Work machine

1. A work machine comprising:

a work implement having a boom, an arm and a bucket;

an operation device that gives an instruction to operate the work implement;

a controller that outputs an operation command to control operation of the work implement in accordance with an operation signal outputted from the operation device;

a construction-target determining device that outputs a designed surface; and

a bucket-position measuring device that measures a bucket-claw-tip position of the bucket, wherein

the work machine includes a load measuring device that measures a drive load of the work implement, and

the controller is configured to:

perform, in order for the bucket to excavate a ground according to the designed surface inputted from the construction-target determining device, a semi-automatic control by calculating a bucket-claw-tip target position and correcting the operation command, such that a velocity of the bucket is restricted more as a deviation between the designed surface and the bucket-claw-tip position measured by the bucket-position measuring device decreases;

acquire, when an excavation work is performed with the bucket, soil-nature information on a basis of the operation command, the bucket-claw-tip position outputted from the bucket-position measuring device, and the drive load outputted from the load measuring device, the soil nature information including a ground-surface height and a unit load which is a load per unit excavation depth;

generate a soil-nature map on a basis of the bucket-claw-tip position and the soil-nature information, the soil-nature map including a height map which indicates a relationship between the bucket-claw-tip position and the ground-surface height and a load map which indicates a relationship between the bucket-claw-tip position and the unit load;

calculate, when the semi-automatic control is performed, an estimated load that is an estimate of an excavation load on a basis of the height map of the soil-nature map, the load map of the soil-nature map, and the bucket-claw-tip target position; and

correct the operation command such that excavation force of the work implement increases as the estimated load increases.

2. The work machine according to claim 1 , wherein

the controller is further configured to:

calculate the ground-surface height at a bucket-claw-tip front-rear/left-right position included in the bucket-claw-tip position on a basis of the height map of the soil-nature map and the bucket-claw-tip front-rear/left-right position;

calculate an excavation depth by subtracting a bucket-claw-tip height included in the bucket-claw-tip position from the ground-surface height;

calculate the excavation load on a basis of the operation command and the drive load; and

calculate the unit load by dividing the excavation load by the excavation depth.

3. The work machine according to claim 1 , wherein

the controller is further configured to:

calculate the ground-surface height at a bucket-claw-tip front-rear/left-right position included in the bucket-claw-tip position on a basis of the height map of the soil-nature map and the bucket-claw-tip front-rear/left-right position; and

update the soil-nature map in a case where a bucket-claw-tip height included in the bucket-claw-tip position falls below the ground-surface height.

4. The work machine according to claim 1 , wherein

the controller is further configured to:

set an initial value of a of the ground-surface height included in the height map of the soil-nature map to a value larger than a maximum reachable height of the work implement; and

set an initial value of the unit load included in the load map of the soil-nature map to a value equal to or smaller than zero.

5. The work machine according to claim 1 , wherein

the controller is further configured to:

calculate the unit load at a bucket-claw-tip target front-rear/left-right position included in the bucket-claw-tip target position on a basis of the load map of the soil-nature map and the bucket-claw-tip target front-rear/left-right position;

calculate a ground-surface height at the bucket-claw-tip target front-rear/left-right position on a basis of the height map of the soil-nature map and the bucket-claw-tip target front-rear/left-right position;

calculate an excavation depth by subtracting a bucket-claw-tip target height included in the bucket-claw-tip target position from the ground-surface height; and

calculate the estimated load by multiplying the unit load and the excavation depth.

6. The work machine according to claim 1 , wherein

the controller is further configured to correct a boom operation command included in the operation command so as to actuate the boom to a boom-lowering side as the estimated load increases.

7. The work machine according to claim 1 , further comprising:

a display device, wherein

the controller is further configured to:

generate height-map display information on a basis of the height map of the soil-nature map and the bucket-claw-tip position;

generate load-map display information on a basis of the load map of the soil-nature map and a bucket-claw-tip front-rear/left-right position included in the bucket-claw-tip position; and

output the height-map display information and the load-map display information to the display device.