Case study
Braced Excavation Basal Heave Case Study
Civil engineering case study on braced excavation basal heave, hydraulic uplift, soft-clay base stability, piezometric head, stop-work decisions, sequence change, and validation evidence.
A braced excavation can look controlled at the wall line while the base is becoming unsafe. Struts may keep wall deflection within trigger values, yet the excavation bottom can heave because the soft clay below the base has insufficient undrained resistance or because upward groundwater pressure reduces the effective confinement.
This case study follows a hypothetical deep excavation where the monitoring team observes upward movement at the base after rainfall and dewatering changes. The case is written for engineering education. It shows how a civil engineer should connect basal heave, hydraulic uplift, piezometric head, construction sequence, stop-work authority, and validation evidence.
The central question is:
Can excavation continue because wall movement and strut loads remain below red triggers, or must the next stage be stopped because base stability is no longer credible?
The correct decision is to stop the next excavation stage until the base-stability mechanism is understood and corrected. Wall movement is only one part of excavation safety.
Case Context
A two-level braced excavation is being constructed beside utilities and an existing masonry building. The excavation is in soft clay with a confined water-bearing layer below the planned base. The support system is a soldier-pile and lagging wall with walers and internal struts.
| Item | Value or observation |
|---|---|
| Current excavation depth | 10.5\ \text{m} |
| Unit weight of removed soil | 18.5\ \text{kN/m}^3 |
| Temporary surcharge near edge | 18\ \text{kPa} |
| Verified undrained shear strength below base | 34\ \text{kPa} |
| Screening bearing factor for base heave | N_c=5.7 |
| Minimum required basal-heave factor of safety | 1.5 |
| Confined piezometric head above excavation base | 6.0\ \text{m} |
| Clay plug thickness above confined layer | 2.8\ \text{m} |
| Saturated unit weight of clay plug | 19.0\ \text{kN/m}^3 |
| Water unit weight | 9.81\ \text{kN/m}^3 |
The values are simplified. A real basal-heave review must use the approved ground model, construction sequence, wall embedment, excavation width, soil anisotropy, strain-softening behavior, groundwater observations, dewatering influence, surcharge control, and the governing design standard.
Field Evidence
The warning signs are not dramatic at first:
| Evidence | Engineering interpretation |
|---|---|
| survey points on the exposed base rise by 35\ \text{mm} over two shifts | possible basal heave or rebound beyond predicted elastic recovery |
| piezometers below the base show a confined head increase after rainfall | upward water pressure has increased |
| wall inclinometers remain below red movement trigger | wall stability alone does not prove base stability |
| lower strut load drops slightly while base survey rises | support force may be redistributing as the base moves |
| small cracks appear in the working platform | deformation is reaching the construction surface |
The observation is a coupled geotechnical problem. A monitoring table that checks only wall deflection could miss the governing failure mode.
Basal-Heave Screening
Use a simplified undrained basal-heave screen:
where:
- N_c is a screening bearing factor for the base mechanism;
- s_u is undrained shear strength below the excavation base;
- \gamma H is the overburden stress removed by excavation depth;
- q is surcharge near the excavation.
Compute the driving stress:
Compute the undrained resistance term:
The screening factor of safety is:
This is below the required value:
The calculation is simplified, but it is not a small shortfall. The current excavation state should be treated as outside the accepted design basis.
Hydraulic Uplift Check
The confined water pressure below the clay plug also needs checking. The upward water pressure is:
The downward total stress from the clay plug is:
The simplified uplift factor of safety is:
Again, the result is below unity. The base is not only weak in undrained bearing; it is also vulnerable to upward hydraulic pressure.
Engineering Decision
The next excavation stage should be held. The decision basis is:
- observed base heave is trending upward;
- the basal-heave screen gives FS_{heave}=0.91;
- the hydraulic uplift screen gives FS_{uplift}=0.90;
- the confined head increased after rainfall and dewatering changes;
- continuing excavation would reduce confinement further;
- wall movement below a red trigger does not remove the base failure mode.
The field instruction should be explicit:
Stop further excavation in the affected bay, remove nonessential surcharge, restrict personnel exposure at the base, stabilize drainage and piezometric control, and require engineering review before any deeper cut.
This is a construction-stage safety decision, not only a calculation update.
Corrective Sequence
The reviewed correction combines operational and physical controls:
- remove temporary surcharge from the excavation edge;
- stop excavation at the current level until the base response stabilizes;
- install relief wells to reduce the confined head;
- place a controlled working platform and localized treated plug in the critical bay;
- verify the treated plug strength before release;
- define a staged excavation sequence with smaller depth increments;
- increase base survey and piezometer reading frequency after rainfall.
After correction, the reviewed values for the release bay are:
| Item | Corrected value |
|---|---|
| effective reviewed excavation depth for release stage | 10.5\ \text{m} |
| controlled surcharge near edge | 5\ \text{kPa} |
| verified equivalent undrained resistance in treated plug | 55\ \text{kPa} |
| confined head after relief wells | 3.2\ \text{m} |
| clay plug thickness retained | 2.8\ \text{m} |
| added working-platform equivalent stress | 16\ \text{kPa} |
Recalculate basal heave:
This clears the screening requirement:
Recalculate uplift. Upward pressure after relief wells:
Downward stress from clay plug and working platform:
The corrected condition is no longer controlled by the same immediate uplift screen, but the excavation should still proceed by hold points because soil improvement, piezometric control, and construction tolerance are variable field controls.
Monitoring Interpretation
The monitoring review should combine base movement, piezometric head, wall movement, and strut load:
| Quantity | Before correction | After correction target |
|---|---|---|
| base heave trend | 35\ \text{mm} in two shifts | stable or reducing rate |
| confined head above base | 6.0\ \text{m} | \le 3.2\ \text{m} for release |
| surcharge near edge | 18\ \text{kPa} | \le 5\ \text{kPa} |
| basal-heave screen | 0.91 | 1.57 |
| uplift screen | 0.90 | 2.20 |
A single green wall-deflection reading cannot override adverse base and groundwater indicators. The excavation should be released only when the governing mechanism is controlled and the data trend supports the revised model.
RPN Screen
A simple risk-priority-number screen can document why the response escalated:
Before correction:
| Factor | Value | Rationale |
|---|---|---|
| Severity S | 10 | Basal instability can expose workers, utilities, and adjacent structures to sudden ground movement. |
| Occurrence O | 4 | Soft clay, surcharge, deep excavation, and elevated head make the mechanism credible. |
| Detection D | 5 | Wall monitoring may not detect base failure early unless base and piezometer data are reviewed together. |
After relief wells, surcharge removal, treated plug verification, and staged release:
| Factor | Value | Rationale |
|---|---|---|
| Severity S | 10 | Consequence remains high if the mechanism returns. |
| Occurrence O | 2 | Physical controls reduce likelihood. |
| Detection D | 2 | Base survey and piezometer hold points improve detectability. |
The RPN does not approve excavation by itself. It records why the failure mode became better controlled after the engineered intervention and monitoring update.
Validation Evidence
The release package should include:
| Evidence item | Why it matters |
|---|---|
| piezometer trend before and after relief wells | proves uplift pressure was reduced |
| base survey trend | confirms heave has stabilized or slowed within the release criterion |
| treated-plug strength records | supports the revised s_u value used in the calculation |
| surcharge exclusion inspection | confirms the lower q value is real on site |
| excavation stage hold-point record | prevents the sequence from drifting back to the failed condition |
| wall and strut monitoring | verifies that correcting base stability did not create another support problem |
| rainfall and pumping log | explains groundwater changes and validates the response |
| independent engineering review | confirms the calculation model and field evidence are consistent |
The closeout should state what excavation depth is released, which bay or chainage is covered, which groundwater head limit controls the next stage, and who can stop work if the trend reverses.
Engineering Lessons
The first lesson is that a braced excavation is not safe just because the wall has not reached a red deflection trigger. Base stability can govern before wall strength or serviceability appears critical.
The second lesson is that surcharge, excavation depth, undrained strength, and groundwater head interact. A modest field change in any one of them can remove the margin in soft ground.
The third lesson is that upward movement at the base should be treated as diagnostic evidence, not as harmless rebound, until the mechanism is checked. Trend, rate, groundwater, and sequence matter more than a single isolated survey reading.
The final lesson is that release after a basal-heave warning requires evidence. Relief wells, surcharge control, plug verification, base survey, piezometer trend, and hold-point records must all support the decision to continue.