Dear All,
When designing structures using computer programs such as CSI SAP2000 or ETABS or any other equivalent software, generally, we input the subgrade modulus in the form of kN/m3. This value can be obtained from the famous Bowles (1996) equation of 40*ultimate bearing capacity. This value corresponds to an allowable settlement of 2.5 cm. However, nowadays, even though soil structure cannot be directly integrated, e.g. using PLAXIS+ETABS or PLAXIS+SAP2000, is it possible to;
- obtain all the loads from the structure and apply it on a constitutive soil model in plaxis, then convert the loads to equivalent pressure (All loads/Foundation area) and divide this value by the surface settlement, then go back to either ETABS or SAP2000 and apply the obtained subgrade modulus, to observe the difference in settlement values.
The main disadvantage might be the increase of settlements with depth, this leads to, as per the assumption above, several subgrade modulus. I also think softwares like PLAXIS 2D/3D can be really helpful in these situations rather than depending on the assumption of bowles.
Note:
- The assumption here is, during the absence of adequate field data.
- This idea might seem a little bit, amateur, please bare with me, as I am trying to learn in this long journey.
For the modulus of subgrade reaction, use Bowles (1996) equation (40*qult) for clay soil.
For sand, it all depends on the relative density of the sand.
Values presented in Bowles (1996) for sand are over-estimated.
I think it is better to divide them by 2.
Bowles' equation is one of the most misunderstood equation as also mistakenly recommended above by Mr. Salem for clays. The two pages in Bowles explains itself. He wanted to give a simple equation based on the settlement based bearing capacity equations for sands. For example, Burland and Burbridge. Those equations calculate the load that corresponds to 25 mm settlement in sand.
Simple rules to follow: (if you can convince your client)
- Forget the 40 x qult equation. Please do.
- Subgrade reaction is load (as pressure) / settlement. So, calculate the settlement using any method you like. If you want to give only one subgrade reaction, divide average pressure to average settlement.
- If someone says the calculated subgrade reaction is very low, ask them "compared to what?" The tables presented in Bowles or any other resources based on the soil type are FOR ONLY PLATE LOADING. In that case, B=0.3.
- Subgrade reaction is E/(B*(1-v*v)) in its simpest form. So, it depends on the B. If Bowles' table says 100.000 for dense sand, it is only for plate loading. For a raft with B=20m, your subgrade modulus will be 5000!
- Don't use 40 x qult.
Mr. Berk Demir , Thank you very much for your reply. Now, for example, in the structural analysis, when the clients´consultants are given a value like 5000, the required reinforcement is typically very high. For a normal typical building, the reinforcement values are high past the typical values, how does one justify this? In its simplest form, with PMT tests available, can one use the Menard pressuremeter modulus, even though this will also result in small values?
If it were easy to perform soil structural analysis using plaxis 3D by defining the foundation, foundation loads and consequently give the resultant forces to the structural group, this definitely would have been a better option.
1.Now, the question rises, for a very small kv value like 5000, this value does not represent the whole soil body (considering the soils within the bulb pressure), how can one overcome this?
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