Environmental Engineering Reference
In-Depth Information
Finished dental concrete horizontal mats should have a roughened, broomed finish
(but not corrugated) to provide a satisfactory bonding surface to embankment material.
The dental concrete should be cured by water or an approved curing compound until
28 days or it is covered by earthfill. Care should be taken to ensure that cracking does
not occur in the dental concrete due to subsequent earthfill placement and compaction
operations. Earthfill operations may not be permitted over dental concrete for a time
interval of 72 hours or more after concrete placement to allow concrete time to develop
sufficient strength to withstand stress caused by earthfill placement operations.
(b) Pneumatically applied mortar or concrete (shotcrete). Shotcrete is concrete or mortar
that is applied pneumatically at high velocity with the force of the jet impacting on the
surface serving to compact the concrete or mortar. Practice today is to use wet mixed
shotcrete because it is easier to control. The quality of shotcrete is highly dependent
upon the skill and experience of the crew applying it, particularly with regard to their
ability to prevent rebound from being entrapped in the shotcrete, control the thick-
ness, prevent feather edges, and ensure adequate thickness over protrusions in irregu-
lar surfaces. There is also a danger of covering unprepared treatment areas because of
the ease and rapidity of placement. Shotcrete should be used beneath impervious zones
only when its use instead of dental concrete can be justified by site conditions. If it is
used, the specifications should be very strict to ensure proper provisions for adequate
quality work. The requirements for concrete quality, layer thickness, feathering and
curing time should be as for dental concrete.
(c) Slush grout. Is a neat cement grout or a sand-cement slurry that is applied to cracks in
the foundation. Slush grout should be used to fill only narrow surface cracks. It
should not be used to cover exposed areas of the foundation other than as described
in 17.3.2, where it is used as a temporary cover over slaking or similar foundations.
Slush grout may consist of cement and water, or sand, cement and water. To ensure
adequate penetration of the crack, the maximum particle size in the slush grout mix-
ture should be no greater than one-third the crack width. The consistency of the slush
grout mix may vary from a very thin mix to mortar as required to penetrate the crack.
The grout shall be mixed with a mechanical or centrifugal mixer and the grout should
be used within 30 minutes after mixing.
Cracks shall be cleaned out and wetted prior to placement of slush grout. Slush
grout may be applied by brooming over surfaces containing closely spaced cracks, or
by troweling, pouring, rodding, or funneling into individual cracks. The requirements
for cement and aggregates should be as for dental concrete.
17.7
ASSESSMENT OF EXISTING EMBANKMENT DAMS
When assessing the likelihood of internal erosion and piping of existing dams, a thorough
assessment should be made of what foundation preparation was carried out. This can be
assessed from construction reports, specifications and most particularly from photographs
taken during construction. While it was common practice to carefully clean up (of loose
material) the cutoff foundation in earlier dam construction, it was not until around the
1960s to 1970s that extensive use was made of dental concrete. Hence many dams built
before then have the potential for loose/softened zones of core material near the contact
between embankment and foundation. These have been detected in some dams using cone
penetration tests through the earthfill.
Large scale irregularities in the foundation have the ability to lead to low stresses near the
crest of the dam. Often the profile is recorded on the drawings showing the results of grouting
and/or on photographs, but as often they are not. On a number of projects construction of haul
roads or access tracks across the core foundation has resulted in benching followed by differ-
ential settlement and cracking of the core. Whether these benches are of concern, often relates
