 | Drilling |
| Maintaining records |
| Screen installation |
| Grouting well casing |
| Measuring-in and levelling |
Soil sampling is covered in Appendix 4.2, water sampling in Appendix 4.3. It has been
the intention to develop the following as a set of minimum requirements that should be met
when drilling. As a consequence, this Appendix can be included as the basic requirements
in connection with submission of tenders for drilling and well construction. Additional
requirements must be formulated in relation to the purpose and nature of individual
investigations.
Statutory Order No. 4 (4 January 1980) on the execution of wells for groundwater /3/ specifies present regulations for drilling, abandoning,
construction, and notification, etc. According to the Statutory Order on design of wells
for groundwater, GEUS must be notified within three months of drilling of any wells
serving this purpose.
The designation ’localisation borings’ is used to denote soil borings
conducted in order to identify and describe contamination of the upper soil strata and
groundwater aquifers near ground level. Such borings are often used during the preliminary
stages of the investigation process.
In connection with very shallow soil borings (1-2 m) , hand drilling is possible. For
this purpose a post-hole auger or a spade drill can be used (Dutch drilling equipment) /1/. Otherwise, shallow exploratory soil borings are usually
carried out using a solid-stem auger or a bucket auger as drilling tool. If geological
conditions permit, the borings can be executed without the use of casing, since this
method can provide adequate level-specific soil samples in connection with shallow borings
(to a depth of approximately 3 m and above groundwater level).
In principle, the dimension of the borehole is discretionary. The choice of dimensions
must be consistent with any desired screen size, including the gravel pack. Normally, a
screen with a diameter of 63 mm (50 mm) is inserted in 6" soil borings, whereas only
a 25 mm piezometer can be placed in 4" soil borings. If screens are installed in an
actual groundwater aquifer, the well casing should always be grouted with bentonite,
typically at the level of any low-permeable strata. Figure 1 features an example of a
screen installation.
Shallow localisation soil borings can be filled back with the excavated material if the
soil borings do not exceed a depth of 3-4 m, or if they do not cut through more than one
groundwater aquifer.
In contrast to the shallow localisation soil borings, casing should always be used
during the drilling of investigation borings. This is due to the fact that these borings
are deeper, and that satisfactory sampling is usually the objective, either of soil or
water or both. The use of casing will prevent cross contamination between the various
strata. Also, the use of casing is necessary when drilling below the groundwater level
Normally, augering is used. However, with non-cohesive soil (sand/gravel) below
groundwater level, the cable tool method will often be used /1/.
When using a cable tool, water is usually added. It is necessary to be aware of this in
connection with subsequent water-sampling, where removal of stagnant water is particularly
important to facilitate representative sampling.
Other drilling techniques may be appropriate to the extent that no soil samples are
required or that an impaired quality and/or quantity of geological information is
acceptable. Such techniques might for include e.g. driven well or drilling with a
hollow-stem auger. Deep investigation borings should not be located in the most
contaminated areas (hot spots) of a contaminated site /2/.
One of the primary targets of investigation soil borings is to ensure the water
sampling over a long period, at specified depths, through a screen. As a consequence, the
dimensions of soil borings must be selected according to the desired screen installation.
Screen dimensions are selected according to the purpose of the soil boring. Typical
associated dimensions and maximum pumping yields are presented in Table 1.
Table 1
List of recommended screen dimensions
Purpose |
Minimum screen
diameter/ mm |
Field measurements or sampling with
suction pump etc. |
25 (21.5 mm inside) |
Sampling with smallest submersible pump
(max yield approx. 2 m3/h) |
63 (52 mm inside) |
Pumping with smaller submersible pumps
(max yield approx. 15 m3/h) |
110 (99.4 mm inside) |
Pumping with larger submersible pumps
(max yield approx. 15-40 m3/h) |
160 (149 mm inside) |
Pumping with larger submersible pump (max
yield approx. 40-80 m3/h) |
225 (203 mm inside) |
Pumping with largest submersible pump
(max yield approx. 40-80 m3/h) |
315 (285 mm inside) |
The connection between the above maximum yield values and the associated minimum
dimensions must be seen as guidelines. The above values are determined using the
dimensions for commonly used submersible pumps (such as GRUNDFOS type). Attention must be
drawn to the fact that pump types exist with different dimensions, and to the fact that
the water head influences pump yield.
Table 2 presents guidelines for the connection between borehole dimensions and screen
dimensions. The recommended screen dimensions should not be exceeded, as this will result
in and insufficient gravel pack and grouting.
The borings must be protected at ground level by a concrete protective casing with a
cap, or by installing dry wells. Wells to deeper groundwater aquifers must be fitted with
lockable seal caps or covers.
Table 2
List of recommended screen dimensions
Borehole diameter |
Screen diameter/ mm |
4" = 100 mm |
25, (50) |
6" = 150 mm |
63, (90) |
8" = 200 mm |
110, (125) |
10" = 250 mm |
160, (140) |
12" = 300 mm |
200, (160, 225) |
16" = 400 mm |
250, (315) |
The designation ’wells’ denotes borings whose construction correspond to
water-supply wells. The primary purpose of these borings is to pump up groundwater from a
groundwater aquifer, whereas soil samples from these borings are only used for geological
assessment and are analysed only if contamination is registered during boring. Borings of
this kind are primarily used for monitoring groundwater, and sometimes for remedial
pumping. With the exception of remedial borings, wells should not be located in the most
contaminated areas (hot spots) of a contaminated site /2/.
When carrying out control wells outside the contaminated site, there is basically
freedom of choice as regards drilling method. This means that rotary drilling or air
drilling, e.g. Odex drilling might be considered. These methods can be advantageous in
connection with solid rock such as carbonate deposits. /1/.
When carrying out rotary drilling, where drilling fluid is added to the borehole, the
chemical content of the fluid should be tested. It is also particularly important to
carefully remove the drilling fluid by developing the well before water sampling takes
place. Rotation drilling should only be utilised outside the contaminated site.
The size of the borehole is chosen in accordance with the intended screen dimension.
With these types of borings, a relatively large screen dimension will be selected, such as
160 mm, 200 mm, or 250 mm. See Table 2.
The borings must have a concrete protective casing as the minimum protection. It may
prove necessary to establish an actual dry well or, as an alternative, an insulated
covering at ground level. Wells reaching lower groundwater aquifers must be reported to
GEUS. Table 3 has an overview over recommended drilling methods.
Table 3
List of recommended drilling methods
|
Localisation soil borings |
Investigation borings |
Wells |
Augering |
x |
x |
x |
Cable tool |
|
x |
x |
Electrical log soil boring |
x |
x |
|
Hollow-stem auger |
x |
x |
|
Driven soil borings |
x |
|
|
Air drilling |
|
|
x |
Rotation drilling |
|
|
x |
During drilling, a record should be maintained stating:
| Site, soil boring no., and date |
| Drilling method |
| Preliminary soil type assessment/classification |
| Recorded signs of contamination (discoloration, odour) |
| Stratum boundaries |
| Sampling depth |
| Drilling depth |
| Screen installation |
| Grouting |
| Water level observations |
The screen installation should be described in a scale drawing. Water level
observations made during soil boring should be indicated in relation to a permanent
measuring point that can be found again later.
Normally, threaded PEH or PVC tubes are used as observation and screen pipes. The
thread facilitates oil-free pipe connection.
Screen installation is carried out by means of slotted tubes packed with gravel. During
screen installation, it can be expedient to place a length of casing to collect
fine-grained material – a silt box.
The screen arrangement must be closed at the bottom using a PVC or wooden plug (not
pressure-impregnated wood).
To account for possible blocking around the casing, soil borings that penetrate
limestone rock or other hard rock must also have screens installed. The screen must be
established near the aquifer layer. The screen installation should represent only one
groundwater aquifer.
The screen can comprise the entire groundwater aquifer or parts thereof. By using short
screens, low dilution and more depth-specific samples are obtained. However, this method
also means that screens will normally be required at more levels, hence more samples for
analysis will be necessary. In localisation borings where no real aquifer layer has been
found, screens of 1-2 m are installed from the bottom of the boring.
By screens in connection with unconfined groundwater tables, the upper side of the
screen should be above the water table to facilitate registration of any oil or similar
substances floating on the water table as well as taking account of fluctuations. The
screen must be packed with cleansed quartz silica sand. The gravel packing must be
carefully monitored to ensure that all cavities between screen and wall of soil boring are
filled in. This monitoring must ensure that the upper side of the gravel packing
constitutes a solid foundation for the necessary grouting in the annular space between
well casing and the wall of the boring above the screen.
When installing screens, screen materials must be selected to fit both the slots on the
screen and the grain size. Emplacing the gravel pack could take place during water
circulation. This ensures optimum packing of the screen gravel. The gravel pack should be
led at least 0.5 m above the screen to avoid bentonite choking the screen. For the same
reason it is advantageous to select finer-grade screen sand for the top 0.5 m. Immediately
upon completion, the soil boring must be pumped clean to achieve optimum effect.
In certain circumstances it may be necessary to add water during soil boring as well as
screen installation. The addition of water may be vital to the quality of soil boring as
well as of screen installation and grouting. It is a requirement that the water used in
connection with drilling and screen installation is uncontaminated tap water.
The purpose of grouting the well casing is to avoid unintentional spreading of
contamination as a result of the soil boring. If contaminated soil strata with clay
substrata are drilled through, it is important to carry out grouting with bentonite at the
clay stratum. It must always be calculated on that natural barriers (e.g. stratification
of the formation) will be destroyed during soil boring. These barriers are best restored
by grouting along the entire length of hole without screens. Bentonite is used for
grouting, whether bentonite powder or granulate. Grouting with "clay balls"
alone will not suffice.
Granulate must always be poured into water. For this reason, water must be added to the
space between casing and wall of soil boring if granulate is used above the groundwater
table. An alternative is bentonite powder, which is stirred in a mixing vessel to a
porridge-like consistency of bentonite slurry.
When grouting at ground level, the bentonite can be poured from ground level. When
grouting at greater depth and beneath the groundwater level, liquid bentonite must be led
down through a pipe (observation pipe) or pumped down using a special pump. In most cases
the use of bentonite granulate is an alternative, possibly in the form of expanding
bentonite. Granulate has the advantage of facilitating easier measurement of the location
and thickness of the grouting during the grouting process. On the other hand, better
contact between drilling hole and casing is obtained by using liquid bentonite /2/.
The location of the grouting depends on geological/hydrogeological conditions. The
following figures show 4 main types of geological conditions with indications of correct
location of groutings. In connection with shallow investigation soil borings (Figure 1) it
is acceptable to fill the hole back up with extracted material (there should, however,
always be a minimum seal of 1 m). The reason that this applies to shallow soil borings in
particular is that soil boring has been carried out in the top soil strata and in
groundwater aquifers near ground level. If these strata are already contaminated, the
extracted material will not add any new contamination. If the top soil strata are
uncontaminated, the backfill will likewise be uncontaminated.
Figure 1
Minimum seal of localisation boring with screens installed in aquifer near ground
level.
Figure 2
Grouting of deep wells with screens installed in aquifer near ground level
Figure 3
Grouting of deep investigation well to a deep lower aquifer (overlying gravel strata
in hydraulic contact with the limestone).
Figure 4
Grouting of deep investigation soil boring/well to deep lower aquifer (lower aquifer
and aquifer near ground level are separate).
In connection with soil borings at contaminated sites it is necessary to consider the
question of disposal of the extracted material. Regardless of the degree of contamination
it is often unacceptable to leave the material at the investigated site.
Since disposing of contaminated soil can be costly, an aim during boring should be to
separate uncontaminated soil from soil which is deemed to be contaminated. It would be
advantageous to keep soil extracted from individual soil borings separate with an aim to
subsequent separation of contaminated and uncontaminated soil. If there is any doubt
whether or not soil is contaminated, it should be considered contaminated and treated
accordingly.
Uncontaminated soil can be deposited at a landfill. However, contaminated soil must be
turned in for treatment at a central processing plant. Consequently, final decisions as
regards disposal must wait, pending the results of soil analyses.
All soil borings should be charted on a map and all soil borings with screens installed
should also be levelled out. Borings are charted in relation to buildings or in a system
of co-ordinates. In a minority of cases it may be necessary to level localisation borings
in a relative datum system. In this case, a retrievable relative reference point is used.
This relative fixed point is assigned the datum +180, preventing any doubt as to whether
this is a relative or absolute datum. In connection with levelling the datum is set at
ground level with a degree of accuracy of 0.1 metres and at observation point with a
degree of accuracy of 0.01 metres (whenever possible, this should be to the highest point
of the top of casing to avoid confusion). Marking the point of measurement facilitates
recognition if the casing is subsequently shortened.
All borings must be safely abandoned when they are no longer needed. The guidelines in
S. 15 of the Ministry of Environment and Energy Statutory Order No. 4, January 1980 must
be followed when abandoning and grouting obsolete borings /3/.
The owner of the boring is responsible for ensuring that abandoning procedure is carried
out.
References
| /1/ |
Hvam, T. Markundersøgelsesmetoder (‘Field
Investigation Methods’). DGF-bulletin 5, September 1990.
[Tilbage]
|
| /2/ |
Baumann, J. Kvalitetssoil boringer (‘Quality Soil
borings’). Geologisk Nyt 4/1996.
[Tilbage]
|
| /3/ |
Bekendtgørelse nr. 4 af 4. januar 1980 om udførelse af
soil boringer efter grundvand (‘Statutory Order No 4, 4th of January 1980 on
executing soil borings for groundwater’.) The Ministry of Environment and Energy.
[Tilbage] |
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