L02 · Foundation · 2–3 hours
Hydrostatic pressure
Draw the water-pressure diagram on a vertical face and calculate the resultant and its line of action per metre width.
Tool: Draft · Lesson: Drafted awaiting review · Example: Drafted awaiting review · Workbook: Built awaiting check · Assessment: Drafted awaiting approval
Outstanding before release: Engineering checker to confirm CDC A3 cl. 3.8.3.1 values (10.00 / 10.30 kN/m³) against the PDF page and sign off v0.1.0.
Sources
| Role | Source | Note |
|---|---|---|
| Primary | LTA CDC A3 cl. 3.8 GROUND AND WATER LOADS | Ground and water loads (mapping awaiting checker verification) |
| Primary | LTA CDC A3 cl. 3.8.3 (design groundwater levels) | Design groundwater levels — the head H must come from here / project criteria (mapping awaiting checker verification) |
| Primary | LTA CDC A3 cl. 3.8.3.1 (groundwater and seawater density) | Water density values (mapping awaiting checker verification) |
| Supporting | Hydrostatics: p = γw·z (fluid statics, any standard fluid-mechanics text) | Method source (mapping awaiting checker verification) |
"Primary" marks the governing clause for this exercise; "contextual" marks related clauses; "supporting" marks the design-method or data source outside the CDC. A CDC chapter mapping does not mean the CDC contains the equation used.
Prerequisites: F01 Design basis and calculation register · Topics: Loads and actions, Retaining structures, Soil and groundwater
Lesson
Lesson draft v0.1 — awaiting trainer and technical review.
Water pressure governs many underground and retaining-structure checks in Singapore, where groundwater is often near the surface. Before a wall or slab can be designed, someone must draw the water-pressure diagram and give its resultant. A reviewer will first ask where the design water level came from.
Learning outcomes
- Draw the hydrostatic pressure diagram on a vertical face.
- Calculate base pressure, resultant per metre width and its line of action.
- Identify the source of the design water level and the water unit weight.
- Recognise when hydrostatic assumptions do not apply (seepage, flow, inclined faces).
Model, assumptions and limitations
In still water with a free surface at atmospheric pressure, pressure increases linearly with depth: p(z) = γw·z.
On a vertical face of height H below the free surface, the diagram is a triangle. Base pressure p = γw·H. Resultant per metre width F = ½·γw·H², acting at H/3 above the base (the centroid of the triangle).
The head H is not something the tool chooses. CDC A3 3.8.3 deals with design groundwater levels; the project criteria set the level for each design situation.
Water unit weight: CDC A3 cl. 3.8.3.1 (as read on 1 Nov 2026, awaiting checker confirmation) gives 10.00 kN/m³ for groundwater and 10.30 kN/m³ for seawater. The worked example below uses 9.81 kN/m³ as a stated training assumption so you can see how the choice changes the result.
Worked example — vertical face, 3.0 m head (assumed for training)
- H = 3.0 m from free surface to base
- γw = 9.81 kN/m³ (assumed for training)
- Vertical plane face; atmospheric pressure at top; no seepage
| Step | Substitution | Result |
|---|---|---|
| Base pressure | p = γw·H = 9.81 × 3.0 | 29.43 kPa |
| Resultant | F = ½·γw·H² = 0.5 × 9.81 × 9.0 | 44.145 kN/m |
| Line of action | ȳ = H/3 = 3.0/3 | 1.0 m above base |
| Same case with CDC A3 groundwater value 10.00 kN/m³ | p = 30.0 kPa; F = 45.0 kN/m | +1.9% |
Excludes seepage and project design factors.
Common mistakes
- Taking H from the ground surface when the design water level is different.
- Using F = γw·H² (forgetting the ½).
- Placing the resultant at H/2 instead of H/3 above the base.
- Using 9.81 without checking the governing criteria.
Interpretation questions
- How much does the resultant change if the design water level rises by 0.5 m? Why is it more than proportional?
- What additional analysis is needed if water is flowing under the wall?
Technical English task
A checker writes: "Confirm design groundwater level and density adopted." Draft a reply giving the level, its source clause and the density used.
Burmese explanation: awaiting reviewed translation.
Calculator
Purpose: Hydrostatic pressure distribution, resultant force and line of action on a vertical plane face, per metre width.
Model: Static water, free surface at atmospheric pressure, vertical plane face, head H from free surface to base. p(z) = γw·z; resultant F = γw·H²/2 acting at H/3 above the base.
Units: SI only: H and z in m, γw in kN/m³, pressure in kPa (kN/m²), force in kN per m width. Ranges: 0 < H ≤ 100 m; 0 ≤ z ≤ H; γw 9.5 to 10.5 kN/m³ for project values (outside → outside supported scope).
Equations: p(z) = γw · z ; p_base = γw · H ; F = ½ · γw · H² ; ȳ = H / 3 (above base)
Not covered: Seepage, flow and pore-pressure gradients; Inclined or curved faces; Layered fluids or pressurised surfaces; Wave, dynamic or tidal effects; Design water level selection (user must take the level from project criteria, e.g. CDC A3 cl. 3.8.3); Partial factors and combinations
Version 0.1.0 Draft
Not available for use. Not yet released for use. Status: Draft. Engineering checker to confirm CDC A3 cl. 3.8.3.1 values (10.00 / 10.30 kN/m³) against the PDF page and sign off v0.1.0.
Excel workbook
The workbook is available with the calculator once this version is released.
Exercises
L02-EX1 — Basement wall water pressure
Drafted — awaiting approval of the brief, rubric and pass mark before submissions open.