How does Ground Water Recharge Reduce Foundation Settlement?

Dewatering provides us a consolidated and stable soil deposit which ensures us lessfoundation settlement. It is not practical to consolidate soil deposit upto 100%; if it is done, no settlement will be occurred.

When pore water pressure reduces, the effective stresses is increased and when it is done under a existing structure which is designed for estimated settlement after a time period, it is detrimental to foundation and structure supported by it. Thus fast settlement and obviously (if dewatering not done all around the foundation) differential settlement will be happened as settlement is more likely to occur near adjacent dewatering side.

In such case two types of remedy can be taken

• Provide a cut-off


• Artificial recharging to ground water

The cut-off can be done by 

• Slurry wall


• Sheet pile


• Grouted ground


• Frozen ground

Our concern is here artificial recharging of ground. In this mitigation measure, water is injected into ground by using recharge wells in controlled way. Water from dewatering source or from other sources can be transported to recharge location (considerable distance from dewatering point should be maintained). The water should be investigated for treatment before recharging.

The artificial recharge is provided to restrict or limit influence zone of dewatering. If should keep in mind that ground water level can be lower not only under installation of dewatering well but also under other method or construction work allowing gravity flow like excavation near foundation.

The influence of drawdown depends on geological properties; the water available for recharge is also important. Increasing ground water level in this way practiced in Shanghai, resulting successful reduction in settlement velocity.

Both water and summer irrigation is determined depending on artificial recharge. An average quantity of artificial recharge is 20000000 m3/year; in last 30 years 0.589 billion m3 water was recharged. In Shanghai only within two years accumulated settlement at city center was found 0.23 m (between 1956’s &1960’s) even after different settlement mitigation measure.

Now consider about cost involved in artificial recharge; the cost involved in this mitigation projects may even be more than repairing of damage building, grouting or underpinning. Not only cost but also some other practical and environmental problems may involve such as:

1. Prediction of permeability may be wrong and more cost may involve than expected, especially when aquifer soil is unsaturated before recharge.


2. A correctly determined permeability may also involve more cost as wall of wells or wall of trenches may be clogged while pumping as pumped water may contain fines.


3. Designed water head may practically not be possible to produce in some cases.

Construction Sequence of Raft or Mat Foundation

Site selection, geotechnical investigation and structural analysis and proportioning of raft or mat foundation are done in design phase. Construction phase includes

• Site preparation

• Clearing construction site

• Excavation

• Fill placement 

• Providing stiffening beams 

• Providing vapor barriers

• Placement of reinforcing steel

• Pouring concrete

• Site finishing

• Follow up during or after construction 

SITE PREPARATION:

The construction site should be maintained in such way that minimum moisture variation is occurred by providing sufficient drainage to improve runoff of rainwater which facilitates to reduce subsequent differential settlement. Well drained site provides a dry working conditions producing firm surface.

A layer of granular fill of up to 1 foot thick is very effective to improve trafficability providing a temporary road for easy and quick transportation of materials and mechanized equipment into or out of construction site. The advantage of providing fill is manifolds; with improved trafficability, it helps

• To have improved grade of better drainage
• To provide surcharge loading on foundation soil beneath it
• To arrest swell pressure of soil generated due to wetting for long period.

It is recommended to complete site preparation before wet season. Soil preparation is a continuous process until site is protected by foundation to reduce ill effect of wetting and drying cycle due to seasonal variation. Sometimes lime with or without cement mixed with surface soil improve trafficability of problematic soil. 

CLEARING CONSTRUCTION SITE:

During clearing site, removal of vegetation and trees is often leave depressions. Such depressions, trenches and holes are often filled with soil compacted at natural moisture content to reach density of in-situ natural soil and graded to initial level of ground.

Cutting of soil, if required, is kept minimum as it reduces overburden pressure on soil underneath over the cut area, cutting also reduces pore water pressure of the soil. When soil underneath is of cohesive type (i.e. less permeable), it takes considerable time to reach an equilibrium condition consistent with surrounding soil. Long-term time dependent heave and rebound, as a consequence, may occur which may aggravate differential settlement over several years.

EXCAVATION:

Roads, facilities or other neighbor properties in vicinity to excavation site must be checked prior commencing excavation. Slopes are usually supported by horizontal or inclined braces against sheet pile, any types of piles, drilled shafts, with reinforcement or by anchoring ground.

Excavations work should be finished to desired depth as early as possible. The exposed soil should be protected from wetting-drying cycle. A concrete layer may be provided to protect final cut level; as an alternative 10-mil thick polyethylene sheet can be used. Sometimes some type of coatings is used but if they are sticky should be avoided.

During and after excavation, seepage of underground water or rainfall may produce ponding which may seep through underlain soil and may reduce shear strength. In this regard, pumping may be required to drain out properly this water.

Excavation boundary should be selected carefully to place and remove formwork after concreting. The dewatering system and provision for placing utility should be considered during excavation.

Excavation depth may be exceeded desired depth only when unexpected materials have to be replaced by fill of desired property and depth. But optimum excavation is very important as it increase cost of excavation work together with cost of drainage and protection of slope.

FILL PLACEMENT

The fill materials should have low plasticity but should be of cohesive nature. They are compacted to achieve desired density to provide adequate bearing capacity. Usually collapsible soils, expansive soils or soils having low bearing capacity are replaced by desired fill of designed density.

Generally when filling depth exceeds 8 ft., the feasibility of shallow foundation like column footing or combined footing have to be checked. Fill materials are also placed around foundation perimeter as pavement, ramp, water or septic tank etc. may be required to place fully or partially on perimeter filling.

The expected fill materials are sand and gravel having less percentage of fine particles. Organic, peat, silts having high plasticity or silty sands are usually unaccepted as fill materials.

In case of construction of ribs for raft foundation, it is required to provide trenches, cohesive properties of fill materials facilitate us to minimize requirement of formwork. Such trenches also required for different utility lines. The most important thing is that it reduces seepage of water from surface to underneath stratum.

Seepage into relatively impervious stratum may lead to raise in water table which may result long-term differential settlement, if soil underneath is of collapsible or expansive type.

Now regarding compaction effort, the usual expectation for cohesive materials is not less than 90% of their optimum density (as per ASTM D1557). A uniform depth of each layer is required to ensure uniform fill in horizontal direction which offer minimum differential settlement of mat foundation. In next post we will discuss in details sequence of excavation of excavation and fill placement.

PROVIDING STIFFENING BEAM

The cohesive granular materials used as fill facilitates to make trenches for stiffening beams. Trenches machine usually capable to provide minimum width of 12” and maximum depth of 3’ below grade. To accommodate reinforcing steel for the beams, a least width of 18” is considered enough. 

PROVIDING VAPOR BARRIER:

Moisture may enter into or come out from concrete; there have two recipient when moisture flow out of concrete, one is soil underneath and other is environment prevails above concrete surface. Either in or out from concrete both are detrimental for concrete.

So we provide a barrier, here named as vapor barrier. In most case, it is plastic film placed beneath slabs and in the trenches, if any. This film acts as barrier against accumulation of sweating moisture into concrete which may reach surface of finished floor through concrete.