below (e.g. the water table lies directly below the
soil) then water will accumulate at the surface, water logging and eventually
flooding the pitch (Figure 4b). This situation is known as a ground water
drainage problem and can be solved by lowering the water table with piped
drainage so that the soil has a greater storage capacity for rain water (Figure
5).
Such
groundwater problems in sports surfaces are actually quite rare. The majority
of waterlogging and flooding of football pitches is actually caused by another
problem entirely – requiring a different solution.
1.3
Infiltration and surface drainage problems
Flow
of water in and out of the soil in Figure 4a assumes that water can get into
the top of the soil in the first place. The entry of water into the top of a
soil is termed infiltration. Typical infiltration rates for different soils and
rootzones used in sports surfaces are shown in Figure 6. The difference between
a high sand content rootzone, such as a 70/30 mix, and a
compacted clay is of several orders of magnitude.
Figure
5 The effect on a groundwater table of the
installation of piped drainage. Note that at the drain
locality, the water table is lowered, increasing the capacity of the soil to
store rainwater and reducing the frequency of flooding. Note also, how
the water table rises in-between the drains – the
height to which it rises is a function of the depth of drains and their
spacing. If the drains are too shallow or too far apart, they can actually
cause flooding – a common indication of poor drainage design
The
infiltration rate (how quickly water enters the soil) is a function of many
factors but is governed by the pore space in the soil – if the soil has an open
structure at the surface water can flow into the soil easily; if the soil is
capped or has small pores, the infiltration rate is reduced (Figure 7).
Low
infiltration rate in sports surfaces is caused by:
·
small pore sizes (eg.
clay soils)
·
compaction (wheeled
traffic)
·
capping (silts or
sliding feet)
·
some types of organic
matter, including thatch
·
or a combination of all of these.
The
inability to get water into a sports surface is one of the most common land
drainage issues and is known as a surface drainage problem.
Figure
6 Typical infiltration rates for different soil media
Consideration
of the above factors shows why clay soils are so susceptible to this problem –
they have very small pores and the surface is easily smeared – providing a water
tight seal over the soil. Installation of groundwater drains such as in Figure
5 will not solve this problem, the water cannot even
get into the soil, let alone exit through the drains. If waterlogged and
flooded pitches are to be avoided then it is necessary to bypass this
impermeable surface completely using a surface drainage (or bypass) system. A
surface drainage system uses bands of higher hydraulic conductivity /
infiltration rate material to allow precipitation to bypass the low
conductivity soil, straight from the surface to a piped drainage system. Such a
scheme is detailed in Figure 8; the low conductivity soil (such as a heavy clay) has a very low infiltration rate and low
hydraulic conductivity.
Figure
7 The effect of particle size on infiltration rate:
(a) coarse particles with wide pores allows water to infiltrate easily; (b)
smaller particles with smaller pores restrict or prevent infiltration
In
this system, some of the water from
precipitation will pass slowly into the low conductivity
soil – providing water to the grass plant. The majority of water, however, will
flow across the surface and through the topdressing layer, into the vertical
sand slits and then down into the collector drain and out to an outfall –
completely bypassing the slowly permeable soil.
The
system requires two key components for it to be effective:
1. A
high hydraulic conductivity connection to the surface – if a vertical slit
becomes capped with fine textured soil, water cannot flow into the system and
it will be redundant. For this reason it is necessary to provide frequent
topdressing and careful devoting of the surface.
2. An adequate collector drain network and outfall – if the water is not taken
away, the material will become saturated rapidly and the system will not
function.
1.4
Review of the key principles in selecting land drainage design
For
the design of effective drainage in natural turf pitches the following points
must be considered:
Figure
8 A cross section and plan view of a sand slit system
as an example of a surface or bypass drainage system for a low conductivity
soil football pitch
1. An
investigation to determine whether the problem is caused by a water table
(groundwater drainage) or a low infiltration rate (surface drainage).
2. An investigation to determine the physical properties of the native soil.
3. Determination of the correct depth and spacing for any piped drainage
infrastructure.
4. Calculation of the correct capacity for any infrastructure.
5. Selection of hydraulically compatible and appropriate materials.
6. Connection to free flowing outfall.
7. Provision for hydraulic connection to the surface for bypass systems
Too many land drainage schemes fail because these points have not been
addressed. The system must be designed to solve the problem in the field. As
discussed above, the most common drainage problem is that of surface drainage
in fine textured (clay) soils.
2 Sand Slit Drainage
If
designed correctly and well maintained, sand slit drainage systems, such as the
one illustrated in Figure 8, will reduce the risk of flooding and fixture
cancellation.
The
benefit of sand slit drainage systems A typical system of 1 m spaced sand slits
(350 m deep) over a perpendicular lateral pipe network of 80 mm diameter
drainage pipe (at a spacing of ~ 6 m and depth of 0.6 m) will provide
sufficient drainage to prevent waterlogging in all but the most extreme
conditions; with the proviso that such a system is regularly topdressed with a
compatible sand. Over a period of years, prevention of revenue loss from
fixture cancellation will offset the cost of installation and maintenance of a
sand slit system. Less tangible benefits might include, improved reputation,
the opportunity to stage higher profile fixtures – such as county tournaments
etc, and improved training and youth development.
The
sand slit system is highly effective and can be installed at a lower cost than
a sand carpet system, where the complete pitch area is excavated and replaced
with a purchased high sand content rootzone. The ongoing maintenance of sand
slits is relatively cheaper and more straightforward than a sand carpet system
too.
Investment
in a sand slit and pipe drainage system that is regularly top-dressed will
provide an effective land drainage scheme that will provide a direct,
significant improvement to any facility where it is installed. As such, funding
bodies can feel reassured that sand slit and pipe drainage will provide an
effective return on investment (again, with the proviso that the facility in
receipt has sufficient resources to both top up and topdress the system).
3 Mole drainage
3.1
The principles of mole drainage
Mole
drainage is achieved by pulling a bullet-shaped plough through the soil to
create a contiguous channel at depth. This channel provides a high hydraulic conductivity
bypass conduit for water flow. The Cranfield University Mole Plough is shown in
Figure 9. It is pulled through the soil at a depth of at least 400 mm, with a
slight grade to encourage water flow.
Figure
9
As the
plough is pulled through the soil, at approximately 1-2 m spacing, a vertical
leg slot is formed, in addition a number of cracks are formed from the foot of
the plough up to the surface as the soil is disturbed. These cracks form the
principal bypass for water flow, connecting the surface to the mole channel
(Figure 10). In agricultural, mole drainage surface heave is encouraged to help
loosen the soil. In the drainage of sports surfaces, however, this is
undesirable as surface disturbance upsets ball roll and can be a trip hazard.
There
is a limited range of soil types that are suitable for mole drainage –
essentially heavy, non-dispersive clay soils. The mole channel is cut by the
foot, but formed by the expander. The expander forces displaced soil into the
walls of the channel. In a sandy soil, the soil would simply collapse following
the expander; in a clay soil, the soil is smeared and has sufficient cohesive
strength to hold the channel open. As the channel dries it sets to form a hard
walled channel, which can last anything from
3.2
The benefit of mole drainage in sports fields
The
benefit of this approach to drainage is cost – there are no materials required.
The principal costs are labour, time, fuel and wearing of parts. There are
reduced costs for drainage pipe and backfill material if a lateral collector
system is required. As discussed above, however, the drainage scheme must be
both low cost and drainage effective.
3.3
The cost of mole drainage in sports fields
A
critical factor in the drainage of sports surfaces is the time taken to return
to play following completion of works. With mole drainage there are three
principal concerns:
1.
Surface disturbance
2. Dangerous leg crack widths
3. Durability and collapse of moles
Surface
disturbance is caused by incorrect mole plough design, technique and soil
conditions. Soil conditions are also responsible for short-lived moles too.
Dangerous leg crack widths are a function of a soil property that affects clay
soils and any drainage scheme. Due to the complex mineralogy of clay soils they
exhibit shrink and swell properties. As the clay particles get wet they expand,
as they dry out they shrink. This property causes the
leg slot crack caused by the installation of the mole to expand as the soil
dries through the summer – this expansion can reach a critical width whereby
the risk of ankle joint or tendon injury is too high and fixtures are cancelled
until the soil becomes wet and swells, closing the crack.
Careful
consideration of the factors must be taken into account when installing mole
drains. The critical requirements are the operating parameters – such as depth
of installation, soil moisture content at installation and post installation
management.
The
article is an extract from the final report of a Football Foundation funded
research project entitled The physical and
financial benefits of mole drainage as an alternative to sand slitting in
slowly permeable soils.
The
research was conducted by:
Cranfield University
Contact and principal author: Dr Iain James
Cranfield Centre for Sports Surfaces
School of Applied Sciences, Building 42a
Cranfield
Bedford MK43 0AL
Tel. No: +44 (0)1234 750111 ext 2736
Email: i.t.james@cranfield.ac.uk
www.cranfield.ac.uk/sas/staff/jamesi.htm
TurfTrax Ground
Management Systems Ltd
Contact: Dr Richard Earl
Unit 1, Highfield Parc
Oakley
Bedford MK43 7TA
Tel. No: +44 (0)1234 821750
Email: richard.earl@turftrax.com
www.turftraxgms.com
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further questions please do not hesitate to contact us