Building Design and Construction Handbook - Merritt - Ventech!

SOIL MECHANICS AND FOUNDATIONS 6.103
TABLE 6.18 Design Considerations for Braced Excavations
Design factor Comments
Water loads Often greater than earth loads on an impervious wall. Recommend
piezometers during construction to monitor water levels. Should also
consider possible lower water pressures as a result of seepage of water
through or under the wall. Dewatering can be used to reduce the water
loads. Seepage under the wall reduces the passive resistance.
Stability Consider the possible instability in any berm or exposed slope. The sliding
potential beneath the wall or behind the tiebacks should also be
evaluated. For weak soils, deep seated bearing failure due to the weight
of the supported soil should be checked. Also include in stability analysis
the weight of surcharge or weight of other facilities in close proximity to
the excavation.
Piping Piping due to a high groundwater table causes a loss of ground, especially
for silty and fine sands. Difficulties occur due to flow of water beneath
the wall, through bad joints in the wall, or through unsealed sheet pile
handling holes. Dewatering may be required.
Movements Movements can be minimized through the use of a stiff wall supported by
preloaded tiebacks or a braced system.
Dewatering
and
recharge
Dewatering reduces the loads on the wall system and minimizes the
possible loss of ground due to piping. Dewatering may cause settlements
and in order to minimize settlements, there may be the need to recharge
outside of the wall system.
Surcharge Construction materials are usually stored near the wall systems. Allowances
should always be made for surcharge loads on the wall system.
Prestressing
of tieback
anchors
Construction
sequence
In order to minimize soil and wall movements, it is useful to remove slack
by prestressing tieback anchors.
The amount of wall movement is dependent on the depth of the excavation.
The amount of load on the tiebacks is dependent on the amount of wall
movement which occurs before they are installed. Movements of the wall
should be checked at every major construction stage. Upper struts should
be installed as early as possible.
Temperature Struts may be subjected to load fluctuations due to temperature differences.
This may be important for long struts.
Frost
penetration
In cold climates, frost penetration can cause significant loading on the wall
system. Design of the upper portion of the wall system should be
conservative. Anchors may have to be heated. Freezing temperatures also
can cause blockage of flow of water and thus unexpected buildup of
water pressure.
Earthquakes Seismic loads may be induced during an earthquake.
Factors of
safety
The following are suggested minimum factors of safety (F) for overall
stability. Note that these values are suggested guidelines only. Design
factors of safety depend on project requirements.
Earth Berms: Permanent, F � 2.0 Temporary, F � 1.5
Cut Slopes: Permanent, F � 1.5 Temporary, F � 1.3
General Stability: Permanent, F � 1.5 Temporary, F � 1.3
Bottom Heave: Permanent, F � 2.0 Temporary, F � 1.5
Source: NAVFAC DM-7.2, 1982.