Improve Bearing Capacity of Soil

[Muhammad Ameen Ali]

Building two basement+Groumd+Four Floor

How to improve bearing capacity of soil soil from 0.75tsf to 1.8 tsf??

and what technique or method should follow to improve the bearing capatcity for raft footing

[UmarMakhzumi]

You are better off using a different foundation system if your bearing capacity is super low than expected. Have you considered piling? You can use friction piles if loads are low.

Thanks.

[Simple Structures]

There are many factors that need to be considered here, to answer the post properly:

1. Type of soil - cohesive or non-cohesive? Techniques are different for both.

2. There are many techniques for improving soil bearing capacity. What techniques are readily available in the local market?

3. Water table level - impacts bearing capacity is sandy soils;

4. Is ground settlement your concern?

5. Working in metric units:  kN & m;     0.75 tsf (80kN/sqm) to 1.8tsf (190kN/sqm): You must consider the "overburden removed". Two basements means (say 7-8m dig), therefore overburden (soil) removed would be 18kN/cum times say 7m = 126kn/sqm (1.3 ton per sq ft pressure removed). i.e. bearing pressure at the depth would be Pb+1.3tsf ...close to what you want. .... the key being water table or finding a low strength soil like "peat", in such or similar low strength ground cases  ground improvement or piling solution would be needed - don’t undervalue the expertise of a geotechnical engineering specialist - consult them always!

6. Immediately after the dig protect the formation level surface by blinding with 75mm lean mix concrete or say 100mm of small stone gravel sub-base;

7. Raft footings - depending on thickness and stiffness, tend to then spread load over a large area, and would limit settlement etc. Not sure why a raft is needed in this case, as technically you have a 6-storey building - unless ground is very poor?

8. You have six/seven storey building - say superimposing a total load of 75kNsqm - 0.8ton per sq ft -  (load obviously concentrated on column locations)!

Hope this helps?

 

[Muhammad Ameen Ali]

On 12/29/2019 at 8:46 AM, UmarMakhzumi said:

You are better off using a different foundation system if your bearing capacity is super low than expected. Have you considered piling? You can use friction piles if loads are low.

Thanks.

Piling is impossible due to site conditions. 

[Muhammad Ameen Ali]

36 minutes ago, Simple Structures said:

There are many factors that need to be considered here, to answer the post properly:

1. Type of soil - cohesive or non-cohesive? Techniques are different for both.

2. There are many techniques for improving soil bearing capacity. What techniques are readily available in the local market?

3. Water table level - impacts bearing capacity is sandy soils;

4. Is ground settlement your concern?

5. Working in metric units:  kN & m;     0.75 tsf (80kN/sqm) to 1.8tsf (190kN/sqm): You must consider the "overburden removed". Two basements means (say 7-8m dig), therefore overburden (soil) removed would be 18kN/cum times say 7m = 126kn/sqm (1.3 ton per sq ft pressure removed). i.e. bearing pressure at the depth would be Pb+1.3tsf ...close to what you want. .... the key being water table or finding a low strength soil like "peat", in such or similar low strength ground cases  ground improvement or piling solution would be needed - don’t undervalue the expertise of a geotechnical engineering specialist - consult them always!

6. Immediately after the dig protect the formation level surface by blinding with 75mm lean mix concrete or say 100mm of small stone gravel sub-base;

7. Raft footings - depending on thickness and stiffness, tend to then spread load over a large area, and would limit settlement etc. Not sure why a raft is needed in this case, as technically you have a 6-storey building - unless ground is very poor?

8. You have six/seven storey building - say superimposing a total load of 75kNsqm - 0.8ton per sq ft -  (load obviously concentrated on column locations)!

Hope this helps?

 

Thankyou Sir for your suggestion

i answers below your all points according to site condition. 

1. partly cohesive soil.

2. At site Soil compaction possible.

3. water table depth about 500feet. 

4.Yes

5. Already 30feet excavation was done long time ago, there is no secant piles for the retaintion of letarel soil pressure, if i go for piling .

and at that 30feet bearing capacity is 0.75tsf.

7. Yes this building is supported by Raft footing, there is no other option.

8. how you can total load on column is 0.8tsf

Sir i am searching for the solution that how to compact the soil to increase the bearing pressure upto atleast 1.5tsf and what is the methodology of compaction.

 

[Simple Structures]

1. Engineers need to be specific on soils - 'partly cohesive' can mean many things. Is there a site investigation report ? i.e have boreholes been undertaken to identify soil types etc below the excavated surface? One should always have a geotechnical / soils report for the site before undertaking any ground-works, such as compaction. Get a soils site investigation done first! Depth of boreholes ( i would suggest 5, 4 near corners and one in middle) below raft should be at least equal to width of raft;

In this case as the overburden has been removed for sometime, the soil may have heaved (up) and this will settle as building load is applied! See option below with layer of 400mm small stone replacement;

2. After SI compaction options: 1, Dynamic compaction from height (could cause vibration to adjacent structures!); 2. Impact compaction special compaction adaption fixed to excavator; 3. rollers etc; 4.  I would removed 400mm of top soil and put in stone compacted by roller, placed in 100mm layers; This will provide a good hard surface locally.

3. OK, water table is low...good... don't forget it will rain and if soil has too much clay you will get a perched water table around the basement!

4. Very difficult to judge if all future settlement will be taken out by compaction technique; use of raft footings will help limit this or the settlement will be uniform;

5. Basement walls will resist any lateral pressure - secant wall is only need for an excavation where site is tight and excavation slopes are not possible?

6. -

7. Raft ok

8....blank load on raft .... at column 0.8tsf (say 75kN/sqm) ... if columns are at say 6m centres then typical INTERNAL column load would be: 75x7x7 = 3675kN (or 0.8 x 21 x 21 = 350t) - for example!

[kHURRAM ALI]

stone column and vibro compaction are the techniques use in my city (KARACHI), but the technique depends on soil condition