L12 Foundation Systems

Added: 2026-05-31

[00:00:00]now for our final topic this week we will be talking about a variety of different Foundation Systems and we'll start off by breaking down our Foundation Systems into two different types we have shallow foundations and deep foundations and typically shallow foundations will be for buildings with 10 stories or less and a soil bearing capacity greater than three cups per square foot if we do not meet either of these criteria that is if we have more than 10 stories or we have soil bearing capacity of less than two Kips per square foot we will be looking at Deep Foundation Systems and as for the behavior of these two systems shallow Foundation Systems rely entirely on soil bearing hence why we need a soil bearing capacity of at least three Kips per square foot deep Foundation Systems on the other hand will rely on soil bearing as well as friction so now let's talk about a couple different types of Foundation Systems and let's start with shallow foundations first we have the typical spread footing and these spread footings are isolated underneath each column typically the column will be placed concentrically inside the spread footing and the spread footing will be square in size however if there happens to be architectural constraints or more commonly MEP restraints as in we could have a pipe that's running too close to this footing and so we need to push the footing to one side creating an eccentricity to the footing we may end up with a footing that is rectangular in shape next we have strip footings which just like spread footings are concentrically placed beneath Foundation walls and note that strip footings are not the same as retaining wall footings which typically will have a longer length on one side of the wall and I'll be posting a separate video that explains why that is lastly with shallow foundations we have matte slabs and matte slabs are basically where we take all of our individual column spread footings and connect them together to create one large matte slab underneath the entire building and typically we use math slabs when there are concerns over differential settlement that is if we were to have a pocket of loose soil with very low bearing capacity say right in the middle of the building here but everywhere else we have sufficient bearing capacity greater than three Kips per square foot in that case if we were to use individual spread footings under each of these columns this one footing here in the middle would settle more than the others and so we would end up with cracking likely inside the building and so instead we connect all of the spread footings together and create this mat slab which helps to mitigate those concerns over differential settlement also if there happens to be a high water table underneath this building for example if we are designing a deep basement and the water table ends up being above the bottom of that basement it would be beneficial to design a mats lab as that would create a continuous barrier beneath the building now moving on to deep foundations first type that we have here are piles and piles can either be drilled or driven into the ground or they can be auger cast in these first two cases where the pile is drilled or driven the pile itself has already been cast and formed and we take it and drive it into the ground using some type of heavy machinery in the case of auger cast we use a piece of equipment to drill down into the ground and dig out a hole and then as that drill is withdrawn we inject grout or concrete into the void creating an auger cast pile these piles can be grouped together under one pile cap in sets of one two three or more and they resist the loads placed on top of them both through bearing at the end and through friction in the case of very very tall skyscrapers we typically Drive these piles all the way down to bedrock where the bearing capacity is very very very high note that these piles can either be steel or concrete in the case of Steel piles we of course need to provide a good amount of Corrosion Protection so that the pile will last over time also with deep foundations we have drilled piers and the key differences here are that drilled Piers will typically only be single drilled Piers they won't often be grouped together also they tend to have larger diameters and therefore they tend to rely more on bearing than piles and so since they rely more on bearing peers will often have a bell at the bottom to increase the surface area and increase the bearing capacity and so those are our two primary types of deep Foundation Systems now if we have a relatively short building say only four stories but we have very low soil bearing capacity we aren't necessarily restricted to using deep Foundation Systems there is actually one other Last Resort we can use and that is improving the existing soil and there are a variety of different methods that we can use to improve the existing soil so that we can then use shallow Foundation Systems and the geotechnical engineer will have to provide recommendations as to the cost of either improving the soil conditions versus the cost of Simply designing a deep Foundation system so the first method for improving existing soil conditions is surface compaction and that's where we run a heavy piece of Machinery like this over top of the soil to compress the soil and compact it and improve its bearing capacity next we have drainage methods or in other words draining more of the water out of the soil and again improving its bearing capacity next we have vibration methods and this is typically done where we have granular soils like sand and here we vibrate the sand in order to decrease the amount of voids between the sand and as we're removing those voids we can also vibrate stone columns down into the sand in locations where we know we are going to place spread and strip footings further increasing the bearing capacity of the soil in that location next we have pre-compression and consolidation in the pre-compression and consolidation procedure the soil under a proposed building is pre-loaded to consolidate the soil and thus either increase the bearing strength of the soil or decrease the settlement of the soil under building loads or both and finally we have grout injection and this is where we inject chemical grouts into the soil to increase soil stabilization and we also fill the voids within the soil to increase bearing capacity and again we do this in locations where we know we are going to place our spread footings so those are the five primary ways that we improve existing soil bearing capacities and lastly in this video we are going to briefly talk about how to size spread footings so if we are given a column with a load of 50 kips and we have a soil bearing capacity of four Kips per square foot what size should we assign to the footing underneath this column and to do this procedure is actually quite simple the footing area that's required is equal to the column load divided by the soil bearing capacity so we get 50 kips divided by four cups per square foot and that is equal to 12 and a half square feet now we need a dimension for this spread footing and we have footing area so in order to get the actual footing size the last step we need to take is we need the square root of this number and that's of course because the area of a square is equal to a times B and if we have a square a is equal to B and so the area is simply a squared so here we get 3.53 feet and we're going to round that up to a four by four footing and so that is how we size a spread footing and that is all that we have time for for this week for our discussion on concrete and Foundation Systems I will post one final follow-up video on retaining walls for you to look at and that video will answer the question of whether it is more efficient from a structural engineer's perspective to design a retaining wall with a greater length on that side where there is soil or to design retaining wall with a footing that has a longer length and it's important to note here that from a constructability standpoint this design on the right is actually preferred and that is because to form the retaining footing in this case on the left we would have to remove more soil in order to form this portion of the footing whereas in this case we would only need to cut back this amount of soil to form this area of the footing so from a constructability standpoint the design on the right is preferred however from an efficiency standpoint it's actually going to be the design on the left that is preferred and again the video will go into greater detail of why this is but for now that's all that I have for you for this week next week we will be wrapping up the entire course with our discussion on wood and masonry which will be a very very quick discussion I don't think we're really going to have time for sizing those kinds of elements we're just going to do a quick overview of those two materials and then of course we will have our final exam and note that I'm not going to put anything on the final exam regarding wood and masonry because I don't think that it would be fair to have that type of material on the final exam given that we probably won't have time to provide feedback to you with an assignment for those two materials and so the content for the final exam will end with the next video however I may have some bonus questions on wood and masonry in the final exam so with that good morning good afternoon and good night and I'll see you next time