Browse Forums Owner Builder Forum 1 Oct 27, 2019 8:21 pm Hi, I am in the middle to excavation and wanting to do the formwork. My approved pals show slab on ground. So 200mm slab on ground with footings. I started with the pool and noticed that hard rock was not found until below below the planned FFL levels. Therefore the structural engineer advised to Cylindrical pier the slab to rock. I am now concerned that the remainder of the house may have to be the same? Initial geotechnical report didn't have enough tests due to existing dwelling on site.. So questions: 1) How best to determine natural rock (I'm thinking drill a hole) - Any other ideas..? 2) Any ideas cost per 450 x 450 pit with 3 n16 reinforced bars poxyset with galvanised chemset and concrete poor about 1000m deep? Able to provide indication of workers needed? I'm thinking 1 foreman (on the drill / excavator) and 1 labourer with drill plus materials... Not sure how long it should take to drill a hole... Im Sydney based (Upper north shore) Help appreciated Re: Foundation slab on ground question - help 2Oct 31, 2019 7:08 am MrChelseaFC Hi, I am in the middle to excavation and wanting to do the formwork. My approved pals show slab on ground. So 200mm slab on ground with footings. I started with the pool and noticed that hard rock was not found until below below the planned FFL levels. Therefore the structural engineer advised to Cylindrical pier the slab to rock. I am now concerned that the remainder of the house may have to be the same? Initial geotechnical report didn't have enough tests due to existing dwelling on site.. So questions: 1) How best to determine natural rock (I'm thinking drill a hole) - Any other ideas..? 2) Any ideas cost per 450 x 450 pit with 3 n16 reinforced bars poxyset with galvanised chemset and concrete poor about 1000m deep? Able to provide indication of workers needed? I'm thinking 1 foreman (on the drill / excavator) and 1 labourer with drill plus materials... Not sure how long it should take to drill a hole... Im Sydney based (Upper north shore) Help appreciated So I have a price $70 per linear m for pier and concrete fill and it’s now essentially a discovery activity and where I need to pier whilst working with structural engineer.... Re: Foundation slab on ground question - help 3Nov 03, 2019 9:38 am Try this quick calc in your head concrete $200m3/10 = $20 where 1 m3 concrete costs $200/M3 you will get approx. 10 m of 300mm diam. piles add pump You can buy an petrol auger ( good for 2m) online for $300 and then sell it on gumtree for Half or hire one on a dingo BTW $70/m is a bit steep, DIY and save imo Designer,Engineer (Civil,Const & Envir),Builder,Concrete & Masonry Contract.Struct Repairs Re: Foundation slab on ground question - help 4Nov 09, 2019 2:31 pm StructuralBIMGuy Try this quick calc in your head concrete $200m3/10 = $20 where 1 m3 concrete costs $200/M3 you will get approx. 10 m of 300mm diam. piles add pump You can buy an petrol auger ( good for 2m) online for $300 and then sell it on gumtree for Half or hire one on a dingo BTW $70/m is a bit steep, DIY and save imo Are you able to clarify the difference please between: 1) Bulk excavation and Detailed Excavation? 2) In detailed excavation would all footing and peiring / concrete piles be excavated to allow for concrete poor? Thanks Re: Foundation slab on ground question - help 5Nov 10, 2019 8:30 am Generally housing is quite simple, If you detail and plan it properly you will only need one lot of earthworks, occasionally on complicated sites you will have stages for different RLs etc, retaining, step ups/down,etc eg Here Slab and formwork costs Bulk excavations/levelling are done by machinery,get your FFLs levels correct aim for a balance point (cut=fill) Detailed excavations are done by the concretor so are 300mm piles. On some footing engineering details you can pour piles and footings together, I prefer to split them on larger complicated houses as above Also consider other factors ie material handling, accessibility, Control joints, pump Vs Barrows,etc OT, I don't do 3D Models (BIM) for Housing Siteworks tho my software is capable, it's not necessary imo Tho it would be good to illustrate procedures/processes, data and integration of on site tools with methodology. It's ironic people turn to google searches for info, but google will never be able to analyse nor certify any images I put up cheers chris Designer,Engineer (Civil,Const & Envir),Builder,Concrete & Masonry Contract.Struct Repairs Re: Foundation slab on ground question - help 6Jan 19, 2020 1:37 pm MrChelseaFC Hi, I am in the middle to excavation and wanting to do the formwork. My approved pals show slab on ground. So 200mm slab on ground with footings. I started with the pool and noticed that hard rock was not found until below below the planned FFL levels. Therefore the structural engineer advised to Cylindrical pier the slab to rock. I am now concerned that the remainder of the house may have to be the same? Initial geotechnical report didn't have enough tests due to existing dwelling on site.. So questions: 1) How best to determine natural rock (I'm thinking drill a hole) - Any other ideas..? 2) Any ideas cost per 450 x 450 pit with 3 n16 reinforced bars poxyset with galvanised chemset and concrete poor about 1000m deep? Able to provide indication of workers needed? I'm thinking 1 foreman (on the drill / excavator) and 1 labourer with drill plus materials... Not sure how long it should take to drill a hole... Im Sydney based (Upper north shore) Help appreciated Hi, First time I've posted on this site but I'll have a stab at answering your questions: '1) How best to determine natural rock (I'm thinking drill a hole) - Any other ideas..?' You could do, a geotech investigation for a house should include a minimum of 3 boreholes/test locations. The petrol auger or dingo suggested may work fine. Depending on your location you might find stiff to hard residual soils or colluvium over the bedrock and the petrol-driven auger might not have the power drill through hard clay soils. If this occurs you will not have proven bedrock so might not work. The Dingo would likely be the better option-I'm not sure what the torque is like on the standard post hole machines though. I think they maybe work in sands but may be slow in stiff to hard clay so be worth checking hire company how they go. Alternatives to consider: If you are not anticipating more than 1.0-1.5m of soil before bedrock you could hire a jackhammer and long chisel points along with a pair of scissor shovels. Then find the nearest 'working' backpackers hostel and find a couple of robust guys that are after some labouring work. It might sound a bit odd but it often works out well and considerably cheaper. After a few test around the place you might find it makes sense to get them to dig the piers out. If you find the digging is just too tough/slow or rock deeper than anticipated consider: Hiring a bit of equipment called a Dynamic Cone Penetrometer (DCP) and is a pretty common bit of kit used for residential jobs around north Sydney. They are made for pavement testing and are a manual tool which drives a probe into the ground via a drop hammer, basically a fancy post driver. They are quick to complete and generally get through any soils to at least weak rock. Finding a place that hires them could be tricky-if you call a geotech company up and ask if they hire out a DCP they will probably say no. You might be able to bet a labourer/field assistant and a DCP for $100 an hour or you could phone up and ask if you can hire a DCP for a few hours that day, will collect, return, provide insurance against damage and pay cash. If bedrock is shallow (<2m) 10-15 tests are easily achievable in a few hours to enable bedrock profile. You do need to do something to get an idea of the ground otherwise you could find yourself installing piers in a Class S site. Perhaps I'm getting the wrong end of the stick but it seems this is the current state of things: No useful geotechnical information is available but the initial design for some reason assumed bedrock a short distance below the existing ground surface under the house. Excavation elsewhere on-site indicated bedrock was 200mm deeper than anticipated. On this basis, the footing design went from something suitable for a Class S site to one more characteristic of a P site. Is that right? -'I am now concerned that the remainder of the house may have to be the same?' That may be the case, it may not, what your structural engineer should be asking for is for a useful investigation so the footing options and costs can be assessed ahead of time and not during construction. -'Able to provide indication of workers needed?' I'm thinking 1 foreman (on the drill / excavator) and 1 labourer with drill plus materials... ' That's pretty standard. If you knew how many piers and to what depth/diameter was required you could make some sort of firm plan. 'Not sure how long it should take to drill a hole' Clay over rock will be minutes. However: If you encounter sand you might have to support the pier holes by installing a liner. You will need to plan for the weather-rainfall may lead to instability in the holes and seepages across the bedrock surface would need to be controlled if encountered. I'm assuming the site is flat but if the bedrock surface is variable you will have a hard time cleaning the base out to an acceptable level. If the bedrock surface slopes, further measures may be required to resist lateral movement. Cobbles/boulders in pier sidewalls may also slow site works up. Probably none of those will be encountered but they are a few of the more common reasons for delays. 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