Browse Forums General Discussion 1 Apr 10, 2014 9:25 pm I have been reading the topics on slab heave and want to make sure our build gets off to the right start. Our engineers report says that pre-wetting the block is mandatory for our block. The house was demolished in February and there has been very little rain since. However the builder says that pre-wetting is not required. Our soil is very reactive and is a Class E-D, P (TREE) site. The trees have been removed but there will still be roots underground. The engineers report stated that the area should be watered with a sprinkler for 2 hours each day for 10 days before earthworks start. Footing type is reinforced concrete raft footings continuously trenched or piered to 200 mm into natural ground. Here is a pic with more technical details Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅ Have others been in this situation? What is the impact if the soil is not pre-wet before they start earthworks? Can you make the soil too wet? Re: Pre-wetting the block before earthworks? Yes or no? 2Apr 10, 2014 10:43 pm Not a good idea.I assume the reason for watering the soil is to re hydrate from the drying effect of the trees.The site should be very dry from the tree drying out the soil and as moisture returns over a long time the clay will swell and cause heave to your slab.Water for 2 hours for 10 days is hardly worth it the swelling effect will take a long time to filter through beyond your build time. Piers should be recommended and they should be deep 1.5 times ys which is the reactive zone of the soil ,they use this in the classification calculations. It all means deep piers and possibly a void former under the slab to allow for heave. Trying to wet the site up is a bit hit or miss, demolition sites usually have large moisture variation across the site, better to a properly engineer design footing system. There is some information on trees impact in the appendix of the australian standards as2870 Re: Pre-wetting the block before earthworks? Yes or no? 3Apr 11, 2014 1:58 am The report states that wetting the soil is mandatory but where does this originate from? If it is mandatory, then you have no option. The report doesn't state what type of sprinkler or the required hourly flow rate. Where would the water come from? If they are talking about a garden sprinkler, you are looking at about 1,000 lph. 2 hours a day for 10 days = 20,000 litres (20 cubic metres) less evaporation. You might get 16,000 litres soak down. Your region (and block size) has not been stated but reading some other posts, it seems that you are in Adelaide. Adelaide has had about 20 mm of rain the last 3 days. Do the referenced notes factor recent rainfall? How many litres does 20 mm of rain equate to on your block? I would have thought that such a recommendation would only be based on a very recent geotechnical site examination. I note that the report references other notes but those notes have not been linked. Maybe they reference a geotechnical site report. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Pre-wetting the block before earthworks? Yes or no? 4Apr 11, 2014 7:01 am Thanks for the replies.
To answer a few of the questions
Here is what the engineers report says on pre-wetting of soils Quote: PRE-WETTING OF SOILS 4.1 On Class H or E sites pre-wetting of soil under slabs is most advantageous, especially if construction occurs in summer or autumn. In some cases pre-wetting of the site will be mandatory, but in all cases (except as noted in Clause 3.1) it is a desirable procedure aimed at reducing the future heave of reactive clays. Similarly, pre-wetting of the site is mandatory where a new structure is proposed to be constructed over a site which had previously been: 1. Occupied by a building with suspended ground floors (e.g. timber floors, concrete slabs on brick build up, etc). 2. Sites which have had significant amounts of established vegetation or even a single large tree. 3. Part of open cropping land. 4.2 Pre-wetting can best be achieved by watering the site as follows: 1. Prior to the placement of underfloor fill mark out the area of the proposed residence (+1 m around the perimeter). 2. Apply water to the area by using garden sprinklers for a minimum of 2 hours daily for 10 to 14 days immediately prior to the commencement of construction. Alternatively, a garden drip irrigation system may be used with drippers evenly spaced along the hose. The flow rate required depends on the site coverage of the building (+1 m around the perimeter) and equates to 1 L/m2 per hr for 8 to 10 days with the system running 24 hours a day. For example, if an average building and perimeter paving area of 200 m2 is required to be covered, set the irrigation system at a flow rate of 200 L/hr. This rate should ensure minimum run-off. 3. Provide underfloor fill within 3 days of the completion of pre-wetting. Notes: 1. Regular checking of the wetting of the site is required to ensure an even water distribution. 2. Care must be taken to ensure that the pre-wetting does not impact detrimentally on adjacent sites or structures. 4.3 As a general rule the pre-wetting exercise will be deemed to be effective if moisture has penetrated at least to a depth of 1 m below the surface. This can be checked by the provision of exploratory holes. Re: Pre-wetting the block before earthworks? Yes or no? 5Apr 11, 2014 7:05 am insider Not a good idea.I assume the reason for watering the soil is to re hydrate from the drying effect of the trees. I think it could be because there was previously a house on this site and to try and re-hydrate that area. Re: Pre-wetting the block before earthworks? Yes or no? 6Apr 11, 2014 7:08 am SaveH2O Where would the water come from? Either the sky or a sprinkler system Not sure if the text in my post above answers this question. Quote: Your region (and block size) has not been stated but reading some other posts, it seems that you are in Adelaide. Adelaide has had about 20 mm of rain the last 3 days. Do the referenced notes factor recent rainfall? How many litres does 20 mm of rain equate to on your block? Block size is 866sqm. However the pre-wetting is only recommended to cover the building envelope + 1 meter Quote: I would have thought that such a recommendation would only be based on a very recent geotechnical site examination. I note that the report references other notes but those notes have not been linked. Maybe they reference a geotechnical site report. The report was from Spring last year. Re: Pre-wetting the block before earthworks? Yes or no? 7Apr 11, 2014 11:08 am StGeorgeBuild SaveH2O Where would the water come from? Either the sky or a sprinkler system Not sure if the text in my post above answers this question. I was thinking that it would be either supplied from a neighbouring property or trucked in...overlooking the obvious that water is already connected. It was a long day. You could be on Bay of Biscay and the worst time to build is at the end of a drought. The sub surface soil moisture content under the house would more than replicate drought conditions. During the drought, Adelaide suburbs had the highest % and worst instances of foundation subsidence and house cracking of all the capital cities. I have files from 6-7 years ago with the suburb by suburb statistics and the % of affected houses in different Adelaide suburbs. No doubt your recommendations were influenced by case studies from that period. The soil's matric conductivity will slowly disperse the water. If you do a search for "matric suction" on this forum, you will bring up a couple of threads where I posted about matric suction and the influence of trees on sub soil moisture content. From memory, I also described the method that geotechnical engineers use to rehydrate the sub soil moisture content and their use of Neutron Moisture Metres to facilitate distribution where most needed. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Pre-wetting the block before earthworks? Yes or no? 8Apr 11, 2014 12:04 pm Yeah we already have mains water connected I believe it is Bay of Biscay soil. So I'm still not sure if I should be querying the builder if pre-wetting is really needed? What would the impact be if I don't pre-wet the block? Re: Pre-wetting the block before earthworks? Yes or no? 9Apr 11, 2014 1:29 pm StGeorgeBuild What would the impact be if I don't pre-wet the block? If it's a mandatory requirement and the decision comes back to you, then I suppose that it could possibly give the builder a get out clause if you did have problems later on and you had ignored the engineer's report. I assume that the builder has not put his overriding recommendation in writing. Others would be better able to answer this question and the subject of the builder's liability if he also ignored the report. Also, how would you prove that the mandated requirement was followed apart from pre and post NMM tests? The report however does state: "4.3 As a general rule the pre-wetting exercise will be deemed to be effective if moisture has penetrated at least to a depth of 1 m below the surface. This can be checked by the provision of exploratory holes." You should be able to hire a NMM but who's responsibility is it to do the tests and would you need the services of a geotechnical engineer for verification? A lot was learned about building design as a result of the drought. Faced with choosing between an engineers recommendation and the stated mandatory requirement Vs a builder's opinion, I would choose the former. Just ask the engineer for case study references, they are certain to be the result of professional geotechnical studies and recommendations. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Pre-wetting the block before earthworks? Yes or no? 10Apr 12, 2014 12:22 am Yes you have to comply with what the engineer says otherwise they have a loop hole. But I still question the wisdom of watering for 10 days and then expecting the clay to magically rebound instantly and then stop conveniently for the build. Anyone who knows these types of clays will realize the clay is going to rebound for a long time and unpredictably. The engineer should take this into account and design footings to suit the situation and not try to change the situation to suit footings. Re: Pre-wetting the block before earthworks? Yes or no? 11Jun 05, 2014 8:53 pm Hi I am in similar situation with StGeorgeBuild - p class e-d and required to prewet with engineers inspections for the slab process. Slab will grillage raft and 900deep and 250,wide- 120 thick will sl92,mesh. In regards to the prewet and also another topic about slab heave. House will be hopefully demolished by end of the month. I was thinking to not commence building until winter is over as it 'may' wet the soil through natural means along with the 2 hour prewet for 2 weeks (unlessI am wrong) though I had been advised if we can leave the block (after demo) to go through a full season that will be ideal but will be costly with mortgage. Cheers boosta Re: Pre-wetting the block before earthworks? Yes or no? 12Jun 05, 2014 10:00 pm I would agree with leaving it for 12 months a full cycle of the seasons.If it was my block I would do a soil test after demo and ask for moisture samples to be taken then 12 months later second soil test with moisture samples and you will be able to see if it has rehydrated.Some people may think that is over kill be the soil is really bad with a abnormal moisture condition ,moisture samples are pretty cheap. We've had the offer of a short term tenant whilst waiting for CDC/DA home approval and demolition for our knock down rebuild. It would achieve a pretty low rent as it's… 0 13758 The fastest thing a builder will do is bank your cheque, those systems work perfectly with lightning speed, everything else is slow burn. Just the way it is. 1 8707 Just to makea point about this, an approach that some people have found sucessful in negotiating these rises down, Is to provide some workings to the builder, specifying… 4 81723 |