1. Global check (VTV-2006) - Liquefaction
The global check for liquefaction as implemented in D-Flow Slide is shown in Figure 1 below. It is based on the assessment schema described in VTV-2006.
Figure 1: Assessment schema for liquefaction (current schema in D-Flow Slide)
For the coming VTV (2013), a new schema has been proposed, see Figure 2 below.
Figure 2: Assessment schema for liquefaction (proposal)
Step 1 : Would liquefaction lead to damage on levee?
The damaging criterion liquefaction (geometrical check) is met if at the so-called assessment level is situated within the slope (i.e. on landward side of) the so-called observation profile, in accordance with Annex 9.4 of the VTV-2006.
1) Determination of the observation profile
The required margin (the horizontal part from the outside toe of the dike) depends on the presence of a revetment and is determined by the relation:
M = 2 Hvw + 1.5 (H - Hvw) in case no revetment is present
M = Max [ Mbestorting ; 2 Hvw + 1.5 (H - Hvw) ] in case a revetment is present
where:
H is the channel depth in case no revetment is present and H = Honbest in case a revetment is present
Hvw is the thickness of the sensitive layer
Honbest is the depth of the channel below the revetment
Mbestorting is the horizontal projection of the length of the revetment.
In case no slope protection is present, if over the entire channel depth softening sensitive sand is present, this margin is thus equal twice the fictive channel depth (M = 2 H).
The inclined part of the observation profile in line with the horizontal portion. The inclinaison of the slope depends on the channel depth:
- 1:15 if the fictive channel depth is less than 40 m (H < 40 m)
- 1:20 if the fictive channel depth is more than 40 m (H ≥ 40 m)
2) Determination of the assessment level
The assessment level is the lower boundary of the liquefiable sand layer, but has a minimum and maximum of H/3 and H/2 above the bottom of the channel respectively.
3) Comparison of the observation profile with the existing profile at the assessment level
The damaging criterium is not met if the liquefaction point SZV is situated landwards of the observation point Ssign.
where Ssign is the intersection point between the observation profile (from the foreshore) and the assessment level and
SZV is the intersection point between the existing profile and the assessment level.
Step 2: Artificially underwater installed and non-compacted sandy foreshore? (answer is always NO)
By lack of information, D-FLOW SLIDE assumes that no artificially underwater installed and non-compacted sandy foreshore is present (step 2 of the Global check).
Step 3: Criterion on slope protection met?
Directly in front of the toe of the dike or on the outside of the toedumping or to the outward end of a dumping, the slope 1:2.5 may not be cut by the profile. This is not checked because the answer has no influence.
Step 4: Liquefaction possible based on geometry only?
The occurrence of a liquefaction is possible if one of the two following conditions is met:
- The average slope is steeper than or equal to 1:4, over a height of at least 5 m;
- The total slope (channel edge-channel bottom) is on average steeper than or equal to 1:7.
