BS ISO 16134:2020 download.Earthquake-resistant and subsidence – resistant design of ductile iron pipelines.
4 Earthquake-resIstant design
4.1 SeIsmic hazards to burled pipelines
In general, there are several main causes olseismic hazards to buried pipelines:
a) ground displacement and ground strain caused by seismic ground shaking;
h) ground deformation such as a ground surface crack, ground subsidence and lateral spread induced by liquefaction;
c) relative displacement at the connecting part with the structure, etc.
d) ground displacement and rupture along a fault zone.
Since the ductile iron pipe has high tensile strength as well as the capacity for expansion/contraction and deflection from Its joint part, giving it the abIlity to follow the ground movement during the earthquake, the stress generated on the pipe body is relatively small. Few ruptures of pipe body have occurred during earthquakes in the past. It Is therefore important to consider whether the pipeline can follow the ground displacement and ground strain without slipping out of joint when considering its earthquake resistance. The internal hydrodynamlc surge pressures Induced by seismic shaking are normally small enough not to be considered.
4.2 Qualitative design considerations
4.2.1 General
To increase the resistance of ductile iron pipelines to seismic hazards, the following qualitative design measures should be taken Into consideration.
a) Provide pipelines with expansion/contraction and deflection capability.
EXAMPLE Use of shorter pipe segments, special joints or sleeves and anti.silpout mechanisms according to the anticipated intensity or nature of the earthquake.
b) Lay pipelines In a firm foundation.
c) Use smooth back fill materials.
NOTE Polyethylene sleeves and special coating are also effective In special cases.
d) Install more valves.
4.2.2 Where high earthquake resistance is needed
It Is desirable to enhance the earthquake resistance of parts connecting the pipelines to structures and when burying the pipes in
a) soft ground such as alluvium,
b) redaimed ground,
c) filled ground.
suddenly changing soil types (geology) or topography. sloping ground, near revetments, liquefiable ground, and/or
h) near an active fault.
4.3 Design procedure
To make earthquakereslstant design for ductile iron pipelines:
a) select the piping route;
b) Investigate the potential for earthquakes and ground movement;
c) assume probable earthquake motion (seismic Intensity);
d) undertake earthquake resistance calculation and safety checking
e) select joints.
Solid/firm foundations should be chosen for the pipeline route.
When investigating earthquakes and ground conditions, take into account any previous earthquakes in the area where the pipeline is to be laid.
4.4 Earthquake resistance calculations and safety checking
When checking the resistance of pipelines to the effects of earthquakes, the calculation shall be carried out for the condition In which the normal load (dead load and normal live load) Is combined with the influence of the earthquake.
The pipe body stress, expansion/contraction value of joint, and deflection angle of joint are calculated by the response displacement method. Earthquake resistance is checked by comparing these values with their respective allowable values. The basic criteria are given in Tablel.
A flowchart of earthquake resistance determination and safety checking is shown in Figu.r .1. The basic formulae only for earthquake resistance calculation are given in iS. A detailed example of calculation is given in Annex A.
5 DesIgn for ground deformation by earthquake
5.1 General
Large scale ground deformation such as ground cracks, ground subsidence and lateral displacement near revetments and inclined ground can be generated by liquefaction during an earthquake, Since such ground deformations can affect the buried pipeline, it is necessary to consider this possibility and to take it into account in the pipeline design.
5.2 EvaluatIon of possIbility of liquefaction
The possibility of liquefaction shall be evaluated for soil layers when the following conditions are present;
a) saturated soil layer s25 rn from the ground surface; bJ average grain diameter, Dsa. 510 mm;
c) content by weight of small grain particles (with grain diameter 50,075 mm) s30 %.
The possibility of liquefaction can be evaluated by calculating the liquefaction resistance coefficient. F1, using Formula (7):
where
R Is the dynamic shear strength ratio Indicating the resistance to Liquefaction;
L is the ground shear stress ratio during an earthquake, which indicates the generated shear stress in ground due to the earthquake.
When F1 1,0, the layer is considered to be liquefied.
A detailed example of the evaluation of liquefaction assessment is given in Annex C.
5.3 Checking basic resistance
For ground deformation such as lateral displacement and ground subsidence induced by liquefaction, the basic resistance of the pipeline shall be checked by observing whether it can absorb the ground movement by the expansion/contraction and deflection ofoints.
Forground deformation in pipe axis direction, the safety of the pipeline shall be checked by Formula t8). When E, exceeds 6 (E, >6k) then the pipeline can absorb the ground displacement and has been safely designed For ground deformation in its axis direction,
E, is total amount of expansion/contraction of joint, in metres (in);
8, is ground displacement in pipe axis direction, in metres (m);
is the amount of expansion/contraction of the joint, in per cent (%) of the pipe length;
n is the number of joints;
I is the pipe length, in metres (m);
f is the reduction ratio of the amount of expansion/contraction of the joint for the ground displacement (= 0,5);
tG Is the ground strain in pipe axis direction, in per cent (%).
When E, does not exceed J , all ioints expand to the joint’s capacity, then the safety of the joint’s slip-out resistance against friction force between pipe and soil shall be checked by Eornuja.
(9)
where
Is the joint’s slip-out resistance, in kilonewtons (kN);
D Is the outside diameter of buried pipeline, in metres (m);
a Is reduction factor of friction force;
Is friction force per unit area between pipe and soil, in kilopascals (kPa).
The examples of safely checking including the case olpipe perpendicular direction are given In Annex 0.
6 Design for ground subsidence in soft ground (e.g. reclaimed ground)
6.1 Calculating ground subsidence
When burying pipes In soft ground, the amount of ground subsidence Is estimated by calculating the increased earth pressure at the bottom of the trench in considering the weight of pipes, the weight of water in the pipes and the earth pressure of back-fill, using Formulae (10). (11) and U-Zi.