Ground is not the same everywhere
The first thing that determines how a building behaves against water is the ground it sits on. A building founded on rock and one founded on an alluvial plain cannot be protected with the same membrane. The Sakarya plain, central Adapazari and the Kocaeli gulf line all sit on thick alluvial layers.
Three characteristics of alluvial ground
- High water table: at most locations the groundwater level sits above basement level. That means constant hydrostatic pressure on the retaining wall.
- Variable permeability: clay and sand bands are stacked irregularly. Water drains quickly at one point and pools a few metres away. For that reason borehole data taken from a single point can be misleading.
- Settlement and liquefaction: water saturated sand layers can liquefy during an earthquake. Controlling the water around the foundation does not directly reduce that risk, but keeping the groundwater regime stable matters for how the structure behaves.
What changes in practice?
Under these conditions drainage is not a preference, it is part of the system. The following differ in application:
- A single membrane layer is not enough; a double layer and a system certified for pressurised water are required
- The drainage pipe is laid at foundation level and always with a fall
- Natural discharge is not possible on most plots, so a sump and lifting pump are needed
- A backup power supply should be planned for the pump, because heavy rain and power cuts often arrive together
- Backfill material must be free draining; excavated clay must not be put back
The seismic dimension
Sakarya, Kocaeli, Yalova and Duzce are in a first degree seismic zone. Moisture reaching reinforcing steel causes corrosion, the steel cross section is reduced, and the building's capacity during an earthquake falls. IZODER also defines waterproofing as part of protecting buildings against earthquakes. Along this line waterproofing is not a comfort item but an investment that determines the service life of the structural system.
What we look at during a survey
- The groundwater level and layer profile in the geotechnical survey report
- The position of the plot relative to surrounding levels, and where the water is coming from
- The state of neighbouring buildings' basements, whether similar problems occur along the same line
- A map of efflorescence, rust staining and cracking in the existing building
- Discharge options, whether there is natural fall or a pump will be required
A price given before these five items are clarified does not reflect the real scope of the work.
On the plain, water comes from below rather than above
This is the fundamental difference that separates drainage work in the region from other provinces. On sloping ground water reaches the building from one direction, from above, and the solution is an intercepting line on the uphill side. On the Sakarya and Kocaeli plain, water rises from within the ground, from below.
That difference changes the whole design. Intercepting line logic does not work here because there is no direction to intercept. What is needed is perimeter drainage that runs entirely around the building, sits just above foundation level and continuously relieves the pressure.
How alluvial ground affects drainage
Alluvial soils are a mixture of sand, silt and clay. The fine particles in that mixture are the real risk to a drainage system, because they travel with the water and blind the filter.
Selecting the filter geotextile is therefore critical in the region. The pore structure of the fabric must suit the fine particle size in the ground. A fabric woven too tightly will not pass water, one woven too openly passes fine particles and clogs the gravel. The outcome of both errors is the same: a system that loses its function within a few years.
Washed gravel matters for the same reason. Gravel containing dust and fines blinds the filter from the inside even where the fabric has been chosen correctly.
The discharge problem
Because the ground on the plain is flat, it is often impossible to find a point where the drainage line can discharge under gravity. This is the most frequently overlooked dimension of work in the region.
The collected water has to go somewhere. Where there is no natural discharge, a pumping chamber is installed. When selecting a pump, reliability matters as much as flow rate, because a pump that fails during heavy rain takes the whole system out of service. Duty and standby arrangements and an alarm system are considered where the basement is in critical use.
Where the pipe end is left open the water returns around the foundation and the system behaves as if it were never built.
What is different about industrial buildings in Kocaeli
On the Kocaeli side, large factory and warehouse buildings are common alongside housing. Drainage design differs in two respects in these buildings.
The first is scale. A large footprint increases the length of the perimeter line and therefore the flow collected. Pipe diameter and discharge capacity are selected accordingly.
The second is roof water. The rainwater flow from large roofs is high and must never be connected to the foundation drainage line. Where that connection is made the line fills on wet days and raises the pressure instead of reducing it.
Retrofitting drainage to an existing building
A significant part of the building stock in the region was built without foundation drainage. When adding it later the sequence is as follows.
Access for excavation is assessed first: is there room to excavate around the building, can a safe distance to neighbouring structures be maintained? Where there is, the outer wall face is opened, cleaned, repaired as required and waterproofed. The drainage line, gravel, geotextile and protection board then follow. The line is tested with water to verify the fall. Backfill is placed and compacted in layers using screened material.
Where excavation is not possible the approach changes: active flow is cut off by injection, a system that works on the negative side is applied to the wall face, and a collection channel with a pumping chamber at the foot of the wall inside the basement reduces the pressure.