Wet basement problems can be caused by a number of conditions but depending on severity, generally are related to a lack of, or insufficient, exterior protection of foundation walls or penetrations of the wall.
Contrary to common thought, both poured concrete and more particularly, masonry block walls, by themselves are still permeable materials. All foundation walls require a protective coating on the exterior to prevent water permeating through the material. In the past, this was accomplished with coatings of dense cement parging or a liquefied asphaltic compound. Parge is subject to cracking or delamination and asphaltic coatings can fail with aging. This alone can lead to moderate dampness issues.
Aside from the obvious leaking of water running across a basement floor, there are other indicators of leakage issues. Most basements will have some level of higher humidity than upper levels but excessive humidity can indicate a moderate problem. More severe issues may be indicated by staining or efflorescence on open masonry or concrete walls. Efflorescence is leaching of salts from the concrete or block due to passage of water thru the material. Worse yet might be the presence of mold on organic materials such as wood or drywall. Black mold can be very dangerous to health and should be mitigated as soon as possible.
A more serious problem, of significant leaking into a basement from surface flow sources, is usually the result of some penetration of the wall such as cracks, utility penetrations or other failures in the wall. Foundation cracking may be due to differential settlement of the footings (unconsolidated soil under the base of wall), poor concrete mix, aged mortar in Masonry block, etc.. Moisture in the soils surrounding the foundation can lead to premature deterioration of masonry block in particular, a not uncommon problem in older homes. Significant moisture problems most often require excavation in the suspected areas of leaks (if they cannot be identified from the interior) and sealing on any penetrations and application of modern and more durable waterproofing compounds. In extreme cases, where foundation settlement is suspected leading to major cracks, installation of remedial foundation piers may be required before any waterproofing work is performed, but can be one continuous operation as both require excavation along the foundation wall.
As noted regarding soils, back fill around foundations is usually less dense than surrounding native soils. Water that may be present near the foundation therefore, finding the path of least resistance, may flow directly down the exterior of the wall and in clayey soils, may be trapped for long periods before finally draining into surrounding soils. Over time, poorly compacted soils around the house may also slump.. homeowners may notice this.. and such slumping can hold water which can contribute to basement dampness or leak problems. A good rule of thumb is that grade should slope away from a basement wall at least 6 inches vertically within 10 feet, and then be drained away. This is not always the case in existing homes or possible and may require measures to improve drainage away from the home or divert flows away from the foundation.