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5-
Special Purpose Projects
In recent years there has been an increase in the number of underground
projects carried out for special purposes that were previously less com-
mon. Case histories of such projects as underwater taps, steep inclines,
and deep underground chambers are so few and the data so limited that
~ data
were obtained tor shafts. However, the geotechn~cal factors are equally
important to site selection, design, and construction of these projects.
Construction methods have been changing in recent years, and there
is now a distinct trend to mechanizing the excavation process. The use
of mechanized excavators requires data that may not have been needed for
older excavation processes. Mechanized construction requires a large
capital investment by a contractor, and delays become costly. Thus,
avoiding delays by means of a site investigation can easily recoup the
cost of the investigation.
It must be recognized that the specific parameters to be investi-
gated vary with both site and use. It must also be recognized that a
project comprises several phases--site selection, facility design, con-
struction, and operation and maintenance--and appropriate data should be
collected and analyzed to support the requirements of each of the indi-
vidual phases.
they were not collected for this study; by the same token, limited
access to them; the
DEEP UNDERGROUND CH - BED ID SINS
These two categories are jointly discussed for two reasons. The first
is that most underground chambers, as later defined, require some type
of shaft for access to them; the second is that (as just noted) very
little of the case history data obtained by the subcommittee deals with
either ~ ~~
forts to obtain such data. Consequently, most of the conclusions and
recommendations are extrapolations from data obtained on shallower exca-
vations, tempered by the personal knowledge of the subcommittee members.
In general it may be stated that the geological, geotechnical, and
hydrological information that can be obtained with existing techniques
in the course of siting a deep underground chamber will be sufficient
for the supporting shafts. However, that information must be used
second is that (as
just
deep shafts or seep underground chambers, Respite vigorous er-
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properly to achieve effective design, appropriate bidding, and successful
construction of the supporting shafts.
Deep Underground Chambers
Deep underground projects that require large chambers usually serve one
or more of the following purposes: to extract a resource, to store a
resource, to dispose of waste, to house equipment, or to protect against
hazards. Whether the pr incipal use for the excavation is civil, mili-
tary, or commercial, there will be a requirement to optimize its loca-
tion in light of the intended use and required performance. Site selec-
tion is controlled by the required performance and use, with limited
latitude available to the user for other considerations.
Factors in Site Selection
Chambers excavated for resource recovery are sited on the basis of ex-
ploration that has defined the dimensions of the resource body and the
hazards and exigencies associated with extraction. Chambers excavated
for resource storage or waste disposal are sited, insofar as possible,
to maximize their ability to provide containment and isolation and to
avoid or minimize conditions adverse to safe performance. Chambers for
protection are usually sited according to the nature of the threat and
what is to be protected.
Waste disposal in particular has required the development, still
evolving at present, of a closely constrained siting process. This pro-
cess favors explicit delineation of disqualifying conditions, adverse
conditions, and favorable conditions, each of which is def ined in terms
of the user's ability to demonstrate the likelihood of safe performance
as mandated by regulatory agencies. A waste-disposal chamber must meet
the containment and isolation stipulations during both its operational
(i.e., filling) and functional (i.e., disposal) lifetimes. Given the
long-term toxicity of many of the wastes produced by our society, the
abi 1 ity to s i te, design, and eons true t f ac il i t ies that mus t pe r f orm
safely for many generations poses a signif icant challenge.
Chambers excavated to house equipment derive their site selection
criteria from the operational conditions. Some chambers must withstand
vibration alone; some, vibration and heat; others, vibration and mois-
ture.
Uniform (predictable) rock bodies and groundwater systems, long-term
tectonic stability, and a benign geochemical environment are important
considerations for all storage cavities. In all three categories, the
disqualifying criteria must be specified prior to exploration activities
so that the drilling and logging activities may be planned properly.
Exploration activities must also provide information required for design
and construction, allowing input to updated designs which incorporate
realistic cost and schedule estimates.
Site-Specific Considerations
After choice of a site, for which the controlling criteria should be in-
fluenced by geotechnical data, the specific considerations that must be
addressed in a chamber for resource recovery are similar to those for
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shafts (see next section). The exploration phase for a storage chamber
is somewhat different.
For a storage chamber, the information acquired must confirm the use
of a volume of rock-surrounded space for containment; yet the explora-
tion activities to obtain that information must not significantly reduce
that containment capability. Storage caverns must not encounter faults,
breccia pipes, Elastic dikes, or karst features that can permit the
transmission of contained material within or near their confines. Con-
sequently, the exploratory drilling program for a storage chamber must
thoroughly evaluate the candidate sitets) without creating migration
pathways that cannot be properly sealed. The Ethology of the rock mass
surrounding a storage cavern must be compatible with the material to be
stored. The geochemistry of the host rock must be such that the rock
does not interact adversely with the material to be stored under the
conditions of storage. The strength of the host rock must be sufficient
to withstand the maximum potential adverse conditions. Such adverse
conditions include the potential maximum stresses caused by seismic ac-
tivity, thermal load, hydrogeologic differential, in-situ stresses, and
stress differential. AS a part of the geologic site investigation, all
of these conditions must be evaluated through careful laboratory and in-
situ tests prior to actual emplacement. Depending on the depth and geo-
logic complexity of the proposed facility, an exploratory shaft and test
chamber may be the only practical method of obtaining the quality of in-
situ data needed for final design.
Storage chambers for a single-phase, nontoxic material that gener-
ates no heat require relatively simple preliminary test programs.
Chambers for the emplacement of equipment involve similar site-
specific considerations as those that apply to sinking shafts. These
chambers may also serve as underground civil or military structures and,
as a result, their design and support measures tend to be conservative
compared to those used solely in mining practice. This is because the
chambers are intended for both continuous and "permanent" use rather
than the more limited periods of use customary in mining.
Geology and Construction
The methods of construction for an underground chamber must be chosen so
as to yield a functional product while being compatible with the mate-
rial properties of the host rock. Depending on the material to be exca-
vated, a chamber can be mined either by conventional drill-and-blast
methods or by mechanical mining, or both. The excavation can proceed in
either an axial or radial direction, or in stages in which the two di-
rections interchange or alternate. Because of the large size and shape
of most chambers, excavation usually proceeds from the top down in sev-
eral stages. For the purpose of economy, the handling of broken or
fragmented material should be minimized. Both drill-and-blast and me-
chanical methods of mining should take care not to produce a combination
of natural and induced fractures yielding unstable roof conditions in
the host rock. Blasting operations must be strictly evaluated to ensure
that structural integrity and isolation capability are not adversely
affected.
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Shafts
Shafts may be either vertical or inclined from the vertical. The degree
of inclination is, by definition, generally steeper than 45 degrees from
the horizontal. (Slopes of less than 45 degrees define the "steep in-
cline," discussed in the next section.) Cross sections may be rectan-
gular or circular, or variations thereof; the vertical length is great
compared to the cross-sectional dimension. Shafts may be lined with a
variety of materials or left unlined in highly competent ground.
Shafts may be constructed for access or egress, material hoisting or
lowering, ventilation, mucking, or water conveyance. Many shafts ful-
fill a combination of several purposes; design is governed by projected
end users) and geotechnical considerations.
Factors in Site Selection
In most cases' geotechnical factors dictate the location of an under-
ground facility such as a nuclear-waste repository or a powerhouse.
However, shaft site may be prescribed by the position of such an accom-
panying underground facility, rather than from an optimum geotechnical
standpoint. Nevertheless, every attempt should be made to site the
shaft so as to reduce the impact of undesirable geotechnical features,
within the constraints imposed by the location of the accompanying fa-
cility.
As noted in the previous section, the voluminous data essential to
proper siting of a large underground chamber (or repository complex) are
generally adequate for appropriate shaft design and construction, If
properly used. In cases where the shaft location is essentially prede-
termined by the accompanying facility, there may be no low-cost approach.
For example, the data may reveal the presence of a number of obstruc-
tions to low-cost shaft sinking and may dictate approaches such as blind
shaft drilling or raise boring (vs "conventional" sinking) . However,
the geologic, hydrologic, and geotechnical data should be defined well
enough so that unknowns are minimized, if not totally avoided.
When the end use of a shaft is not specific to a chamber or reposi-
tory, as in the case of resource recovery, the required design and con-
struction data are less likely to be available. In those cases the
trial site selection should be made after an analysis of geotechnical,
environmental, legal, economic, and utility factors. Where geotechnical
knowledge is lacking, surface mapping, seismic surveys, and data from
boreholes should be employed to pinpoint the trial site.
Site-Specific Considerations
At the point that verification of the trial site commences, the geotech-
nical {including hydrologic) considerations become paramount. The scope
of the geotechnical investigation is determined by several factors:
o End use of the shaft
· Typefs) of geology anticipated
· Water conditioned anticipated
· Nature and depth of overburden
Shaft depth
Sinking method (and alternatives) under consideration
Information available from other nearby boreholes or openings.
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In order of priority, the most important data to be obtained pertain
to the water conditions that will be encountered, and to the character-
istics of the soil and rock that the shaft must penetrate. This is
because water, or water in combination with unconsolidated ground, has
historically presented the greatest impediment to conventional shaft
sinking. The shaft exploration should include, as a minimum, a cored
borehole or holes near the shaft site or within its area,, because geo-
logic and hydrologic conditions may vary considerably over a short dis-
tance, even in flat-lying sedimentary formations or strata.
With nonhorizontal geologic features, and especially in areas where
potentially near-vertical features occur, more than a vertical hole is
required. It is also important to drill an angled hole or holes
{depending on depth) in an effort to detect unfavorable features such as
parallel faults, dikes, or shear zones.
Deep shaft exploratory holes drilled without directional techniques
may eventually deviate from the vertical enough to make their informa-
tion ambiguous, and hence less relevant. Although this may not be a
problem in uncomplicated geologic settings, it is recommended that
straight-hole drilling techniques be used where difficult conditions are
expected and specific data are required.
In all cases, the core should be carefully logged and a geologic
section prepared. (Electric logging may be helpful in preparing useful
information.) Tests to be conducted are tailored to the specific situa-
tion, but some of the more essential determinations are the following:
· Potential water inflow and groutability (or freezability) at
various elevations, when drilling fluid is lost or water is sus-
pected.
· Swelling ground, when the potential is expected.
· Standing water level (which should be recorded).
· Chemical composition and pH of the water.
· Orientation of the exploratory holes.
· Rock and water temperatures, in deep shafts or in shafts where
freezing or extra ventilation may be required (or where personnel
may subsequently have to work).
When the foregoing have been determined (and their importance cannot
be overemphasized), it may appear time- and cost-effective to change the
approach from a "conventionally" sunk (drill-and-blast) shaft to a
drilled shaft or shafts. One example of an environment where a drilled
shaft presents an attractive alternative is in the more heavily water-
bearing areas of the Grants (New Mexico) uranium belt.
Geology and Construction
A thorough and systematic geologic/hydrologic/geotechnical investigation
is fundamental to time- and cost-effective shaft construction. Proper
exploration will predefine the problems and will permit an engineer's
estimate to accurately reflect the actual construction costs.
The determinations noted above will confirm whether certain alterna-
tives may work. Additional tests should then be made, depending on the
desired or anticipated construction methods. For example, if dewatering
or Repressurization is contemplated, additional boreholets) and related
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tests should be carried out. The relationship between appropriate geo-
logical information and selection of shaft construction methods is am-
plified in the following discussion of freezing, grouting, ventilation,
mechanical mining, and drilling.
When freezing is contemplated, groundwater velocity and direction of
movement and in-situ water content are important; laboratory tests to
determine the properties of the frozen material should be conducted.
Gas content and the potential weakening of a freeze wall by dissolution
of gas should be addressed. In cases where freezing is to be used,
ultrasonic readings should be taken between freeze holes before ground
freezing starts, in order to provide a comparative background for later
ultrasonic testing to detect windows in the freeze wall.
When grouting is anticipated, laboratory tests should be conducted
to determine the ground characteristics related to grout-curtain design
and grout selection, such as grain size and distribution, porosity, and
permeability. The feasibility of pregrouting should also be studied.
Pregrouting is becoming increasingly important as down-the-hole mining
machines become more commonly employed in shaft sinking, considering the
relatively greater capital cost involved in stand-by time.
In exploration for deep shafts, temperature measurements are essen-
tial to the selection of mining method, need for and design of freeze
walls, and design of ventilation during both construction and operation.
When mining by mechanical methods is considered, physical specimens
should be tested to provide data for estimating rates of penetration and
cutter costs. Ground having a potential for balling of the bit or cut-
ters should be characterized carefully in cases where surface-based or
down-the-hole boring is contemplated.
When drilling in the form of raise bore and slash or full face is
employed, the angles of formation intersection and behavior of the core
holes will indicate potential accuracy problems. For drilling, the
ability to control potential sloughing or wall failure with mud becomes
important. The ability to provide appropriate control should be ad-
dressed at the geotechnical investigation stage in consultation with mud
experts. The potential for sloughing in a raise bore should be assessed
geologically, because the raise bore and slash method can be very ad-
versely affected by sloughing.
The potential for high in-situ stress in the rock should be eval-
uated for effect on construction methods. In extreme cases, prestres-
sing and benching may be indicated instead of full-face blasting.
STEEP INCLINES
A steep incline is a tunnel or shaft constructed on a slope; roughly de-
fined, it includes angles between about 15 degrees and 45 degrees from
the horizontal. The method of construction is almost always drill-
and-blast, although the use of TBMs is becoming more common. The actual
construction can proceed either upward ("positive angle" ~ or downward
~ "negative angle". Incline is an acceptable term for both, but downward
construction is sometimes referred to as a decline.
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As is true for shaft sinking by drill-and-blast, inclines are among
the most difficult, hazardous, and costly underground excavations. Geo-
logical conditions will affect water handling, roof support, and the
shape of the opening. Particular attention to roof conditions is re-
quired, as inclines frequently intersect the bedding planes of a strati-
fied rock formation at low angles, which creates a situation prone to
roof falls. In weakly bonded sediments, a rectangular opening may be
more desirable than a circular cross section because a circular configu-
ration may encourage the cusps, or corners, to fall out.
Water inflows create difficult problems for incline construction,
and efforts to stem flows by grouting, freezing, or pumping down the
water table are frequently required. However, if construction proceeds
upward, some inflow of water may be considered an advantage if it can be
employed to remove the muck.
Factors in Site Selection
l
An incline has inherent advantages and disadvantages in site selection
that are similar to those that affect a tunnel. During site investiga-
tions there is usually some discretion permitted in routing the incline
to avoid the worst ground conditions. A thorough geological profile of
the immediate area is therefore warranted. Particular attention to
water flow and the dip of the strata will minimize two of the major haz-
ards of slope construction.
Site-Specific Considerations
In most cases, once the course of an incline has been determined from
general site geological studies, a limited site-specific study has been
the normal approach. Coring of additional, closely spaced boreholes
along the specific route is unusual, unless perhaps a particularly crit-
ical zone is suspected. Boring an inclined hole along the proposed axis
of the incline is a highly desirable procedure, once the location and
bearing have been determined. Such a boring eliminates the need for
probe holes during construction. The feasibility of this approach, how-
ever, is limited by the length of the incline.
A more usual approach is to prepare contingency clauses in the con-
struction contract and handle special problems if and when they arise.
Without an inclined exploratory boring, the progress of the incline can
be delayed as the excavation approaches an anticipated fault zone. In
this case, a probe drill can be bored ahead of the face to determine the
exact nature of the zone; then grouting and/or stabilization methods can
be employed if necessary.
Roof support methods are probably the most frequent deviation from
the construction specifications. The bonding of formations is perhaps
the most critical rock characteristic in determining proper roof support
in an incline. This characteristic is also one of the more difficult to
predict on the basis of core samples. As construction progresses,
instrumentation to assess roof and rib creep or floor heave is war-
ranted, unless similar excavations have determined otherwise.
Geology and Construction
The direction, geology, purpose, and shape of the completed incline (or
decline) influence the methods of construction. Methods include drill-
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and-blast, tunnel borers, road headers, raise drills, or box-hole drills.
Each method has benefits and limitations.
On a relatively short incline, raise drills or box-hole drills may
be considered. Although modern raise drills are capable of pulling
raises as far as 3,000 ft. it becomes very difficult and hazardous to
trip the reamer head at the slopes considered here (15 to 45 degrees
from the horizontal). The raise drill is therefore limited to what can
be achieved on a single pull. If machine boring or excavating is con-
templated, site investigations require an assessment of both rock
strength and abrasiveness.
A box-hole drill operates in the same excavation mode as a raise
drill, except that the head is pushed up from below rather than pulled
down from above. This process is slower and more costly than raise
drilling, and is limited to a length of about 600 ft. It is, however, a
mechanical method of achieving a steep, blind incline.
Tunnel boring machines have found limited use in downward construc-
tion but have been successful in upward construction. On a downward
slope, these machines have difficulty removing the cuttings from the
face. Full-face borers have been used in upward slopes, including one
at 45 degrees. Muck can be removed by flushing with the aid of gravity
in such cases. However, compared to traditional horizontal rates, prog-
ress of the machine is slow and costs are higher.
Drill-and-blast is still the most common method of incline construc-
tion. Because the rates of mechanized methods are slow, the lower capi-
tal investment of the dr ill-and-blast method is attractive . Muck remov-
al is also simplified with free access to the face by loading equipment.
Inclines driven by the drill-and-blast method typically use specialized
suspended-track climbers which provide drilling and ground support in-
stallation platforms. Depending on the slope angle, the inclines can be
self-mucking and are free-draining. Drill-and-blast also offers versa-
tility in shape of the tunnel (i.e., the cross-sectional shape can vary
with depth).
A method which in some cases competes with drill-and-blast is a
roadheader. These mobile excavators offer the flexibility and access to
the face equivalent to drill-and-blast techniques, while providing the
safety and smooth surfaces of a mechanical excavation. The major weak-
ness to date has been the inability of the roadheaders to economically
cut hard or abrasive rocks. Recent developments in water-jet-enhanced
picks and a mobile unit employing rolling cutters may increase the pro-
portion of mechanically excavated inclines.
The purpose of the incline also affects the construction method and
is similarly related to geological conditions. Tubular shapes are de-
sirable for aqueducts, whereas a flat floor is required for operating
vehicles. In certain applications, smooth walls and minimum disturbance
of surrounding rock can be compelling requirements. Therefore, the
geological studies of slope sites will also have a profound influence on
the choice of construction method within the constraints imposed by end
use.
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UNDERWATER TAPS
An underwater tap is a subsurface connection between a body of water and
an onshore installation. Taps may be required for either the withdrawal
or discharge of water by the installation.
The applications for underwater taps are growing more numerous, and
will continue to increase within the United States as water and power
resources are developed. Hydroelectric power will be extracted with low-
head generating technology from dams and sites previously ignored. Pow-
er plants require large quantities of water for cooling. The trend is
for waste water from urban sources to be discharged into the sea by con-
trolled diffusers rather than using only rivers or open drainage.
Because of the expanding need and its implications for the future, site
selection procedures and planning of underwater taps based on geological
study are increasingly important.
Frequently the tap is constructed between a vertical, or steeply in-
clined shaft and a horizontal tunnel. A direct lateral tap into a body
of water is avoided if possible because of the difficulty in judging the
breakthrough cut. The final cut must be taken with utmost care because
there is minimal opportunity to correct this tap after the workings are
flooded. The tap should be made as nearly normal to the bottom of the
water body as possible to assure uniform thickness of the final plug.
Factors in Site Selection
In a project involving an underwater tap, location of the tap must be
considered in relation to the other project facilities. In some instan-
ces, such as an offshore intake or discharge, considerable latitude in
location is possible. In others, such as a powerhouse intake, the tap
location may be dictated largely by adjacent facilities.
In either case, as much geological data as possible should be ob-
tained, including at least the use of overt lights, photography, and a
study of the surrounding formations. Cores should be taken and exam-
ined. Seismic examination may also be considered, as tap construction
depth is generally shallow. A complete geological profile should be de-
veloped, with particular attention to faults, weathered rock, blocky
ground, and parameters for determining water inflow {e.g., permeability,
discontinuity spacing, and hydrostatic head). Usually a tap will in-
volve drill-and-blast excavation; therefore, RQD, abrasiveness, and com-
pressive strength are significant factors.
It is important to remember that construction of a tap often crosses
two interfaces and involves two or three dissimilar construction methods.
The geological interfaces are the water-sediment boundary and the sedi-
ment-underlying rock boundary. Construction through the water-sediment
boundary may involve piling construction with excavation by dredge,
pump, or clam. At the rock layer, a blind drill using reverse circula-
tion for muck removal may be involved. From the opposite direction, a
tunnel or incline construction is probable. Geological considerations
will probably dictate the method and type of equipment selected for tap
construction, and will probably affect its location as well.
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Site-Specific Considerations
Once a specific site is selected, the complexities of design and con-
struction of an underwater tap demand that a thorough site investigation
be conducted prior to initiating final design or construction. The on-
shore site must be investigated by geological mapping, and core drilling
is needed to eliminate as many uncertainties as possible. Thorough off-
shore studies are required as well. Cored boreholes are essential;
cores can be drilled in the subsurface rock using barges or platforms.
Inspection by divers, underwater photography, and soil sampling are use-
ful for determining the water-sediment and sediment-rock interfaces.'
It is particularly important to establish the occurrence and charac-
ter of faults and intercepting joints or fractures that could conceiv-
ably conduct water. The strength of the rock and definition of the
strata are needed to ensure structural integrity of the completed in-
stallation. Determination of the applicable rock properties will en-
hance the selection of construction equipment and control of its opera-
ting cost.
In the case of an underwater tap, a second opportunity for specific
site investigation occurs when construction nears the point of break-
through. Given the penalty for error if the crucial tap is performed
carelessly, this opportunity should not be ignored. From a tunnel,
probe drills, with packers or operating through a seal, can safely drill
horizontally for about 200 ft or less. If a drilling operation is in-
volved, a probe drill along the axis of the excavation is possible.
Seismic and electromagnetic pulse examination should prove effective;
density changes are among the most readily detectable characteristics
recognized by seismic tests. Electromagnetic (radar) means are some-
times successfully used for identifying aquifers at close range. The
objective of this final study is to accurately identify the rock bounda-
ries and confirm evaluations of the integrity of the rock.
Geology and Construction
Consideration must be given to the methods of geological analysis and
potential treatment in relation to the type of excavation to be em-
ployed. A fissure intersecting the floor of a water body may not be a
deterrent to blind drilling, but it could be calamitous in a hand-
excavated tunnel. Treatment such as grouting or ground stabilization
must be compatible with the planned construction method.
Methods of creating the final tap vary with end use, but generally
f all into four categories.
· The vertical structure is built on bed rock offshore. A
caisson may be built to surface. Bed rock below the structure is
drilled down to a point where it will intersect the approaching tunnel.
This cut can be "wets or "dry." If the offshore structure permits, the
vertical column can be pumped out and the f inal cut is dry. The fa-
cility is then flooded by removing a portion of the vertical structure.
o A reverse procedure can be employed, in which the tunnel por-
tion is completed first and the connection is made by drilling into it.
An underwater caisson is built on bed rock above the tunnel and the con-
necting hole is drilled, sometimes through the caisson and into the tun-
nel. This procedure is used where a diffuser outflow or manifold tap is
employed.
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· The connection may be made by dr illing upward from the tun-
nel. A raise drill can be employed from a drilling platform. Again,
wet or dry connections are pass ible .
· The caisson built offshore on bed rock may contain a plug.
After contact with the caisson is made from the tunnel, either below or
horizontally, the plug is removed by explos Ives. Such a hydraulic bulk-
head can even be dewatered by pumping so that a liquid explos ive can be
poured in and detonated remotely. When blasting the final plug wet, a
sump or trap may be built downstream to catch and permanently store the
rock from the final blast or spread it evenly along the floor of the
tunnel.
45
-
Representative terms from entire chapter:
construction methods
