Seepage of Ground Water to Underground Basements

Our office has a lot of experience in handling contract work losses in general and, among those, handling seepage of underground water to underground basements. All that in the framework of the contract works policies for construction projects insurance.

Hereunder is our reference to the subject of the seepage of underground water, the large expenses involved in the need to overcome the seepage of the water, and reasons that make the handling complicated for insurers following such occurrences.

B. Underground Basements and the Problem of sealing them

Underground basements are usually used for parking lots and/or warehouses/ tenant warehouses in resident houses or different rooms for machines/systems/water supplies etc. It is usually a large space of tens and hundreds of square meters that is located underground and must be sealed from entrance of liquids into it/ into the building. The problem is even more severe when the construction of the project is done near a source of water (seaside/ lake/river), because the underground water level is higher, and therefore, the majority of the basements are below underground water level and are exposed to a high hydrostatic[r1] [1][r2] [RT3] [r4] [RT5]  water pressure.

In construction projects of underground basements their digging is done following the casting of the reinforcement walls, the slurry walls, or the walls from piles that are meant to keep the integrity of the surrounding and prevent the collapse/ the slide of sand/earth to the dug hole. Following the digging, a sealing system is created for the purpose of sealing / separating between the reinforcement wall and the lower floors, and the walls of the underground construction. The sealing suit is done in one of the two typical following methods:

1.     A sealing suit that includes a sealing layer from a bitumen sealing material that is implemented by spraying on a layer of thin concrete that is poured above the subsoil and above the reinforcement walls. The lower floor is cast above the bitumen sealing layer, usually a large raft from reinforced concrete, and a reinforced concrete wall is also built by pouring the cement that constitutes the inner circumferential wall of the basement.

2.     A sealing suit based on bitumen layers which are sealing layers made of bitumen- a thermoplastic product that softens at heat and formed from distillation of crude oil, which is installed on the infrastructure via heating flame with a gas burner. The heat of the flame melts the bitumen layer and as a result, the layer sticks to the infrastructure and the next/neighboring layer in order to form a sealing suit.

Some projects utilize combining combination of both of these methods and also by a use of additional materials/components.

During the creation of the ceiling suit other components are used in order to mitigate the seepage of water such as water stoppers- a material which inflates when it touches water that is put in the stiches between the floor and the walls and which looks like a stripe/a roll of gum.

The sealing suits for the underground basements are planned in the framework of the project planning as several experts/planners deal with the subject and are required to supervise the creation of the sealing suit as part of their role as project planners. In the framework of the program and the bill of quantities for the construction of the building, a section or a part that relates to sealing is included and there is a detailed plan and a bill of quantities of materials and required works that are performed in the framework of the construction of the building.

We would like to note hereby that the sealing system/suit is hidden after it is done and is located behind the peripheral walls of the underground basements, therefore all inspections and/or maintenance and/or repair is actually impossible.

In the course of the construction of the buildings below underground water level there is a possibility of seepage of underground water through the sealing suit that is foreseen and taken into account in the framework of the construction. For that purpose an injector piping system within the stich between the floor and the wall is usually planned. If there is a leak or seepage of underground water into the underground basement, an injection of a polyurethane material via the pipes is executed. The polyurethane material is used for sealing after it is created by mixing two (2) components, a base and a catalyst which cause a process of inflation and firming of the polyurethane as it comes in contact with water.

When construction works of an underground basements are performed below underground water level there is a need to perform a lowering of the underground water level- a process of pumping the water from within the site in order to lower the hydrostatic pressure and allow the execution of the works underground. The lowering of the underground water level is performed by an installation of several water well pumps in the site from which underground water are pumped and removed from the site (the water are usually poured to a draining system in the area or back to the water source). The stop of the pumping/lowering of the underground water level is preformed gradually as the construction in the site progresses so that the hydrostatic pressure on the underground part of the building gradually rises. There are two (2) main points which indicate when to slow down or stop the lowering of the underground water level, a process which one is interested to perform as soon as possible due to economic considerations:

1.     The completion of the sealing suit around the underground basements.

2.     The weight of the completed building is higher than the floating forces activated by the underground water that operate to pull the buildings and its foundation from the ground.

C. Seepage Losses and the Financial Expenses as a Result

Buildings that include construction of underground basements below underground water level, even at the time of the halt/gradual stop of the lowering of the underground water level, water may enter into the underground structure/the basements. The seepage of water can occur to different extents; small seepages- which are manifested by a stain/wetness on the peripheral wall of the basement and which cause is a specific fault in the sealing suit- and up to the extent of water seeping in large quantities and a total collapse of the whole sealing system.

In case of a localized wetness, the subject will usually be handled in a small extent and with a small expense, and usually without the involvement of the insurers, by an injection of a polyurethane material via the injector pipes that were installed in the building and/or through the injector valves. In cases where large quantities of water are seeping in, the solution is a long process of injection of large amounts of polyurethane material, tens, hundreds and even thousands of kilograms of sealing materials- a process that leads to a great financial claim. The high costs are a result of the pricing of the injection works which are preformed according to the number of the injected kilograms, meaning that the cost of work and perishable materials is loaded on every kilogram of injected material. In addition, we would note that the end of the injection is determined according to the situation on site – the injection is stopped after the actual seepage of water which cannot be predicted in advance, stops. In addition, there are additional costs of works for the maintenance and the management of the site by the main contractor. all this is without going in the subject of future loss of revenue that may be caused as a result of the delayed completion of the project which is sometimes insured.

We will note that the injection works are actually a improvement/ addition to the original sealing suit/system of the building (the one that was planned and created in the original) because there is no possibility to inspect and/or fix the sealing system as explained herewith.

In some cases, there is a consideration to the injections of sealing materials following seepage of underground water in the original planning of the sealing suit and in the bill of quantities derived from it. The amounts which appear in the bill of quantities are in the extent of tens to several hundreds of kilograms- the expected amount of injections following small seepage of water or of water entering in specific points. Most times, when an insurance Policy is activated and a financial claim is submitted, injections of a higher extent for handling seepage of water in higher volumes are in question.

Another possible way of handling seepage of underground water to underground basements is constructing drain tunnels in the bottom of the inner wall of the basement, between it and the reinforcement walls, and adding pumps in order to transfer the water which continue to enter in small amounts, from the tunnel to the draining system of the building, and sometimes hiding it behind the wall located in parallel to the basement wall. This solution is the last measure that is taken if the injection do not succeed to completely overcome or lower the seepage of the underground water to a reasonable level, but it has several disadvantages. The two (2) main ones are as follows:

1.     A decrease/a reduction of the area of the basement and damaging the parking lot and/or warehouses and/or rooms area in it – subtracting usable space.

2.     An addition of a system and additional costs of maintenance and management of the buildings in its lifetime such as costs of electricity of the pumps, costs of the maintenance of the added system (the tunnel/ the wall) etc.

The source and cause to seepage of underground water are usually faulty works, faulty planning, faulty materials or a combination of the aforementioned reasons as follows:

In addition, we would like to note that since sealing of large areas in a relatively long period oftime, it is impossible to eliminate the possibility of a fault that is not discovered at the time of execution or the supervision of the works, or even an intentional damage to the sealing system/ suit, as a major part of the risk.

D. Summary

In the above document, we refer to the subject of the seepage of underground water and the large expenses involved in its solution.

We will note that from our experience in the field of surveying building construction projects with parts or basements below underground water level and handling losses of underground water seepage, the subject of mitigating the exposure is problematic and even when many precautions are taken, they do not fully eliminate the risk.

[1]  A pressure which is created by a flowing liquid or gas, which is found in a static balance (rest), under   the influence of the gravity force (mass force) as opposed to pressures created by the influence of flows, waves or whirlpools.