I. Factors Affecting Groundwater Level

As we know, groundwater activity has three elements: recharge, runoff, and discharge. Here we divide them into two categories: groundwater recharge and discharge.

The groundwater level in a region or site is influenced by two aspects: the first is groundwater recharge, including infiltration of surface water such as rain, snow, rivers, and lakes, lateral runoff recharge, seepage recharge from upper and lower aquifers, and human activities (water injection); the second is discharge, including lateral runoff discharge, seepage discharge to upper and lower aquifers, and human activities (pumping).

When groundwater recharge exceeds discharge, the groundwater level tends to rise; otherwise, it falls. Under certain conditions, a relative equilibrium is reached, forming a stable groundwater level.

Through certain calculation models, it is possible to predict changes in groundwater level.

II. Determination of Anti-Uplift Design Water Level

For a specific construction project, many factors affect the anti-uplift design water level. In my opinion, establishing a calculation model to predict the anti-uplift design water level should mainly consider the following factors:

1. Surface Water Infiltration Recharge

Surface water infiltration is an important source of groundwater recharge and a significant factor affecting groundwater level changes, including atmospheric rainfall and snowfall, and infiltration from surface rivers and lakes. It is an important factor in predicting the anti-uplift design water level. Among these, river and lake water volumes and infiltration are also closely related to rainfall and snowfall. Meteorological and hydrological predictions during the service life of a construction project are relatively difficult, usually inferred from historical meteorological and hydrological data, such as predicting based on rainfall events with a 50-year or longer return period.

2. Groundwater Runoff Recharge and Discharge

Groundwater runoff recharge and discharge also significantly affect groundwater level changes and are one of the important factors in predicting the anti-uplift design water level.

3. Artificial Pumping and Injection

Artificial groundwater extraction, including domestic water and industrial/agricultural water use, is an important factor in predicting the anti-uplift design water level. Some of this water may return to the ground through infiltration, but most is discharged as wastewater outside the assessment area. Construction dewatering also causes a large amount of groundwater to be extracted, most of which does not return to the assessment site's groundwater, significantly affecting the groundwater level in the short term.

Artificial groundwater recharge, including artificial groundwater reinjection, like construction dewatering, significantly affects the groundwater level in the short term. Artificial reservoirs usually have a certain impact on nearby groundwater levels.

4. Geological Conditions

Geological conditions such as stratum lithology and geotechnical characteristics are the basis for groundwater occurrence and runoff. Among them, the stratum permeability coefficient is an important indicator of the impact on groundwater recharge, runoff, and discharge, especially for short-term rainfall infiltration.

5. Groundwater Level Status

The current stable groundwater level is the dynamic equilibrium of groundwater under the influence of various factors and is an important factor in predicting the anti-uplift design water level. Groundwater level changes in the past 3 to 5 years reflect the dynamic trend of groundwater under various factors and have a significant impact on predicting the short-term anti-uplift design water level. The historical highest groundwater level is the equilibrium water level under extreme differences between groundwater recharge and discharge and is one of the factors for predicting the long-term anti-uplift design water level.

6. Foundation Position and Construction Impact

The foundation position, i.e., the underground excavation depth, changes the seepage conditions of groundwater at the site. The backfill material and compaction degree of the foundation trench significantly affect the infiltration path or amount of surface water and, under specific conditions, can markedly affect the anti-uplift capacity of the underground foundation.

7. Current Topography and Its Changes

The current groundwater level is the equilibrium of various factors under the current topography, and site leveling or even changes in surrounding topography may affect groundwater occurrence and groundwater level. In this regard, in addition to topographical changes caused by construction, the potential impact of regional planning on the construction project should also be considered.

8. Design Service Life of the Construction Project

The prediction of the anti-uplift design water level is an assessment for the entire life cycle of the construction project, and the design service life of the construction project is also an important factor in the assessment.

In addition to the above objective factors for predicting the anti-uplift design water level during the service life of the structure, the final determination of the anti-uplift design water level should also consider factors such as the importance of the construction project, its functional use, and safety requirements.

Of course, many of these factors will change, such as rainfall and topography. In other words, the anti-uplift design water level is predicted under certain conditions and has boundary conditions.

III. Risk Warnings for Design, Construction, and Use

Since the anti-uplift design water level is predicted under certain boundary conditions, there is a risk that the predicted anti-uplift design water level may become invalid if the boundary conditions are exceeded. For example, if the prediction is based on a 50-year return period rainfall but the actual rainfall exceeds that, or if site leveling changes the topography and alters surface water or groundwater recharge, runoff, and discharge conditions.

Therefore, to ensure the validity of the predicted anti-uplift design water level, risk warnings or usage requirements should be provided for later design and construction to ensure the project remains within the assumed boundary conditions. For example, requirements should be specified for the backfill material, compaction degree, and permeability coefficient of the foundation trench, as well as requirements for construction excavation, groundwater control, and usage.

It should be noted here that current relevant specifications mainly require the compaction degree of foundation trench backfill, with no permeability requirements. When graded sand and gravel are used as backfill material, even if the compaction degree is qualified, under heavy rainfall, surface water can infiltrate into the foundation trench, causing a rapid increase in groundwater pressure beneath the foundation base, leading to uplift and cracking of the foundation slab, especially when surface and underground drainage at the site is poor.

IV. Brief Analysis of Recent Cases

From several recent judicial cases of construction project uplift and underground structure damage, the following common characteristics are observed:

1. During investigation, the groundwater level was below the foundation slab, and the strata above the base were mainly aquitards;

2. All occurred after heavy rainfall;

3. In all cases, water infiltrated from the foundation trench outside the foundation.

The cause of slab uplift and damage is that surface water infiltrated into the foundation trench, increasing groundwater pressure beneath the foundation slab, which has been recognized by all responsible parties. This situation is commonly called the "water basin effect," which is significantly different from the commonly perceived uplift and damage caused by rising site groundwater levels.

Regarding which responsible party should be held accountable for this situation, experts in the relevant industry have very different views. Court rulings also vary greatly; some judge the investigation unit to bear primary responsibility, while others judge the design unit to bear primary responsibility, and there are significant differences between first-instance and second-instance judgments.

This likely requires a cognitive process, and it is believed that over time, the responsibility issue will gradually become clear.

V. Summary

1. Many factors affect the prediction of the anti-uplift design water level;
2. The predicted anti-uplift design water level has boundary conditions, and beyond these conditions, the predicted value cannot be guaranteed;

3. To ensure the validity of the predicted anti-uplift design water level, requirements must be imposed on later design, construction, and use.

It is believed that all responsible parties in construction projects should fully recognize the importance and complexity of anti-uplift design issues, have more risk awareness, and engage in more communication and consultation, so that similar engineering accidents can be avoided.