Underground Construction - August 2020 - 33

EDITION 60 | AUG 2020 | NASSCO.ORG
stress in the pipe walls as the soil pressure around
the pipe is converted to passive from active support
pressures. As long as there is adequate side support the
pipe now functions in much the same manner as a flexible
pipe. The second change that can offset that benefit
over time is that soil transfer can occur through a fracture
which wouldn't be able to occur through a crack.
In PACP coding a grade 5 severity is assigned to rigid
pipes with 5% deformation. The severity rating is based
on the ability for soil migration to occur moving forward
and not a concern for the overall structural stability.
Based on field-testing by Trott et al, it was found that
circular concrete and clay tile sewers are routinely
structurally stable with 5-10% deformation. Beyond 10%
deformation, however, the sewer becomes increasingly
more susceptible to failure from a random event (e.g. a
rain event, a 3rd party excavation, etc.). The concern over
future deterioration relates to the rate of soil migration
around the pipe.
The rate of soil migration is impacted by the size of the
defect; hydraulic conditions including the location of
the water table, and the frequency and magnitude of
surcharge; and the type of soil, including its gradation for
cohesionless soils and plasticity index for cohesive soils.
Based on collapse studies the SRM provides guidance on
the risk or relative rate of ground loss by considering each
of these supplemental factors as noted in the chart to
the right.
The significance of this is that the same severity of defect
in many sewers has subtly different ramifications based
on the nature of the conditions in and around the pipe.
The severity of the defect is well captured in standard
PACP observations. The supplementary information can
often be obtained by a review of regional soils records
and hydraulic studies on the sewer system. With the
accompanying illustrations they can be used to put into
better context what the rate of deterioration is likely to be.

Sewer Defect Size
Severe defects (>1/2")
Large defects (1/4"-1/2")
Medium defects (1/16"-1/4")
Small defects (<1/16")

HIGH
RISK OF
GROUND
LOSS
LOW

Hydraulic Conditions
(Groundwater level)
Above sewer
Close to sewer
Below sewer

HIGH
RISK OF
GROUND
LOSS
LOW

Exposure to Surcharged Conditions
Frequent, high magnitude
Frequent, low magnitude
Occasional, high magnitude
Occasional, low magnitude
Never

HIGH
RISK OF
GROUND
LOSS
LOW

Soil Type
Non-plastic silts, silty fine
sands, or fine sands
Medium to coarse sands
Low plasticity clays
Fine to medium gravels
Well graded sandy gravels
Medium to high plasticity
clays
All clays, where sewer
constructed by tunneling

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HIGH

RISK OF
GROUND
LOSS

LOW
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Underground Construction - August 2020

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