Struc­tur­al Foun­da­tion Dam­age Caused by Improp­er Backfill

Dec 22, 2013 • By Matthew Stock with Michael Likvan.

The con­struc­tion of a home begins with dig­ging a hole. It may be a deep hole to accom­mo­date a full-depth foun­da­tion or a shal­low one for a crawl space. It may even be a few small holes for foot­ings for a house that is to be built on a slab.

The deep­er the foun­da­tion is to go, the wider the hole must be to allow the exca­va­tion equip­ment to reach the required depth. Once the foot­ings have been poured and the foun­da­tion has been built, either by lay­ing con­crete block or pour­ing con­crete into a sys­tem of forms, the hole has to be filled in around it in a process called back­fill­ing. This process, even when done well, can be prob­lem­at­ic for the foun­da­tion in the future; when done poor­ly, prob­lems are almost guaranteed.

How Back­fill Con­tributes to Foun­da­tion Damage

The exca­va­tion described above usu­al­ly takes place in undis­turbed soil that may have been untouched for decades or maybe even hun­dreds of years. That soil has com­pact­ed as much as it ever will and forms a sta­ble base for build­ing. How­ev­er, the exca­va­tion cre­at­ed a ​“bowl” of undis­turbed soil in which the foun­da­tion will sit and which is pret­ty effec­tive in retain­ing water.

Also, once an exca­va­tor digs it up, the soil removed from the hole breaks apart and becomes loose again and, when it is used to back­fill around the foun­da­tion must be com­pact­ed while it is being placed in the exca­va­tion. Even the most painstak­ing job of com­paction, how­ev­er, will not com­press the soil equal­ly with the undis­turbed soil; there­in lies the problem.

Water in the Bowl

The orig­i­nal bowl that result­ed from exca­va­tion still remains under the foun­da­tion and the less­er-com­pact­ed back­filled soil. When the water table ris­es or heavy rains or snowmelt cause the bowl to fill with water and the back­fill to sat­u­rate, water at the bot­tom of the bowl cre­ates upward pres­sure on the foun­da­tion, known as hydro­sta­t­ic pres­sure. This can cause seep­age in the base­ment through floor cracks or the cove joint.

In a shal­low foun­da­tion, the water in the bowl can freeze and cause the foun­da­tion to ​“heave,” or lift upward. If the water is with­drawn from the sat­u­rat­ed soil by trees and plants seek­ing hydra­tion dur­ing a drought, the soil will com­pact and the foun­da­tion will crack and drop into the void that created.

In a deep­er foun­da­tion, heav­ing is not like­ly to take place because its bot­tom is below the ​“frost line,” the low­est lev­el in the soil that freez­ing takes place. In the Chica­go area, the frost line is at approx­i­mate­ly 42 inch­es below the sur­face, a fact well known to any­one who’s built a deck or a fence or installed a mailbox.

How­ev­er, the area of the back­filled bowl that sur­rounds the foun­da­tion is also sub­ject to sat­u­ra­tion and when the clay soil com­mon to the Chica­go area absorbs water it swells, which cre­ates lat­er­al pres­sure against the foun­da­tion walls. This can cause non-struc­tur­al cracks in a poured con­crete foun­da­tion that will seep water but it can also cre­ate struc­tur­al cracks in con­crete and cause the foun­da­tion wall to tip or rotate inward, which requires repair and sta­bi­liza­tion.

In a mason­ry foun­da­tion, this pres­sure caus­es mor­tar joints to crack and walls to bulge or bow inward in the mid­dle. Left unre­paired, the blocks will even­tu­al­ly begin to shift out of line. Either of these con­di­tions also requires repair and stabilization. 

When any of these events occurs, whether caused by improp­er back­fill or oth­er con­di­tions, the home­own­er will need the advice and assis­tance of a qual­i­fied foun­da­tion repair con­trac­tor. At U.S. Water­proof­ing, our foun­da­tion repair experts make use of engi­neer­ing data and the lat­est proven tech­niques and mate­ri­als to repair and sta­bi­lize foun­da­tions per­ma­nent­ly with a min­i­mum of incon­ve­nience. Why not ask us for a free con­sul­ta­tion?

Tags: structural foundation damage, foundation damage backfill, backfill damage foundation

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