Dynamic compaction method for foundation reinforcement

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Formulator of the dynamic compaction method for foundation reinforcement: China Construction Sixth Engineering Bureau; Approving authority: China State Construction Engineering Corporation; Method code: GF/206023—94; Main author: Mao Hongyuan. 1 Characteristics and scope of application... 2 2 Objectives and requirements for reinforcement... 2 3 Construction preparation... 3 4 Trial compaction... 5 5 Determination of construction parameters... 5 6 Construction process... 7 7 Inspection of the reinforcement effect achieved by dynamic compaction... 8 8 Project examples... 8 1 Characteristics and scope of application The dynamic compaction method for foundation reinforcement has a wide range of applications; it can be used for collapsible loess, gravelly soil, sand, ordinary cohesive soil, soft soil, as well as various types of fill soils derived from industrial or domestic waste. For unsaturated soil foundations, dynamic compaction yields significant reinforcement effects; dumping gravel, (steel slag, slag) into flow-plastic silt and then using dynamic compaction to compress the silt also produces good results. The dynamic compaction method generally uses hammers with a weight of 80 kN (8 tons) or more, which are lifted to a height of over 6 meters before falling freely to compact the soil intensively. It is a foundation reinforcement method that not only enhances the strength of the foundation and reduces its compressibility, but also improves its resistance to seismic liquefaction and eliminates the collapsibility of the soil. The equipment used in this method is simple, quality control is easy, and its application range is wide. The effectiveness of strengthening saturated soil foundations depends crucially on drainage; for example, saturated sandy soils have good permeability, allowing excess pore water pressure to dissipate easily, and thus consolidation occurs rapidly after ramming. For saturated clays or silty soils, careful consideration must be given due to their poor permeability. Strengthening the foundation using dynamic compaction can replace certain pile foundations, saving steel, wood, and cement. It is fast, effective, and requires low investment, making it an economical and simple method for foundation reinforcement. 1.1 Optimal construction conditions for dynamic compaction: (1) The treatment depth generally should not exceed 15 m ； (2) The groundwater level should be 2–3 meters below the ground surface ； (3) The material to be tamped is preferably composed of coarse-grained soil ； (4) The construction site should be at a sufficient safety distance from existing buildings, generally preferably more than 10 m ； (5) For saturated soft soil, a thick layer of fill material such as gravel or sand should be laid on the ground surface. 2 Purposes and requirements for reinforcement: Different foundations and projects have varying requirements for reinforcement: 2.1 The main objectives are to improve the bearing capacity of the foundation and eliminate uneven deformation. 2.2 For earthquake-liquefied foundations, liquefaction should be eliminated after reinforcement. Different liquefied foundations have different requirements. 2.2.l For saturated sandy soil foundations, liquefaction is considered to be absent when the value of the standard penetration blow count N63.5 after reinforcement is greater than the value N’ calculated using the following formula: N’ = N′[1 + 0.125(ds – 3) – 0.05(dw – 2)] (1) Where: N’ is the critical standard penetration blow count for liquefaction of the sandy soil at a depth of ds, with the distance from the outdoor ground surface to the groundwater level being lm ； N’——The critical number of hammer blows for sand liquefaction when ds=3m and dw=2m. When the design seismic intensity is 7, 8, or 9 degrees, the corresponding values are 6, 10, and 16 respectively ； ds — depth at which the saturated sand is located (m) ； dw — distance from the room floor to the groundwater level. 2.2.2 The light sub-clay can be calculated using the following formula: N″＝14.5＋0.5ds—2.4dw—Mc. (2) In the formula: Mc – clay content, expressed as a percentage. Non-liquefaction occurs when the actual SPT blow count N63.5 is greater than the calculated value N″. 2.3 For the reinforcement of collapsible loess foundations, it is necessary to eliminate the collapsibility property; the collapsibility coefficient shall be calculated using the following formula: (3) Where: δs —— collapsibility coefficient ； hp – The stable settlement height (in cm) of soil samples with their natural moisture and structure, when subjected to a certain pressure ； hP′ —— the height (cm) of the soil sample after stabilization under pressure, once it has settled under the influence of water infiltration ； ho——the original height of the soil sample. When the settlement coefficient δs of the foundation after dynamic compaction reinforcement is < 0.015, settlement is eliminated. 2.4 Stabilization of weak soil foundations focuses on improving the strength of the foundation soil and reducing deformation. 3 Construction Preparation 3.l Equipment Preparation 3.1.1 Rammer 3.l.l.l Materials for the rammer The rammer can be made of steel, or it can consist of a steel shell filled with steel rebar ends, steel sand, or concrete. Rammer heads made of cast iron and cast steel are small in size, have a low center of gravity, excellent stability, limited rebound, and high efficiency. Large-tonnage ones can be made as prefabricated cast steel ram hammers to facilitate transportation. 3.1.1.2 Shape of the ram: The ram can be cylindrical, square, pear-shaped, spherical, or other shapes. To reduce or eliminate the adhesion between the soil and the ram during its lifting, thereby avoiding an increase in the crane’s load, the ram should be equipped with 4 to 6 exhaust holes that run from the bottom to the top of the ram. 3.l.l.3 Ram hammer size: The static pressure per unit area of the ram hammer can be selected according to the following principles: (l) The static pressure per unit area of the ram hammer should be between 25 and 40 kPa ； (2) When the ramming energy level is below 2000 kN•m, the impact energy per unit area should be 300–500 kN•m/m2 ； (3) When the ramming energy level is above 3000 kN·m, the ramming energy per unit area should be in the range of 600–800 kN·m/m2 ； (4) For sand and gravelly soil, etc., a hammer with a smaller area should be used ； For soft soils, silt, and silty soils, hammers with a larger surface area should be used ； For light argillaceous clays, loess, etc., hammers of medium size should be used. 3.1.1.4 Weight of the ram The optimal weight of the ram can be determined using the following formula: M = (l + K/2)AγR (4) Where: M – weight of the ram (t) ； K — coefficient of effective reinforcement influence depth ； γ —— Natural unit weight of the foundation soil ； A —— Bottom area of the rammer (m2) ； R — Radius of the bottom of the ram. When using equation (4), the following points should be noted: (1) When the bottom surface of the ram is curved, its base area A is the projected area ； The rammer radius R corresponds to the radius of the projected circle ； (2) The natural unit weight γ of the foundation soil is the weighted average within the designed effective reinforcement depth range of the foundation soil prior to dynamic compaction treatment ； (3) The coefficient K representing the effective reinforcement influence depth is taken as 0.5 to 0.7 here. 3.1.1.5 Placement of exhaust holes on the ram: (l) The total area of the exhaust holes on the ram is approximately 15% of the projected area of the ram ； (2) The exhaust holes should be evenly distributed and run vertically; their number should be 4 to 6 ； (3) The center of the exhaust hole is generally at the midpoint of the radius of the rammer’s projection plane ； (4) If manufacturing permits, it can also be designed with a narrower top and a wider bottom in a trumpet shape ； (5) For concrete hammers with steel casings or ramming hammers used for strengthening soft soil foundations such as saturated clay, the pore diameter can be increased appropriately to facilitate exhaust and hole cleaning. 3.l.2 Lifting equipment 3.1.2.l The lifting equipment should be selected based on the weight of the ram. 3.1.2.2 Lifting equipment may include crawler cranes, tire cranes, and specialized tripod rigs for dynamic compaction; crawler cranes should be given priority, with a lifting capacity of generally not less than 15 tons. 3.1.2.3 When the lifting capacity of the crane is insufficient to meet the ramming requirements (i.e., hammer weight × distance from the hammer to the center of the crane), gantry supports can be added within a certain range ； It can also be constructed using two cranes combined to form a \"double-crane combination crane\". When using tire-mounted ramming, it should be carried out in conjunction with gantry support. 3.2 Site Preparation 3.2.1 The construction site should be leveled and compacted using a bulldozer, so as to withstand the weight of cranes and rammers. 3.2.2 Measurement and setting out: determine the ramming area and the locations of the ramming points. 3.2.3 When there are buildings in the vicinity, vibration isolation ditches should be dug at a certain distance from the dynamic compaction site. 3.2.4 Prepare the necessary equipment such as bulldozers, automatic uncoupling devices, rammer, theodolites, levels, pulleys, steel wires, and rope clips. 3.3 Labor organization (per shift) (l) 1 foreman and technician ； (2) 1 measurement and settlement recorder ； (3) 1 person for on-site operation supervision and tower level holding ； (4) Link 1–2 people ； (5) 1 crane operator. 4 Test Ramming 4.1 Tasks of Test Ramming 4.1.1 Based on the engineering geological survey report and the requirements for reinforcement, select a representative test area at the construction site; the area of this area should be no less than 20×20 m2 ； The test area can also be selected on the ground that needs to be reinforced. 4.1.2 Ramming is carried out using the selected rammer with different numbers of blows (or at different heights). 4.1.3 According to the test requirements, pore water pressure probes shall be installed at various locations and at different depths within the test area; monitoring of pore water pressure before and after ramming shall be carried out to determine the interval time between ramming sessions. 4.1.4 Conduct settlement observations and record them accurately. 4.1.5 Organize the results of tests such as static cone penetration testing to determine the number of ramming passes and the number of blows per pass. 5 Determination of construction parameters 5.1 Effective reinforcement range 5.1.1 Reinforcement range (see Figure 1) 5.1.1.1 According to the modified Menard formula, the depth and width of reinforcement can be calculated using the following equations: (5) b = 2htgθ + l (6) Where: h – effective reinforcement depth (m) ； K — effective depth correction coefficient, with a value ranging from 0.3 to 0.9 ； b——Effective reinforcement width (m) ； θ —— pressure diffusion angle, generally taken as θ=22º～30º ； M — Hammer weight (t) ； H — Drop distance of the rammer (m). 5.1.1.2 Based on the propagation characteristics of waves and the soil’s ability to absorb energy, the reinforcement radius (i.e., the width and depth of reinforcement) rs can be determined using the following formula (see Figure 2). (7) In the formula: rs —— effective reinforcement radius (m) ； M — Hammer weight (t) ； H — Drop distance of the rammer (m) ； Vp — longitudinal wave velocity (m/sec) ； α —— the energy absorption coefficient of soil ； Ki—a coefficient greater than 1, usually ranging from 3 to 5. The energy absorption coefficient α of soil: Table 1 shows the values of α for different soil conditions (in s/m): Soft, saturated silty sand, subclay, light subclay, clay – 0.01–0.03; Very wet subclay, clay – 0.04–0.06; Slightly wet and dry light subclay, subclay – 0.07–0.10; Hard plastic clay and medium-dense boulders, gravel – 0.0875–0.115; Plastic clay and medium-dense coarse sand, gravel – 0.1–0.125. Note: This formula is taken from the article “Vibration Waves and the Mechanism of Dynamic Compaction” by Zuo Mingqi, published in the third issue of the Journal of Geotechnical Engineering in 1986, section 5.2. Intermission time: The interval between successive rounds of dynamic compaction should be determined based on the time it takes for the pore water pressure to dissipate. Strengthen the monitoring of pore water pressure during the interval periods; the time required for the pore water pressure to decline from its peak value to a stable level constitutes a reasonable interval period. The impact of intermittent time can be ignored for sandy soil. 5.3 Layout of ramming points 5.3.1 For foundations with a large area or made of cohesive soil, a square grid layout is recommended. 5.3.2 For strip foundations, the “dot-line interlocking” method can be used for layout. 5.3.3 For column footings, spot ramming is recommended; for projects requiring greater reinforcement depth, spot ramming can be carried out in more than two passes depending on the thickness of the soil layer, with a full ramming pass being performed in the last pass. 5.3.4 For sandy soils or gravel-filled foundations as well as soil-with-gravel fill materials, the continuous ramming method can be used to determine the locations for ramming. 5.4 Determination of the number of ramming blows The number of ramming blows is determined based on the specified criteria that must be met after the foundation has been strengthened. Ramming requires maximum vertical compression and minimum lateral movement. 5.4.l For unsaturated soil or fill, the number of ramming blows per point can be taken as the average of the sums of the settlements from the last two blows, provided that this value is less than 4 cm. 5.4.2 For saturated cohesive soil, the pore water pressure should reach its maximum value, which is equal to the weight of the soil itself: Ut = 9.80665×10⁻²γ·ho (8) Where: Ut – pore water pressure (kPa) ； γ —— Unit weight of soil (g/cm3) ； ho——thickness of the overlying soil layer (cm) ； Or liquefaction may occur to control the number of ramming blows. 5.5 Number of ramming passes: The principle for determining the number of ramming passes is based on the thickness of the compressed layer, soil conditions, and the required settlement amount. The thicker the soil compression layer, the finer the soil particles, and the higher the moisture content, the more ramming passes are required ； When the ramming settlement reaches 80–90% of the calculated final settlement, it is considered that ramming is complete. 6 Construction Techniques 6.1 Construction procedure for dynamic compaction (see Figure 3) Construction preparation – Survey prior to compaction; Construction plan 1; Inspection after compaction I; Experimental construction; Construction plan 1; Formal construction; Inspection after compaction II; Project acceptance; Results; Additional compaction; Construction inspection; Trial compaction (to verify various design parameters and determine the relevant construction parameters accordingly); (Design plan) 6.2 Precautions for compaction work 6.2.1 Mark out the compaction points strictly according to the design specifications and carry out compaction ； Construction shall be carried out in accordance with the parameters determined through trial ramming, such as the number of ramming blows, interval time, number of passes, hammer weight, and drop height. 6.2.2 If a square hammer is used, manual guidance shall be provided. 6.2.3 During full ramming, the position of the crane should be adjusted continuously to prevent the formation of a fan-shaped area centered on the crane, which could affect the effectiveness of the ramming. 6.3 Safety precautions during construction 6.3.1 In dynamic compaction operations, the ram is automatically disengaged; therefore, high levels of concentration and unified command are essential at the construction site, with no room for chaos. The entire site must be under the full responsibility of a crane operator. 6.3.2 Both crane operators and bulldozer operators must concentrate on following the instructions of the lifting supervisor and must not act recklessly; once the ram is lifted, no one is allowed to pass under the crane boom. 6.3.3 Crane operators must strictly adhere to safety operating procedures. 6.3.4 If the ram does not disengage automatically even after rising to a height exceeding that at which disengagement occurs, the lifting supervisor shall immediately issue a stop signal and lower the ram in order to identify the cause and take corrective action. 6.3.5 Before ramming, attention should be paid to the impact of surrounding high-voltage lines on the crane and cables; a certain distance should also be maintained from nearby buildings, structures, underground ducts or pipelines in industrial areas, utility poles, etc. An investigation should be carried out in conjunction with the design department to jointly determine the scope of dynamic compaction reinforcement. Vibration isolation measures should be taken for buildings adjacent to precision electronic equipment in order to eliminate the hazards caused by vibrations. 6.3.6 The ramming site should be appropriately watered as appropriate to prevent stones and water from splashing around and dust from rising; a wire mesh protective screen should be installed in front of the crane operator’s windshield. 7. Detection of dynamic compaction reinforcement effects: After dynamic compaction, it is necessary to assess the reinforcement effects of the foundation. Testing methods such as static cone penetration testing, load plate testing, SPT, vane shear testing, and side pressure testing can be employed, with generally at least one such test being carried out. 8 Engineering Examples 8.1 The new casting workshop and office building at Tianjin Masteel Plant were constructed on former pond and low-lying areas, where the surface layer of mixed fill soil was 3–7 meters thick; this soil consisted mainly of debris from demolished buildings, household waste, slag, iron shavings, and small iron pieces. The total area of the house foundations that have been strengthened through dynamic compaction is 5,686 m2. During construction, a W-1001 type crawler crane is used. The ram is constructed with a shell made of 20 mm thick steel plate; the interior of the ram is divided into nine compartments using steel plates, which are filled with steel sand. The weight of the ram is 9.4 tons, the dropping height is 11.2 meters, and the impact energy per strike is 1032 kN·m. First, carry out ramming until full, with 3–4 blows per point ； Then, ramming is carried out continuously along the width of the foundation trench in the building’s foundation section, with a spacing of 3.3 m between each ramming point; 8 to 10 ramming blows are given per point, such that the average of the settlement amounts from the last two blows is ≤ 4 to 6 cm. For this construction project, both dynamic and static instruments from the Wuhan Institute of Geotechnics, Chinese Academy of Sciences, were used to measure the pore water pressure ； Four Type 65 micro-vibrators were used for wave velocity measurement, and the K30 foundation bearing capacity testing vehicle developed by the China Academy of Railway Sciences was also employed for testing. After dynamic compaction, the bearing capacity of the foundation increased by 67%. More importantly, the compacted fill foundation exhibits good uniformity, resolves the issue of negative frictional resistance, and meets the strength and deformation requirements for house foundation design. The project has been in operation for 5 years, with good performance. This project saved an investment of about 400,000 yuan. 8.2 The dynamic compaction method is used to reinforce soft and slightly clayey soil foundations, eliminating liquefaction. The Sino-Hungarian joint broiler farm is located in the western suburbs of Tianjin, on saline-alkali wasteland. The 8 silos to be constructed are 22 m high, with a total weight of 900 t per silo. Foundation reinforcement requirements: to increase bearing capacity and meet the requirement that the foundation not liquefy in earthquakes of magnitude 8. The saturated silty clay layer at depths of 0–4 m beneath the ground surface in this site constitutes a severely liquefiable foundation. The bearing capacity of all strata from 0 to 8m is very low. In dynamic compaction construction, a bulldozer is first used to remove a 60-cm-thick layer of surface soil, followed by spot compaction and full compaction operations. The rammer weighs 9 tons, the dropping height is 10.5 m, and the spacing between rams is 3.4 m. One pass of ramming is carried out, with 8 strikes per point; for full ramming, 3 strikes are given per point. Evaluation of the reinforced foundation: 8.2.l Foundation bearing capacity 8.2.1.1 For the layer of soft sub-clay at a depth of 0–4m, the value of [R] was around 100 kPa before dynamic compaction; after reinforcement via dynamic compaction, this value increased to 240 kPa ； 8.2.1.2 For clay layers with a thickness of 4.1–6.8 m, [R] before dynamic compaction = 110 kPa, and [R] after dynamic compaction = 150 kPa. 8.2.2 Seismic liquefaction issue: After dynamic compaction, and through testing as well as application of the formulas specified in relevant standards, the foundation is considered non-liquefiable under earthquakes of intensity 8. The project was constructed by the Fifth Company of China State Construction Sixth Engineering Bureau.