When designing pile foundation structure on the ground with stone quite early (less than 40m) as in the coastal and island areas: Quang Ninh, Nha Trang, Da Nang, … There is a bearing pile structure. The development of the coastal resort real estate segment, according to It is the common use of bearing pile structure. Below shares one Some practical experiences about:
💎Calculation of structural design
💎Note and handle situations in construction
💎 (Karst) cave and effects to bearing piles
and do not forget the gift🎁 attached : A spreadsheet for engineering design.
Necessity
Topic related to the foundation, clearly affecting the pocket money invest💶 Already.
Pile not only against the stone surface as the name, but often drilling In the rock, a piece is called the mount. Due to the huge cost of drilling into the rock, it is easy to see Design consulting needs to forecast Exactly the depth of the pile. Drill enough deep into the rock to ensure load.
Moreover, limestone foundation is common in the North, almost sure Meet Karst cave. Therefore the depth forecasting and processing of pile construction, ensuring safety, avoiding waste of money drifting into the cave, even more necessary.
Principles
The bearing pile is defined as the type of pile transmission to the background Mainly through the tip of the pile. The tip of the pile leaned on the foundation is very hard compared to the soil layers on the body Pile. Therefore all types of piles leaning on the stone background are bearing piles. In case of closing piles In the less compressed soil layer, it is also a pile. When calculating the design can be ignored without considering the side resistance (friction) of the soil. Except for the case of friction, capital Reduce compressive load capacity of piles. Distinguished from conventional piles (The standard is called a hanging pile), is a pile of transmission to the ground through the body and tip
🔍”less compressed soil layer ”: is The ground is in the form of rough and sandy debris in a medium and tight state, the soil is sticky Hard state, water saturation, deformed module $E_o\le$ 50MPa
Vietnamese design code on pile structure TCVN 10304: 2014, clear instructions for calculation formula The load capacity of the bearing pile:
Ultimate load capacity $R_{c,u}=\gamma_cq_bA_b$, where as:
$\gamma_c=1$
$A_b$ is the area of cross -sectional pile nose
$q_b$ is the resistance intensity of the ground under the bearing pile tip,with:
– Closed piles, pressed piles: $q_b = $20MPa
– Deposit: depending on the depth of the mount in $l_d$, the condition is that the stone is not weathering, does not interfere Weak glass, karst cave …
+ $l_d< 0,5$m: $q_b=R_m$
+$l_d\geqslant0,5$m: $q_b=R_m(1+0,4\frac{l_d}{d_f})\leqslant3R_m$
$R_m=\frac{R_{c,m,n}}{1,4}$
$R_{c,m,n}=R_{c,n}K_s$
$K_s$ is the intensity reduction coefficient, depending on the quality index RQD, according to Table 1 of the standard
$R_ {c, n}$ is the standard value limit of an axle compressive resistance of the rock in a state saturation state, determined according to the results of the whole stone sample Laboratory block.
❓What is RQD?
Is the stone quality index, abbreviated for Rock Quality Designation. RQD means assessing the level of cracking, and is a quality indicator of stone. The bigger the RQD as possible (Table 1 of the standard). RQD was surveyed by the survey Geology determined at the request of design consultancy. How to determine:
RQD (%) = The total length of the stone samples taken in the dimension longer than 10cm / total drill length
If the stone is ordained or softened, the nose is located in, $r_ {c, m, n}$ must be taken according to the test results of the compressed table or according to the results of testing the pile with static load.
Calculated load capacity is determined by formula (7.1.11) Familiarity of the standard:
$$n_{c, d} = \frac{\gamma_o}{\gamma_n}{r_ {c, k}}{\gamma_k}$$
– All cases are limited to $q_b$ not more than 20MPa. To mobilize the largest pile load according to the ground, the designer calculates so that $q_b =$ 20MPa.Then select the material (concrete, reinforcement) to achieve the corresponding load capacity. Bear The load of large piles allows reducing the number of piles.
– The depth of the rock is more or less affecting the load capacity of the pile, as the presence of $l_d$ in the formula $q_b$ as above.
– The structural engineer should balance depth, pile materials, bearing strength of the pile load, and followed by the number of piles to get the optimal problem on the background cost foundation with construction loads.
Unlike conventional friction piles:
– The coefficient of $\gamma_k$ is usually equal to 1.4 for bearing piles (1.6 with single bored piles). According to the case a) instead of b) with regular friction piles (7.1.11 of the code)
– The bored pile places the same reinforcement from top to bottom, no Reduce like a friction pile, because ignoring the friction of the pile body, the compression force is transmitted from the top.
– Conditions for sedimentation at the bottom of the pile when constructing bored piles, bearing piles More severe: The thickness of the sediment is not more than 5cm (pile hanging is 10cm). According to 12.2.1, TCVN 9393: 2012 “Bored piles – construction and acceptance code”
🔍Karst is a process that occurs when water On the face and water in the ground, contact with stone is likely to be dissolved as limestone For a long time. Soluble water (chemical erosion), erosion (mechanical erosion) Rolled rocks easily dissolved and formed underground caves, called Karst cave. The cave may be empty (like Phong Nha Ke Bang) or a small cave containing mud lenses Due to the processing process.
Harm in construction:
High risks encountering problems when drilling and drilling – Losing solution boring (bentonite) and lost concrete into Karst cave. Maybe this loss will be very large. Even causing the bottom of the hole, losing the drilling bit .
Harm to the budget💶is clearly visible, because most of cases It must be drilled deep in the rock, crossing the karst caves to lean the stake in the stone background Good. Therefore, the investor of the project on limestone areas should be prepared mentally The cost of building the foundation exceeded the estimate.
In general, in practice, it is unpredictable. The father of Karst cave, needs to pay attention when drilling piles, if you meet the cave, you need to handle it immediately. Ask the person who stands and the technical direction must be very experienced. Under possible conditions, then drill before: carry out a geotechnical investigation to determine the scale of Karst cave in locations near the borehole area with Karst.
Construction design engineer should comply with Article 13 TCVN 10304: 2014. Carefully collect enough information, to make sure the stake has to pass the karst caves To lean on the stone layer meeting the requirements of bearing force (not weathering or leprosy). Try Imagine under the tip of the pile still there How unpredictable harm is.
The structural engineers also noted when making a drilling task Geological killing, to assess the physical and mechanical properties of stone layers. Especially if there is a lime stone, need to know the distribution of Karst cave. From there, there are enough decisive basis The depth of the pile used for works in the construction design process. Note the following parameters also affect pocket money💶 For the survey:
🔹Number of Geo investigation plots: According to Appendix D TCVN 9363: 2012, there are no less than 3 explorations for A separate house and not less than 3-5 points for a cluster of houses. In the case of thick Especially need to delineate the distribution of Karst cave, the layout may be smaller than 20m.
🔹Geo investigation depth: Drilling into the stone capable of bearing the building load a minimum of 5m, according to The regulations of TCVN 9363: 2012, or ensures the settlement. The holes in the stone if encountered The crushed strip due to fault or karst cave should drill into the original stone layer Do not wind at least 3m. Stop drilling in greater depth in the two numbers above.Refer to Article 13.3 of TCVN 10304: 2014.
🔍“Stone is capable of loading The project ”: is a stone at a depth of little or very little crack The minimum RQD of the rock reaches RQDMin. RQD Min depends on the prediction calculation Bear pile load according to the above formulas of the designer to obtain $q_b$ as needed, through $k_s$. (For example, at least $k_s = 0.6$, RQDmin = 75%)
In addition to the survey holes to sample rocks, it is necessary to supplement The drill holes to explore depth. The purpose is to supplement the geological data of the stone layer and the distribution of Karst cave, only starting to take samples when touching stone. Even if the Karst cave is complicated, it must be made in the just position of each pile before construction.
The project in Ha Long City – Quang Ninh, structural design bearing piles with D1000mm diameter, pitcher into a good stone at least 1.0m. Design single pile load capacity is 700 tons. Ground reaches limestone from 22m depth; there are many complex Karst caves. The first few piles were concrete piles when poured. Hence design advice The decision to drill the stone probe at the position of each pile before drilling.
Some of the karst cave probe positions are up to 12m and then come Stone background sufficient load. It is not clear if there are many pine caves. The structural engineer takes measures to use steel wall tube down when drilling, to prevent bentonite and concrete drifting out of the pile hole. The deepest bored pile comes 43m through all karst caves.
