DETERMINING MODULUS OF SUB-GRADE REACTION (K VALUE)

Transcription

1 Test Procedure for DETERMINING MODULUS OF SUB-GRADE REACTION (K VALUE) TxDOT Designation: Tex-125-E Effective Date: August SCOPE 1.1 This method evaluates the stiffness of subgrade soils in either the compacted condition or the natural state. 1.2 The data developed in the static load test is used in the calculation of the modulus of subgrade reaction in Section 6.1, which is a modification of ASTM D Data from the triaxial compression test on compacted specimens and the estimated Potential Vertical Rise (PVR) are used in estimating a modulus of subgrade reaction from Figure 1 as outlined in Section The values given in parentheses (if provided) are not standard and may not be exact mathematical conversions. Use each system of units separately. Combining values from the two systems may result in nonconformance with the standard. CONSTRUCTION DIVISION 1 5 LAST REVIEWED: SEPTEMBER 2014

2 Figure 1 A Graphic Presentation of K Values for all Compactable Sub-Grades 2. APPARATUS 2.1 Loading device, such as a truck, trailer, tractor-trailer, an anchored frame, or other structures loaded with sufficient weight to produce the desired reaction on the surface under test. The supporting points (wheels, in the case of a truck or trailer) should be at least 2.4 m (8 ft.) from the circumference of the largest diameter bearing plate being used. 2.2 Hydraulic jack assembly, with a spherical bearing attachment, capable of applying and releasing the load in increments. The jack should have sufficient capacity for applying the maximum load required and shall be equipped with an accurately calibrated gauge that will indicate the magnitude of the applied load. 2.3 Two or more dial gauges, graduated in units of 0.03 mm (0.001 in.), capable of recording a maximum settlement of 25.4 mm (1 in.) 2.4 Deflection beam, upon which should be mounted: dial gauges; a 63.5 mm (2.5 in.) standard black pipe; and a mm (3 3 1/4 in.) steel angle, or equivalent, at least 4.9 m (16 ft.) long, and resting upon supports located at least 2.4 m (8 ft.) from the bearing plate or the nearest wheel or supporting leg CONSTRUCTION DIVISION 2 5 LAST REVIEWED: SEPTEMBER 2014

3 2.5 Miscellaneous tools, including a thermometer and spirit level. 2.6 Set of circular steel bearing plates, not less than 25.4 mm (1 in.) in thickness, machined so they can be arranged in pyramid fashion to ensure rigidity, with diameters ranging from mm (6 30 in.) Note 1 A minimum of four different plate sizes is recommended for pavement design or evaluation purposes. For evaluation purposes alone, a single plate may be used, provided its area is equal to the tire contact area corresponding to what may be considered as the most critical combination of conditions of wheel load and tire pressure. For providing data indicative of bearing index (for example, the determination of relative subgrade support throughout a period of a year), a single plate of any selected size may be used. 3. PROCEDURE 3.1 Carefully center a bearing plate of the selected diameter under the jack assembly Set the remaining plates of smaller diameter concentric with, and on top of the bearing plate Set the bearing plate level in a thin bed of a mixture of sand and plaster of Paris, of plaster of Paris alone, or of fine sand, using the least quantity of materials required for uniform bearing. 3.2 Where unconfined load tests are to be made at a depth below the surface, remove the surrounding material to provide a clearance equal to one and one-half bearing plate diameters from the edge of the bearing plate For confined tests, the diameter of the excavated circular area shall be just sufficient to accommodate the selected bearing plate. 3.3 Place a sufficient number of dial gauges, located and fixed in position to indicate the average settlement of the bearing plate. 3.4 After the equipment has been properly arranged with all of the dead load (jack, plates, etc.) in place, quickly seat the bearing plate and assembly and release a load sufficient to produce a settlement of not less than 0.3 mm (0.01 in.) or more than 0.5 mm (0.02 in.), as indicated by the dials When the dial needles come to rest following release of the load, reseat the plate by applying half of the recorded load that produced the mm ( in.) settlement When the dial needles have again come to rest, set each dial accurately at its zero mark. 3.5 Apply loads at a moderately rapid rate in uniform increments. The magnitude of each increment should be small enough to permit the recording of a sufficient number of loadsettlement points (not less than six) to produce an accurate load-settlement curve. CONSTRUCTION DIVISION 3 5 LAST REVIEWED: SEPTEMBER 2014

4 3.5.1 After each increment of load, wait until a rate of settlement of not more than 0.03 mm (0.001 in.) per minute has been maintained for three consecutive minutes Record load and settlement readings for each load increment Continue this procedure until the selected total settlement has been obtained, or until the load capacity of the apparatus has been reached, whichever occurs first. t this point, maintain the load until an increased settlement of not more than 0.03 mm (0.001 in.) per minute for three consecutive minutes occurs. Record the total settlement Release the load to the load at which the dial gauges were set at zero. Maintain this zerosetting load until the rate of recovery does not exceed 0.03 mm (0.001 in.) per minute for three consecutive minutes Record settlement at the zero-setting load From a thermometer suspended near the bearing plate, read and record the air temperature at half-hour intervals. 4. TEST RECORD 4.1 In addition to the continuous listing of all load, settlement, and temperature data as prescribed under Section 3, make record of all associated conditions and observations pertaining to the test, including the following: date list of personnel weather conditions time of beginning and completion of test any irregularity in routine procedure any unusual observations made during the test any unusual conditions observed at the test site. 5. CALCULATION AND PLOTTING OF LOAD-SETTLEMENT RELATIONSHIPS 5.1 Plot the load intensity in kpa (psi) for each increment against the corresponding settlement in mm (in.) (See Figure 2.) 5.2 Plot the recovery after full release of load. 5.3 Correction should be made for the zero settlement point, taking into account the dead weight of the equipment and the seating load. 5.4 From this graph, the relation of load and total settlement for that load and the relation of elastic and permanent settlement for the maximum load used may be obtained. CONSTRUCTION DIVISION 4 5 LAST REVIEWED: SEPTEMBER 2014

5 Figure 2 Field Loading Bearing Test 6. MODULUS OF SUBGRADE REACTION 6.1 Method 1 On-Site Static Load Test: The as-tested modulus of subgrade reaction is calculated from the load settlement curve at a convenient point, usually 1.3 mm (0.05 in) deflection from the relation The load intensity in kpa (psi) is divided by mm (in.) of settlement (deflection). This modulus of subgrade reaction K is in kpa/mm (psi/in.), usually designated kg/m 3 (pcf). 6.2 Method 2 Laboratory Static Load Test: To determine the subgrade reaction when triaxial class (Tex-117-E) and PVR (Tex-124-E) are known: First, enter subgrade strength class at the bottom of the graph represented in Figure Then, go up to the curve of PVR, and go along the horizontal line to the thickness of subbase, then go up to the slanting K value of the first horizontal line Read the subgrade reaction ( K value) on the horizontal line at the left axis. CONSTRUCTION DIVISION 5 5 LAST REVIEWED: SEPTEMBER 2014