Awell-built basement give s

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1 The first steps in building highquality poured wall foundations Excavating and constructing footings BY R. T. BUCK BARTLEY THE BARTLEY CORPORATION SILVER SPRING, MARYLAND Awell-built basement give s the homeowner a big b o n u s l ow cost, energ y- efficient living space. And i t s not hard to build. A basement wall is simply a retaining wall. Bu t it won t work properly unless all the components shown in Fi g u re 1 are in place. Leave out some of the components or do the work poorly, and yo u ve created a continuing s o u rce of aggra vation for the homeowner and a series of headaches for the builder. A poured-wall contra c- tor can help the builder avoid these headaches by using two skills: observation and communication. Look, ask, and listen Eve ry stage of basement cons t ruction, from digging the hole to backfilling re q u i res a good eye. If something doesn t look right, question it. Get down in the hole and find out what s going on. Observ i n g w h a t s happening on the jobsite helps to avoid mistakes instead of having to correct them later. Communication is as import a n t as observation. Poor concrete is seldom a problem in the poured wall b u s i n e s s. But poor communication can kill you. To do a job well and make a profit, yo u ve got to communicate well with the home builder. Some builders give you a set of Figure 1. A high-quality poured wall basement requires well-built footings and walls, an adequate waterproofing system, and careful backfilling. It must sit high enough on the lot to permit surface water to drain away from the house. p rints and tell you exactly what they need for the foundation. With othe r s, you have to ask about many details they have neglected to mention. Are there any step-down are a s in the house? Is it a walkout basement? Is the optional fireplace included in this installation? It s equally important for you to talk with surve yo r s, other subcont ra c t o r s, and with your own footing and wall crews at each stage of the basement building pro c e s s. St a rt with the sub who can have a dire c t effect on how easy your job is the e xc a va t o r. Excavation work Site pre p a ration and basement e xc a vation are the first steps in basement construction. The exc a- vator clears and strips the site, then

2 SQUARING THE CORNERS OF A FOUNDATION Figure 2. Place offset hubs that extend two of the building lines 10 feet beyond the building edge. Use these stakes to re-establish a primary building line after the foundation hole is dug. Figure 3. Solid ramps at the corners may be needed to provide enough height to pour concrete directly into wall forms. s u rve yors stake out the foundation b o u n d a ry. It s a good idea to place offset hubs that extend two of the p ri m a ry building lines 10 feet beyond the building edge (Fi g u re 2). These stakes are used to re - e s t a b l i s h a pri m a ry building line after the foundation hole is dug. They also can be shot with a level and mark e d with the amount of cut or fill needed at each of the building corn e r s. Be f o re digging, make sure there a re no utility lines in the area to be e xc a vated. In most urban are a s, by calling a special number you can h a ve a re p re s e n t a t i ve from the gas and electric companies check the site for buried lines. A check for telephone or television cables also may be needed. Decide ahead of time who is responsible for getting the utility reps to the site and who pays if there s a fee. Usually the builder pays for this serv i c e. Ba rt l e y s crews all carry calculators and use the two-tape method for squaring corn e r s. One person working alone can establish all the corn e r s. After the first two corners are marked with nails along the stringline bet ween hubs, the worker calculates the diagonal measure m e n t as the square root of the sum of the squares of the sides. The calculator allows entering the side dimensions in feet and inches and gives the diagonal measurement in inches. With one tape hooked to the nails at each corner on the stri n g l i n e, the intersection of the diagonal measurement and the length of the other side marks the next point. T h e re are seve ral other ways of establishing corn e r s. One is to m e a s u re to the other corners at an estimated right angle, then m e a s u re the diagonals. If the corners are square, the diagonals will be equal. If the diagonals a re n t equal, the corner mark e r s a re moved to make them equal. Another method of squari n g the corners is using measurements for a (or ) ri g h t t ri a n g l e. Me a s u re 4 feet from one of the corners under the stri n g- line and dri ve a nail there. Then hook one tape to that nail and one tape to the corner nail. The intersection of a 5-foot diagonal and a 3-foot leg marks the other leg of the right tri a n g l e. Gi ve the exc a vator specific guidelines for the work to be done and explain what you expect the finished hole to look like. We use a pri n t e d sheet that outlines the following req u i re m e n t s. Site pre p a ra t i o n Re m ove all trees within 15 feet of the future house. Figure 4. To put rebar into the footings, drive short sections of bars into the ground and tie the bars to them or place bars on the surface and push them in with a shovel. St rip topsoil from the area of cons t ruction and grade to slightly be l ow finished gra d e. Finished g rade should slope away from the e xc a vation. Build a berm to dive rt s u rface water from the hole in case it rains before the footings and walls go in. Stockpile re m oved material in a mound close enough to the exc a - vation for easy backfilling but without re s t ricting truck access d u ring basement construction. Many exc a vators want to spread the dirt around the hole to fill low a reas and to speed up backfilling. We ask them not to do this so our t rucks and the concrete trucks can dri ve on undisturbed soil. Basement exc a va t i o n Do slab-on-grade portions and c rawl spaces first, then basement e xc a va t i o n. Re m ove earth to at least 24 inches (36 inches is pre f e r red) beyo n d the exterior face of each outside foundation wall. For side banks higher than 5 feet,

3 slope the bank above 5 feet at a 45 angle to minimize the chance of a cave - i n. Access for trucks and equipment Build a solid roadway to and a round the basement exc a vation b e f o re digging the hole. Solid ramps at the corners may be needed to provide enough height to pour directly into wall f o rms from the concrete trucks ( Fi g u re 3). L e a ve room around the exc a va t e d basement for concrete trucks to be positioned perpendicular (not p a rallel) to the footings or walls. After the exc a vator is done, check the completed hole. He re s where a good eye is essential. Is the hole m o re than 6 feet deep? Find out w h y. An exc a vation that s too deep can be a major problem. A house built too low on the site will have poor drainage and possibly a we t b a s e m e n t. Is the finished work level? If it s not yo u ll need extra labor or extra c o n c re t e. Keeping the top of the footing level may re q u i re an 8-inch thickness of concrete on one end and a 12-inch thickness on the o t h e r. If you find the hole is dug too deep or the subgrade isn t level, tell the builder about any extra charg e s for labor or concrete to correct the p roblem. In a hole that s dug too d e e p, the basement floor contra c t o r also will need more gra vel to place beneath the floor. Also check for poor soil when yo u look at the finished hole. Get a feel for what good soils in your area look and feel like so you can spot potential bad ones. Check for unstable soils (sands, silts, water-logged mat e ri a l s, or compacted fills) on the side banks. You may need to cut back the banks at even a milder slope than 45 for safety re a s o n s. Make sure the hole is a safe workplace before sending your footing c rew into it. Figure 5. To set the first building line for laying out footings, string a line from one offset hub to the other after the hole is dug. Note in this photo that the excavator has done a good job of sloping the side bank to prevent cave-ins. Planning the footings Review the plan with your footing c rew foreman. Be sure he understands the pri n t s, house ori e n t a t i o n, and any special conditions such as walkout areas where frost footings a re needed. Find out if a pump or c rane is needed on the pour. If the site is muddy you may need a loader to assist concrete trucks that get stuck. De t e rmine who s re s p o n s i b l e for supplying the loader. Be alert for any special conditions that affect the footings: Bad soil conditions might cause e xc e s s i ve foundation settlement or cracked footings. Be d rock near the surface might cause differential settlement if footings are partly on soil and p a rtly on ro c k. Sp rings or a high water table may mean a potential water problem in the basement and low soil s t rength in the wet are a. If any of these problems are app a rent, get advice from a specialist. You may need further soil testing and a new design that might re q u i re wider footings, re i n f o rced footings, or grade beams. Most codes don t re q u i re re i n- f o rced footings, but wheneve r Figure 6. Building corners are marked with a 30-penny nail and spray paint. Then stakes and a stringline at the corners mark the back edge of the outside form. The stringline is 6 inches off the corner. With inchthick side form lumber this leaves a inch projection of the footing beyond the wall. t h e re s any doubt about soil stre n g t h I encourage the builder to have us add two #4 bars in the footing about 3 inches above the base. You can d ri ve short sections of #4 bars into the ground and tie the bars to them or insert the bars in the fresh conc rete right after it s placed (Fi g u re 4). Even if the footing cra c k s, the bars will hold it together and pre vent une ven settlement. Forming the footings Lay out the footings by first setting one of the building lines using the surve yo r s hubs. St ring a line f rom one hub to the other (Fi g u re 5), then measure the offset distance and plumb down for the first corn e r of the foundation. Set the other corner on the line from that point. My c rews then use the two-tape method (see box) to locate the other corn e r s. If we can, we like to get the corners laid out in late afternoon the day before we pour the footings. Then if the hole hasn t been dug right we find out before the whole

4 footing crew shows up. With the corners laid out early we re also less likely to run out of time to pour concre t e after the footings are dug. It s fru s- t rating for a crew to get a heavy ra i n on open footing exc a vations and h a ve to do some of the work ove r. After the corners are marked, set the outside footing form s. There are many ways to do this too. My cre w s set stakes 6 inches outside the corners and string a line betwe e n stakes (Fi g u re 6). Then they set the back edge of the outside form to that line. For a 2x4 form, that leave s a inch projection of the footing b e yond the wall. Codes in our are a re q u i re footings to project 4 inches on each side of the wall. The extra 1 2 inch is a safety factor. I d rather use a little extra concrete than have a h a rd-nosed inspector reject my Figure 7. Stakes for inside forms are set so the outside edges of the side forms are 20 inches apart for 17-inchwide footings. The worker uses a carpenter s level to set grade for the inside stakes. footing because it s too narrow. The crew lays the side form lumber along the stringline so they l l k n ow where stakes are needed, then d ri ves the stakes to gra d e. They use a builder s level or laser level to set the stakes. It takes more time to drive stakes to grade but my crews do it because it s easier to strike off the c o n c rete when the footings are p o u red. Some crews just mark the f o rm elevation on the sides of the stakes and don t bother to dri ve them to gra d e, especially in ro c k y soil. Either way work s. It s not terribly important to precisely hit the plan elevation for the f o o t i n g s. You can usually be off by an inch or two without causing p ro b l e m s. I tell my crews to shade it to the high side. I d always ra t h e r h a ve the footing slightly too high than too low. After setting the outside form s, stakes for the inside forms are driven so outside edges of the side f o rms are 20 inches apart. This give s a 17-inch-wide footing for an 8- inch-thick wall. The worker uses a c a r p e n t e r s level to set grade for the inside stakes (Fi g u re 7). Earth-formed versus lumber-formed footings Our soils are often stable enough to dig a straight and clean eart h t rench for the footing. Some contra c- tors use a backhoe for this and don t use form lumber at all. They dri ve nails in the bank to mark gra d e, or d ri ve stakes to grade in the footing a rea as a guide for striking off the c o n c re t e. It s much harder for cre w s to build a level footing with this method so we don t use it. A leve l footing is the key to fast wall form i n g. In some are a s, the soils are n t coh e s i ve enough for eart h - f o rm e d footings so the contractor uses 2x8 lumber or plywood to form the full depth of the footing. We use a combination of lumber forms and eart h f o rming where the soil will hold an e d g e. The crews set 2x4 side form s, then dig the remaining depth of the footing. This ensures that the bottom of the footing rests on undisturbed bearing soil and keys the footing into the natural soil. For walkout basements in our a rea, the bottom of the footing for the exposed wall has to be at least 6 inches below frost depth. We could keep footing thickness constant and pour a stepped footing for the exposed wall, but that req u i res longer form panels for that wall. We prefer to just make the footing deeper (Fi g u re 8). Putting in pier pads Pier pads sometimes don t get as much attention as they should. Figure 8. For walkout basements, the bottom of the footing for the exposed wall has to be below frost depth. Making the footing deeper instead of stepping it eliminates the need for longer wall form panels on that wall. They carry heavy loads and need to be sized pro p e r l y. Codes in our are a re q u i re minimum pier pad dimensions of 24x24x12 inches but we d o n t build pier pads smaller than 30 inches on a side. Re q u i red elevation for the top of the pier pad depends on the height Figure 9. Workers spade the footing concrete with shovels to compact it. Codes generally don t require more than 2500-psi concrete for the footings.

5 Figure 10. The footing surface doesn t have to be floated or troweled. A keyway is formed by dragging a 2x4 along the surface leaving a groove about inches deep. of the pipe column that the contra c- tor uses. Make sure to ve rify the column height with the contractor bef o re setting the pad eleva t i o n. Inspection before the footing pour The footing exc a vation and forming must be inspected before conc rete is placed. We re re q u i red to g i ve the building inspector a 24- hour notice. The county where we w o rk permits us to hire pri vate inspectors because the county has limited personnel. The good inspectors check bearing strength of the soil and also look for proper sizing and depth of footings and pads as s h own on approved plans and speci f i c a t i o n s. Placing the concrete Codes generally don t re q u i re m o re than 2500-psi concrete for the f o o t i n g s. A 5-bag mix works fine. We use a fairly stiff 3- to 5-inch-slump mix (Fi g u re 9) and spade it thoroughly with shove l s. Crews strike off the top of the footing with a short 2x4 using the side forms to ensure that the surf a c e is level. We make no effort to slick the footing surface with a float or t rowel. I like a level but ro u g h - a p- p e a ring surface because I think it g i ves a better bond to the wall. Howe ve r, a rough surface makes form setting a little harder so some cont ractors prefer to float the footing. We form a keyway by dragging a 2x4 along the surface and leaving a g ro ove about inches deep (Fi g u re 10). Then we re s c reed to re m ove m a t e rial pushed up along the g ro ove. Other contractors embed a b e veled 2x4 in the footing to form the keyway, and some use re b a r d owels to give the needed shear res i s t a n c e. The pro t ruding dowe l s w o r ry me because they re a safety h a z a rd. I don t want workers or visitors to the jobsite impaling thems e l ves if they fall. Some contra c t o r s loop the dowels to form a hairpin s h a p e, but these are still a tri p p i n g h a z a rd. The day after footings are placed, a different crew sets the wall form s. We ll discuss forming methods in the next art i c l e. P U B L I C AT I O N# C Copyright 1989, The Aberdeen Gro u p All rights re s e r v e d