Civil Engineering Reference
In-Depth Information
typically provide design recommendations such as the minimum footing dimensions, embed -
ment requirements, and allowable bearing capacity values. These recommendations would
normally be included in a soils report. Appendix D presents an example of a geotechnical
engineering report.
An example of typical wording for a bearing material that is not expected to be weak-
ened by the earthquake is as follows:
The subject site consists of intact Mission Valley formation (siltstone and sandstone)
bedrock. For the static design condition, the allowable bearing pressure for spread footings is
8000 lb/ft
2
(400 kPa) provided that the footing is at least 5 ft (1.5 m) wide with a minimum of
2-ft (0.6-m) embedment in firm, intact bedrock. For continuous wall footings, the allowable
bearing pressure is 4000 lb/ft
2
(200 kPa) provided the footing is at least 2 ft (0.6 m) wide with
a minimum of 2-ft (0.6-m) embedment in firm, intact bedrock. It is recommended that the struc-
tures be entirely supported by bedrock.
Because of cut-fill transition conditions, it is anticipated that piers will be needed for the
administrative building. Belled piers can be designed for an allowable end-bearing pressure of
12,000 lb/ft
2
(600 kPa) provided that the piers have a diameter of at least 2 ft (0.6 m), length of
at least 10 ft (3 m), with a minimum embedment of 3 ft (0.9 m) in firm, intact bedrock. It is rec-
ommended that the geotechnical engineer observe pier installation to confirm embedment
requirements.
In designing to resist lateral loads, passive resistance of 1200 lb/ft
2
per foot of depth (200
kPa per meter of depth) to a maximum value of 6000 lb/ft
2
per foot of depth (900 kPa per meter
of depth) and a coefficient of friction equal to 0.35 may be utilized for embedment within firm
bedrock.
For the analysis of earthquake loading, the above values of allowable bearing pressure and
passive resistance may be increased by a factor of one-third. This material is not expected to
be weakened by the earthquake-induced ground motion.
An example of typical wording for a bearing material that is expected to be weakened
by the earthquake is as follows:
The subject site consists of a 10-ft-thick upper layer of cohesive soil that is underlain by a
15-ft-thick layer of submerged loose sand. Based on our analysis, it is anticipated that the 15-
ft-thick sand layer will liquefy during the design earthquake. Since the site is essentially level,
lateral movement due to a liquefaction-induced flow failure or lateral spreading is not antici-
pated to occur. In addition, the upper 10-ft-thick clay layer should be adequate to prevent liq-
uefaction-induced ground damage (i.e., sand boils, surface fissuring, etc.).
It is our recommendation that the lightly loaded structures be supported by the 10-ft-thick
upper cohesive soil layer. For the design condition of lightly loaded shallow foundations, the
allowable bearing pressure is 1000 lb/ft
2
(50 kPa). It is recommended that the shallow footings
be embedded at a depth of 1 ft (0.3 m) below ground surface and be at least 1 ft (0.3 m) wide.
It is anticipated that piles or piers will be needed for the heavily loaded industrial building.
The piles or piers should be founded in the unliquefiable soil stratum which is located at a depth
of 25 ft. The piles or piers can be designed for an allowable end-bearing pressure of 4000 lb/ft
2
(200 kPa), provided that the piles or piers have a diameter of at least 1 ft (0.3 m) and are embed-
ded at least 5 ft (1.5 m) into the unliquefiable soil strata. It is recommended that the geotech-
nical engineer observe pile and pier installation to confirm embedment requirements. The piles
or piers should also be designed for downdrag loads during the anticipated earthquake-
induced liquefaction of the loose sand layer.
In designing to resist lateral loads, the upper 10-ft-thick clay layer can provide passive
resistance of 100 lb/ft
2
per foot of depth (equivalent fluid pressure). For seismic analysis, the
underlying 15-ft-thick sand layer should be assumed to have zero passive resistance.
The above values of allowable bearing pressure and passive resistance should not be
increased for the earthquake conditions. As previously mentioned, the loose sand layer from a
depth of 10 to 25 ft below ground surface is expected to liquefy during the design earthquake
(i.e., weakened soil conditions).
