Weep Holes in Retaining Walls -Types, Functions and When it is Required
Weep holes are provided in masonry walls, retaining walls, underpasses, wing walls and other ground draining structure. Functions and advantages of weep holes are discussed.
Weep holes provide an opening that allows the drainage of any moisture that may come from back of the wall through penetration, capillary action or leakage.
Weep holes installed in brick masonry also serve as ventilators to allow air to the back of the wall to prevent mildew, dry rot and damp which in turn affect the life and performance of the building.
Weep holes are typically found in outer masonry of cavity walls. They can also be found above windows and doors in cavity walls. They are provided at regular intervals to permit escape of moisture. The recommended distance between weep holes is 450 mm.
From engineering point of view, weep holes relieve hydrostatic pressure or water pressure on walls. This reduces the structural design demand of the water and earth pressure by reducing thickness as well as reinforcement requirement.
With this, weep holes also reduce the Buoyancy and uplift on the structure making it possible to construct lighter structure without any uplift related stability issues.
Functioning of Weep Holes
If a structure is situated about the water table then there is no possibility of water to accumulate behind the wall. So, weep holes are not required.
But when the structure is situated below the water table and extra pressure of water acts on the structure which is more than the saturated pleasure or the earth pressure, weep holes are required.
Case-1: Water Table is Below the Structure – No Weep Holes Required
In the figure-1 the water table is below the structure so only earth pressure is considered while designing of structure.
Fig.1: Water Table is Below the Structure – No Weep Holes Required
Case-2: Water Table is above the Structure -No Weep Holes Provided
In the figure-2, water table is above structure. When water is mixed with soil, the saturated pleasure or earth pressure becomes submerged weight, which is lesser than that of saturated pressure but the water pressure is greater than that of saturated pressure.
So, in this type of structure the soil and water pressure are to be considered while designing.
Fig.2: Water Table is above the Structure -No Weep Holes Provided
Case-3: Water Table is above the Structure and Weep Holes are Provided
If the water table is above the structure, but weep holes are provided in structure. As seen as in the figure the major water pressure is eliminated using weep holes, the pressure due to water is relieved by the holes. The height of the placement of holes plays an important role. As the height of the weep hole, more is the pressure exerted by the water on the structure.
Fig.3: Water Table is above the Structure and Weep Holes are Provided
Weep holes are provided using sand gravel trains to ensure proper functioning of this holes. The number of holes is provided depending on the size of structure and height of water table. The height at which weep holes is provided depends on the maximum possible height of water formulation or natural groundwater table.
Types of Weep Holes
The type of weep holes depends on the nature of use of the holes:
1. Open Head Joint Weep Holes
These type of weep holes are formed by eliminating mortar from the vertical joint between bricks. These walls are of same size as the typical joint spacing.
The typical space between Open hands joint is done at 21 inches interval. This is also done by using plastic weeps baffled structure to prevent rain from penetrating the holes and preventing insert from entering the cavity. A drip is included at the front lip to aid drainage.
2. Cotton Rope Wicking Weep Holes
These type of weep holes are formed by using a rope of up to 12 inches in length. This is placed in the joints and the other end is extended up to the cavity wall.
The water from the back of the wall is adsorbed by the cotton and the on the other side it is evaporated. This process is slow when compared to ordinary weep holes.
3. Tubes Weep Holes
Tubes weep holes are formed by using hollow plastic or metal tubes. They are spaced around 16 inches apart. This tubes are placed at a slight angle to allow water egress. It must be made sure that the angle is not too steep or too flat.
Disadvantages of Weep Holes
Blockage of Weep Holes due to Trash Mortar
When holes are installed in the brick masonry. The mortar squeezes out between the successive courses of bricks and drop down in the weep holes. This can result in partially or totally blockage of weep holes by trash mortar.
Pest Entry through Weep Holes
Weep holes can provide access to the pest such as rodents and insects. Entry point to the internal of the building can be through downlights ventilator fans and holes of plumbing and electric purposes. To overcome this problem, Baffled vent can be installed.
Obstruction in Airflow
To overcome the above problems of pest entry and trash mortar and to even make the weep holes more pleasing, weep hole may not provide the necessary flow of air to properly ventilate the internal brick wall.