Current through Register 1531, September 27, 2024
(1)
General. Design
and construction of dams shall comply with 302 CMR 10.14. Design and
construction standards that are not included in 302 CMR 10.14, shall conform to
design procedures established by: The U.S Army Corps of Engineers, the U.S.
Bureau of Reclamation, the U.S. Natural Resources Conservation Service and
other generally accepted engineering practices and principles. Where specific
site conditions may exist which warrant appropriate changes in the following
design and construction criteria, the Commissioner shall review and approve the
design.
(2)
Foundations
and Abutments. The foundations and abutments investigation shall
consist of borings, test pits, and other subsurface exploration necessary to
assess the soil, rock, and groundwater conditions.
(3)
Construction
Materials. Specifications for construction materials shall
establish minimum acceptance criteria so that anticipated design properties are
achieved. If the use of onsite borrow materials is specified, exploration,
testing, and calculations shall be performed to indicate that there are
sufficient quantities of material available that meet the design
criteria.
(4)
Surveys. Surveys shall be made with sufficient
accuracy and scale to locate the proposed construction and to define the volume
of the storage in the reservoir. The downstream area shall be investigated in
order to delineate the area of potential damage in case of failure. Locations
of centerlines, and other horizontal and vertical control points, shall be
shown on a map of the site.
(5)
Hydrologic Investigation. The drainage area shall be
determined. Present land use shall be considered in determining the runoff
characteristics of the drainage area. All hydrologic assumptions and design
calculations shall be included in the report.
(6)
Spillway Design.
(a) The spillway system shall have a capacity
to pass a flow resulting from a design storm, as indicated in the following
table, unless the applicant provides calculations, designs and plans to show
that the design flow can be stored, passed through, or passed over the dam
without failure occurring.
SPILLWAY DESIGN FLOOD DESIGN STORM
|
Hazard Potential
|
Size
|
Existing Dams
|
New Dams
|
|
Low
|
small
|
50 year
|
100 year
|
|
intermediate
|
50 year
|
100 year
|
|
large
|
100 year
|
100 year
|
|
Significant
|
small
|
100 year
|
500 year
|
|
intermediate
|
100 year
|
500 year
|
|
large
|
500 year
|
1/2 PMF
|
|
High
|
small
|
500 year
|
PMF
|
|
intermediate
|
1/2 PMF
|
PMF
|
|
large
|
1/2 PMF
|
PMF
|
(b)
Vegetated earth or unlined emergency spillway(s) will be approved when
computations indicate that it will pass the design flood without jeopardizing
the safety of the structure. The risk of recurring storms, excessive erosion,
and inadequate vegetative cover will be considered acceptable in such a
spillway when its average frequency of use is predicted to be no more than
indicated in the following table.
EMERGENCY SPILLWAY FREQUENCY TABLE
|
Hazard Potential
|
Size
|
Existing Dams
|
New Dams
|
|
Low
|
small
|
25 years
|
25 years
|
|
intermediate
|
25 years
|
25 years
|
|
large
|
25 years
|
25 years
|
|
Significant
|
small
|
25 years
|
50 years
|
|
intermediate
|
25 years
|
50 years
|
|
large
|
50 years
|
50 years
|
|
High
|
small
|
50 years
|
100 years
|
|
intermediate
|
50 years
|
100 years
|
|
large
|
100 years
|
100 years
|
(c)
The Department recognizes that the relationships between valley slope and
width, total reservoir storage, drainage area, and other hydrologic factors
have a critical bearing on determining the safe spillway design flood. Rational
selection of a safe spillway design flood for specific site conditions based on
quantitative and relative impact analysis is acceptable. The spillway may be
sized so that the increased downstream damage resulting from an overtopping
failure of the dam (i.e., the selected spillway design
capacity has been exceeded) would not be significant when as compared with the
damage caused by the flood in the absence of dam overtopping failure. In
lieu of quantitative and relative impact analysis, the
preceding table shall be used as spillway design criteria.
(d)
Lined Spillways and
Channels. The design report shall include design data criteria for
open channel, drop, ogee, and chute spillways and other spillway types that
include crest structures, walls, channel linings, and miscellaneous details.
All masonry or concrete structures shall have joints that are relatively water
tight and shall be placed on foundations capable of sustaining applied loads
without undue deformation. Provisions must be made for handling leakage from
the channel or under seepage from the foundation which might cause saturation
of underlying materials or uplift against the undersurfaces.
(7)
Conduits. A gate or controlled conduit shall be
provided to drain each reservoir.
(a) Any new
and/or existing conduit design shall include the computation of the minimum
time required to drain the reservoir.
(b) All pipe conduits shall convey water at
the design velocity without damage to the interior surface.
(c) Protection shall be provided to prohibit
unsafe seepage along conduits through the dam, abutments, and foundations. The
specific design for seepage protection along conduits shall be shown in the
drawings and specifications.
(d)
Adequate allowances shall be incorporated in the design to compensate for
differential settlement and possible elongation of the pipe conduit.
(e) Trash racks shall be installed at the
intake of conduits to prevent clogging the conduit.
(f)
Pipe Conduit
Materials.
1. Pipe conduits shall
be designed to support the total external loads in addition to the total
internal hydraulic pressure without leakage.
2.
Reinforced or Prestressed
Concrete Pipe Conduits.
a. All
conduits shall be designed and constructed to remain watertight under maximum
anticipated hydraulic pressure and maximum probable joint opening, including
the effects of joint rotation and extensibility.
b. Provisions for safe movement of the barrel
shall be provided at each joint in the barrel and at the junction of the barrel
and riser or inlet. Cradles shall be articulated if constructed on a yielding
foundation.
c. The owner's engineer
shall submit the final design details of the proposed pipe to be used for all
significant and high hazard potential dams.
3.
Corrugated Metal Pipe
Conduits.
a. Corrugated metal
pipe shall not be used in any dam, except for special cases where the design
engineer can adequately demonstrate satisfactory performance. Any exemption
which allows their use must be issued in writing by the Commissioner.
4.
Dissipating
Devices. All gates, valves, conduits and concrete channel outlets
shall be provided with an energy dissipater designed and constructed to control
erosion and prevent damage to the embankment or the downstream outlet or
channel.
(g) In the case
when an alternative method(s) of drawdown is requested, the proponent shall
submit with the permit application reasons why a waiver should be granted
(i.e., contaminated sediment, funding issues, complexity of
construction). The request for waiver shall demonstrate that the water in
storage can be moved out of the reservoir by mechanical means. The project
design report shall include a detailed description of the pumps, siphons,
etc. that would be necessary to remove the stored water in a
reasonable period of time and maintain the reservoir in a dry state if
necessary. A detailed drawdown plan must be included in the design, that
identifies the volume of water in storage, the rate of inflow under average
inflow conditions, identification of pump equipment, or other means necessary
to remove stored water and maintain a drawdown condition, the time it will take
to lower the water level, etc. The alternative drawdown plan
shall be included in the required Operation and Maintenance Manual (O&MM)
and in the Emergency Action Plan (EAP), if required.
(h) In the case where an existing conduit is
in poor condition (i.e., severely deteriorated, structurally
compromised, leaky) and the condition could compromise the structural stability
of the existing dam, the design report shall address the compromised conduit
condition (relining, slip lining, grouting or other feasible means) and bring
the existing conduit to safe and good condition.
(8)
Seepage Control.
(a) All dams shall be designed and
constructed to prevent the development of instability due to excessive seepage
forces, uplift forces, or loss of materials in the embankment, abutments,
spillway areas, or foundation. Seepage analyses for design shall identify areas
having high internal uplift or exit gradients.
(b) The design shall include an embankment
internal drainage system, a zoned embankment, a foundation cut-off, an upstream
blanket, a sufficiently wide homogeneous section, or other methods to protect
against instability from excessive seepage forces or high hydraulic
gradients.
(c) For high hazard
potential dams, a flow net analysis shall be made to determine the location of
the phreatic surface, flow lines, and equipotential lines within the embankment
and its foundation. These analyses may be based on graphical construction,
electrical or liquid analogs, soil prototype methods, or other generally
accepted methods. The flow net and stability analysis shall be the maximum
water storage elevation. Possible fluctuations in tail water elevation shall be
included in the analysis. The flow net and seepage analysis shall be included
in the final design report.
(d)
Piezometers for confirming the location of the phreatic surface assumed for
seepage and slope stability analyses shall be considered by the design engineer
for low and significant hazard potential dams and shall be required for high
hazard potential dams. Where piezometers are required, their design, depths and
locations shall be provided in the final design report.
(9)
Structural Stability and
Slope Protection.
(a) Design and
construction of dams to assure structural stability shall be consistent with
accepted engineering practice. The scope and degree of precision that will be
required for a specific project will depend on the conditions of the site and
the damage potential of the proposed structure. Consideration in design for
structural stability shall include, but are not necessarily limited to, the
following:
1. The hazard potential of the dam
under present downstream conditions and under conditions which would likely
develop during the life of the reservoir;
2. Foundation bearing capacity,
compressibility, and permeability; the extent and reliability of the site
investigation; and the predictability of the site and foundation
conditions;
3. The reliability of
construction materials, such as borrow soils, in terms of sufficient volume to
complete construction without unanticipated interruption and in terms of
predictability of physical properties such as strength, permeability, and
compressibility;
4. Durability of
construction materials;
5.
Construction conditions at the site;
6. The degree of quality control to be
exercised during construction;
7.
Pore pressure build-up during construction;
8. The rate of filling the reservoir and the
rate of possible reservoir drawdown;
9. Tailwater conditions and the impact of
drawdown;
10. Possible effects of
landslides and subsurface solution activity on the structural stability of the
dam and spillway structures; and
11. The extent of the proposed use of
piezometers and other devices which will be used to monitor the completed dam
and the means of access for inspections.
(b) Slope stability analysis shall be
considered by the design engineer for all embankment dams, or as required by
the Commissioner, and is required for high hazard potential dams. Where slope
stability analysis is required, documentation in the final design report, such
analysis shall include the design cross section(s) showing the soil parameters
assumed for analysis, the location of the phreatic surface assumed for
analysis, stability computations, and the location and computed safety
factor(s) for the most critical circle(s) or failure wedge(s).
(c) Minimum factors of safety are listed in
the following table. Final accepted factors of safety may depend upon the
degree of confidence in the engineering data available. In selecting a minimum
acceptable factor of safety, an evaluation should be made on both the degree of
conservatism with which assumptions were made in choosing soil strength
parameters and pore water pressures, and the influence of the method of
analysis used.
1.302 CMR 10.14(8)(c) shall not
be applicable to embankments on clay shale foundations, soft sensitive clays,
or materials with large strength loss under stresses.
2. For embankments over 50 ft. high on
relatively weak foundations, a minimum factor of safety of 1.4 shall be used.
SLOPE STABILITY ANALYSIS MINIMUM FACTORS OF SAFETY
|
Loading Conditions
|
Minimum Factor of Safety Analyzed
|
Slope to be
|
|
End of construction condition
|
1.3
|
upstream and downstream
|
|
Sudden drawdown from maximum pool
|
>1.1*
|
upstream
|
|
Sudden drawdown from spillway crest or top of
gates
|
1.2
|
upstream
|
|
Steady seepage with maximum storage pool
|
1.5
|
upstream and downstream
|
|
Steady seepage with surcharge pool
|
1.4
|
downstream
|
|
Earthquake (for steady seepage conditions with
seismic loading using seismic coefficient method)
|
>1.0
|
upstream and downstream
|
* The factor of safety shall not be less than 1.5 when drawdown
rate and pore water pressures developed from flow nets are used in the
stability analyses and where rapid drawdown is a normal operating condition as
with pumped storage reservoir.
(d) Foundation bearing capacity and sliding
base analysis shall be considered for all dams and are required for high hazard
potential dams. Where bearing capacity or sliding base analysis is required,
documentation of assumptions, computations, and safety factors shall be
included in the final design report.
(e) Resistance of appurtenant structures
against flotation uplift shall be provided for all dams. If the structures are
anchored by dead weight alone, the buoyant weight shall be used for analysis.
If the structures are anchored to soil or rock, the minimum factor of safety
for that portion of the resistance provided by soil or rock anchorage shall be
2.0 unless the design engineer provides a thoroughly documented basis for using
a lower safety factor.
(f) For
concrete, masonry, or other similar dams of relatively narrow cross section,
resistance against overturning and sliding under maximum design loading
conditions shall be considered; overturning and sliding stability computations
shall be required for significant and high hazard potential dams.
(g) The anticipated reservoir and tailwater
drawdown conditions shall be considered in all stability computations and shall
be included in the design documents provided in the final design
report.
(h) The slopes shall be
protected against erosion by wave action, and the crest and downstream slope
shall be protected against erosion due to wind and rain. Riprap and other
erosion protection shall be provided over the full range in stages between the
lowest drawdown elevation and at least two feet above maximum water storage
elevation. Exemptions for specific site conditions such as special use slowly
rising reservoirs or waste storage facilities may be approved in writing by the
Commissioner upon written request by the Applicant.
(i) All significant and high hazard potential
dams shall be designed to withstand seismic accelerations of the following
intensities: Zone 1 = 0.025 g., Zone 2 = 0.05 g., Zone 3 = 0.15. Zones refer to
"Geologic Hazard Maps".
(j)
Loading Combinations. The following conditions and
requirements are suitable in general for gravity dams of intermediate size.
Loads which are not indicated such as wave action or any unusual loadings
should be considered where applicable.
Case I: Usual Loading Combination-Normal Operating
Condition. The reservoir elevation is at the normal pool, as
governed by the crest elevation of an overflow structure or the top of the
closed spillway gates, whichever is greater. Normal tailwater is used. If
applicable, horizontal silt pressure should also be considered.
Case II: Unusual Loading Combination-Flood
Discharge. The projected inflow design flood up to and including
the Probable Maximum Flood, if appropriate, that results in reservoir and
tailwater elevations that exert the greatest head differential and uplift
pressure upon the structure shall be used. However, unusual conditions, such as
high tailwater, shall be examined on a case by case basis as it is possible
that the worst case loading condition exists under other than extreme
floods.
Case IIA: Unusual Loading Combination-Ice
Case. I loading plus ice pressure, if applicable. Generally ice
pressure will not be a factor in the stability analyses, but may affect the
operation, or structural integrity of flash boards and spillway gates.
Case III: Extreme Loading Combination-Normal
Operating with Earthquake. Case I loading except that the inertial
force due to the earthquake acceleration of the dam, and the increased
hydrostatic forces due to the reservoir reaction on the dam are added.
(k)
Stability
Criteria. Specific stability criteria for a particular loading
combination shall be dependent upon the type of analysis being done
(i.e. foundation or concrete analysis), the degree of
understanding of the foundation-structure interaction and site geology, and, to
some extent, on the method of analysis.
1. For
new dams, preliminary analyses shall be based upon more conservative criteria
than final designs. As the design process progresses, the designer has
available more sophisticated and detailed foundation information and material
testing results. Therefore, when the unknowns associated with the preliminary
designs are reduced by the final design stage, lower safety factors may be
acceptable.
2. For existing dams,
assumptions used in the analysis shall be based upon construction records and
the performance of the structures under historical flood loadings. In the
absence of available design data and records, site investigations shall be
conducted to verify all assumptions.
3. Recommended safety factors shall apply to
the calculations of stress and the shear-friction factor of safety within the
structure, at the rock/concrete interface and in the foundation. Safety factors
shall be determined using the gravity method of analysis.
|
RECOMMENDED FACTORS OF SAFETY
Dams having a high or significant hazard
potential.
|
|
Loading Condition
|
Factor of Safety
|
|
Usual
|
3.0
|
|
Unusual
|
2.0
|
|
Extreme
|
>1.0
|
|
Dams having a low hazard potential.
|
|
Loading Condition
|
Factor of Safety
|
|
Usual
|
20
|
|
Unusual
|
1.25
|
|
Extreme
|
>1.0
|
(10)
Design Life of a
Dam. The selection of materials and equipment to be used in a dam
and all of its appurtenant features shall either be based on sufficient quality
and durability to function satisfactorily throughout the design life or to
provide for safe and economical replacement within the design life span. The
design life of a dam shall be the period of time the dam can be expected to
perform effectively as planned. The design life of a dam shall be determined by
the following:
(a) The time required to fill
the reservoir with sediment from the contributing watershed;
(b) The durability of appurtenances and
materials used to construct the dam; and
(c) The time required to perform the specific
function for which the dam was designed.
(11)
Additional Design
Requirements.
(a) All elements of
the dam shall conform to good and generally accepted engineering practice. The
safety factors, design standards, and design references that are used shall be
included in the final design report.
(b) Monitoring or inspection devices may be
required by the Commissioner for use by the inspectors or owners during
construction and filling and after completion of construction. The Commissioner
may also require that such monitoring or inspection devices, existing or
installed by requirement, be read and documented at specified intervals and
copies of such be forwarded to his or her office.
(12)
Construction
Schedule. The applicant shall submit a construction schedule that
includes:
(a) Suggested techniques and work
force to be used to demonstrate that the dam will be constructed according to
the plans and specifications;
(b)
An estimated time to complete the construction activities;
(c) Techniques to be used to divert the
stream flow to prevent interference with construction; and
(d) The extent and method of quality
control.
(e) A determination of the
likelihood of seasonal or winter shut down and any provisions or requirements
to ensure safe dam operations during shut down period.
(13)
Proposed Changes In
Design. The owner shall notify the Commissioner in writing of any
proposed changes in design, plans, and specifications that will affect the
stability of the dam. Rationale and analysis supporting the proposed changes
must be provided. Approval shall be in the form of a written addendum to the
Chapter 253 Permit and must be obtained prior to installation.
(14)
As-built Plans.
Two complete sets of as-built plans shall be submitted to the Commissioner
within 30 days of completion of the project.
(15)
Engineer's
Certification. The registered professional civil engineer who has
inspected the construction of the dam, shall submit a written statement bearing
his or her professional seal that the dam and all appurtenances have been
built, repaired, altered, or removed in conformance with the plans,
specifications, and drawings approved by the Commissioner and that the dam is
in compliance with
302 CMR
10.00. For repairs accomplished, the certification
shall be for the repairs only.
(16)
Acceptable Design: Procedures and Technical
References. The following represent acceptable design procedures
and references:
(a) The design procedures,
manuals and criteria used by the United States Army Corps of
Engineers;
(b) The procedures,
manuals, and criteria used by the United States Natural Resources Conservation
Service (formerly US Soil Conservation Service);
(c) The procedures, manuals, and criteria
used by the US Bureau of Reclamation; and
(d) Other procedures that are approved by the
Commissioner.
(17)
Granting of Final Approval. Unless the Commissioner
has reason to believe that the dam, on completion, is unsafe or not in
compliance with any applicable requirement, regulation, or law, or of any
condition or specification contained within the Permit, upon completion of
construction and upon receipt of the engineer's statement, the Commissioner
shall issue a final Certificate of Compliance certifying that the work has been
completed in conformance with plans, specifications, drawings and conditions of
the permit, subject to such terms as deemed necessary for the protection of
life and property.
