6m Deep Basement
6m Deep Basement
(OP)
I have a 6m deep single level basement to design for a three story concrete framed building. basement footprint is 11,000 m2. The ground is sand, water table is at approx 4m.
The wall will be built on the plot boundary so no external excavation or ground tie backs will be possible. For the retention system my initial thought is a secant pile wall. A steel sheet piled wall is also an option, but waterproofing this is a concern. I assume this would have to be done by welding the joints?
For a secant wall, i assume there are options for the water proofing - admixtures in either the piles or the cast in place internal wall. could tanking be applied to the face of the piles prior to pouring the wall as another option?
Also have a concern regarding constructing foundations for the temporary props in the excavation with ground water present.
Does anyone have any experience with this and can offer some thoughts or advice? would be much appreciated.
Thanks in advance
The wall will be built on the plot boundary so no external excavation or ground tie backs will be possible. For the retention system my initial thought is a secant pile wall. A steel sheet piled wall is also an option, but waterproofing this is a concern. I assume this would have to be done by welding the joints?
For a secant wall, i assume there are options for the water proofing - admixtures in either the piles or the cast in place internal wall. could tanking be applied to the face of the piles prior to pouring the wall as another option?
Also have a concern regarding constructing foundations for the temporary props in the excavation with ground water present.
Does anyone have any experience with this and can offer some thoughts or advice? would be much appreciated.
Thanks in advance






RE: 6m Deep Basement
For your case, in my area a steel sheet pile would be uneconomical due to the high retained height, and a diaphragm wall would probably be an overkill for that low water pressure of only 2m (this is also much more expensive in my area) so I'd also go for a secant pile wall (80 or 100cm diameter piles), as you suggested. Additionally, if you have pile foundations for the building, the pile equipment is already there. For lateral support, depending of course on plot boundaries and on the position of building foundations, I suggest the use of either the inverted method (executing floor slab bands first and excavate underneath) or propping the walls to the foundations executed in the central area of the plot. If toe passive pressure for the wall is a concern, a thick jet-grouting watertight slab may be executed to form a strut on the wall toe, keeping the internal forces on the wall low, providing a more or less watertight base to work on and reducing water flow to the excavation.
I was once involved in a preliminary design for a 10m excavation for a large plot basement for a 25 storeys tower where the water table was near the surface and there was also a 6m difference between the ground and the -2 floor. The geotechnical conditions were pretty bad at the top, with landfill, alluvial soils and thick mud deposits overlying cdg and bedrock layers. At the time, we proposed thick diaphragm walls penetrating all the way down to the bedrock to resist the water and soil pressures and to provide adequate foundation for the bottom levels' slabs. There was also present, of course, a thick watertight bottom slab.
As we couldn't use tiebacks either since the plot was a reclamation area in a lake, the lateral support for the excavation was to be provided with the concrete slabs that would constitute the ground and -2 levels (we call it the inverted method). After the installation of the diaphragm wall and tower columns pile foundations, part of the ground floor slab was concreted leaving large areas near the centre of the plot unconcreted to allow for excavation equipment to dig under the completed slab. The slab bands adjacent to the wall were supported vertically by the wall and by temporary steel columns inserted into the pile foundations for the tower. These were later encapsulated with concrete to form the tower columns.
I enclose a picture of the construction sequence we proposed at the time (sorry for the captions in Portuguese).
RE: 6m Deep Basement
RE: 6m Deep Basement
RE: 6m Deep Basement
Mike Lambert
RE: 6m Deep Basement
RE: 6m Deep Basement
LonnieP is right.
After wall construction and excavation has begun, can the 11,000 m2 area inside the wall be continuously dewatered to an elevation below the bottom of the excavation? Without dewatering, in sand you could get groundwater flow beneath the wall then up into the excavation.
www.SlideRuleEra.net
www.VacuumTubeEra.net
RE: 6m Deep Basement
RE: 6m Deep Basement
Also look at the available space you have to surrounding structures and the required clearance you need for each.
Both options would need to be propped during construction though I would expect the secant would need one level and the sheet piles could need 2 levels of props until the basement is installed.
In the uk sheet piles are almost always cheaper to install but they are not as good at taking vertical loads as secant pile walls and may need intumescent paint if the they are supporting structure. Welding the clutches after the basement slab is poured is the usual method.
If you are in the uk then you should look at the basement waterproofing code as the starting point as this will often dictate your options for the particular basement use.
There is alsways the option of pouring a waterproof concrete liner wall to the inside of the sheet piles as a structural waterproofing layer.
Steel intensive basements is a good guide for sheet piled basement walls in the uk.
Comments above regarding groundwater are very good ones, if you have clay not too far down it may be worth seeing if you can sheet down to it and provide a 'cut off' wall keeping most of the water out.
RE: 6m Deep Basement
RE: 6m Deep Basement
RE: 6m Deep Basement
I do not think that a soldier pile wall would be feasible when excavating below the water table in sand.