Although it doesn’t appear to be, concrete is very porous. Most concrete mixes contain more water than needed to set the concrete. As it dries, this water moves to the surface and evaporates leaving microscopic capillaries throughout the concrete. They are very small, much smaller than a human hair, but much larger than water molecules which continuously pass through when the capillary pores get wet again.
Dampness from soil that is saturated after a rain seeps against the basement wall. Capillarity in the concrete draws the moisture on though to the inside. Capillarity will also draw water up from moist soil at the water table as much as 16 gallons per day for a 1000 square foot house. This water can then enter the footings and floor from below.
It is usually possible to keep moisture away from the foundation and basement by digging drainage ditches. If your house is on sloping ground, these should be positioned so as to intercept the water flowing downhill and take it around the house. The important thing is to intercept the water and carry it away from the walls of the house.
The extent and depth of the digging depends on the amount of water, the source, and the amount of money and labor one is willing to spend to solve the problem. Following is an example.
Install plastic sheeting in a modified french-drain. Dig a trench about three feet wide around the house walls, line the trench with a vapor impermeable material like polyethylene film (plastic sheeting). Coat the wall with asphalt. Press the sheeting to the wall and along the base of the trench. Slope the trench away from the house at lease ? inch per foot. Install PVC slotted plastic drainpipe at the end of the sheeting to carry off the water toward the low side of the property. Be careful not to dig so far as to undermine the foundation of the house wall. For houses with a basement, about 1 foot against the house, sloping down from there, should be fine.
Make sure the bottom of the trench slopes away from the house with no dips or hills in it with a steady slope so water will not stand in the slotted plastic PVC drainpipes. Extend the gravity flow PVC drainpipes at least 12 feet from the house. Fill with a layer of coarse gravel, cover with a weed control fabric and then cover with a layer of soil. Slope the soil gently away from the wall.
If it is not possible to have gravity flow from the level of the PVC drain tile, then a sump pump must be used to remove the water. Water cannot be collected in the drain tiles without an appropriate way to remove it.
As always, I wish you good fortune and success solving your waterproofing problems. Take a deep breath and try not to get discouraged if things don?t go quickly and easily, as is often the case. Don?t take it personally. Remember, nearly every solution which now exist was acquired by trial and error by someone who was frustrated before you. Persevere, you?ll get the job done and feel very good about it.